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In vitro activity of grepafloxacin and 25 other antimicrobial agents against Streptococcus pneumoniae: correlation with penicillin resistance
CLINICAL THERAPEUTICS®/VOL.20, NO. 6, 1998
In Vitro Activity of Grepafloxacin and 25 Other Antimicrobial Agents Against Streptococcuspneumoniae: Correlation with Penicillin Resistance Clyde Thornsberry, PhD, 1 Penny T. Ogilvie, MS, 1,z H. Preston Holley, Jr., MD, 3 and Daniel F. Sahm, PhD 4 1MRL Pharmaceutical Services, Brentwood, Tennessee, 2Home Healthcare Laboratories of America, Franklin, Tennessee, ZGlax~ Wellcome Inc., Research Triangle Park, North Carolina, and 4MRL Pharmaceutical Services, Herndon, Virginia
ABSTRACT Strains of Streptococcus pneumoniae from the United States that were susceptible, intermediately resistant, or highly resistant to penicillin were tested for susceptibility to 26 anfimicrobial agents that have been used or considered for the treatment of patients with pneumococcal infections. The drugs tested included penicillins, one penicillin/beta-lactamase inhibitor combination, cephalosporins, macrolides, a lincosamide, fluoroquinolones, and four miscellaneous drugs (vancomycin, rifampin, tetracycline, and trimethoprimsulfamethoxazole). The activities of the penicillins and macrolide agents were similar, but the activities within the cephalosporin and fluoroquinolone classes were often dissimilar. For the fluoroquinolones, the order of in vitro activity, from most to least active, was grepafloxa0149-2918/98/$19.00
cin, sparfloxacin, levofloxacin, ciprofloxacin, and ofloxacin. Increased resistance to penicillin in the pneumococcal isolates studied correlated with increased resistance to other penicillins, cephalosporins, macrolides, clindamycin, tetracycline, and trimethoprimsulfamethoxazole but did not correlate with increased resistance to the fluoroquinolones, rifampin, or vancomycin. These findings may be helpful to health professionals selecting empiric therapy for respiratory tract infections involving S pneumoniae. Key words: pneumococci, antibiotic resistance, in vitro susceptibility, grepafloxacin, penicillin. INTRODUCTION
Streptococcus pneumoniae is a major pathogen in a variety of infections, particularly those affecting the respiratory tract, and penicillins have been the therapy of choice for these infections for many 1 179
CLINICAL THERAPEUTICS®
decades. 1 Although penicillin resistance was reported in S pneumoniae as early as the 1960s, the prevalence of penicillinresistant pneumococci in the United States remained below 5% through the 1980s. 2,3 In the 1990s, however, the proportion of penicillin-resistant isolates of S pneumoniae increased greatly and became a major clinical concern. In the winter of 1996-1997, a US surveillance study of pneumococcal isolates 4 determined that high-level resistance to penicillin (minimum inhibitory concentration [MIC], >2 p,g/mL) was observed in 14% of clinical isolates and that intermediate resistance (MIC, 0.12 to 1 ~g/mL) was detected in 20%. This study also demonstrated that penicillin resistance was correlated with nonsusceptibility to cephalosporins and macrolides but not to fluoroquinolones. The goals of the present study were to examine the in vitro activity of 26 antimicrobial agents against strains of S pneumoniae that were susceptible, intermediately resistant, or resistant to penicillin and to assess the degree of correlation of penicillin resistance with resistance to other antimicrobial agents.
MATERIALS AND METHODS
Bacteria We selected 369 recent (1995-1997) strains of S pneumoniae that had been isolated from patients treated at health care institutions in all geographic regions of the United States. We were careful to select strains that represented susceptible, intermediately resistant, and resistant categories, but the number of isolates in each category did not reflect the relative prevalence of these categories in the United 1180
States. Of the 369 strains, 121 (32.8%) were susceptible to penicillin (MIC, <0.06 ~g/mL), 150 (40.7%) had intermediate resistance to penicillin (MIC, 0.12 to 1 I~g/mL), and 98 (26.6%) had high-level resistance to penicillin (MIC, >2 ixg/mL). The isolates studied had been identified by standard methods and tested for susceptibility to penicillin by broth microdilution using the method recommended by the National Committee for Clinical Laboratory Standards (NCCLS). 5,6All isolates had been stored at -75 °C or colder before testing. When tests were scheduled to be performed, the frozen isolates were thawed and subcultured to blood agar once or twice to obtain sufficient growth, and these subcultures were used for susceptibility testing.
Antimicrobial Agents Representing both oral and parenteral formulations, the 26 anfimicrobial agents chosen for this study are either used currently or have been considered for the treatment of patients with pneumococcal infections. The panel of anfimicrobial agents tested included 2 penicillins (penicillin and amoxicillin), 1 penicillin/beta-lactamase inhibitor combination (amoxicillin/clavulanate), 10 oral and parenteral cephalosporins (cephalothin, cefaclor, loracarbef, cefuroxime, cefprozil, cefotaxime, ceftriaxone, cefpodoxime, cefixime, and ceftibuten), 3 macrolides (erythromycin, clarithromycin, and azithromycin), 1 lincosamide (clindamycin), 5 fluoroquinolones (ciprofloxacin, ofloxacin, levofloxacin, sparfloxacin, and grepafloxacin), and 4 miscellaneous drugs (vancomycin, rifampin, tetracycline, and trimethoprim-sulfamethoxazole). These antimicrobial agents were obtained from their respective manufacturers in the form of powders suitable for susceptibility testing.
C. THORNSBERRYET AL.
Susceptibility Testing The MIC, which is the lowest concentration of an antimicrobial agent that inhibits visible bacterial growth, was determined for each of the 26 antimicrobial agents using broth microdilution as recommended by the NCCLS. 6 The medium was cation-adjusted Mueller-Hinton broth supplemented with 2% to 5% lysed horse blood. The susceptibility test trays were prepared by Microtech Medical Systems (Aurora, Colorado). The final inocula were approximately 5 x 105 colony-forming units/mL. The inoculated trays were incubated in ambient air for 20 to 24 hours at 35 °C. For quality control purposes, the reference strain S pneumoniae American Type Culture Collection 49619 was included in each test. All antimicrobial agents were tested simultaneously for each isolate. RESULTS Table I shows the MIC data for the study strains stratified according to NCCLS category of penicillin susceptibility (susceptible, intermediately resistant, or resistant). The activity of many of the antimicrobial agents was correlated with the activity of penicillin (Tables I and II). Penicillinsusceptible isolates of S pneumoniae tended to be susceptible to other antimicrobial agents, but strains with intermediate penicillin resistance and highly resistant strains were likely to show resistance to many other antimicrobial agents, a tendency that appeared to be associated with the level of penicillin resistance. For example, for cephalothin, penicillin-susceptible strains had an MIC50 (50% of isolates inhibited at that concentration) of 0.12 Ixg/mL and an MIC90 (90% of isolates in-
hibited at that concentration) of 0.5 wg/mL, but penicillin-resistant isolates had an MIC50 of 8 I~g/mL and an MIC9o of 32 I~g/mL (Table I). The MIC50 and MIC90 for penicillin-intermediate strains were 1 and 8 i~g/mL, respectively. This relationship was typical of other penicillins, the cephalosporins, the macrolides, clindamycin, tetracycline, and trimethoprimsulfamethoxazole but not of the fluoroquinolones, vancomycin, or rifampin (Table II). Of the fluoroquinolones, the MICs0 and MIC90 for grepafloxacin were 0.12 and 0.25 I~g/mL, respectively, for all three categories of penicillin susceptibility, signifying that all isolates were susceptible to grepafloxacin (MICs <0.5 I.~g/mL). For those drugs for which resistance was associated with penicillin resistance and for which an increase in MIC90 could be calculated from discrete data (ie, using no < or > values), the MIC90s of penicillinresistant strains were 5 to 7 log2 concentrations higher than those of the penicillinsusceptible strains. For example, the MIC90 of cefotaxime increased from 0.12 Ixg/mL for penicillin-susceptible strains to 4 ~g/mL for penicillin-resistant strains; the MIC90 for cefprozil increased from 0.25 wg/mL to 32 I~g/mL. The MICs for the three penicillin compounds (penicillin, amoxicillin, and amoxicillin/clavulanate) were within ±1 log2 concentration (Table I). The MICs for the cephalosporins, however, were often dissimilar (Tables I and III). The most active cephalosporins were the two third-generation parenteral compounds, cefotaxime and ceftriaxone, which had MIC9os of 4 Ixg/mL for penicillin-resistant strains. For strains that were not susceptible to penicillin (MIC _>0.12 I~g/mL), the MIC90s for several of the cephalosporins (cefaclor, loracarbef, cefixime, and ceftibuten) were greater than 1181
CLINICAL THERAPEUTICS*
Table I. S u s c e p t i b i l i t y o f s e l e c t e d isolates o f
Streptococcuspneumoniae to
26 a n t i m i c r o -
bial agents.** % of Isolates Antimicrobial Agent
No. of Isolates
MIC (p~g/mL) Range
Penicillin All 369 0.008-8 Pen s 121 0.008--0.06 Pen I 150 0.12-1 Pen R 98 2-8 Amoxicillin All 369 <0.03-16 Pen s 121 <0.03-2 Pen I 150 <0.03-2 Pen g 98 <0.03-16 Amoxicillin/clavulanate All 369 <0.03-8 Pen s 121 <0.03-2 Pen ~ 150 <0.03-4 Pen R 98 <0.03-8 Cephalothin All 369 0.06-64 Pen s 121 0.06-2 Pen I 150 0.25-16 Pen R 98 2-64 Cefaclor All 369 0.12->32 Pen s 121 0.12-4 Pen I 150 0.12->32 Pen R 98 0.5->32 Loracarbef All 369 0.25->32 Pen s 121 0.25->32 Pen x 150 0.25->32 Pen R 98 0.5->32 Cefuroxime All 369 0.03->32 Pen s 121 0.03->32 Pen I 150 0.06-32 Pen R 98 0.06-->32 Cefprozil All 369 0.03->32 Pen s 121 0.06-8
MIC50
Intermediately MIC9o
Susceptible
Resistant
Resistant
0.25 0.015 0.5 2
2 0.06 1 4
32.8 100 0.0 0.0
40.7 0.0 100 0.0
26.5 0.0 0.0 100
0.25 <0.03 0.25 1
2 0.06 1 4
77.0 97.5 89.3 32.7
9.5 1.7 9.3 19.4
13.5 0.8 1.4 47.9
0.25 <0.03 0.25 1
2 0.06 1 8
60.4 96.7 70.0 1.0
25.8 2.5 28.7 50.0
13.8 0.8 1.3 49.0
1 0.12 1 8
16 0.5 8 32
--* ----
2 0.25 2 >32
>32 1 >32 >32
----* ----
4 0.5 4 >32
>32 2 >32 >32
-----* ----
0.5 0.03 1 4
8 0.25 4 32
--~ ----
0.5 0.12
16 0.25
--* --
B
m
m
B
m
m
m
m
m
m
N
m
m
m
m
m
n
m
m
m
m
m
m
m
Continued
1182
C. THORNSBERRY ET AL.
Table I. ( c o n t i n u e d ) % of Isolates Antimicrobial No. of Agent Isolates
MIC (l~g/mL) Range
MICs0
Intermediately MIC9o
Susceptible
Resistant
Resistant
Cefprozil (continued) Pen I Pen R Cefotaxime All Pen s Pen I Pen R Ceftriaxone All Pen s Pen I Pen R Cefpodoxime All Pen s Pen I Pen R Cefixime All Pen s Pen I Pen R Ceftibuten All Pen s Pen I Pen R Erythromycin All Pen s Pen I Pen R Clarithromycin All Pen s Pen I Pen R
150 98
0.03-32 0.06->32
0.5 8
8 32
---
369 121 150 98
<0.015-8 <0.015-1 <0.015-4 0.5-8
0.25 <0.015 0.25 1
2 0.12 1 4
--~ ----
369 121 150 98
<0.015-8 <0.015-1 <0.015-4 0.03-8
0.25 0.03 0.25 1
2 0.12 I 4
--~ ----
369 121 150 98
<0.015->32 <0.015-2 0.03->32 _<0.015->32
0.5 0.03 0.5 2
4 0.12 2 16
--~ ----
369 121 150 98
_<0.25-->16 4 <0.25->16 -<0.25 -<0.25-2_16 8 <0.25-2_16 ->16
_>16 1 ->16 ->16
--~ ----
369 121 150 98
-<0.015--2_16 ->16 <0.25-->16 4 0.5-->16 _>16 2-_>16 _>16
_>16 _>16 _>16 _>16
--* ----
369 121 150 98
_<0.03->64 _<0.03->64 <0.03->64 -<0.03->64
0.06 _<0.03 0.06 4
64 2 >64 >64
60.2 86.0 5%3 32.7
0.8 0.0 2.0 0.0
39.0 14.0 40.7 67.3
369 121 150 98
_<0.03->64 -<0.03->64 _<0.03->64 -<0.03->64
0.06 _<0.03 0.06 4
16 1 64 >64
60.7 86.8 58.7 31.6
1.6 0.0 1.3 4.1
37.7 13.2 40.0 64.3
m
m
q
m
B
m
m
m
m
B
q
h
B
m
B
m
B
m
B
m
b
m
m
B
m
m
Continued
1183
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Table I. (contimgxl) % of Isolates Antimicrobial No. of Agent Isolates
MIC (Ixg/mL) Range MlCso MIC9o
Intermediately Susceptible Resistant Resistant
Azithromycin All Pens Pen ~ PeP
Clindamycin All Pen s
Pen~ Pen R Ciprofloxaein All Pen s Pen I Pen s
Ofloxacin All Pens PenI Pen R Levofloxacin All Pen S Pen I Pen R Sparfloxacin All Pen s PenI Pen R Grepafloxacin All Pen s Pen I Pen s
Vancomycin All Pen s PenI Pen R
369 121 150 98
<0.03->64 0.06->64 <0.03->64 0.06->64
0.12 0.12 0.25 8
64 2 >64 >64
60.4 86.0 58.0 32.7
1.6 1.7 1.3 2.0
38.0 12.3 40.7 65.3
369 121 150 98
<0.015->32 0.03->32 0.03->32 ~0.015->32
0.06 0.06 0.06 0.06
2 0.06 32 >32
88.1 97.5 85.3 80.6
0.3 0.0 0.7 0.0
11.6 2.5 14.0 19.4
369 121 150 98
0.03->8 0.09>8 0.1~8 0.124
0.5 0.5 0.5 ~5
1 1 2 1
369 121 150 98
0.5->8 0.~8 0.5->8 0.54
1 1 1 1
2 2 2 2
98.4 98.4 98.0 99.0
1.1 0.8 1.3 1.0
0.5 0.8 0.7 0.0
369 121 150 98
0.25->8 0.54 0.25->8 0.25-2
0.5 0.5 0.5 0.5
1 1 1 1
99.5 ~.2 ~.3 1~
0.3 0.8 0.0 0.0
0.2 0.0 0.7 0.0
369 121 150 98
0.06-2 0.1~1 0.06-1 0.12-2
0.25 0.25 0.25 0.25
0.25 0.5 0.25 0.25
98.1 100 98.2 97.8
1.6 0.0 1.8 1.5
0.3 0.0 0.0 0.7
369 121 150 98
0.034.5 0.06--0.5 0.06--0.5 0.034.5
0.12 0.12 0.12 0.12
0.25 0.25 0.25 0.25
100 100 100 100
0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0
369 121 150 98
0.1~1 0.1~1 0.254.5 0.254.5
0.25 0.25 0.25 0.25
0.5 0.5 0.5 0.5
100 100 100 100
__§
m
m
m
m
B
B
Continued 1184
C. THORNSBERRY ET AL.
Table I. (continued) %ofIsolates Antimicrobial No. of Agent Isolates
MIC (wg/mL) Range MICs0 MIC90
Susceptible
Intermediately Resistant Resistant
Rifampin All 369 _<0.015->16 Pen s 121 ~0.015->16 Pen I 150 _<0.015->16 Pen R 98 ~0.015-0.25 Te~acycline All 369 _<0.5->16 Pen s 121 _<0.5->16 Pe~ 150 _<0.5->16 Pen R 98 ~0.5->16 Tfimethopfim-sulfamethoxazole All 369 ~0.12->4 Pen s 121 g0.12->4 Pen I 150 ~0.12->4 Pen R 98 _<0.12->4
0.03 0.03 0.03 0.03
0.03 0.03 0.03 0.03
98.6 99.2 97.3 100
0.3 0.0 0.7 0.0
1.1 0.8 2.0 0.0
~0.5 A0.5 ~0.5 _<0.5
>16 1 >16 >16
74.0 92.6 68.7 59.2
1.6 0.8 0.7 4.1
24.4 6.6 30.6 36.7
1 0.25 1 4
>4 1 >4 >4
48.2 85.1 40.7 14.3
15.2 7.4 19.3 18.4
36.6 7.5 40.0 67.3
MIC = minimum inhibitory concentration; MICs0= MIC at which 50% of isolates are inhibited; MIC90 = MIC at which 90% of isolates are inhibited. *National Committee for Clinical Laboratory Standards (NCCLS) breakpoints were used. *Data are presented for all isolates and for isolates that were penicillin susceptible (PenS), of intermediate penicillin resistance (PenI), and penicillin resistant (PenR). *Most of the cephalosporins and ciprofloxacin do not have NCCLS-recommendedinterpretive breakpoints. We compared the cephalosporins by MICs0s and MICgos and by cumulative distribution of MICs (see Table 1~). §The breakpoints for vancomycin do not recognize intermediately resistant or resistant strains.
the highest concentration tested. The MIC distribution data (Table HI) indicate that the activities of the cephalosporins varied, and that cefpodoxime and cefuroxime were the most active members of this antimicrobial class. These two agents were also more active than the other oral cephalosporins against strains with intermediate resistance to penicillin (Table I). The activities o f the three macrolides (erythromycin, clarithromycin, and azithromycin) are shown in Tables I and IV. A l t h o u g h distribution within the categories o f susceptibility was quite similar
for the three macrolides, the MIC90 for c l a r i t h r o m y c i n was 16 Ixg/mL but 64 ixg/mL for erythromycin and azithromycin. As shown in Table IV, the difference in MIC90s was due to very small differences in the MICs. In addition, the MIC90s for all three macrolides were _>64 v~g/mL for isolates with intermediate resistance and high resistance to penicillin. Increased penicillin resistance in S pneumoniae was c o r r e l a t e d with increased resistance to these macrolides, but even the penicillinsusceptible strains had 12% to 14% resistance to the three drugs.
1185
CLINICAL THERAPEUTICS®
Table II. Correlation of penicillin resistance in Streptococcuspneumoniae with resistance to other antimicrobial agents. Correlated* Amoxicillin Amoxicillin/clavulanate Cefaclor Cefixime Cefotaxime Cefpodoxime Cefprozil Ceftibuten Ceftriaxone Cefuroxime Cephalothin Loracarbef Azithromycin Clarithromycin Erythromycin Clindamycin Trimethoprim-sulfamethoxazole Tetracycline
Not correlatedt Ciprofloxacin Grepafloxacin Levofloxacin Ofloxacin Sparfloxacin Rifampin Vancomycin
*Strainswith increasedresistanceto penicillingenerallyhave increasedresistanceto these antimicrobialagents. tNo correlationwith increasedpenicillinresistancewas observedfor these antimicrobialagents. The correlation with penicillin resistance was also evident in the activity of clindamycin, with MIC90s of 32 and >32 ~g/mL for isolates with intermediate resistance and high resistance to penicillin, respectively, compared with an MIC90 of 0.06 p~g/mL for penicillin-susceptible strains. The correlation between penicillin resistance and the activity of tetracycline was seen in MIC90s of >16 Ixg/mL for isolates with intermediate resistance and high resistance to penicillin, compared with an MIC90 of 1 Ixg/mL for penicillin-susceptible strains. For trimethoprim-sulfamethoxazole, the correlation with penicillin resistance was seen in both MICs0s and MIC90s. As shown by the MIC50 and MIC90 data (Table I) and the distribution of MICs 1186
(Table IV) for the fluoroquinolones, the in vitro activity was different for each drug in this class. The order of activity, from most to least active (as measured by MICs), was grepafloxacin, sparfloxacin, levofloxacin, ciprofloxacin, and ofloxacin. Following NCCLS-recommended breakpoints, it was found that resistance of S pneumoniae to fluoroquinolones was rare, with the exception of ciprofloxacin. Increased resistance to penicillin did not affect the activity of the five fluoroquinolones, vancomycin, or rifampin (Tables I and II), as indicated by MICs that were essentially the same for strains that were susceptible and resistant to penicillin. None of the isolates tested were resistant to vancomycin (all MICs <1 Ixg/mL) or grepafloxacin (all MICs <0.5 I~g/mL).
C. THORNSBERRY ET AL.
t",,I
o=
oO
II
Z
r~
t~
¢q
8. Q 0~0 t'xl t'q
t'xl ¢ q
¢.q
.=
u~u~uuuu 1187
CLINICAL THERAPEUTICS*
Table IV. Cumulative percentage of strains of Streptococcus pneumoniae (N = 369), by minimum inhibitory concentration (MIC) of fluoroquinolones and macrolides. Cumulative Percentage of Strains, by MIC Antimicrobial Agent Grepafloxacin Sparfloxacin Levofloxacin Ciprofioxacin Ofloxacin Erythromyein Azithromycin Clarithromycin
<0.03 0.06 0.12 0.25 1
16
78 26 2
33 44
57 6 58
59 50 59
0.5
1
2
97 100 90 98 1 62 15 66 4 60 61 60 60 61 62
100 97 92 76 63 62 65
99 97 98 67 64 72
4
8
16
32
64
>64
100" 100" 100" 78 85 70 79 81 88
89 86 91
89 89 91
90 90 92
100 100 100
*Percentageof strains equal to or greater than this MIC.
DISCUSSION AND C O N C L U S I O N S Strains of S pneumoniae having decreased susceptibility to penicillin were reported occasionally in the 1960s and 1970s but were not considered a clinical problem affecting the choice of penicillin as the preferred therapy for pneumococcal infections. 2 Although the number of strains with intermediate resistance to penicillin increased in the United States in the 1980s, their prevalence was <5%, which was not considered to be of public health significance. 3 In the 1990s, however, the prevalence of S pneumoniae strains that were not penicillin susceptible increased sharply. 4,7-9 In the early 1990s, most of the strains with diminished susceptibility to penicillin had an intermediate level of resistance; more recently, however, there have been significant increases in the prevalence of highly resistant strains. 4,8 Of S pneumoniae strains isolated from adults in the United States during the winter of 1996-1997, 66% were susceptible to penicillin, 20% had intermediate resis1188
tance to penicillin, and 14% had a high level of resistance to penicillin. 4 The findings from 1996--1997 suggested that highlevel resistance had more than doubled since the previous year. 9 The increase in penicillin resistance has created concern about the treatment of patients with pneumococcal infections) One of the findings of earlier studies was that some of the antimicrobial agents that might be selected to treat pneumococcal infections--for example, the penicillins, cephalosporins, and macrolides-were encountering increased resistance that correlated closely with increased penicillin resistance. 4,7-9 Penicillin-susceptible strains tended to be susceptible to these agents, but strains not susceptible to penicillin had various levels of resistance to these other agents. The fluoroquinolones and vancomycin were the exceptions to these correlations with penicillin resistance. 4 We extended these studies to include 26 antimicrobial agents that might be used to treat patients with pneumococcal infec-
C. THORNSBERRY ET AL.
tions. Our results show that isolates of S pneumoniae having increased resistance to penicillin may also have increased resistance to other penicillins, the cephalosporins, the macrolides, clindamycin, tetracycline, and the sulfonamides but not to the fluoroquinolones, vancomycin, and rifampin. Penicillin-resistant strains of S pneumoniae have altered penicillin-binding proteins that may directly affect the activity not only of penicillins but also of cephalosporins and thus can account for the cross-resistance observed between penicillins and cephalosporins in these strains, m In the case of the other classes of antimicrobial agents in which resistance is associated with penicillin resistance, penicillin-binding proteins are not linked to reduced activity. Instead, resistance to macrolides, clindamycin, tetracyclines, and sulfonamides is thought to be due to the acquisition of other resistance genes. For example, a common mechanism of resistance to macrolides and clindamycin is a mutation that results in a profile known as MLS resistance, because the ribosome is insensitive to all macrolides, lincosamides, and streptogramins. The gene encoding such resistance has also been identified on a transposon. 11 The acquisition of this and other transposons could result in multiple resistance, but only resistance to penicillins and cephalosporins would be related to altered penicillin-binding proteins. The reason penicillin-resistant strains may be more likely to acquire and maintain these genes is unknown, but the acquisition of additional resistances increases the likelihood of creating selective pressures that could result in more rapid dissemination of these multiply-resistant strains.
Because S pneumoniae is a leading cause of community-acquired respiratory infections and because the newer fluoroquinolones are active against penicillinresistant pneumococci, the newer fluoroquinolones have been advocated for therapy of S pneumoniae infections. 12Vancomycin has been advocated for the same reasons. Although rifampin resistance is not correlated with penicillin resistance in S pneumoniae, rifampin is not generally indicated for respiratory infections, because resistance may develop rapidly when it is used as a single therapeutic agent. Our data show that increased penicillin resistance in S pneumon/ae is correlated with increased resistance to other penicillins, the cephalosporins, the macrolides, clindamycin, tetracycline, and t r i m e ~ - s u l f a m e t h o x a zole but not with resistance to fluoroquinolones, vancomycin, and rifampin. No isolates were resistant to vancomycin or grepafloxacin. Based on MICs, the order of activity of the fluoroquinolones, from most to least active, was grepafloxacin, sparfloxacin, levofloxacin, ciprofloxacin, and ofloxacin. ACKNOWLEDGMENTS This study was supported by a grant from Glaxo Wellcome Inc., Research Triangle Park, North Carolina. Grepafloxacin is licensed by Glaxo Wellcome PLC from Otsuka Pharmaceutical Co., Ltd., Tokyo, Japan. The authors thank Geriann Piazza, MA, for editing assistance and Angela M. Feher for assistance in preparing drafts of the manuscript.
Address correspondence to: Clyde Thomsberry, Phi), MRL Pharmaceutical Services, 7003 Chadwick Drive, Suite 235, Brentwood, TN 37027. 1189
CLINICAL THERAPEUTICS*
REFERENCES 1. Klein JO. Selection of oral antimicrobial agents for otitis media and pharyngitis. Infect Dis Clin Pract. 1995;4(Suppl 2): $88-$94. 2. Hansman D, Bullen MM. A resistant pneumococcus. Lancet. 1967;2:264-265. Letter. 3. Spika JS, Facklam RR, Plikaytis BD, Oxtoby MJ. Antimicrobial resistance of Streptococcus pneumoniae in the United States. J Infect Dis. 1991;163:1273-1278. 4. Thornsberry C, Ogilvie P, Kahn J, et al. Surveillance of antimicrobial resistance in Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis in the United States in 1996-1997 respiratory season. Diagn Microbiol Infect Dis. 1997;29:249-257. 5. Baron EJ, Murray PR. Bacteriology. In: Murray PR, Baron EJ, Pfaller MA, et al, eds. Manual of Clinical Microbiology, 6th ed. Washington, DC: American Society for Microbiology; 1995:246--662.
6. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically. Document M7-A4. 4th ed. Villanova, Pa: National Committee for Clinical Laboratory Standards; 1997.
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7. Doern GV, Brueggemann A, Holley HP Jr, Rauch AM. Antimicrobial resistance of Streptococcus pneumoniae recovered from outpatients in the United States during the winter months of 1994 to 1995: Results of a 30-center national surveillance study. Antimicrob Agents Chemother. 1996;40:1208-1213. 8. Thornsberry C, Brown SD, Yee YC, et al. Increasing penicillin resistance in Streptococcus pneumoniae in the U.S. Infect Med. 1993; 12(Suppl):S 15-$24. 9. Thornsberry C, Burton PH, Vanderhoof BH. Activity of penicillin and three thirdgeneration cephalosporins against U.S. isolates of Streptococcus pneumoniae: A 1995 surveillance study. Diagn Microbiol Infect Dis. 1996;25:89-95. 10. Zighelboim S, Tomasz A. Penicillin-binding protein of multiply antibiotic-resistant South African strains of Streptococcus pneumoniae. Antimicrob Agents Chemother. 1980;17:434 4A.2. 11. Klugman KP. Pneumococcal resistance to antibiotics. Clin Microbiol Rev. 1990;3: 171-196. 12. Bartlett JG, Breiman RF, Mandell LA, File TM. Community-acquired pneumonia in adults: Guidelines for management. Clin Infect Dis. 1998;26:811-838.
In Vitro Activity of Grepafloxacin and 25 Other Antimicrobial Agents Against Streptococcuspneumoniae: Correlation with Penicillin Resistance Clyde Thornsberry, PhD, 1 Penny T. Ogilvie, MS, 1,z H. Preston Holley, Jr., MD, 3 and Daniel F. Sahm, PhD 4 1MRL Pharmaceutical Services, Brentwood, Tennessee, 2Home Healthcare Laboratories of America, Franklin, Tennessee, ZGlax~ Wellcome Inc., Research Triangle Park, North Carolina, and 4MRL Pharmaceutical Services, Herndon, Virginia
ABSTRACT Strains of Streptococcus pneumoniae from the United States that were susceptible, intermediately resistant, or highly resistant to penicillin were tested for susceptibility to 26 anfimicrobial agents that have been used or considered for the treatment of patients with pneumococcal infections. The drugs tested included penicillins, one penicillin/beta-lactamase inhibitor combination, cephalosporins, macrolides, a lincosamide, fluoroquinolones, and four miscellaneous drugs (vancomycin, rifampin, tetracycline, and trimethoprimsulfamethoxazole). The activities of the penicillins and macrolide agents were similar, but the activities within the cephalosporin and fluoroquinolone classes were often dissimilar. For the fluoroquinolones, the order of in vitro activity, from most to least active, was grepafloxa0149-2918/98/$19.00
cin, sparfloxacin, levofloxacin, ciprofloxacin, and ofloxacin. Increased resistance to penicillin in the pneumococcal isolates studied correlated with increased resistance to other penicillins, cephalosporins, macrolides, clindamycin, tetracycline, and trimethoprimsulfamethoxazole but did not correlate with increased resistance to the fluoroquinolones, rifampin, or vancomycin. These findings may be helpful to health professionals selecting empiric therapy for respiratory tract infections involving S pneumoniae. Key words: pneumococci, antibiotic resistance, in vitro susceptibility, grepafloxacin, penicillin. INTRODUCTION
Streptococcus pneumoniae is a major pathogen in a variety of infections, particularly those affecting the respiratory tract, and penicillins have been the therapy of choice for these infections for many 1 179
CLINICAL THERAPEUTICS®
decades. 1 Although penicillin resistance was reported in S pneumoniae as early as the 1960s, the prevalence of penicillinresistant pneumococci in the United States remained below 5% through the 1980s. 2,3 In the 1990s, however, the proportion of penicillin-resistant isolates of S pneumoniae increased greatly and became a major clinical concern. In the winter of 1996-1997, a US surveillance study of pneumococcal isolates 4 determined that high-level resistance to penicillin (minimum inhibitory concentration [MIC], >2 p,g/mL) was observed in 14% of clinical isolates and that intermediate resistance (MIC, 0.12 to 1 ~g/mL) was detected in 20%. This study also demonstrated that penicillin resistance was correlated with nonsusceptibility to cephalosporins and macrolides but not to fluoroquinolones. The goals of the present study were to examine the in vitro activity of 26 antimicrobial agents against strains of S pneumoniae that were susceptible, intermediately resistant, or resistant to penicillin and to assess the degree of correlation of penicillin resistance with resistance to other antimicrobial agents.
MATERIALS AND METHODS
Bacteria We selected 369 recent (1995-1997) strains of S pneumoniae that had been isolated from patients treated at health care institutions in all geographic regions of the United States. We were careful to select strains that represented susceptible, intermediately resistant, and resistant categories, but the number of isolates in each category did not reflect the relative prevalence of these categories in the United 1180
States. Of the 369 strains, 121 (32.8%) were susceptible to penicillin (MIC, <0.06 ~g/mL), 150 (40.7%) had intermediate resistance to penicillin (MIC, 0.12 to 1 I~g/mL), and 98 (26.6%) had high-level resistance to penicillin (MIC, >2 ixg/mL). The isolates studied had been identified by standard methods and tested for susceptibility to penicillin by broth microdilution using the method recommended by the National Committee for Clinical Laboratory Standards (NCCLS). 5,6All isolates had been stored at -75 °C or colder before testing. When tests were scheduled to be performed, the frozen isolates were thawed and subcultured to blood agar once or twice to obtain sufficient growth, and these subcultures were used for susceptibility testing.
Antimicrobial Agents Representing both oral and parenteral formulations, the 26 anfimicrobial agents chosen for this study are either used currently or have been considered for the treatment of patients with pneumococcal infections. The panel of anfimicrobial agents tested included 2 penicillins (penicillin and amoxicillin), 1 penicillin/beta-lactamase inhibitor combination (amoxicillin/clavulanate), 10 oral and parenteral cephalosporins (cephalothin, cefaclor, loracarbef, cefuroxime, cefprozil, cefotaxime, ceftriaxone, cefpodoxime, cefixime, and ceftibuten), 3 macrolides (erythromycin, clarithromycin, and azithromycin), 1 lincosamide (clindamycin), 5 fluoroquinolones (ciprofloxacin, ofloxacin, levofloxacin, sparfloxacin, and grepafloxacin), and 4 miscellaneous drugs (vancomycin, rifampin, tetracycline, and trimethoprim-sulfamethoxazole). These antimicrobial agents were obtained from their respective manufacturers in the form of powders suitable for susceptibility testing.
C. THORNSBERRYET AL.
Susceptibility Testing The MIC, which is the lowest concentration of an antimicrobial agent that inhibits visible bacterial growth, was determined for each of the 26 antimicrobial agents using broth microdilution as recommended by the NCCLS. 6 The medium was cation-adjusted Mueller-Hinton broth supplemented with 2% to 5% lysed horse blood. The susceptibility test trays were prepared by Microtech Medical Systems (Aurora, Colorado). The final inocula were approximately 5 x 105 colony-forming units/mL. The inoculated trays were incubated in ambient air for 20 to 24 hours at 35 °C. For quality control purposes, the reference strain S pneumoniae American Type Culture Collection 49619 was included in each test. All antimicrobial agents were tested simultaneously for each isolate. RESULTS Table I shows the MIC data for the study strains stratified according to NCCLS category of penicillin susceptibility (susceptible, intermediately resistant, or resistant). The activity of many of the antimicrobial agents was correlated with the activity of penicillin (Tables I and II). Penicillinsusceptible isolates of S pneumoniae tended to be susceptible to other antimicrobial agents, but strains with intermediate penicillin resistance and highly resistant strains were likely to show resistance to many other antimicrobial agents, a tendency that appeared to be associated with the level of penicillin resistance. For example, for cephalothin, penicillin-susceptible strains had an MIC50 (50% of isolates inhibited at that concentration) of 0.12 Ixg/mL and an MIC90 (90% of isolates in-
hibited at that concentration) of 0.5 wg/mL, but penicillin-resistant isolates had an MIC50 of 8 I~g/mL and an MIC9o of 32 I~g/mL (Table I). The MIC50 and MIC90 for penicillin-intermediate strains were 1 and 8 i~g/mL, respectively. This relationship was typical of other penicillins, the cephalosporins, the macrolides, clindamycin, tetracycline, and trimethoprimsulfamethoxazole but not of the fluoroquinolones, vancomycin, or rifampin (Table II). Of the fluoroquinolones, the MICs0 and MIC90 for grepafloxacin were 0.12 and 0.25 I~g/mL, respectively, for all three categories of penicillin susceptibility, signifying that all isolates were susceptible to grepafloxacin (MICs <0.5 I.~g/mL). For those drugs for which resistance was associated with penicillin resistance and for which an increase in MIC90 could be calculated from discrete data (ie, using no < or > values), the MIC90s of penicillinresistant strains were 5 to 7 log2 concentrations higher than those of the penicillinsusceptible strains. For example, the MIC90 of cefotaxime increased from 0.12 Ixg/mL for penicillin-susceptible strains to 4 ~g/mL for penicillin-resistant strains; the MIC90 for cefprozil increased from 0.25 wg/mL to 32 I~g/mL. The MICs for the three penicillin compounds (penicillin, amoxicillin, and amoxicillin/clavulanate) were within ±1 log2 concentration (Table I). The MICs for the cephalosporins, however, were often dissimilar (Tables I and III). The most active cephalosporins were the two third-generation parenteral compounds, cefotaxime and ceftriaxone, which had MIC9os of 4 Ixg/mL for penicillin-resistant strains. For strains that were not susceptible to penicillin (MIC _>0.12 I~g/mL), the MIC90s for several of the cephalosporins (cefaclor, loracarbef, cefixime, and ceftibuten) were greater than 1181
CLINICAL THERAPEUTICS*
Table I. S u s c e p t i b i l i t y o f s e l e c t e d isolates o f
Streptococcuspneumoniae to
26 a n t i m i c r o -
bial agents.** % of Isolates Antimicrobial Agent
No. of Isolates
MIC (p~g/mL) Range
Penicillin All 369 0.008-8 Pen s 121 0.008--0.06 Pen I 150 0.12-1 Pen R 98 2-8 Amoxicillin All 369 <0.03-16 Pen s 121 <0.03-2 Pen I 150 <0.03-2 Pen g 98 <0.03-16 Amoxicillin/clavulanate All 369 <0.03-8 Pen s 121 <0.03-2 Pen ~ 150 <0.03-4 Pen R 98 <0.03-8 Cephalothin All 369 0.06-64 Pen s 121 0.06-2 Pen I 150 0.25-16 Pen R 98 2-64 Cefaclor All 369 0.12->32 Pen s 121 0.12-4 Pen I 150 0.12->32 Pen R 98 0.5->32 Loracarbef All 369 0.25->32 Pen s 121 0.25->32 Pen x 150 0.25->32 Pen R 98 0.5->32 Cefuroxime All 369 0.03->32 Pen s 121 0.03->32 Pen I 150 0.06-32 Pen R 98 0.06-->32 Cefprozil All 369 0.03->32 Pen s 121 0.06-8
MIC50
Intermediately MIC9o
Susceptible
Resistant
Resistant
0.25 0.015 0.5 2
2 0.06 1 4
32.8 100 0.0 0.0
40.7 0.0 100 0.0
26.5 0.0 0.0 100
0.25 <0.03 0.25 1
2 0.06 1 4
77.0 97.5 89.3 32.7
9.5 1.7 9.3 19.4
13.5 0.8 1.4 47.9
0.25 <0.03 0.25 1
2 0.06 1 8
60.4 96.7 70.0 1.0
25.8 2.5 28.7 50.0
13.8 0.8 1.3 49.0
1 0.12 1 8
16 0.5 8 32
--* ----
2 0.25 2 >32
>32 1 >32 >32
----* ----
4 0.5 4 >32
>32 2 >32 >32
-----* ----
0.5 0.03 1 4
8 0.25 4 32
--~ ----
0.5 0.12
16 0.25
--* --
B
m
m
B
m
m
m
m
m
m
N
m
m
m
m
m
n
m
m
m
m
m
m
m
Continued
1182
C. THORNSBERRY ET AL.
Table I. ( c o n t i n u e d ) % of Isolates Antimicrobial No. of Agent Isolates
MIC (l~g/mL) Range
MICs0
Intermediately MIC9o
Susceptible
Resistant
Resistant
Cefprozil (continued) Pen I Pen R Cefotaxime All Pen s Pen I Pen R Ceftriaxone All Pen s Pen I Pen R Cefpodoxime All Pen s Pen I Pen R Cefixime All Pen s Pen I Pen R Ceftibuten All Pen s Pen I Pen R Erythromycin All Pen s Pen I Pen R Clarithromycin All Pen s Pen I Pen R
150 98
0.03-32 0.06->32
0.5 8
8 32
---
369 121 150 98
<0.015-8 <0.015-1 <0.015-4 0.5-8
0.25 <0.015 0.25 1
2 0.12 1 4
--~ ----
369 121 150 98
<0.015-8 <0.015-1 <0.015-4 0.03-8
0.25 0.03 0.25 1
2 0.12 I 4
--~ ----
369 121 150 98
<0.015->32 <0.015-2 0.03->32 _<0.015->32
0.5 0.03 0.5 2
4 0.12 2 16
--~ ----
369 121 150 98
_<0.25-->16 4 <0.25->16 -<0.25 -<0.25-2_16 8 <0.25-2_16 ->16
_>16 1 ->16 ->16
--~ ----
369 121 150 98
-<0.015--2_16 ->16 <0.25-->16 4 0.5-->16 _>16 2-_>16 _>16
_>16 _>16 _>16 _>16
--* ----
369 121 150 98
_<0.03->64 _<0.03->64 <0.03->64 -<0.03->64
0.06 _<0.03 0.06 4
64 2 >64 >64
60.2 86.0 5%3 32.7
0.8 0.0 2.0 0.0
39.0 14.0 40.7 67.3
369 121 150 98
_<0.03->64 -<0.03->64 _<0.03->64 -<0.03->64
0.06 _<0.03 0.06 4
16 1 64 >64
60.7 86.8 58.7 31.6
1.6 0.0 1.3 4.1
37.7 13.2 40.0 64.3
m
m
q
m
B
m
m
m
m
B
q
h
B
m
B
m
B
m
B
m
b
m
m
B
m
m
Continued
1183
CLINICAL THERAPEUTICS®
Table I. (contimgxl) % of Isolates Antimicrobial No. of Agent Isolates
MIC (Ixg/mL) Range MlCso MIC9o
Intermediately Susceptible Resistant Resistant
Azithromycin All Pens Pen ~ PeP
Clindamycin All Pen s
Pen~ Pen R Ciprofloxaein All Pen s Pen I Pen s
Ofloxacin All Pens PenI Pen R Levofloxacin All Pen S Pen I Pen R Sparfloxacin All Pen s PenI Pen R Grepafloxacin All Pen s Pen I Pen s
Vancomycin All Pen s PenI Pen R
369 121 150 98
<0.03->64 0.06->64 <0.03->64 0.06->64
0.12 0.12 0.25 8
64 2 >64 >64
60.4 86.0 58.0 32.7
1.6 1.7 1.3 2.0
38.0 12.3 40.7 65.3
369 121 150 98
<0.015->32 0.03->32 0.03->32 ~0.015->32
0.06 0.06 0.06 0.06
2 0.06 32 >32
88.1 97.5 85.3 80.6
0.3 0.0 0.7 0.0
11.6 2.5 14.0 19.4
369 121 150 98
0.03->8 0.09>8 0.1~8 0.124
0.5 0.5 0.5 ~5
1 1 2 1
369 121 150 98
0.5->8 0.~8 0.5->8 0.54
1 1 1 1
2 2 2 2
98.4 98.4 98.0 99.0
1.1 0.8 1.3 1.0
0.5 0.8 0.7 0.0
369 121 150 98
0.25->8 0.54 0.25->8 0.25-2
0.5 0.5 0.5 0.5
1 1 1 1
99.5 ~.2 ~.3 1~
0.3 0.8 0.0 0.0
0.2 0.0 0.7 0.0
369 121 150 98
0.06-2 0.1~1 0.06-1 0.12-2
0.25 0.25 0.25 0.25
0.25 0.5 0.25 0.25
98.1 100 98.2 97.8
1.6 0.0 1.8 1.5
0.3 0.0 0.0 0.7
369 121 150 98
0.034.5 0.06--0.5 0.06--0.5 0.034.5
0.12 0.12 0.12 0.12
0.25 0.25 0.25 0.25
100 100 100 100
0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0
369 121 150 98
0.1~1 0.1~1 0.254.5 0.254.5
0.25 0.25 0.25 0.25
0.5 0.5 0.5 0.5
100 100 100 100
__§
m
m
m
m
B
B
Continued 1184
C. THORNSBERRY ET AL.
Table I. (continued) %ofIsolates Antimicrobial No. of Agent Isolates
MIC (wg/mL) Range MICs0 MIC90
Susceptible
Intermediately Resistant Resistant
Rifampin All 369 _<0.015->16 Pen s 121 ~0.015->16 Pen I 150 _<0.015->16 Pen R 98 ~0.015-0.25 Te~acycline All 369 _<0.5->16 Pen s 121 _<0.5->16 Pe~ 150 _<0.5->16 Pen R 98 ~0.5->16 Tfimethopfim-sulfamethoxazole All 369 ~0.12->4 Pen s 121 g0.12->4 Pen I 150 ~0.12->4 Pen R 98 _<0.12->4
0.03 0.03 0.03 0.03
0.03 0.03 0.03 0.03
98.6 99.2 97.3 100
0.3 0.0 0.7 0.0
1.1 0.8 2.0 0.0
~0.5 A0.5 ~0.5 _<0.5
>16 1 >16 >16
74.0 92.6 68.7 59.2
1.6 0.8 0.7 4.1
24.4 6.6 30.6 36.7
1 0.25 1 4
>4 1 >4 >4
48.2 85.1 40.7 14.3
15.2 7.4 19.3 18.4
36.6 7.5 40.0 67.3
MIC = minimum inhibitory concentration; MICs0= MIC at which 50% of isolates are inhibited; MIC90 = MIC at which 90% of isolates are inhibited. *National Committee for Clinical Laboratory Standards (NCCLS) breakpoints were used. *Data are presented for all isolates and for isolates that were penicillin susceptible (PenS), of intermediate penicillin resistance (PenI), and penicillin resistant (PenR). *Most of the cephalosporins and ciprofloxacin do not have NCCLS-recommendedinterpretive breakpoints. We compared the cephalosporins by MICs0s and MICgos and by cumulative distribution of MICs (see Table 1~). §The breakpoints for vancomycin do not recognize intermediately resistant or resistant strains.
the highest concentration tested. The MIC distribution data (Table HI) indicate that the activities of the cephalosporins varied, and that cefpodoxime and cefuroxime were the most active members of this antimicrobial class. These two agents were also more active than the other oral cephalosporins against strains with intermediate resistance to penicillin (Table I). The activities o f the three macrolides (erythromycin, clarithromycin, and azithromycin) are shown in Tables I and IV. A l t h o u g h distribution within the categories o f susceptibility was quite similar
for the three macrolides, the MIC90 for c l a r i t h r o m y c i n was 16 Ixg/mL but 64 ixg/mL for erythromycin and azithromycin. As shown in Table IV, the difference in MIC90s was due to very small differences in the MICs. In addition, the MIC90s for all three macrolides were _>64 v~g/mL for isolates with intermediate resistance and high resistance to penicillin. Increased penicillin resistance in S pneumoniae was c o r r e l a t e d with increased resistance to these macrolides, but even the penicillinsusceptible strains had 12% to 14% resistance to the three drugs.
1185
CLINICAL THERAPEUTICS®
Table II. Correlation of penicillin resistance in Streptococcuspneumoniae with resistance to other antimicrobial agents. Correlated* Amoxicillin Amoxicillin/clavulanate Cefaclor Cefixime Cefotaxime Cefpodoxime Cefprozil Ceftibuten Ceftriaxone Cefuroxime Cephalothin Loracarbef Azithromycin Clarithromycin Erythromycin Clindamycin Trimethoprim-sulfamethoxazole Tetracycline
Not correlatedt Ciprofloxacin Grepafloxacin Levofloxacin Ofloxacin Sparfloxacin Rifampin Vancomycin
*Strainswith increasedresistanceto penicillingenerallyhave increasedresistanceto these antimicrobialagents. tNo correlationwith increasedpenicillinresistancewas observedfor these antimicrobialagents. The correlation with penicillin resistance was also evident in the activity of clindamycin, with MIC90s of 32 and >32 ~g/mL for isolates with intermediate resistance and high resistance to penicillin, respectively, compared with an MIC90 of 0.06 p~g/mL for penicillin-susceptible strains. The correlation between penicillin resistance and the activity of tetracycline was seen in MIC90s of >16 Ixg/mL for isolates with intermediate resistance and high resistance to penicillin, compared with an MIC90 of 1 Ixg/mL for penicillin-susceptible strains. For trimethoprim-sulfamethoxazole, the correlation with penicillin resistance was seen in both MICs0s and MIC90s. As shown by the MIC50 and MIC90 data (Table I) and the distribution of MICs 1186
(Table IV) for the fluoroquinolones, the in vitro activity was different for each drug in this class. The order of activity, from most to least active (as measured by MICs), was grepafloxacin, sparfloxacin, levofloxacin, ciprofloxacin, and ofloxacin. Following NCCLS-recommended breakpoints, it was found that resistance of S pneumoniae to fluoroquinolones was rare, with the exception of ciprofloxacin. Increased resistance to penicillin did not affect the activity of the five fluoroquinolones, vancomycin, or rifampin (Tables I and II), as indicated by MICs that were essentially the same for strains that were susceptible and resistant to penicillin. None of the isolates tested were resistant to vancomycin (all MICs <1 Ixg/mL) or grepafloxacin (all MICs <0.5 I~g/mL).
C. THORNSBERRY ET AL.
t",,I
o=
oO
II
Z
r~
t~
¢q
8. Q 0~0 t'xl t'q
t'xl ¢ q
¢.q
.=
u~u~uuuu 1187
CLINICAL THERAPEUTICS*
Table IV. Cumulative percentage of strains of Streptococcus pneumoniae (N = 369), by minimum inhibitory concentration (MIC) of fluoroquinolones and macrolides. Cumulative Percentage of Strains, by MIC Antimicrobial Agent Grepafloxacin Sparfloxacin Levofloxacin Ciprofioxacin Ofloxacin Erythromyein Azithromycin Clarithromycin
<0.03 0.06 0.12 0.25 1
16
78 26 2
33 44
57 6 58
59 50 59
0.5
1
2
97 100 90 98 1 62 15 66 4 60 61 60 60 61 62
100 97 92 76 63 62 65
99 97 98 67 64 72
4
8
16
32
64
>64
100" 100" 100" 78 85 70 79 81 88
89 86 91
89 89 91
90 90 92
100 100 100
*Percentageof strains equal to or greater than this MIC.
DISCUSSION AND C O N C L U S I O N S Strains of S pneumoniae having decreased susceptibility to penicillin were reported occasionally in the 1960s and 1970s but were not considered a clinical problem affecting the choice of penicillin as the preferred therapy for pneumococcal infections. 2 Although the number of strains with intermediate resistance to penicillin increased in the United States in the 1980s, their prevalence was <5%, which was not considered to be of public health significance. 3 In the 1990s, however, the prevalence of S pneumoniae strains that were not penicillin susceptible increased sharply. 4,7-9 In the early 1990s, most of the strains with diminished susceptibility to penicillin had an intermediate level of resistance; more recently, however, there have been significant increases in the prevalence of highly resistant strains. 4,8 Of S pneumoniae strains isolated from adults in the United States during the winter of 1996-1997, 66% were susceptible to penicillin, 20% had intermediate resis1188
tance to penicillin, and 14% had a high level of resistance to penicillin. 4 The findings from 1996--1997 suggested that highlevel resistance had more than doubled since the previous year. 9 The increase in penicillin resistance has created concern about the treatment of patients with pneumococcal infections) One of the findings of earlier studies was that some of the antimicrobial agents that might be selected to treat pneumococcal infections--for example, the penicillins, cephalosporins, and macrolides-were encountering increased resistance that correlated closely with increased penicillin resistance. 4,7-9 Penicillin-susceptible strains tended to be susceptible to these agents, but strains not susceptible to penicillin had various levels of resistance to these other agents. The fluoroquinolones and vancomycin were the exceptions to these correlations with penicillin resistance. 4 We extended these studies to include 26 antimicrobial agents that might be used to treat patients with pneumococcal infec-
C. THORNSBERRY ET AL.
tions. Our results show that isolates of S pneumoniae having increased resistance to penicillin may also have increased resistance to other penicillins, the cephalosporins, the macrolides, clindamycin, tetracycline, and the sulfonamides but not to the fluoroquinolones, vancomycin, and rifampin. Penicillin-resistant strains of S pneumoniae have altered penicillin-binding proteins that may directly affect the activity not only of penicillins but also of cephalosporins and thus can account for the cross-resistance observed between penicillins and cephalosporins in these strains, m In the case of the other classes of antimicrobial agents in which resistance is associated with penicillin resistance, penicillin-binding proteins are not linked to reduced activity. Instead, resistance to macrolides, clindamycin, tetracyclines, and sulfonamides is thought to be due to the acquisition of other resistance genes. For example, a common mechanism of resistance to macrolides and clindamycin is a mutation that results in a profile known as MLS resistance, because the ribosome is insensitive to all macrolides, lincosamides, and streptogramins. The gene encoding such resistance has also been identified on a transposon. 11 The acquisition of this and other transposons could result in multiple resistance, but only resistance to penicillins and cephalosporins would be related to altered penicillin-binding proteins. The reason penicillin-resistant strains may be more likely to acquire and maintain these genes is unknown, but the acquisition of additional resistances increases the likelihood of creating selective pressures that could result in more rapid dissemination of these multiply-resistant strains.
Because S pneumoniae is a leading cause of community-acquired respiratory infections and because the newer fluoroquinolones are active against penicillinresistant pneumococci, the newer fluoroquinolones have been advocated for therapy of S pneumoniae infections. 12Vancomycin has been advocated for the same reasons. Although rifampin resistance is not correlated with penicillin resistance in S pneumoniae, rifampin is not generally indicated for respiratory infections, because resistance may develop rapidly when it is used as a single therapeutic agent. Our data show that increased penicillin resistance in S pneumon/ae is correlated with increased resistance to other penicillins, the cephalosporins, the macrolides, clindamycin, tetracycline, and t r i m e ~ - s u l f a m e t h o x a zole but not with resistance to fluoroquinolones, vancomycin, and rifampin. No isolates were resistant to vancomycin or grepafloxacin. Based on MICs, the order of activity of the fluoroquinolones, from most to least active, was grepafloxacin, sparfloxacin, levofloxacin, ciprofloxacin, and ofloxacin. ACKNOWLEDGMENTS This study was supported by a grant from Glaxo Wellcome Inc., Research Triangle Park, North Carolina. Grepafloxacin is licensed by Glaxo Wellcome PLC from Otsuka Pharmaceutical Co., Ltd., Tokyo, Japan. The authors thank Geriann Piazza, MA, for editing assistance and Angela M. Feher for assistance in preparing drafts of the manuscript.
Address correspondence to: Clyde Thomsberry, Phi), MRL Pharmaceutical Services, 7003 Chadwick Drive, Suite 235, Brentwood, TN 37027. 1189
CLINICAL THERAPEUTICS*
REFERENCES 1. Klein JO. Selection of oral antimicrobial agents for otitis media and pharyngitis. Infect Dis Clin Pract. 1995;4(Suppl 2): $88-$94. 2. Hansman D, Bullen MM. A resistant pneumococcus. Lancet. 1967;2:264-265. Letter. 3. Spika JS, Facklam RR, Plikaytis BD, Oxtoby MJ. Antimicrobial resistance of Streptococcus pneumoniae in the United States. J Infect Dis. 1991;163:1273-1278. 4. Thornsberry C, Ogilvie P, Kahn J, et al. Surveillance of antimicrobial resistance in Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis in the United States in 1996-1997 respiratory season. Diagn Microbiol Infect Dis. 1997;29:249-257. 5. Baron EJ, Murray PR. Bacteriology. In: Murray PR, Baron EJ, Pfaller MA, et al, eds. Manual of Clinical Microbiology, 6th ed. Washington, DC: American Society for Microbiology; 1995:246--662.
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