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Clinical importance of cefoxitin-resistant Bacteroides fragilis isolates
DIAGNMICROBIOLINFECT DIS 1987;7:119-126
119
ANTIMICROBIAL STUDIES
Clinical Importance of Cefoxitin-Resistant Bacteroides fragilis Isolates Virginia M. Bieluch, George J. Cuchural, David R. Snydman, Sherwood L. Gorbach, and Francis P. Tally*
Resistance to cefoxitin among species of the Bacteroides fragilis group of organisms has remained low (8%-10%) in a multicenter nationwide survey. However, a statistically significant increase in the percentage of B_:.fragilis group organisms resistant to cefoxitin was found at Tufts-New England Medical Center from 1981 to 1982. Non fragilis species accounted for most of the resistance. The presence of cefoxitin resistance in B_:.fragilis isolates correlated with resistance to other antibiotics. The presence of cefoxitin-resistant B. fragilis group organisms also correlated with the presence of other cefoxitin-resistant bacteria. No difference could be detected in therapeutic outcome of patients with cefoxitin-sensitive or cefoxitin-resistant B._:. fragilis group organisms, regardless of treatment with cefoxitin or other antibiotics. INTRODUCTION Cefoxitin has been u s e d w i d e l y to treat m i x e d surgical and gynecologic infections because its antimicrobial s p e c t r u m encompasses facultative organisms as well as Bacteroides fragilis group organisms. Due to the c o m p l e x i t y of antimicrobial susceptibility testing of anaerobic bacteria, most clinical laboratories do not r o u t i n e l y perform such tests. Therefore, clinicians must rely on p u b l i s h e d data. A nationwide survey of B. frogilis group organisms was undertaken at the Tufts-New England Medical Center in cooperation with eight other centers, beginning in 1981, to determine the antimicrobial susceptibility of these isolates. Nationwide, the rate of resistance of B. fragiJis group organisms to cefoxitin d i d not change significantly from 1981 (8%) to 1982 (10%) (Tally et al., 1983; Cuchural et al., 1984). The rate of cefoxitin resistance at New England Medical Center, however, increased from 14% to 30% during this period, a statistically significant increment (p = 0.012). Because of this dramatic increase, a retrospective analysis of the patients infected w i t h susceptible and resistant strains of B. fragilis group organisms was u n d e r t a k e n to define the risk factors and clinical significance of cefoxitin resistance in vitro. From the Infectious Diseases Division, Department of Medicine (V.M.B., G.J.C., D.R.S., S.L.G.), and Clinical Microbiology Laboratory, Department of Pathology (F.P.T.), Tufts University School of Medicine, and New England Medical Center Hospitals, Boston, MA. *An abstract of this work was published in the Program and Abstracts of the 23rd Interscience Conference on Antimicrobial Agents and Chemotherapy (abstract 164). Address reprint requests to: Dr. Virginia M. Bieluch, Departments of Medicine and Microbiology, Medical College of Ohio, C.S. 10008, Toledo, OH 43699. Received November 12, 1986; revised and accepted February 25, 1987.
© 1987 Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
0732-8893/87/$03.50
120
V.M. Bieluch et al.
MATERIALS AND METHODS
Identification and Susceptibility Testing Identification of B. fragilis group organisms was performed in the Tufts Anaerobe Laboratory by methods outlined by the Virginia Polytechnic Anaerobe Manual (Holdeman et al., 1977). Susceptibility testing of these isolates was carried out using an agar dilution technique with a Steer's ~.g/ml replicator (Tally et al., 1975). Strains having an MIC above 16 were classified as resistant. Only the first isolate of a given Bacteroides spp. from a patient was counted in the analysis. However, six patients had more than one Bacteroides spp. resistant to cefoxitin isolated during their hospitalization.
Collection of Data A retrospective analysis was made of the hospital charts of those patients from whom susceptible and resistant B. fragilis group isolates were recovered in 1981 and 1982. Any patient who had a resistant Bacteroides spp, was counted only in the resistant group, even if other susceptible isolates were also isolated. All charts of the 22 patients with resistant organisms and charts from 57 of the 65 patients with susceptible isolates were available for review. Data collected by means of a standardized questionnaire included age, length of hospitalization, sex, hospital versus community acquisition of infection, polymicrobial nature of infection, previous surgery, presence of other cefoxitin resistant isolates, previous antibiotic therapy, and underlying illnesses. Standard definitions of the National Nosocomial Infections study were used in determination of hospital versus community acquisition of infections (Centers for Disease Control, 1972). The category of other underlying diseases was defined to include diabetes mellitus, steroid therapy, immunosuppression, inflammatory bowel disease, cardiovascular disease, peripheral vascular disease, cancer, or severe neurologic dysfunction. Antibiotic utilization data was available from October 1982 through the end of 1983 and was obtained from the records of the hospital pharmacy. Antibiotic distribution methods prior to that time were handled in a different manner in the Intensive Care Units and total utilization data were not available at the time of manuscript preparation.
Outcome Analysis Empiric antibiotic therapy was defined as that chosen at the time of diagnosis by the physicians involved in the patient's care. A patient was considered to have failed empiric antibiotic therapy if he/she had a rapidly downhill course, despite antibiotic therapy of at least 48 hr duration, failed to improve after having received at least 48 hr of therapy, or required a change in antibiotic therapy because of isolation of a resistant organism. Patients were considered cured if there was resolution of the signs and symptoms of their infection and no recurrence. Patients were considered improved if they had received at least 48 hr of therapy and there was improvement in the signs and symptoms of infection, although further surgery may have been required for cure, the infection may not have been completely cured, or it may have recurred. Patients whose antibiotics were changed because of suspected adverse effects were considered separately.
Clinical Importance of Cefoxitin-Resistant Isolates
121
TABLE 1. Species Differences for Cefoxitin Resistance 1981
1982
Species
Number resistant/Total (%)
Number resistant/Total (%)
B. fragilis B. ovatus-thetaiotaomicron B. vulgatus
3/47 6/30 1/4 2/6 1/6 0/3
(6) (20) (25) (33) (17) (0)
2/27 10/17 3/6 3/7 0/2 0/1
(7) (59) (50) (43) (0) (0)
13/96
(14)
18/60
(30)
B. distasonis B. uniformis B. species Total
Statistical Analysis Statistical analysis was carried out by means of Chi squares for categorical variables, and t-test on means for c o n t i n u o u s variables (Colton, 1974}.
RESULTS The rate of cefoxitin resistance among m e m b e r s of the B. fragilis group of organisms at T u f t s - N e w England Medical Center rose from 14% in 1981 to 30% in 1982 (p = 0.012; Table 1). Non-fragilis species were more likely to be resistant to cefoxitin than B. frogilis. In 1982, the rate of resistance among B. fragilis (7%) was significantly lower than the rate f o u n d in the non-fragilis species (48%; p = 0.0015). Similarly, the rate of resistance of Bacteroides ovatus-thetaiotaomicron (59%) during 1982 was significantly higher than that of the other species (p = 0.006). Table 2 lists the species of the B. frogilis group and the sites of isolation of the resistant and susceptible organisms. Proportionally, more resistant organisms were associated w i t h a b d o m i n a l infections and fewer w i t h blood stream and skin soft tissue infections, w h e n c o m p a r e d w i t h cefoxitin-sensitive organisms. In 1982, the cefoxitin-resistant B. fragilis group isolates at our hospital were significantly more often resistant to other antibiotics tested than cefoxitin-sensitive strains (Table 3). These other antibiotics i n c l u d e d c e p h a l o s p o r i n s (moxalactam, cefotaxime, cefoperazone), the u r e i d o p e n i c i l l i n , piperacillin, as well as the non-[3lactam agent, c l i n d a m y c i n . A n analysis of the clinical characteristics of the patients with susceptible and resistant organisms was performed. A l t h o u g h there were too few patients to dem-
TABLE 2. Sites of Isolation*
Site Abdomen Blood Skin and soft tissue Other Unknown *Data for 1981 and 1982 combined.
Cefoxitin-resistant isolates (%} (n = 31) 23 1 6 1 0
(74) (3) (19) (3) (0)
Cefoxitin-susceptible isolates (%) (n = 125) 62 16 35 7 5
(50) (13) (28) (6) (4}
V.M. Bieluch et al.
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TABLE 3. Crossresistance to Other Antibiotics A m o n g Cefoxitin-Resistant and Susceptible B. frailis Group Organisms in 1982 Percent resistant Cfx-resistant
Cfx-susceptible
B. fragilis group (%)
B. fragilis group (%)
Antibiotic
(n = 18)a
(n = 42) 0
Chi-square
p-value
Moxalactam Cefotaxime Cefoperazone Piperacillin Clindamycin Tetracycline
72 100 100 33 22 72
5 43 43 3 2 52
30.58 14.28 14.28 9.81 6.49 2.04
<0.001 <0.001 <0.001 <0.005 <0.025 <0.5
°Maximum number of isolates in each group.
onstrate statistical significance, there were trends toward certain clinical characteristics in the patients with cefoxitin-resistant isolates. Patients with cefoxitin resistant B. fragilis group isolates were older (54 versus 47 yr), had undergone more recent surgery (23% versus 13%), had more surgery at any time (55% versus 33%), had longer hospitalizations (35 versus 28 days), had received previous antibiotics more often (41% versus 32%), and had more u n d e r l y i n g illnesses (73% versus 58%), than did patients without such organisms (Table 4). The only significant finding was that the presence of cefoxitin-resistant B. fragilis group organisms was correlated with the presence of other cefoxitin-resistant aerobic and anaerobic bacteria at the same site of infection. Figure 1 reveals the cefoxitin utilization data from T u f t s - N e w England Medical Center from October 1982 through December 1983. Toward the end of 1983 a downward trend in the utilization of cefoxitin was noted. A review of the therapeutic outcomes of patients with cefoxitin-sensitive or resistant B. fragilis group organisms revealed no differences, regardless of whether cefoxitin or other antibiotics were used as empirical treatment (Table 5). Patients with resistant organisms failed cefoxitin t h e r a p y 30% of the time and all other antibiotics 33% of the time. Patients w i t h cefoxitin-sensitive B. fragilis group organisms failed cefoxitin therapy 41% of the time, and other antibiotics 23% of the time. DISCUSSION Although the n a t i o n w i d e rate of resistance of B. frogilis group organisms to cefoxitin has r e m a i n e d low, there was a statistically significant increase in cefoxitin-resistance in this group of organisms from 1981 to 1982 at T u f t s - N e w England Medical Center. Most of the resistance was attributable to the non-fragilis species, B. ovatus-thetoiotaomicron, Bacteroides vulgotus, and Bocteroides distasonis. Few cefoxitin-resistant organisms were isolated from the blood stream, probably reflecting the low incidence of B. fragilis among the resistant organisms. The one statistically significant clinical characteristic detected in our patients with cefoxitin-resistant B. fragilis group isolates was the correlation with the presence of other cefoxitin-resistant aerobic and anaerobic bacteria. This, c o m b i n e d with significant cross-resistance to other antibiotics among the cefoxitin-resistant B. fragilis group isolates is of concern, because it m a y render these infections difficult to treat. A m o n g patients with cefoxitin-resistant B. fragilis group isolates there were trends toward older age, more previous surgery, longer hospitalization, more previous an-
Clinical Importance of Cefoxitin-Resistant Isolates
123
TABLE 4. Clinical Characteristics of Patients with Cefoxitin-Resistant and Susceptible B. fragilis Group Isolates*
Mean age (years +- S.E.) Hospital days (+- S.E.) Survivors Sex:Male Female Hospital acquired Community acquired Polymicrobial One organism Previous related surgery Ever: Yes No Previous related surgery 2mo:Yes No Other cfx-resistant organisms Yes No Previous antibiotics Yes No Unknown Previous cefoxitin Yes No Underlying illnesses Yes No
Resistant (n = 22)
Susceptible (n = 57)
p-value
53.95 +- 17.18 34.63 +- 30.11
47.51 _+ 21.17 28.25 -+ 50.74
NS NS
14 (64%) 8 (36%) 9 (41%) 13 (59%) 22 (100%) 0
30 27 19 38 48 9
(53%) (47%) (33%) (67%) (84%) (16%)
NS
12 10
(55%) (45%)
19 38
(33%) (67%)
NS
5 17
(23%) (77%)
7 50
(13%) (88%)
NS
15 7
(68%) (32%)
20 37
(35%) (65%)
<0.02
9 13 0
(41%) (59%)
18 38 1
(32%) (67%) (2%)
NS
2 20
(9%) (91%)
5 52
(9%) (91%)
NS
16 6
(73%) (27%)
33 24
(58%) (42%)
NS
NS NS NS
"1981 and 1982 data combined.
FIGURE 1. Cefoxitin utilization data from New England Medical Center from October 1982 through December 1983 revealed a decreased usage of this drug in the later half of 1983. 5.000
E O~
4,000
.=_. X
3.000
0 2,000
0 :~ O
1.ooo
E <¢
Oct - Dec 1982
J a n - Mar 1983
Apr - J u n 1983
Jul - Solo 1983
Oct ~ Dec 1983
V.M. Bieluch et al.
124
TABLE 5. Outcome of Empiric Antibiotic Therapy in Patients with CefoxitinResistant and Susceptible B. fragilis Group Isolates Cefoxitin-resistant B. fragilis group Number Cured or improved Failed Not assessable Total Cured or improved Failed Not assessable Total
7 3 1 11
Percentage assessable
Cefoxitin therapy (70%} (30%)
X2 = 0.039 Other antibiotic therapy 6 (67%) 3 (33%) 2 11 X2 = 0.026
Cefoxitin-susceptible B. fragilis group Number
Percentage assessable
13 9 3 25
(59%} {41%}
17 5 10 32
(77%} (23%)
tibiotic therapy, and more underlying illness. It may well be that the number of patients included in this analysis was too small to attain clinical significance. Similarly, Drusano et al. found that patients infected with cefoxitin-resistant facultative and aerobic bacteria had received prior antibiotics more often, had longer durations of hospitalization prior to study, and had a greater incidence of multiple organ failure at the time of study than patients lacking cefoxitin-resistant organisms (Drusano et al., 1982). These observations most likely reflect the emergence of resistant organisms among seriously ill patients. The high failure rates reported in this study have several possible explanations. Tufts-New England Medical Center is a tertiary referral center and often receives, in transfer, patients with a poor prognosis who have failed therapy elsewhere. Furthermore, our hospital lacks an obstetric service, which would have relatively healthy patients with mixed aerobic and anaerobic bacterial infections. This latter group of patients would be expected to have a better prognosis and if included in randomized studies, would result in a lower failure rate. In another study comparing cefoxitin to clindamycin-aminoglycoside as therapy for mixed anaerobic-aerobic bacterial infections, failure rates of 40%-50% were found in patients with intraabdominal infections treated with either regimen (Drusano et al., 1982). In contrast, among patients with genital tract infections, the failure rate was 2% (DiZerega et al., 1979). The 30% failure rates seen in patients with cefoxitin-resistant B. fragilis group isolates regardless of the type of antibiotic therapy may reflect the poorer prognosis of these patients who tended to have other clinical characteristics, which would predict a poor clinical outcome. The 41% failure rate among patients with cefoxitinsusceptible isolates treated with cefoxitin is more difficult to explain. As many of the patients treated with other antibiotics received combination therapy, and some patients treated with cefoxitin received only single drug therapy, one might speculate that single drug therapy may not be adequate in some cases of mixed aerobic and anaerobic bacterial infections. As this study was designed to look at cefoxitin-resistance in B. fragilis group organisms, no conclusions can be made concerning the importance of other cefoxitin-resistant organisms, including facultative/aerobic gramnegative rods, in the poor response rate of these patients.
Clinical Importance of Cefoxitin-Resistant Isolates
125
A trend in increased usage of cefoxitin in our hospital began during the period of this analysis and continued into 1983. The increase in cefoxitin-resistant B. fragilis group isolates, paralleling the increased usage of this antibiotic, may indicate that some of these infections were acquired nosocomially. Indeed, when usage of cefoxitin decreased in 1983, the rate of resistance of B. fragilis group organisms at New England Medical Center decreased to 13% in 1983. Cefoxitin's excellent activity against B. fragilis group organisms has been attributed to its resistance to hydrolysis by the [3-1actamases commonly produced by these organisms (Darland and Birnbaum, 1977; Tally et al., 1979; Neu, 1983). The most common mechanism of resistance to cefoxitin in this group of organisms is failure of this antibiotic to penetrate through the outer membrane of the cell (Olsson et al., 1979), and this impermeability is likely a major factor accounting for resistance in the isolates reported here. However, high levels of B. fragilis ~-lactamase activity have been demonstrated in cefoxitin-resistant isolates and at these high levels, may contribute to resistance, despite cefoxitin's relatively high degree of stability to this enzyme (G.J. Cuchural et al., Abstr. Annu. Meet. Am. Soc. Microbiol. 1985, 133 (A 52) p. 114). Recently, several groups of investigators have demonstrated hydrolysis of cefoxitin by other ~-lactamases produced by B. fragilis strains (Yotsuju et al., 1983; Cuchural et al., 1986a; Cuchural et al., 1986b). Investigation of one of the cefoxitin-resistant strains from this study, TAL3636, showed that both limited outer membrane permeability and hydrolysis of cefoxitin by a novel f3-1actamase both play a role in the cefoxitin-resistance manifested by this organism (Cuchural et al., 1986b). Although many questions concerning the importance of resistance to cefoxitin in B. fragilis group organisms remain to be answered, this study demonstrates an association between the presence of cefoxitin-resistant B. fragilis group organisms and other cefoxitin-resistant facultative-aerobic Gram-negative rods. Also, cefoxitin resistance is more common in non-fragilis Bacteroides spp. and is associated with resistance to other antibiotics, both ~-lactam and non-[3-1actam. Furthermore, the clinical findings in these patients suggest that infections with these organisms occur in a more seriously ill population. As treatment of mixed aerobic and anaerobic bacterial infections is multifactorial, involving both surgical and medical modalities, any complicating factors, such as antibiotic resistance, may render such infections difficult to eradicate. Therefore, attempts must be made to optimize therapy to maximize chances of a successful outcome.
REFERENCES
Centers for Disease Control (1972) Outline for Surveillance and Control of Nosocomial Infections. Atlanta: CDC. Colton T (1974) Statistics in Medicine. Boston: Little, Brown and Company, pp. 138-143, 175-179. Cuchural GJ, Tally FP, Jacobus NV, Gorbach SL, Aldridge KE, Cleary TJ, Finegold SM, Hill GB, Iannini PB, O'Keefe JP, Pierson CL (1984) Antimicrobial susceptibilities of 1,292 isolates of the Bacteroides fragilis group in the United States: Comparison of 1981 and 1982. Antimicrob Agents Chemother 26:145. Cuchural GJ, Tally FP, Storey JR, Malamy MH (1986a) Transfer of B-Lactamase-associated cefoxitin resistance in Bacteroides fragilis. Antimicrob Agents Chemother 29:918. Cuchural GJ, Malamy MH, Tally FP (1986b) [3-Lactamase-mediated imipenem resistnce inBacteroides fragilis. Antimicrob Agents Chemather 30:645. Darland KP, Birnbaum J (1977) Cefoxitin resistnace to beta-lactamase: A major factor for susceptibility of Bacteroides fragilis to the antibiotic, Antimicrob Agents Chemother 11:725.
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DiZerega G, Yonekura L, Roy S, et al. (1979) A comparison of clindamycin-gentamicinand penicillin-gentamicinin the treatment of post-cesarean section endomyometritis. Am J Obstet Gynec 134:238. Drusano GL, Waren JW, Saah AJ, Caplan EJ, Tenney JH, Hansen S, Granados J, Standiford HC, Miller EH (1982) Prospective randomized controlled trial of cefoxitin versus clindamycin-aminoglycoside in mixed anaerobic-aerobic infections. Surg Gynecol Obstet 154:715. Holdeman LV, Cato EP, Moore WEC (1977) Anaerobe Laboratory Manual, 4th ed. Blacksburg, VA: Virginia Polytechnic Institute and State University. Neu H (1983) [3-1actamase stability of cefoxitin in comparison with other [3-1actamcompounds. Diagn Microbial Infect Dis 1:313. Olsson B, Dornbasch K, Nord CE (1979) Factors contributing to resistance to beta-lactam antibiotics in Bacteroides fragilis. Antimicrob Agents Chemother 15:263. Tally FP, Cuchural GJ, Jacobus NV, Gorbach SL, Aldridge KE, Cleary TJ, Finegold SM, Hill GB, Iannini PB, McCloskey MV, O'Keefe JP, Pierson CL (1983) Susceptibility of the Bacteraides fragilis group in the United States in 1981. Antimicrob Agents Chemother 23:536. Tally FP, Jacobus NV, Bartlett JG, Gorbach SL (1975) Susceptibility of anaerobes to cefoxitin and cephalosporins. Antimicrob Agents Chemother 7:128. Tally FP, O'Keefe JP, Sullivan NM, Gorbach SL (1979) Inactivation of cephalosporins by Bacteroides. Antimicrob Agents Chemother 16:565. Yotsuji A, Minami S, Inoue M, Mitsuhashi S (1983) Properties of novel ~-lactamase produced by Bacteroides fragilis. Antimicrab Agents Chemother 24:925.
119
ANTIMICROBIAL STUDIES
Clinical Importance of Cefoxitin-Resistant Bacteroides fragilis Isolates Virginia M. Bieluch, George J. Cuchural, David R. Snydman, Sherwood L. Gorbach, and Francis P. Tally*
Resistance to cefoxitin among species of the Bacteroides fragilis group of organisms has remained low (8%-10%) in a multicenter nationwide survey. However, a statistically significant increase in the percentage of B_:.fragilis group organisms resistant to cefoxitin was found at Tufts-New England Medical Center from 1981 to 1982. Non fragilis species accounted for most of the resistance. The presence of cefoxitin resistance in B_:.fragilis isolates correlated with resistance to other antibiotics. The presence of cefoxitin-resistant B. fragilis group organisms also correlated with the presence of other cefoxitin-resistant bacteria. No difference could be detected in therapeutic outcome of patients with cefoxitin-sensitive or cefoxitin-resistant B._:. fragilis group organisms, regardless of treatment with cefoxitin or other antibiotics. INTRODUCTION Cefoxitin has been u s e d w i d e l y to treat m i x e d surgical and gynecologic infections because its antimicrobial s p e c t r u m encompasses facultative organisms as well as Bacteroides fragilis group organisms. Due to the c o m p l e x i t y of antimicrobial susceptibility testing of anaerobic bacteria, most clinical laboratories do not r o u t i n e l y perform such tests. Therefore, clinicians must rely on p u b l i s h e d data. A nationwide survey of B. frogilis group organisms was undertaken at the Tufts-New England Medical Center in cooperation with eight other centers, beginning in 1981, to determine the antimicrobial susceptibility of these isolates. Nationwide, the rate of resistance of B. fragiJis group organisms to cefoxitin d i d not change significantly from 1981 (8%) to 1982 (10%) (Tally et al., 1983; Cuchural et al., 1984). The rate of cefoxitin resistance at New England Medical Center, however, increased from 14% to 30% during this period, a statistically significant increment (p = 0.012). Because of this dramatic increase, a retrospective analysis of the patients infected w i t h susceptible and resistant strains of B. fragilis group organisms was u n d e r t a k e n to define the risk factors and clinical significance of cefoxitin resistance in vitro. From the Infectious Diseases Division, Department of Medicine (V.M.B., G.J.C., D.R.S., S.L.G.), and Clinical Microbiology Laboratory, Department of Pathology (F.P.T.), Tufts University School of Medicine, and New England Medical Center Hospitals, Boston, MA. *An abstract of this work was published in the Program and Abstracts of the 23rd Interscience Conference on Antimicrobial Agents and Chemotherapy (abstract 164). Address reprint requests to: Dr. Virginia M. Bieluch, Departments of Medicine and Microbiology, Medical College of Ohio, C.S. 10008, Toledo, OH 43699. Received November 12, 1986; revised and accepted February 25, 1987.
© 1987 Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
0732-8893/87/$03.50
120
V.M. Bieluch et al.
MATERIALS AND METHODS
Identification and Susceptibility Testing Identification of B. fragilis group organisms was performed in the Tufts Anaerobe Laboratory by methods outlined by the Virginia Polytechnic Anaerobe Manual (Holdeman et al., 1977). Susceptibility testing of these isolates was carried out using an agar dilution technique with a Steer's ~.g/ml replicator (Tally et al., 1975). Strains having an MIC above 16 were classified as resistant. Only the first isolate of a given Bacteroides spp. from a patient was counted in the analysis. However, six patients had more than one Bacteroides spp. resistant to cefoxitin isolated during their hospitalization.
Collection of Data A retrospective analysis was made of the hospital charts of those patients from whom susceptible and resistant B. fragilis group isolates were recovered in 1981 and 1982. Any patient who had a resistant Bacteroides spp, was counted only in the resistant group, even if other susceptible isolates were also isolated. All charts of the 22 patients with resistant organisms and charts from 57 of the 65 patients with susceptible isolates were available for review. Data collected by means of a standardized questionnaire included age, length of hospitalization, sex, hospital versus community acquisition of infection, polymicrobial nature of infection, previous surgery, presence of other cefoxitin resistant isolates, previous antibiotic therapy, and underlying illnesses. Standard definitions of the National Nosocomial Infections study were used in determination of hospital versus community acquisition of infections (Centers for Disease Control, 1972). The category of other underlying diseases was defined to include diabetes mellitus, steroid therapy, immunosuppression, inflammatory bowel disease, cardiovascular disease, peripheral vascular disease, cancer, or severe neurologic dysfunction. Antibiotic utilization data was available from October 1982 through the end of 1983 and was obtained from the records of the hospital pharmacy. Antibiotic distribution methods prior to that time were handled in a different manner in the Intensive Care Units and total utilization data were not available at the time of manuscript preparation.
Outcome Analysis Empiric antibiotic therapy was defined as that chosen at the time of diagnosis by the physicians involved in the patient's care. A patient was considered to have failed empiric antibiotic therapy if he/she had a rapidly downhill course, despite antibiotic therapy of at least 48 hr duration, failed to improve after having received at least 48 hr of therapy, or required a change in antibiotic therapy because of isolation of a resistant organism. Patients were considered cured if there was resolution of the signs and symptoms of their infection and no recurrence. Patients were considered improved if they had received at least 48 hr of therapy and there was improvement in the signs and symptoms of infection, although further surgery may have been required for cure, the infection may not have been completely cured, or it may have recurred. Patients whose antibiotics were changed because of suspected adverse effects were considered separately.
Clinical Importance of Cefoxitin-Resistant Isolates
121
TABLE 1. Species Differences for Cefoxitin Resistance 1981
1982
Species
Number resistant/Total (%)
Number resistant/Total (%)
B. fragilis B. ovatus-thetaiotaomicron B. vulgatus
3/47 6/30 1/4 2/6 1/6 0/3
(6) (20) (25) (33) (17) (0)
2/27 10/17 3/6 3/7 0/2 0/1
(7) (59) (50) (43) (0) (0)
13/96
(14)
18/60
(30)
B. distasonis B. uniformis B. species Total
Statistical Analysis Statistical analysis was carried out by means of Chi squares for categorical variables, and t-test on means for c o n t i n u o u s variables (Colton, 1974}.
RESULTS The rate of cefoxitin resistance among m e m b e r s of the B. fragilis group of organisms at T u f t s - N e w England Medical Center rose from 14% in 1981 to 30% in 1982 (p = 0.012; Table 1). Non-fragilis species were more likely to be resistant to cefoxitin than B. frogilis. In 1982, the rate of resistance among B. fragilis (7%) was significantly lower than the rate f o u n d in the non-fragilis species (48%; p = 0.0015). Similarly, the rate of resistance of Bacteroides ovatus-thetaiotaomicron (59%) during 1982 was significantly higher than that of the other species (p = 0.006). Table 2 lists the species of the B. frogilis group and the sites of isolation of the resistant and susceptible organisms. Proportionally, more resistant organisms were associated w i t h a b d o m i n a l infections and fewer w i t h blood stream and skin soft tissue infections, w h e n c o m p a r e d w i t h cefoxitin-sensitive organisms. In 1982, the cefoxitin-resistant B. fragilis group isolates at our hospital were significantly more often resistant to other antibiotics tested than cefoxitin-sensitive strains (Table 3). These other antibiotics i n c l u d e d c e p h a l o s p o r i n s (moxalactam, cefotaxime, cefoperazone), the u r e i d o p e n i c i l l i n , piperacillin, as well as the non-[3lactam agent, c l i n d a m y c i n . A n analysis of the clinical characteristics of the patients with susceptible and resistant organisms was performed. A l t h o u g h there were too few patients to dem-
TABLE 2. Sites of Isolation*
Site Abdomen Blood Skin and soft tissue Other Unknown *Data for 1981 and 1982 combined.
Cefoxitin-resistant isolates (%} (n = 31) 23 1 6 1 0
(74) (3) (19) (3) (0)
Cefoxitin-susceptible isolates (%) (n = 125) 62 16 35 7 5
(50) (13) (28) (6) (4}
V.M. Bieluch et al.
122
TABLE 3. Crossresistance to Other Antibiotics A m o n g Cefoxitin-Resistant and Susceptible B. frailis Group Organisms in 1982 Percent resistant Cfx-resistant
Cfx-susceptible
B. fragilis group (%)
B. fragilis group (%)
Antibiotic
(n = 18)a
(n = 42) 0
Chi-square
p-value
Moxalactam Cefotaxime Cefoperazone Piperacillin Clindamycin Tetracycline
72 100 100 33 22 72
5 43 43 3 2 52
30.58 14.28 14.28 9.81 6.49 2.04
<0.001 <0.001 <0.001 <0.005 <0.025 <0.5
°Maximum number of isolates in each group.
onstrate statistical significance, there were trends toward certain clinical characteristics in the patients with cefoxitin-resistant isolates. Patients with cefoxitin resistant B. fragilis group isolates were older (54 versus 47 yr), had undergone more recent surgery (23% versus 13%), had more surgery at any time (55% versus 33%), had longer hospitalizations (35 versus 28 days), had received previous antibiotics more often (41% versus 32%), and had more u n d e r l y i n g illnesses (73% versus 58%), than did patients without such organisms (Table 4). The only significant finding was that the presence of cefoxitin-resistant B. fragilis group organisms was correlated with the presence of other cefoxitin-resistant aerobic and anaerobic bacteria at the same site of infection. Figure 1 reveals the cefoxitin utilization data from T u f t s - N e w England Medical Center from October 1982 through December 1983. Toward the end of 1983 a downward trend in the utilization of cefoxitin was noted. A review of the therapeutic outcomes of patients with cefoxitin-sensitive or resistant B. fragilis group organisms revealed no differences, regardless of whether cefoxitin or other antibiotics were used as empirical treatment (Table 5). Patients with resistant organisms failed cefoxitin t h e r a p y 30% of the time and all other antibiotics 33% of the time. Patients w i t h cefoxitin-sensitive B. fragilis group organisms failed cefoxitin therapy 41% of the time, and other antibiotics 23% of the time. DISCUSSION Although the n a t i o n w i d e rate of resistance of B. frogilis group organisms to cefoxitin has r e m a i n e d low, there was a statistically significant increase in cefoxitin-resistance in this group of organisms from 1981 to 1982 at T u f t s - N e w England Medical Center. Most of the resistance was attributable to the non-fragilis species, B. ovatus-thetoiotaomicron, Bacteroides vulgotus, and Bocteroides distasonis. Few cefoxitin-resistant organisms were isolated from the blood stream, probably reflecting the low incidence of B. fragilis among the resistant organisms. The one statistically significant clinical characteristic detected in our patients with cefoxitin-resistant B. fragilis group isolates was the correlation with the presence of other cefoxitin-resistant aerobic and anaerobic bacteria. This, c o m b i n e d with significant cross-resistance to other antibiotics among the cefoxitin-resistant B. fragilis group isolates is of concern, because it m a y render these infections difficult to treat. A m o n g patients with cefoxitin-resistant B. fragilis group isolates there were trends toward older age, more previous surgery, longer hospitalization, more previous an-
Clinical Importance of Cefoxitin-Resistant Isolates
123
TABLE 4. Clinical Characteristics of Patients with Cefoxitin-Resistant and Susceptible B. fragilis Group Isolates*
Mean age (years +- S.E.) Hospital days (+- S.E.) Survivors Sex:Male Female Hospital acquired Community acquired Polymicrobial One organism Previous related surgery Ever: Yes No Previous related surgery 2mo:Yes No Other cfx-resistant organisms Yes No Previous antibiotics Yes No Unknown Previous cefoxitin Yes No Underlying illnesses Yes No
Resistant (n = 22)
Susceptible (n = 57)
p-value
53.95 +- 17.18 34.63 +- 30.11
47.51 _+ 21.17 28.25 -+ 50.74
NS NS
14 (64%) 8 (36%) 9 (41%) 13 (59%) 22 (100%) 0
30 27 19 38 48 9
(53%) (47%) (33%) (67%) (84%) (16%)
NS
12 10
(55%) (45%)
19 38
(33%) (67%)
NS
5 17
(23%) (77%)
7 50
(13%) (88%)
NS
15 7
(68%) (32%)
20 37
(35%) (65%)
<0.02
9 13 0
(41%) (59%)
18 38 1
(32%) (67%) (2%)
NS
2 20
(9%) (91%)
5 52
(9%) (91%)
NS
16 6
(73%) (27%)
33 24
(58%) (42%)
NS
NS NS NS
"1981 and 1982 data combined.
FIGURE 1. Cefoxitin utilization data from New England Medical Center from October 1982 through December 1983 revealed a decreased usage of this drug in the later half of 1983. 5.000
E O~
4,000
.=_. X
3.000
0 2,000
0 :~ O
1.ooo
E <¢
Oct - Dec 1982
J a n - Mar 1983
Apr - J u n 1983
Jul - Solo 1983
Oct ~ Dec 1983
V.M. Bieluch et al.
124
TABLE 5. Outcome of Empiric Antibiotic Therapy in Patients with CefoxitinResistant and Susceptible B. fragilis Group Isolates Cefoxitin-resistant B. fragilis group Number Cured or improved Failed Not assessable Total Cured or improved Failed Not assessable Total
7 3 1 11
Percentage assessable
Cefoxitin therapy (70%} (30%)
X2 = 0.039 Other antibiotic therapy 6 (67%) 3 (33%) 2 11 X2 = 0.026
Cefoxitin-susceptible B. fragilis group Number
Percentage assessable
13 9 3 25
(59%} {41%}
17 5 10 32
(77%} (23%)
tibiotic therapy, and more underlying illness. It may well be that the number of patients included in this analysis was too small to attain clinical significance. Similarly, Drusano et al. found that patients infected with cefoxitin-resistant facultative and aerobic bacteria had received prior antibiotics more often, had longer durations of hospitalization prior to study, and had a greater incidence of multiple organ failure at the time of study than patients lacking cefoxitin-resistant organisms (Drusano et al., 1982). These observations most likely reflect the emergence of resistant organisms among seriously ill patients. The high failure rates reported in this study have several possible explanations. Tufts-New England Medical Center is a tertiary referral center and often receives, in transfer, patients with a poor prognosis who have failed therapy elsewhere. Furthermore, our hospital lacks an obstetric service, which would have relatively healthy patients with mixed aerobic and anaerobic bacterial infections. This latter group of patients would be expected to have a better prognosis and if included in randomized studies, would result in a lower failure rate. In another study comparing cefoxitin to clindamycin-aminoglycoside as therapy for mixed anaerobic-aerobic bacterial infections, failure rates of 40%-50% were found in patients with intraabdominal infections treated with either regimen (Drusano et al., 1982). In contrast, among patients with genital tract infections, the failure rate was 2% (DiZerega et al., 1979). The 30% failure rates seen in patients with cefoxitin-resistant B. fragilis group isolates regardless of the type of antibiotic therapy may reflect the poorer prognosis of these patients who tended to have other clinical characteristics, which would predict a poor clinical outcome. The 41% failure rate among patients with cefoxitinsusceptible isolates treated with cefoxitin is more difficult to explain. As many of the patients treated with other antibiotics received combination therapy, and some patients treated with cefoxitin received only single drug therapy, one might speculate that single drug therapy may not be adequate in some cases of mixed aerobic and anaerobic bacterial infections. As this study was designed to look at cefoxitin-resistance in B. fragilis group organisms, no conclusions can be made concerning the importance of other cefoxitin-resistant organisms, including facultative/aerobic gramnegative rods, in the poor response rate of these patients.
Clinical Importance of Cefoxitin-Resistant Isolates
125
A trend in increased usage of cefoxitin in our hospital began during the period of this analysis and continued into 1983. The increase in cefoxitin-resistant B. fragilis group isolates, paralleling the increased usage of this antibiotic, may indicate that some of these infections were acquired nosocomially. Indeed, when usage of cefoxitin decreased in 1983, the rate of resistance of B. fragilis group organisms at New England Medical Center decreased to 13% in 1983. Cefoxitin's excellent activity against B. fragilis group organisms has been attributed to its resistance to hydrolysis by the [3-1actamases commonly produced by these organisms (Darland and Birnbaum, 1977; Tally et al., 1979; Neu, 1983). The most common mechanism of resistance to cefoxitin in this group of organisms is failure of this antibiotic to penetrate through the outer membrane of the cell (Olsson et al., 1979), and this impermeability is likely a major factor accounting for resistance in the isolates reported here. However, high levels of B. fragilis ~-lactamase activity have been demonstrated in cefoxitin-resistant isolates and at these high levels, may contribute to resistance, despite cefoxitin's relatively high degree of stability to this enzyme (G.J. Cuchural et al., Abstr. Annu. Meet. Am. Soc. Microbiol. 1985, 133 (A 52) p. 114). Recently, several groups of investigators have demonstrated hydrolysis of cefoxitin by other ~-lactamases produced by B. fragilis strains (Yotsuju et al., 1983; Cuchural et al., 1986a; Cuchural et al., 1986b). Investigation of one of the cefoxitin-resistant strains from this study, TAL3636, showed that both limited outer membrane permeability and hydrolysis of cefoxitin by a novel f3-1actamase both play a role in the cefoxitin-resistance manifested by this organism (Cuchural et al., 1986b). Although many questions concerning the importance of resistance to cefoxitin in B. fragilis group organisms remain to be answered, this study demonstrates an association between the presence of cefoxitin-resistant B. fragilis group organisms and other cefoxitin-resistant facultative-aerobic Gram-negative rods. Also, cefoxitin resistance is more common in non-fragilis Bacteroides spp. and is associated with resistance to other antibiotics, both ~-lactam and non-[3-1actam. Furthermore, the clinical findings in these patients suggest that infections with these organisms occur in a more seriously ill population. As treatment of mixed aerobic and anaerobic bacterial infections is multifactorial, involving both surgical and medical modalities, any complicating factors, such as antibiotic resistance, may render such infections difficult to eradicate. Therefore, attempts must be made to optimize therapy to maximize chances of a successful outcome.
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DiZerega G, Yonekura L, Roy S, et al. (1979) A comparison of clindamycin-gentamicinand penicillin-gentamicinin the treatment of post-cesarean section endomyometritis. Am J Obstet Gynec 134:238. Drusano GL, Waren JW, Saah AJ, Caplan EJ, Tenney JH, Hansen S, Granados J, Standiford HC, Miller EH (1982) Prospective randomized controlled trial of cefoxitin versus clindamycin-aminoglycoside in mixed anaerobic-aerobic infections. Surg Gynecol Obstet 154:715. Holdeman LV, Cato EP, Moore WEC (1977) Anaerobe Laboratory Manual, 4th ed. Blacksburg, VA: Virginia Polytechnic Institute and State University. Neu H (1983) [3-1actamase stability of cefoxitin in comparison with other [3-1actamcompounds. Diagn Microbial Infect Dis 1:313. Olsson B, Dornbasch K, Nord CE (1979) Factors contributing to resistance to beta-lactam antibiotics in Bacteroides fragilis. Antimicrob Agents Chemother 15:263. Tally FP, Cuchural GJ, Jacobus NV, Gorbach SL, Aldridge KE, Cleary TJ, Finegold SM, Hill GB, Iannini PB, McCloskey MV, O'Keefe JP, Pierson CL (1983) Susceptibility of the Bacteraides fragilis group in the United States in 1981. Antimicrob Agents Chemother 23:536. Tally FP, Jacobus NV, Bartlett JG, Gorbach SL (1975) Susceptibility of anaerobes to cefoxitin and cephalosporins. Antimicrob Agents Chemother 7:128. Tally FP, O'Keefe JP, Sullivan NM, Gorbach SL (1979) Inactivation of cephalosporins by Bacteroides. Antimicrob Agents Chemother 16:565. Yotsuji A, Minami S, Inoue M, Mitsuhashi S (1983) Properties of novel ~-lactamase produced by Bacteroides fragilis. Antimicrab Agents Chemother 24:925.