Comparative activity of gatifloxacin and other antibiotics against 4009 clinical isolates of Streptococcus pneumoniae in the United States during 1999–2000

Diagnostic Microbiology and Infectious Disease 43 (2002) 207–217

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Comparative activity of gatifloxacin and other antibiotics against 4009 clinical isolates of Streptococcus pneumoniae in the United States during 1999 –2000 Roger L. Whitea,*, Kevin A. Enzweilera, Lawrence V. Friedrichb, David Wagnerb, Daryl Hobanc, John A. Bossoa a

Anti-infective Research Laboratory, Medical University of South Carolina, Charleston, South Carolina, USA b Bristol-Myers Squibb, Princeton, New Jersey, USA c International Health Management Associates, Inc., Rolling Meadows, Illinois, USA Received 3 January 2002; accepted 19 March 2002

Abstract The susceptibility of 4009 recent clinical isolates of Streptococcus pneumoniae to gatifloxacin, levofloxacin, ciprofloxacin, penicillin, ceftriaxone and azithromycin was determined. Overall rates of susceptibility to these agents were 99.4, 98.7, 71.2, 55.2, 80.9, and 71.3%, respectively. Resistance to all tested agents was associated with penicillin resistance. Of penicillin nonsusceptible isolates, 36% were resistant. Resistance to the fluoroquinolones was unusual and gatifloxacin generally appeared to be four-fold more active than levofloxacin or ciprofloxacin. Multidrug resistant S. pneumoniae accounted for 6.2% of this sample. The lowest rate of susceptibility to nonfluoroquinolone antibiotics was observed in isolates from the South region of the United States, which appeared to be explained by both the proportion of and the inherently higher MICs of certain types of isolates. © 2002 Elsevier Science Inc. All rights reserved.

1. Introduction Streptococcus pneumoniae remains a leading cause of bacterial respiratory infections. Its gradual decrease in susceptibility to a variety of antibiotics including penicillin, other ␤-lactams, and macrolides has resulted in increased surveillance and even changes in recommendations for empiric therapy of common respiratory tract infections (Bartlett, 2000; Heffelfinger, 2000). A number of large surveillance studies have suggested that the extent of resistance of S. pneumoniae varies with region (worldwide as well as within the United States), patient age, and specimen source (Doern, 1996; Doern, 2001; Felmingham, 2000; Hoban, 2001; Thornsberry, 1999). A number of risk factors for infection with antibiotic resistant S. pneumoniae have been described; among these is recent treatment with antibiotics (Block, 1995; Deeks, 1999). The continuing trend in resistance development warrants continuing assessment of patterns of antibiotic resistance with this pathogen (Sahm, * Corresponding author. Tel.: ⫹1-843-792-8462; fax: ⫹1-843-7921712. E-mail address: [email protected] (R.L. White).

2001; Shlaes, 1997; Whitney, 2000). We describe a study of over 4,000 clinical isolates (1999 –2000) from across the United States, which extends and supplements the results of other similar investigations.

2. Materials and methods Non-duplicate patient isolates of S. pneumoniae (n ⫽ 4750) were collected at 108 institutions in 36 states across the United States from January 1999 through August of 2000. Duplicate isolates were defined as the same isolate (same susceptibility pattern) obtained from the same patient within 5 days. All participating institutions collected and identified consecutive, clinical isolates from patients diagnosed with respiratory tract infections. The following data were collected for each isolate: MIC result, collection and susceptibility test dates, source of isolate, patient gender and age, inpatient or outpatient status, and hospital demographic data (number of beds, hospital type, city and state). Isolate sources were defined as: respiratory (sputum or other), head, eye, ears, nose and throat (HEENT), blood, and other body fluids (e.g., exudate). Hospital types were categorized as

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Table 1 Patient demographics Parameter

Number of Isolates

Percent of Isolatesa

Male Female Outpatient Inpatient Age ⱕ10 11–20 21–30 31–40 41–50 51–64 ⱖ65

2290 1719 1393 2616

57 43 35 65

961 153 162 411 504 699 1119

24 4 4 10 13 17 28

a

Total of percentages may not equal 100 due to rounding

Veterans Administration [VA], university, private, community teaching, community non-teaching, or reference laboratory. MIC testing was performed at each institution by the epsilometer (Etest) method per the manufacturer’s guidelines (AB Biodisk, Solna, Sweden). Isolates were incubated for 18 –24 h at 35°C in 5–7% CO2 on Mueller-Hinton agar supplemented with 5% sheep blood as specified in the National Committee for Clinical Laboratory Standards (NCCLS) guidelines. Quality control testing was performed using ATCC S. pneumoniae control strain 49619. All isolates were tested against penicillin, ceftriaxone, azithromycin, levofloxacin, gatifloxacin, and ciprofloxacin. Concentrations on the Etest strips ranged from 0.002–32 ␮g/ml for the fluoroquinolones and the ␤-lactams and 0.016 –256 ␮g/ml for azithromycin. In a few instances, institutions used an Etest strip with a concentration range of 0.002–32 ␮g/ml for the ␤-lactams and reported results as MICs ⬎ 32 ␮g/ml (n ⫽ 15 for penicillin, n ⫽ 3 for ceftriaxone). Susceptibility categories (susceptible, intermediate, or resistant: S, I, or R) were determined according to NCCLS guidelines for penicillin, ceftriaxone, levofloxacin, and gatifloxacin (NCCLS, 2001). The manufacturer’s suggested breakpoints for Etest strips (susceptible ⱕ 4 ␮g/ml, resistant ⱖ 16 ␮g/ml) were used for azithromycin due to the well known effects of CO2 and subsequent pH changes on azithromycin MICs under these conditions. The NCCLS does not provide interpretive breakpoints for ciprofloxacin and S. pneumoniae. Therefore, for ciprofloxacin, an MIC ⱖ 4 ␮g/ml was defined as resistant (Chen, 1999; Doern, 2001), an MIC ⱕ 1 ␮g/ml was considered susceptible, and values between these concentrations were considered intermediate. An analysis of data with MICs rounded upward to the next common two-fold dilution value (e.g., rounding an MIC of 0.75 ␮g/ml to 1.0 ␮g/ml) was performed. When compared to actual MICs determined by the Etest strips, these rounded MICs resulted in similar susceptibility categorization (%S, I, R). In this analysis, there was no difference in %S for any drug; however, an absolute decrease in %I (and corresponding in-

Table 2 Number of institutions and isolates classified by hospital type, number of beds, region and culture site

Hospital type n ⫽ 100 VA University Reference Lab Private Community teaching Community nonteaching Number of beds 0–99 100–199 200–299 300–399 400–499 500–599 600–699 700–799 800–899 900–999 1000–1499 1500–2000 Region Midwest Northeast South West Culture site Respiratoryb HEENTc Blood Bodily fluids

# of Institutions

# of Isolates

% of Isolatesa

2 36 4 17 8 33

48 1491 193 772 307 1198

1 37 5 19 8 30

1 1 10 8 10 18 10 17 7 8 8 2

45 2 328 332 423 736 389 704 317 335 302 96

21 20 41 18

901 769 1597 742

22 19 40 19

96 66 77 30

2474 368 1107 60

62 9 28 1

1 0.05 8 8 11 18 10 18 8 8 8 2

a

Total of percentages may not equal 100 due to rounding Respiratory consists of isolates from sputum and other c HEENT consists of isolates from the eyes, ears, and throat b

crease in %R) ranging from 0.02% to 4.9% was noted. Therefore, the more precise (non-rounded) Etest MIC values were used for all subsequent analyses. MIC and all demographic data for each isolate were compiled electronically at International Health Management Associates, Inc (IHMA). Although MIC data were available for 4750 isolates, in some instances either demographic data (9.2% of isolates) or MIC data for all study drugs (6.4% of isolates) were incomplete. An analysis revealed that the MIC50, MIC90, mean, and median of all 4750 isolates were not different than for those 4009 isolates with complete data. Therefore, the final analysis assessed 4009 isolates from 100 study sites in 35 states. For these 4009 isolates, MIC50, MIC90, MIC range, and cumulative percent MICs were calculated. Subset analyses were performed based upon the source of the isolate, region of the country determined by U.S. Census categories (Census Regions and Divisions of the United States Map), type of hospital, bed size, patient age, cross resistance among fluoroquinolones, and multidrug resistance. Multidrug resistance was defined, for purposes of this study, as non-susceptibility (I or

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209

Table 3 Susceptibility based on penicillin susceptibility classificationa Antimicrobial

Susceptibility Classification

Penicillin

(n) All All Pen Pen Pen All Pen Pen Pen All Pen Pen Pen All Pen Pen Pen All Pen Pen Pen

Ceftriaxone

Azithromycin

Levofloxacin

Gatifloxacin

Ciprofloxacin

a

S I R S I R S I R S I R S I R

MIC50

0.032 0.016 0.25 1 1.5 1 2 64 1 1 0.75 0.75 0.25 0.25 0.25 0.25 1 1 0.75 0.75

MIC90

1 0.032 0.75 2 256 2 ⬎256 ⬎256 1.5 1.5 1.5 1.5 0.38 0.38 0.38 0.38 2 2 2 2

Range

%S

%I

%R

0.004–⬎32 0.004–1.5 0.008–8 0.016–⬎32 ⱕ0.016–⬎256 ⱕ0.016–⬎256 ⱕ0.016–⬎256 ⱕ0.016–⬎256 0.016–⬎32 0.032–⬎32 0.047–⬎32 0.016–⬎32 0.016–⬎32 0.016–⬎32 0.016–⬎32 0.016–⬎32 0.016–⬎32 0.023–⬎32 0.016–⬎32 0.023–⬎32

(2211) 55.2 80.9 99.9 83.0 12.3 71.3 94.3 55.6 20.8 98.7 98.9 98.8 98.0 99.4 99.5 99.3 98.9 71.2 66.9 76.7 76.4

(1150) 28.7 17.0 0.1 16.2 75.9 3.1 0.5 5.5 7.6 0.7 0.7 0.7 0.9 0.2 0.1 0.3 0.3 25.8 30.3 20.3 20.1

(648) 16.2 2.1 0 0.8 11.7 25.6 5.1 39.0 71.6 0.5 0.4 0.5 1.1 0.4 0.4 0.3 0.8 3.0 2.8 3.0 3.5

Total of percentages may not equal 100 due to rounding

Fig. 1. Cumulative frequency (%) of S. pneumoniae MICs for gatifloxacin, levofloxacin, and ciprofloxacin.

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Fig. 2. Cumulative frequency (%) of S. pneumoniae MICs for azithromycin, ceftriaxone, and pencillin.

R) to penicillin (MIC ⬎ 0.06 ␮g/ml) and intermediate susceptibility or resistance to at least two other members of non-␤ lactam drug classes. Statistical significance was determined using ANCOVA with Fisher’s post-hoc test for the differences in MICs among the different subsets including age, isolate source, inpatient or outpatient status, bed size, hospital type, and region (StatView, Cary,

NC). When using ANCOVA, all analyses utilized individual MIC values. For these analyses, MIC values recorded as greater than or less than/equal to a value were rounded to the next higher or lower E test value (e.g., ⱕ 0.016 was rounded to 0.012). Logistic regression was used to determine odds ratios and 95% confidence intervals for factors associated with multidrug resistance.

Table 4 Multidrug resistance among S. pneumoniae isolatesa Penicillin

Ceftriaxone

Azithromycin

Levofloxacin

Gatifloxacin

Ciprofloxacin

No. of isolates

R I I I R R I R I R I R I R

I S S I R S S I I I S I S I

R R I R R R R I I R R R R R

S S S S S S S S S S I R R I

S S S S S S S S S S S R R S

I I I I I I R I I R R R R I

68 58 18 16 12 11 10 9 6 5 5 3 3 2

a

Multidrug resistance is defined as intermediate or resistant to penicillin and at least two non-beta-lactam antibiotics Phenotypes listed are those containing more than one isolate

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211

Table 5 Factors associated with multidrug resistance Parameter

Category (n)

No. of Multidrug Resistant Isolates

Odds Ratio

95% Confidence Interval

Gender

Male (2290) Female (1719) Respiratory-other (843) Respiratory-sputum (1631) Ears (184) Eyes (93) Throat (91) Blood (1107) Body fluids-exudate (5) a Body fluids-other (55) ⱕ10 (961) 11–20 (153) a 21–30 (162) 31–40 (411) 41–50 (504) 51–64 (699) ⱖ65 (1119) a Community non-teaching (1198) Community teaching (307) Private (772) Reference Lab (193) University (1491) VA (48) a Midwest (901) Northeast (769) South (1597) West (742) Outpatient (1393) a Inpatient (2616)

155 93 65 110 14 5 10 42 0 2 65 7 15 22 27 40 72 63 13 42 14 111 5 50 43 111 44 90 158

1.24 1 2.62 2.28 2.43 1.68 4.24 1.26

0.94, 1.62

a

Collection Source

Age Category

Hospital Type

Region

Location a b

1 0.73 0.43 1 0.58 0.57 0.63 0.76 1 1.08 1.16 1.53 1.70b 2.47 1 1.06 1.45 1.15 1.02 1

0.62, 11.16 0.54, 9.66 0.52, 11.41 0.31, 9.15 0.86, 20.74 0.29, 5.41

0.40, 1.35 0.17, 1.09 0.29, 1.16 0.29, 1.12 0.34, 1.19 0.42, 1.37 0.57, 2.03 0.76, 1.77 0.82, 2.84 1.21, 2.38 0.87, 6.98 0.68, 1.67 1.00, 2.10 0.73, 1.81 0.76, 1.38

Reference category for statistical comparisons p ⫽ 0.0023

3. Results Patient demographics are displayed in Table 1 and study site characteristics and specimen sources are presented in Table 2. Gender was fairly equally represented in the patients of the study population and 65% were inpatients at the time of isolate collection. The majority of isolates were collected from patients ⱕ 10 years of age (24%) or ⱖ 65 years of age (28%). University hospitals collected 37% of all isolates. Forty percent of isolates were collected in the South region. Overall, 62% of isolates were of respiratory origin. Although all MIC values were included in the analysis, an analysis of the subset excluding ␤-lactam MIC values ⬎ 32 ␮g/ml (18 of 4009) revealed identical MIC50 and MIC90 values to those of all isolates (4009). Regarding categorical analysis, these MIC values would be interpreted as resistant to either ␤-lactam irrespective of the Etest strip range utilized. MIC50, MIC90, MIC range, and % S, I, and R for each antibiotic evaluated are presented in Table 3 according to penicillin susceptibility. Overall % susceptibilities were penicillin: 55.2%, ceftriaxone: 80.9%, azithromycin: 71.3%, levofloxacin: 98.7%, gatifloxacin: 99.4%, and ciprofloxacin:

71.2%. In contrast to the results with azithromycin and ceftriaxone, susceptibility to the fluoroquinolones did not appear to vary greatly with penicillin susceptibility category. A small decrease in %S and increase in %I and %R was noted for levofloxacin and gatifloxacin with penicillin resistance. A small increase in %R was also seen for ciprofloxacin. Cumulative MIC results are displayed in Figs. 1 and 2. The differences in MICs among the fluoroquinolones were very consistent across the observed range, with ciprofloxacin and levofloxacin displaying similar values. Gatifloxacin MICs were approximately fourfold lower than the other fluoroquinolones. With the other agents, the range of MICs was much wider than that found with the fluoroquinolones and the distributions were either bimodal (␤-lactams) or trimodal (azithromycin). Of 119 (3.0%) ciprofloxacin-resistant isolates, 97 (81.5%) were susceptible to gatifloxacin (range 0.25–⬎32 ␮g/ml) and 75 (63.0%) were susceptible to levofloxacin (range 0.75–⬎32 ␮g/ml). All gatifloxacin-resistant and levofloxacin-resistant isolates were also resistant to ciprofloxacin. Of the 22 levofloxacin-resistant isolates, all were either intermediate (23%) or resistant (77%) to gatifloxacin (range 1.5–⬎32 ␮g/ml), while all 17 gatifloxacin-resistant isolates were resistant to levofloxacin (range 8 –⬎32 ␮g/

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Table 6 In vitro activity against S. pneumoniae classified by collection source Antimicrobial Agent

Specimen Collection Source (n) All sources (4009) MIC50 MIC90 Range % S/I/Rb Respiratory (2474)c MIC50 MIC90 Range % S/I/Rb HEENT (368)d MIC50 MIC90 Range % S/I/Rb Blood (1107) MIC50 MIC90 Range % S/I/Rb Bodily Fluids (60) MIC50 MIC90 Range % S/I/Rb

Penicillina

Ceftriaxonea

Azithromycin

Levofloxacin

Gatifloxacin

Ciprofloxacin

0.047 2 0.002–256 55/29/16

0.032 1 0.004–⬎32 81/17/2

1.5 256 ⱕ0.016–⬎256 71/3/26

1 1.5 0.016–⬎32 98.7/0.7/0.5

0.25 0.38 0.016–⬎32 99.4/0.2/0.4

1 2 0.016–⬎32 71/26/3

0.047 2 0.002–192 55/30/15

0.032 1 0.004–⬎32 81/17/2

1.5 256 ⱕ0.016–⬎256 71/3/26

1 1.5 0.016–⬎32 98/1/1

0.25 0.38 0.016–⬎32 99.1/0.3/0.6

1 2 0.016–⬎32 69/27/4

0.125 4 ⱕ0.016–256 44/31/25

0.064 1 0.008–8 74/22/4

2 ⬎256 0.016–⬎256 59/5/36

0.75 1.5 0.25–3 99.7/0.3/0

0.25 0.38 0.032–1 100/0/0

0.75 2 0.125–6 73/26/1

0.032 2 0.003–⬎32 60/25/15

0.023 0.75 0.004–⬎32 83/16/1

1.5 256 ⱕ0.016–⬎256 77/2/21

0.75 1.5 0.047–⬎32 99.5/0.3/0.2

0.25 0.38 0.023–24 99.7/0.1/0.2

0.75 2 0.047–⬎32 76/22/2

0.047 3 ⱕ0.016–8 52/30/18

0.032 0.75 0.008–1.5 80/20/0

1.5 256 0.19–⬎256 70/3/27

1 1.5 0.38–⬎32 98/0/2

0.25 0.38 0.094–24 98/0/2

0.75 2 0.38–⬎32 70/28/2

Etest strips with ranges of 0.002–32 ␮g/ml and 0.016 –256 ␮g/ml were used Total of percentages may not equal 100 due to rounding c Respiratory includes sputum and other respiratory isolates d HEENT includes eyes, ears, and throat isolates a

b

ml). Two hundred and forty eight isolates (6.2%) were characterized as multidrug resistant with those phenotypes represented by more than one isolate shown in Table 4. The most common resistance phenotype was resistant to azithromycin and penicillin and intermediate to ciprofloxacin. All multidrug resistant isolates were either intermediate or resistant to ciprofloxacin. Table 5 contains the odds ratios and 95% confidence intervals for factors associated with the multidrug resistant isolates. The only isolate characteristic associated with multidrug resistance in a statistically significant fashion was isolation from a university setting (O.R.: 1.70, 95% C.I.: 1.21, 2.38, p ⫽ 0.0023). However, the likelihood of isolates from the South region being multiresistant approached significance (p ⫽ 0.0503). Comparative in vitro activity of the tested antibiotics against these clinical isolates based upon isolate source/ body site, patient age, hospital type, and geographic region is displayed in Tables 6 –9. In general, MICs were similar for each antibiotic across isolate source categories although some differences were noted. For example, the highest MIC50 values for penicillin, ceftriaxone and azithromycin were noted in HEENT isolates. One-half of HEENT isolates were of middle ear origin and the average age of these patients was 6 years. Subanalysis of HEENT isolates re-

vealed that the MIC50s were higher for middle ear isolates than isolates from the throat or eyes for azithromycin (4sixfold), ceftriaxone (4-eightfold), and penicillin (15–21 fold). Of note, ear isolates exhibited higher MICs of penicillin and azithromycin than those from all other sources (p ⬍ 0.032) except bodily fluids. These differences were not noted with the fluoroquinolones for ear isolates. HEENT isolates had the lowest %S for penicillin, ceftriaxone, and azithromycin. For levofloxacin and gatifloxacin, although the differences were small, HEENT isolates had the highest %S. When MIC50 and MIC90 values were categorized by patient age as shown in Table 7, differences are apparent with penicillin, ceftriaxone, and azithromycin but not the fluoroquinolones. Additionally, when considering all MICs, the non-fluoroquinolones all had statistically higher MICs for the ⱕ10 years age group than for all age groups aged 31 years and above (p ⬍ 0.03; 1-fourfold differences in median MICs), except with penicillin and ages 41–50. Thus, the lowest %S was also observed in the ⱕ 10 years age group. In contrast, when considering all MICs for ciprofloxacin and levofloxacin, the ⱕ10 years age group had significantly lower MICs than three of the higher age groups (p ⬍ 0.02; 1.3-fold differences in median MICs).

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Table 7 In vitro activity against S. pneumoniae classified by patient age Antimicrobial Agent

Age category (no. isolates) ⱕ10 (961) MIC50 MIC90 Range % S/I/Rb 11–20 (153) MIC50 MIC90 Range % S/I/Rb 21–30 (162) MIC50 MIC90 Range % S/I/Rb 31–40 (411) MIC50 MIC90 Range % S/I/Rb 41–50 (504) MIC50 MIC90 Range % S/I/Rb 51–64 (699) MIC50 MIC90 Range % S/I/Rb ⱖ65 (1119) MIC50 MIC90 Range % S/I/Rb a b

Penicillina

Ceftriaxonea

Azithromycin

Levofloxacin

Gatifloxacin

Ciprofloxacin

0.125 3 0.008–256 44/31/25

0.064 1 0.004–⬎32 72/25/3

1.5 ⬎256 0.016–⬎256 62/4/34

0.75 1.5 0.25–4 99.6/0.4/0

0.25 0.38 0.032–0.75 100/0/0

0.75 2 0.19–⬎32 76/22/2

0.047 2 0.012–12 57/31/12

0.032 0.75 0.008–2 85/14/1

1 256 0.047–⬎256 76/1/23

1 1.5 0.38–3 98.7/1.3/0

0.25 0.38 0.125–0.75 100/0/0

1 1.5 0.25–4 70/29/1

0.032 2 0.008–⬎32 57/32/11

0.032 0.75 0.008–⬎32 86/12/1

1.5 256 0.016–⬎256 72/4/24

1 1.5 0.016–3 99.4/0.6/0

0.25 0.38 0.016–1 100/0/0

1 2.0 0.023–⬎32 65/33/2

0.032 2 0.008–⬎32 60/29/11

0.023 0.75 0.008–3 87/12/1

1.5 256 ⱕ0.016–⬎256 77/3/20

1 1.5 0.047–⬎32 97.6/1.4/1

0.25 0.38 0.047–⬎32 98/1/1

1 2 0.047–⬎32 66/30/3

0.032 2 0.008–192 58/31/11

0.023 0.75 0.008–3 85/14/1

1.5 128 ⱕ0.016–⬎256 76/3/21

1 1.5 0.19–⬎32 98.8/0.8/0.4

0.25 0.38 0.064–⬎32 99.6/0.2/0.2

1 2 0.19–⬎32 70/27/3

0.032 2 0.008–⬎32 58/28/14

0.032 1 0.008–6 83/15/2

1.5 256 0.064–⬎256 73/3/24

1 1.5 0.032–⬎32 98.3/0.7/1

0.25 0.38 0.016–⬎32 99/0.1/0.9

1 2 0.016–⬎32 71/26/3

0.032 2 0.002–64 60/25/15

0.023 1 0.004–⬎32 82/16/2

1.5 256 ⱕ0.016–⬎256 74/2/24

1 1.5 0.125–⬎32 98.5/0.7/0.8

0.25 0.38 0.016–⬎32 99.1/0.3/0.6

1 2 0.125–⬎32 71/26/4

Etest strips with ranges of 0.002–32 ␮g/ml and 0.016 –256 ␮g/ml were used Total of percentages may not equal 100 due to rounding

When analyzed by hospital type, both VA and university hospitals reported higher MICs with all fluoroquinolones than the private and community non-teaching hospitals (p ⬍ 0.02; 1–2.6-fold higher median MICs). Although few in number, isolates from VA hospitals (1% of all isolates) exhibited the highest fluoroquinolone and the lowest ␤-lactam MIC50 and MIC90 values. There were no significant differences in MICs among hospital types for penicillin (p ⫽ 0.09) and azithromycin (p ⫽ 0.25). Although not apparent when comparing MIC50 and MIC90 values (Table 8), when considering all ceftriaxone MICs, VA isolate MICs were higher than those from all other sites (p ⬍ 0.05). Ceftriaxone MICs for isolates from community non-teaching sites were higher than for those from private and university hospital sites (p ⫽ 0.03 and 0.002, respectively). To examine the possibility that these differences were due to a small number of highly resistant isolates (ceftriaxone MICs

of ⬎32 ␮g/ml; one from VA and two from community non-teaching sites), re-analysis without these isolates was performed. The statistically significant differences in ceftriaxone MICs between community non-teaching and private and university hospital sites was again found. However, the significant differences between the VA and other sites were no longer found. When analyzed by number of hospital beds, no statistically significant differences were found in MICs for any drug. Only azithromycin showed a difference between inpatient vs. outpatient status with outpatient MICs being greater (p ⬍ 0.001). MIC50, MIC90, and susceptibility category, arranged by geographical region, are displayed in Table 9. The percentage of isolates not susceptible to penicillin, ceftriaxone, and azithromycin was higher in the South than all other regions. In general, nonsusceptibility to the fluoroquinolones was lower in the South, although differences were small. When

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Table 8 In vitro activity against S. pneumoniae classified by hospital type Antimicrobial Agent

Hospital type (no. isolates) Community non-teaching (1198) MIC50 MIC90 Range % S/I/Rb Community teaching (307) MIC50 MIC90 Range % S/I/Rb Private (772) MIC50 MIC90 Range % S/I/Rb Reference Lab (193) MIC50 MIC90 Range % S/I/Rb University (1491) MIC50 MIC90 Range % S/I/Rb VA (48) MIC50 MIC90 Range % S/I/Rb a b

Penicillina

Ceftriaxonea

Azithromycin

Levofloxacin

Gatifloxacin

Ciprofloxacin

0.047 3 0.002–192 54/28/18

0.032 1 0.004–⬎32 80/16/4

1.5 ⬎256 ⱕ0.016–⬎256 72/3/25

0.75 1.5 0.016–⬎32 98.7/1/0.3

0.25 0.38 0.016–4 99.7/0.25/0.08

0.75 2 0.023–⬎32 74/22/4

0.064 4 0.008–64 50/28/22

0.047 1 0.008–3 73/24/3

1.5 256 0.125–⬎256 70/3/27

0.75 1 0.25–⬎32 98/1/1

0.25 0.38 0.094–⬎32 99/0.7/0.3

0.75 1.5 0.25–⬎32 83/16/1

0.032 2 0.008–256 57/28/15

0.023 1 0.004–4 82/16/2

1.5 256 ⱕ0.016–⬎256 70/2/28

0.75 1.5 0.25–⬎32 99.6/0.1/0.3

0.25 0.38 0.016–6 99.8/0.1/0.1

1 2 0.125–⬎32 77/22/1

0.125 3 0.004–8 45/30/25

0.094 1 0.004–4 70/29/1

1.5 256 0.032–⬎256 58/6/36

0.75 1.5 0.38–⬎32 98/0.5/1.5

0.25 0.38 0.125–24 98/0/2

0.75 1.5 0.25–⬎32 76/20/4

0.032 2 0.003–⬎32 57/29/14

0.023 0.75 0.008–4 83/16/1

1.5 ⬎256 0.032–⬎256 73/4/23

1 1.5 0.125–⬎32 98/1/1

0.25 0.38 0.023–⬎32 99.2/0.1/0.7

1 2 0.016–⬎32 65/32/3

0.023 1 0.008–⬎32 71/25/4

0.0195 0.5 0.008–⬎32 94/4/2

2 256 0.5–⬎256 81/0/19

1.5 3 0.75–⬎32 90/6/4

0.38 0.5 0.125–24 96/2/2

2 6 0.75–⬎32 25/60/15

Etest strips with ranges of 0.002–32 ␮g/ml and 0.016 –256 ␮g/ml were used Total of percentages may not equal 100 due to rounding

considering all MICs, isolates from the South had higher MICs than those from all other regions for azithromycin (p ⬍ 0.005) and for the Northeast and Midwest for ceftriaxone (p ⬍ 0.009). Fluoroquinolone MICs did not vary statistically by region with the exception that ciprofloxacin MICs were significantly higher in the Midwest and West regions than in the South (p ⬍ 0.04).

4. Discussion The nonsusceptibility rate of S. pneumoniae reported here to penicillin (45%) is higher than that reported by other investigators. Three other studies found nonsusceptibility rates of 34, 34, and 35%, respectively (Doern, 2001; Hoban, 2001; Thornsberry, 1999). In the recent publication by Doern and colleagues (Doern, 2001), it was reported that, of the penicillin nonsusceptible isolates, over 50% were resistant. This was not the case in the current study; 36% of penicillin nonsusceptible isolates were resistant even though these two studies collected isolates during a similar time

period. Nonetheless, the proportion of nonsusceptible isolates comprising resistant organisms continues to rise and is reason for concern. Multidrug resistance is a further concern; however, the percent of multidrug resistant isolates (6.2%) is less than the 22% previously reported (Doern, 2001). This may simply reflect the smaller number of drugs and drug classes evaluated in our investigation. In Doern’s study, 32 antimicrobials representing 12 drug classes (␤lactams and 11 others) were evaluated and may have been the reason for these differences. Although many isolate characteristics had odds ratios ⬎ 1, the only factor reaching statistical significance was university setting (Table 5). It would be of interest to conduct such an analysis on a larger population of multidrug resistant isolates. In agreement with these other studies, we found an association between penicillin resistance and ceftriaxone and azithromycin resistance. Fluoroquinolone resistance was highest with penicillin-resistant isolates, although the relationship with penicillin resistance is less dramatic than that observed with ceftriaxone and azithromycin (Table 3). Increased ciprofloxacin resistance in penicillin-resistant vs. penicillin-sus-

R.L. White et al. / Diagnostic Microbiology and Infectious Disease 43 (2002) 207–217

215

Table 9 In vitro activity against S. pneumoniae classified by region Antimicrobial Agent

Region (no. isolates) Midwest (901) MIC50 MIC90 Range % S/I/Rb Northeast (769) MIC50 MIC90 Range % S/I/Rb South (1597) MIC50 MIC90 Range % S/I/Rb West (742) MIC50 MIC90 Range % S/I/Rb a b

Penicillina

Ceftriaxonea

Azithromycin

Levofloxacin

Gatifloxacin

Ciprofloxacin

0.032 2 0.008–256 60/26/14

0.023 1 0.004–⬎32 83/15/2

1.5 256 0.012–⬎256 73/3/24

1 1.5 0.38–⬎32 98.9/0.4/0.7

0.25 0.38 0.016–24 99.3/0.2/0.4

1 2 0.19–⬎32 69/29/2

0.032 2 0.003–⬎32 61/25/14

0.023 1 0.008–4 84/14/2

1.5 256 0.032–⬎256 77/3/20

1 1.5 0.19–⬎32 98/1/1

0.25 0.38 0.094–⬎32 98.9/0.3/0.8

1 2 0.125–⬎32 76/22/3

0.064 3 0.002–⬎32 48/33/19

0.064 1 0.004–⬎32 77/20/3

1.5 ⬎256 0.012–⬎256 66/3/31

0.75 1.5 0.016–⬎32 99.1/0.6/0.3

0.25 0.38 0.016–12 99.5/0.3/0.2

0.75 2 0.016–⬎32 74/24/2

0.032 2 0.008–192 59/27/14

0.023 1 0.008–4 83/16/1

1.5 256 0.047–⬎256 76/3/21

1 1.5 0.032–⬎32 98/1/1

0.25 0.5 0.016–⬎32 99.3/0.1/0.6

1 3 0.023–⬎32 81/8/11

Etest strips with ranges of 0.002–32 ␮g/ml and 0.016 –256 ␮g/ml were used Total of percentages may not equal 100 due to rounding

ceptible pneumococci has also been reported by others (Goldsmith, 1998). Our results concerning variances in penicillin resistance in S. pneumoniae depending on such factors as patient age, isolate source, and region of origin within the United States are consistent with the observations of other investigators. As in other studies, we found that the percentage of penicillin, azithromycin and ceftriaxone resistance was highest in the South. This regional variation prompted further analysis in order to determine the reason for this difference. The MICs for azithromycin were higher in the South than for all other regions for isolates in blood, outpatient, community teaching hospitals, and the ⱕ10 years age groups (p ⬍ 0.04). Specifically, isolates from two sources appeared to be mainly responsible for the significant regional differences. These include blood isolates from inpatients at community teaching hospitals (p ⫽ 0.007) and blood isolates from outpatients who were ⱕ10 years old (p ⫽ 0.005). With ceftriaxone, the South had significantly higher MICs for non-sputum respiratory isolates (p ⬍ 0.007) compared to the Northeast and West. Several combinations of isolate characteristics resulted in differences between the South and only one of several other regions in each case. The one isolate group for which the South had greater MICs than both the Northeast and West was non-sputum respiratory isolates in outpatients that were ⱕ10 years old (p ⫽ 0.01). As previously stated, ear isolates had higher non-fluoroquinolone MICs than other sources and the ⱕ10 years age group had higher MICs than several other age groups. The lower fluoroquinolone MICs in this age group may be due to

the low fluoroquinolone use resulting in decreased selective pressure. We found that the South supplied a higher proportion of children ⱕ10 years of age (28% vs. 20 –23%) and included a higher percentage of ear isolates (7%) than other regions (all ⬍ 4%). Therefore, at least in the case of the present study, the higher rate of non-fluoroquinolone resistance in the South may be explained by both the proportion of and the inherently higher MICs of certain types of isolates. Of the fluoroquinolones tested in this study, gatifloxacin possesses the greatest activity against S. pneumoniae on the basis of MIC, although resistance to any of the three tested fluoroquinolones was infrequent. In fact, the MIC of gatifloxacin was generally four-fold lower than that with either levofloxacin or ciprofloxacin. This may have implications both for clinical efficacy and the development of resistance. The clinical efficacy of fluoroquinolone antibiotics is thought to be best related to the pharmacodynamic parameter area-under-the plasma or serum concentration-time curve to MIC, or AUC/MIC ratio (Craig, 1998). The AUC is a measure of drug exposure and is a reflection of the antibiotic’s pharmacokinetics and dosage regimen. Limited in vitro and in vivo data suggest that an AUC/MIC ratio of at least 30 is associated with a favorable response with S. pneumoniae (Ambrose, 2000b; Lacy, 1999; Mattoes, 2001). Thus, if one were to use the MICs (e.g., MIC50 or MIC90) generated in the present study along with the AUC produced by typical doses in a subject with “normal” or “average” pharmacokinetics, single point analysis could be performed allowing comparisons of the fluoroquinolones. A more so-

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phisticated approach in which the entire MIC population is considered along with the range of pharmacokinetic variation represented in a given patient population (i.e., Monte Carlo analysis) would seem much more attractive in projecting the likelihood of achieving desired pharmacodynamic values in clinical practice (Dudley, 2000). Previous analyses of this type revealed that the probability of achieving target AUC/MIC ratios was higher with gatifloxacin than levofloxacin (Ambrose, 2000a; White, 2001). The lower MICs of gatifloxacin may also be of importance given the known mechanisms of resistance to the fluoroquinolones. In contrast to less potent fluoroquinolones, a two-step mutation may be necessary for the development of in vitro resistance to gatifloxacin (Sanders, 2001). In conclusion, the results of the present study confirm that penicillin resistance in S. pneumoniae continues to grow with marked differences in activity seen among antibiotic classes. In contrast to the other antibiotic classes tested in this study, fluoroquinolone resistance remains infrequent in this country. We would agree with others that ongoing surveillance of antibiotic resistance with S. pneumoniae is warranted (Sahm, 2001; Shlaes, 1997; Whitney, 2000).

References Ambrose, P. G., & Grasela, D. M. (2000a). The use of Monte Carlo simulation to examine pharmacodynamic variance of drugs: fluoroquinolone pharmacodynamics against Streptococcus pneumoniae. Diagnostic Microbiology & Infectious Disease, 38, 151–157. Ambrose, P. G., Grasela, D. M., Grasela, T. H., Passarell, J. A., Mayer, H. B., & Pierce, P. F. (2000b). Pharmacodynamics of fluoroquinolones against Streptococcus pneumoniae: analysis of phase III clinical trials. In Abstracts of the 40th Interscience Conference on Antimicrobial Agents and Chemotherapy, Toronto, Canada, September 17–20, Abstract 1387. Bartlett, J. G., Dowell, S. F., Mandell, L. A., File Jr., T. M., Musher, D. M., & Fine, M. J. (2000). Practice guidelines for the management of community-acquired pneumonia in adults. Clinical Infectious Diseases, 31, 347–382. Block, S. L., Harrison, C. J., Hedrick, J. A., Tyler, R. D., Smith, R. A., Keegan, E., & Chartrand, S. A. (1995). Penicillin-resistant Streptococcus pneumoniae in acute otitis media: risk factors, susceptibility patterns and antimicrobial management. Pediatric Infectious Disease Journal, 14, 751–759. Census Regions and. Divisions of the United States. In “US Census Bureau” [www]. Available: URL: http://www.census.gov.geo/www/ maps/. Chen, D. K., McGeer, A., DeAzavedo, J. C., & Low, D. E. (1999). Decreased susceptibility of Streptococcus pneumoniae to fluoroquinolones in Canada. New England Journal of Medicine, 341, 233–239. Craig, W. A. (1998). Pharmacokinetic/pharmacodynamic parameters: rationale for antibacterial dosing of mice and men. Clinical Infectious Disease, 26, 1–12. Deeks, S. L., Palacio, R., Ruvinsky, R., Kertesz, D. A., Hortal, M., Rossi, A., Spika, J. S., DiFabio, J. L. & the Streptococcus pneumoniae Working Group. (1999). Risk factors and course of illness among children with invasive penicillin-resistant Streptococcus pneumoniae. Pediatrics, 103, 409 – 413.

Doern, G. V., Brueggemann, A., Holley Jr., H. P., & Rauch, A. M. (1996). 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. Antimicrobial Agents and Chemotherapy, 40, 1208 –1213. Doern, G. V., Heilmann, K. P., Huynh, H. K., Rhomberg, P. R., Coffman, S. L., & Brueggemann, A. B. (2001). Antimicrobial resistance among clinical isolates of Streptococcus pneumoniae in the United States during 1999 –2000, including a comparison of resistance rates since 1994 –1995. Antimicrobial Agents and Chemotherapy, 45, 1721–1729. Dudley, M. N., & Ambrose, P. G. (2000). Pharmacodynamics in the study of drug resistance and establishing in vitro susceptibility breakpoints: ready for prime time. Current Opinion in Microbiology, 3, 515–521. Felmingham, D., Gruneberg, R. N., & the Alexander Project Group. (2000). The Alexander Project 1996 –1997: latest susceptibility data from this international study of bacterial pathogens from communityacquired lower respiratory tract infections. Journal of Antimicrobial Chemotherapy, 45, 191–203. Goldsmith, C. E., Moore, J. E., Murphy, P. G., & Ambler, J. E. (1998). Increased incidence of ciprofloxacin resistance in penicillin-resistant pneumococci in Northern Ireland. Journal of Antimicrobial Chemotherapy, 41, 420 – 421. Heffelfinger, J. D., Dowell, S. F., Jorgensen, J. H., Klugman, K. P., Mabry, L. R., Musher, D. M., Plouffe, J. F., Rakowsky, A., Schuchat, A., Whitney, C. G., & the Drug-resistant Streptococcus pneumoniae Therapeutic Working Group. (2000). Management of community-acquired pneumonia in the era of pneumococcal resistance. Archives of Internal Medicine, 160, 1399 –1408. Hoban, D. J., Doern, G. V., Fluit, A. C., Roussel-Delvallez, M., & Jones, R. N. (2001). Worldwide prevalence of antimicrobial resistance in Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis in the SENTRY Antimicrobial Surveillance Program, 1997–1999. Clinical Infectious Diseases, 32 (Suppl 2), S81–S93. Lacy, M. K., Lu, W., Xu, X., Tessier, P. R., Nicolau, D. P., Quintiliani, R., & Nightingale, C. H. (1999). Pharmacodynamic comparisons of levofloxacin, ciprofloxacin, and ampicillin against Streptococcus pneumoniae in an in vitro model of infection. Antimicrobial Agents and Chemotherapy, 43, 672– 677. Mattoes, H. M., Banevicius, M., Li, D., Turley, C., Xuan, D., Nightingale, C. H., & Nicolau, D. P. (2001). Pharmacodynamic assessment of gatifloxacin against Streptococcus pneumoniae. Antimicrobial Agents and Chemotherapy, 45, 2092–2097. National Committee for Clinical Laboratory Standards. (2001). Performance Standards for Antimicrobial Susceptibility Testing; Eleventh Informational Supplement: NCCLS document M100 –S11, Vol. 21, No. 1. NCCLS, Wayne, PA. Sahm, D. F., Karlowsky, J. A., Kelly, L. J., Critchley, I. A., Jones, M. E., Thornsberry, C., Mauriz, Y., & Kahn, J. (2001). Need for annual surveillance of antimicrobial resistance in Streptococcus pneumoniae in the United States: 2-year longitudinal analysis. Antimicrobial Agents and Chemotherapy, 45, 1037–1042. Sanders, C. C. (2001). Mechanisms responsible for cross-resistance and dichotomous resistance among the quinolones. Clinical Infectious Diseases, 32 (Suppl. 1), S1–S8. Shlaes, D. M., Gerding, D. N., John Jr., J. F., Craig, W. A., Bornstein, D. L., Duncan, R. A., Eckman, M. R., Farrer, W. E., Greene, W. H., Lorian, V., Levy, S., McGowan Jr., J. E., Paul, S. M., Ruskin, J., Tenover, F. C., & Watanakunakorn, C. (1997). Society for Healthcare Epidemiology of America and Infectious Diseases Society of America Joint Committee on the Prevention of Antimicrobial Resistance: guidelines for the prevention of antimicrobial resistance in hospitals. Clinical Infectious Diseases, 25, 584 –599. Thornsberry, C., Jones, M. E., Hickey, M. L., Mauriz, Y., Kahn, J., & Sahm, D. F. (1999). Resistance surveillance of Streptococcus pneu-

R.L. White et al. / Diagnostic Microbiology and Infectious Disease 43 (2002) 207–217 moniae, Haemophilus influenzae and Moraxella catarrhalis isolated in the United States, 1997–1998. Journal of Antimicrobial Chemotherapy, 44, 749 –759. White, R. L., Enzweiler, K. A., Friedrich, L. V., Lorenz, K. R., Voit, E., Sims, K., Hood, W. C., & Bosso, J. A. Monte Carlo Analysis of Levofloxacin and Gatifloxacin Pharmacodynamics Using Expected. Dosing in a Patient Population with Varying Degrees of Renal Function (RF) and 4,738 Recent Clinical Isolates of Streptococcus pneumoniae

217

(SP), abstract 2073. In Programs and Abstracts of the 41st Interscience Conference on Antimicrobial Agents and Chemotherapy. American Society for Microbiology, Chicago, IL. Whitney, C. G., Farley, M. M., Hadler, J., Harrison, L. H., Lexau, C., Reingold, A., Lefkowitz, L., Cieslak, C., Cetron, M., Zell, E. R., Jorgensen, J. H., & Schuchat, A. (2000). Increasing prevalence of multidrug-resistant Streptococcus pneumoniae in the United States. New England Journal of Medicine, 343, 1917–1924.