Antimicrobial Resistance in Human Oral and Intestinal Anaerobic Microfloras

ANTIMICROBIAL AGENTS AND CHEMOTHERAPY, Aug. 1993, p. 1665-1669 Vol. 37, No. 8 0066-4804/93/081665-05$02.00/0 Copyright X 1993, American Society for Microbiology Antimicrobial Resistance in Human Oral and Intestinal Anaerobic Microfloras CHRISTINA A. STARK,' CHARLOTTA EDLUND,1 SVANTE SJOSTEDT,2 GUN KRISTENSEN,3 AND CARL ERIK NORD"* Departments ofmicrobiology' and Surgery,2 Huddinge University Hospital, Karolinska Institute, Huddinge, and Axelsberg Public Health Centre, Stockholm, 3 Sweden Received 22 June 1992/Accepted 25 May 1993 In the present study we determined the resistance patterns of anaerobic bacteria from human saliva and stool specimens and investigated whether there were significant differences in resistance between outpatients and hospitalized patients, regardless of whether they had received antimicrobial agents. No bacterial strains resistant to ampicillin, piperacillin, cefoxitin, cefuroxime, imipenem, clindamycin, doxycycline, chloramphenicol, or metronidazole were isolated from the saliva samples. However, resistance to ampicillin, cefoxitin, and cefuroxime was found in strains from 70%o of the fecal samples (mainly Bacteroides thetaiotaomicron, Clostridium innocuum, and Bacteroides ovatus). Resistance to both ampicillin and cefuroxime was frequently found in 19%o of the isolated strains (mainly B. thetaiotaomicron, B. ovatus, and Bacteroides vulgatus). No strains that were resistant to imipenem, chloramphenicol, or metronidazole were found. Hospitalization and/or intake of antimicrobial agents was associated with an increase in the relative number of resistant anaerobic intestinal bacteria. The percentage of resistant anaerobic strains encountered, compared with the total number of anaerobic bacteria in the normal fecal microflora, was between 5.2 and 14.8%, with the lower value associated with the outpatient group. Two-thirds of the resistant strains from this group had a relative frequency of less than 1% of the total anaerobic flora, while one-third of the strains were present at a level of greater than 1%; for the hospitalized patients, two-thirds of the strains were present at a level of greater than 1%, and one-third of the strains were present at a level of less than 1% (P < 0.001). Patients who had received antimicrobial agents for 6 days or more (n = ) had an average of 1.6 resistant anaerobic strains each, while patients treated for 3 to 5 days (n = ) had a mean number of 0.87 resistant strains each (P < 0.05). The normal human oral and intestinal microfloras are huge potential reservoirs of resistant organisms which may cause infections in other sites of the body. Bacteria from the intestinal microflora dominate as causative agents in postoperative infections, and resistance among them may seriously complicate the prophylaxis and treatment of intra-abdominal infections (12). A correlation between drug consumption and the emergence of bacterial drug resistance in the normal intestinal microflora of outpatients and hospitalized patients has been reported (1, 6, 8, 19). Those investigations have mainly focused on aerobic bacteria; it has been shown that administration of ampicillin or tetracycline significantly increases the number of resistant gram-negative aerobic microorganisms in the fecal flora, while other agents such as amoxicillin, cefuroxime, imipenem, and metronidazole have been shown to have no impact on the development of resistance patterns (12, 18). Most of the anaerobic microorganisms that cause clinical infections come from the normal oropharyngeal and gastrointestinal microfloras. Since anaerobic bacteria outnumber the aerobic bacteria both in the oral cavity (0:1) and in the intestinal tract (1,000:1), it is of interest to investigate the antimicrobial resistance patterns in the normal anaerobic microfloras from both outpatient and hospitalized individuals Ṫhe aim of the present study was to investigate the resistance patterns in the normal oral and intestinal microfloras of outpatients with no recent history of antimicrobial * Corresponding author. 1665 treatment compared with those in the microfloras of hospitalized patients with and without a recent history of antimicrobial treatment. An effort was made to provide quantitative comparisons of specific drug-resistant populations in all three study groups. MATERIALS AND METHODS Patients. From October 1990 until October 1991, saliva and fecal samples were collected from 150 patients who were equally distributed in the three groups. Group A consisted of 50 patients who visited an outpatient department, who were not treated with antimicrobial agents, and who had not stayed in the hospital during the previous 6 months (31 female and 19 male patients; mean age, 58 years; age range, 24 to 84 years). Group B was made up of 50 inpatients at the Department of Surgery, Huddinge University Hospital, who had not been treated with antimicrobial agents during the previous 6 months ( female and male patients; mean age, 66 years; age range, 27 to 87 years). The mean number of days in hospital was 15.6 range, 6 to 1 days). Group C consisted of 50 inpatients at the Department of Surgery, Huddinge University Hospital, who had been treated with various antimicrobial agents for at least 3 days during the previous 2 weeks and whose treatment was terminated 3 days before the sampling (23 female and 27 male patients; mean age, 60 years; age range, 19 to 86 years). The mean hospital stay was 16.0 days (range, 6 to 90 days), and the mean time of antimicrobial treatment was 5 days (range, 3 to 19 days). For this group, the dominant antimicrobial regimen was cefuroxime (36 patients), and 31 patients in that group were treated with cefuroxime in combination with metron- Downloaded from http://aac.asm.org/ on November 9, 18 by guest

1666 STARK ET AL. idazole. The following other antimicrobial regimens were used as single or combination therapy: trimethoprim (seven patients), sulfonamide (six patients), imipenem (three patients), cefadroxil (two patients), phenoxymethylpenicillin, benzylpenicillin, ampicillin, cloxacillin, pivampicillin, and amdinocillin (pivmecillinam), tobramycin, cefoxitin, aztreonam, doxycyline, metronidazole, and gentamicin (one patient each). There was no significant difference between the three patient groups by sex or age distribution. Sample collection and processing. Saliva and fecal samples from each patient were collected into sterile plastic containers and were immediately transported to the laboratory, where they were frozen at -70 C until they were processed. Processing of saliva samples. A sterile swab was soaked in the saliva and was rinsed in 1 ml of prereduced peptone yeast glucose medium (PYG). Aliquots of 0.1 ml of the resulting suspension were rapidly inoculated onto nine freshly prepared PDM-agar plates (AB Biodisk, Solna, Sweden) containing 5% defibrinated horse blood and nine different antimicrobial agents at defined concentrations. The antimicrobial agents and the corresponding breakpoints used were as follows: ampicillin (Astra, Sodertalje, Sweden),.32 p,g/ml; piperacillin, (Lederle),.64,ug/ml; cefoxitin and imipenem (Merck Sharp & Dohme International, Rahway, N.J.),.32,ug/ml; cefuroxime (Glaxo Ltd., Greenford, Middlesex, United Kingdom),.32,ug/ml; clindamycin (Upjohn),.16,g/ml; doxycycline (Pfizer, Brussels, Belgium),.8,ig/ml; chloramphenicol (Parke-Davis Pharmaceutical Research Division, Warner Lambert Company, Ann Arbor, Mich.).32 jig/ml; and metronidazole (Rhone-Poulenc Rorer, Birkeroed, Denmark),.32 p,g/ml. All agar plates contained gentamicin (16 jig/ml; Roussel Laboratories Ltd., Swindon, United Kingdom) in order to prevent growth of aerobic microorganisms. This concentration of gentamicin was chosen since it inhibits the growth of most aerobic and facultatively anaerobic bacteria without affecting the growth of anaerobic bacteria. Three agar plates containing only gentamicin and blood served as controls in order to quantify the total number of anaerobic bacteria and to control for the absence of aerobic growth. Processing of fecal samples. The fecal samples were mixed and diluted in two steps. A sterile cotton swab was rotated in the center of the sample and was mixed in 4.5 ml of prereduced PYG. From this solution, 0.5 ml was further suspended in 4.5 ml of PYG. From the bacterial suspension thus obtained, samples of 0.1 ml were rapidly applied onto each of nine PDM agar plates with different antimicrobial agents and gentamicin as described earlier and on two control plates, aerobic and anaerobic, and after further dilution (1:0), a second anaerobic control plate was also included. Microbiological analysis. After incubation in anaerobic jars (GasPak) at 37 C for 48 h, the total number of anaerobic colonies growing on each control plate was counted. The resistant microorganisms growing on the antimicrobial plates were enumerated, isolated in pure cultures, and identified by morphological, gas chromatographic, and biochemical tests (14). As a control, all strains isolated were also incubated aerobically, and the strains which grew were disregarded. The MICs of the nine different antimicrobial agents for each strain were determined by the agar dilution method. A final inoculum of 5 to 6 CFU/ml prepared by dilution of a fresh agar culture was applied to the agar plates with a Steers replicator. The agar plates were incubated anaerobically at 37 C for 48 h. The MIC was defined as the lowest concentration which inhibited the growth of microorganisms on the ANTIMICROB. AGENTS CHEMOTHER. agar plates. The control strains used were Bacteroides fragilis NCTC 9343 and Clostndium perfringens ATCC 13124. Statistical methods. The results were analyzed by Student's t test, the chi-square test, and Kolmogorov Smirnov tests. RESULTS Frequency of resistant strains from saliva samples. No anaerobic strains resistant to the antimicrobial agents tested were isolated from the saliva samples of patients in any of the three groups. Frequency of resistant strains from fecal samples. Seventy percent of the patients harbored resistant anaerobic strains in their fecal samples. Two resistant strains or more per fecal sample were isolated from 37% of the patients, and three or more resistant strains were found in 15% of the patients. A total of 194 resistant strains were isolated. In the outpatient group (group A), 41 of the patients carried resistant strains; 34 of the patients in the hospitalized group without antimicrobial treatment (group B) and 31 of the patients in the hospitalized group with antimicrobial treatment (group C) harbored resistant strains. There was no statistically significant difference between the three patient groups in the prevalence of resistant strains or the distribution of patients with different numbers of strains. Differences in the resistance patterns between patient groups. All resistant strains were quantified in relation to the total number of anaerobic microorganisms in each patient. The mean value for each group was determined by comparing the relative frequency for each patient. There was no significant difference between the three patient groups according to the total amount of fecal anaerobic microorganisms. Patients in group A had 5.2% resistant strains (mean value) in their anaerobic fecal flora, patients in group B had 14.8% resistant strains, and patients in group C had 7.5% resistant strains in their fecal microflora. The resistant strains isolated were subdivided into <1% level and 21% level. For the outpatient group (group A), 36% of the strains were present at a relative frequency above the 1% level; for the hospitalized group without antimicrobial treatment (group B), 66% of the strains were present at greater than the 1% level; for the hospitalized group with antimicrobial treatment (group C), 61% of the strains were present at greater than the 1% level. The differences between group A and groups B and C were statistically significant in this respect (P < 0.001). Species prevalence of resistant strains in the fecal samples. Strains from the Bacteroides fragilis group dominated among the isolates (74%). The most commonly isolated bacteria were Bacteroides thetaiotaomicron (27%), Clostndium innocuum (24%), Bacteroides ovatus (19%), and Bacteroides uniformis (9%). The distributions of strains among the different patient groups are shown in Table 1. The outpatient group had a lower percentage of clostridial strains (17%) than either the hospital group without antimicrobial therapy (%) or the hospital group with antimicrobial therapy (21%), although the difference was not statistically significant. Distribution of resistance to different antimicrobial agents. Ampicillin was the antimicrobial agent to which most anaerobic strains showed resistance; this was followed by cefoxitin and cefuroxime (Table 2). No strains were found to be resistant to imipenem, chloramphenicol, or metronidazole. The MICs of the different antimicrobial agents for the strains Downloaded from http://aac.asm.org/ on November 9, 18 by guest

VOL. 37, 1993 RESISTANCE IN HUMAN ANAEROBIC MICROFLORAS 1667 TABLE 1. Distribution of resistant anaerobic strains isolated from fecal samples from patients in groups A, B, and C Microorganism No. of strains/patient group A B C Total Bacteroides thetaiotaomicron 21 8 11 40 Clostridium innocuum 11 15 36 Bacteroides ovatus 12 6 28 Bacteroides uniformis 6 5 7 18 Bacteroides distasonis 6 3 6 15 Bacteroides vulgatus 7 3 4 14 Bacteroides fragilis 6 3 4 13 Bacteroides caccae 5 4 3 12 Bacteroides spp. 2 0 1 3 Prevotella melaninogenicus 1 2 0 3 Prevotella eggerthii 1 0 1 2 Prevotella oris 1 0 1 2 Clostridium ramosum 1 0 1 2 Prevotella bivius 0 0 1 1 Prevotella loeschii 0 0 1 1 Clostridium perfringens 0 1 0 1 Clostridium sartagoforum 1 0 0 1 Clostridium spp. 0 0 1 1 Clostridium subterminale 1 0 0 1 Total 82 54 58 194 isolated from the three groups of patients are shown in Fig. 1. Nine strains from the B. fragilis group (mostly B. distasonis and B. fragilis) were found to be highly resistant to ampicillin (MIC,.1,024,ug/ml) and were isolated from all three patient groups. Of 42 clostridial strains isolated, 41 were resistant to cefoxitin. There was no significant difference between the patient groups regarding the MICs for the strains isolated from the patients. Multiresistant strains in the fecal samples. Multiresistant strains were defined as those that were resistant to two or more antimicrobial agents. A total of 58 multiresistant strains were recovered in the fecal samples, with no significant difference between the three patients groups. B. thetaiotaomicron was the most commonly isolated species (Table 3). Multiresistant clostridia were found only in the hospitalized group treated with antimicrobial agents. The most common combination of multiresistance was ampicillin with cefuroxime (36 strains), while resistance against a betalactam agent and a non-beta-lactam agent occurred in 9 TABLE 2. Distribution of strains resistant to different antimicrobial agents in the three patient groups Antimicrobial agent No. of strains/patient group A B C Ampicillin 31 31 Cefoxitin 33 28 17 Cefuroxime 32 24 Piperacillin 4 5 2 Imipenem 0 0 0 Doxycycline 11 4 4 Clindamycin 1 1 2 Chloramphenicol 0 0 0 Metronidazole 0 0 0 Total 112 82 76 Uc z cn IL 0 m z 0 I AMPICILLIN El_~~~~~PIEACLI ~CFOII, PIPERA~~~~OXCYCLINE 2 4 8 16 32 64 128 256 512 24 48 4096 8192 MINIMUM INHIBITORY CONCENTRATION gig/ml FIG. 1. Distribution of MICs of different antimicrobial agents for resistant strains. strains. No multiresistance involving imipenem, clindamycin, chloramphenicol, or metronidazole was found. Impact of time spent in hospital. When comparing the length of the hospital stay between the patients in the hospitalized groups, no difference regarding the prevalence of resistant strains or the number of bacteria was observed. Impact of antimicrobial treatment. The majority of patients in group C were treated with cefuroxime (36 patients). These patients were not specifically predisposed to an increase in the number of bacteria resistant to cefuroxime or to any beta-lactam agent in general. Neither was any statistically significant correlation found between patients receiving beta-lactam agents and the occurrence of beta-lactam resistance. Four of six patients receiving antimicrobial agents not belonging to the beta-lactam group harbored resistant strains. For patients who were treated with antimicrobial agents for 6 days or more, 9.9% of the anaerobic fecal microfloras of these patients were resistant to the antimicrobial agents tested, while for patients who received anti- Downloaded from http://aac.asm.org/ on November 9, 18 by guest

1668 STARK ET AL. TABLE 3. Distribution of multiresistant anaerobic strains isolated from the fecal samples of 150 patients Bacterial species No. strains of Bacteroides thetaiotaomicron... 23 Bacteroides ovatus... 9 Bacteroides vulgatus...... 8 Bacteroides uniformis... 7 Bacteroides distasonis... 5 Clostridium innocuum... 2 Bacteroides caccae...... 1 Bacteroides fragilis...... 1 Prevotella loeschii... 1 Prevotella melaninogenicus... 1 microbial treatment for a shorter period of time (<6 days), 5.8% of their fecal anaerobic microfloras were resistant. Patients treated for 6 days or more had an average of 1.6 resistant strains each, while patients treated for less than 6 days had a mean number of 0.87 resistant strains each (P < 0.05). DISCUSSION Anaerobic infections often originate from microorganisms from the patient's own microflora, and many antimicrobial agents that are active against these organisms are now available. However, an increasing number of treatment failures caused by anaerobic oacteria that are resistant to antimicrobial agents have been reported (3, 15). The results of the present study show that the frequency of resistant anaerobic microorganisms in the oropharyngeal tract is low in Sweden. This is in accordance with a study by Heimdahl et al. (), who investigated the occurrence of 1-lactamaseproducing Bacteroides species in the oral cavity in relation to penicillin therapy. Only a few beta-lactam-resistant Bacteroides strains were isolated from saliva samples in that study, and most patients harboring resistant strains had recently been treated with penicillin or ampicillin. The breakpoints of penicillin and ampicillin used in that study were.4,ug/ml; compare this with a breakpoint of 232,ug/ml for ampicillin in the present study. Bacteroides species are the most common pathogens in oral infections, and most strains are susceptible to clindamycin, metronidazole, and chloramphenicol (4,, 11). In the current study, the prevalence of patients with resistant anaerobic strains in their intestinal tracts was high (70%), with no significant difference between the three patient groups. The results of the present study show that although outpatients seem to have a higher number of resistant strains per person, the relative amount of resistant anaerobic bacteria in feces is lower in outpatients compared with that in hospitalized patients. In the outpatient group, one-third of the resistant strains made up more than 1% of the total anaerobic microflora, while in both hospitalized groups, two-thirds of the resistant strains constituted more than 1% of the total anaerobic fecal flora. Strains from the B. fragilis group were isolated most frequently, as has been reported previously (1, 3, 12). B. thetaiotaomicron and B. ovatus were the most commonly isolated members of the B. fragilis group. In previous studies, B. ovatus and then B. fragilis were the most frequently isolated anaerobic resistant strains from intestinal microfloras (1, 12). The antimicrobial susceptibility of the B. fragilis group in the present study ANTIMICROB. AGENTS CHEMOTHER. correlates well with those found in recent investigations in Canada and Europe (12, 16). The known mechanisms of resistance to beta-lactam antibiotics in anaerobic bacteria are elaboration of,3-lactamases, which inactivate the agent; alteration of the number or type of penicillin-binding proteins, which affects the affinity of the proteins for the antibiotics; and blocked penetration of the agent into the active site via alteration of the bacterial outer membrane porins (7, ). The most important factor in beta-lactam resistance appears to be the production of P-lactamases, which have been found in various Bacteroides, Fusobacterium, and Clostndium species (15). Thus, resistant anaerobic species in the normal microflora may play a role as indirect pathogens because of their P-lactamase production. Resistance to ampicillin, cefoxitin, and cefuroxime were often found in the fecal isolates. The relative percentage of strains resistant to ampicillin was higher in the hospitalized group receiving antimicrobial treatment than in the other two patient groups, although the difference was not statistically significant. Levy et al. (13) observed a statistically significant increase in the number of strains resistant to ampicillin among hospitalized patients with a recent history of unspecified antimicrobial treatment. A distinct correlation between local drug consumption and prevalence of resistant aerobic and anaerobic fecal microorganisms has been reported for ampicillin and doxycycline (12, 13). Resistance to cefoxitin occurred at the same frequency as resistance to ampicillin and cefuroxime, which is notable, since cefoxitin has generally been considered effective against species of the B. fragilis group. These findings are in accordance with those of Bourgault et al. (2), who reported that resistance to cefoxitin among the B. fragilis group in Canada has increased from 2 to 26% over the past 6 years. The resistance mechanism for cefoxitin is still poorly understood, but Piddock and Wise (17) have shown that resistant strains from the B. fragilis group have altered outer membrane porins. Resistance to carbapenems such as imipenem is very rare, although,b-lactamase-producing B. fragilis strains resistant to imipenem have been described (5, 9). The increased use of cefuroxime and metronidazole has not been correlated with the emergence of drug-resistant aerobic or anaerobic intestinal bacteria (12, 13). Since the most common drug regimen in the present study was cefuroxime, which was often used in combination with metronidazole, it is not surprising that no major differences between the hospitalized patient groups were found. However, patients who had antimicrobial treatment for 6 days or more had a significantly increased number of resistant anaerobic strains in their fecal microfloras compared with the numbers in patients who were treated for shorter periods of time. Patients who had been treated in the hospital for at least 6 days (groups B and C) had a significantly higher relative number of resistant microorganisms in their anaerobic intestinal microfloras compared with the numbers in outpatients. These results correlate well with those from the study by Levy et al. (13), in which an increased percentage of the total microfloras of hospitalized patients was resistant. This may be a result of the hospital stay, with large numbers of resistance determinants being present in strains in the environment, or it may be related to the health status of the patient. There are many problems involved in correlating in vitro susceptibility results with clinical success or failure in individual cases. Appropriate surgical manipulations, the microenvironment at the site of infection, host defense factors, the age and health of the patient, and the in vitro Downloaded from http://aac.asm.org/ on November 9, 18 by guest

VOL. 37, 1993 RESISTANCE IN HUMAN ANAEROBIC MICROFLORAS 1669 susceptibilities of the causative microorganisms are all important factors that have a significant impact on the outcome (8, 21). In conclusion, the anaerobic intestinal microflora contains a large number of resistant anaerobic bacteria that mainly belong to the B. fragilis group. It seems that hospitalization or intake of antimicrobial agents promotes an increase in the relative number of resistant anaerobic microorganisms isolated from feces. REFERENCES 1. Andaker, L., P.-A. Kling, and L. G. Burman. 1987. Antibiotic consumption and faecal bacterial susceptibility in surgical inpatients. Acta Chir. Scand. 153:411-416. 2. Bourgault, A.-M., F. Lamothe, D. J. Hoban, M. T. Dalton, P. C. Kibsey, G. Harding, J. A. Smith, D. E. Low, and H. Gilbert. 1992. Survey of Bacteroides fragilis group susceptibility patterns in Canada. Antimicrob. Agents Chemother. 36:343-347. 3. Brook, I. 1989. Anaerobic bacterial bacteremia: 12-year experience in two military hospitals. J. Infect. Dis. 160:71-75. 4. Crook, D. W., G. J. Cuchural, Jr., N. V. Jacobus, and F. P. Tally. 1988. Antimicrobial resistance in oral and colonic bacteroides. Scand. J. Infect. Dis. Suppl. 57:55-64. 5. Cuchural, G. J., Jr., M. H. Malamy, and F. P. Tally. 1986.,-lactamase-mediated imipenem resistance in Bacteroides fragilis. Antimicrob. Agents Chemother. :645-648. 6. Ellner, P. D., D. J. Fink, H. C. Neu, and M. F. Parry. 1987. Epidemiologic factors affecting antimicrobial resistance of common bacterial isolates. J. Clin. Microbiol. 25:1668-1674. 7. Finegold, S. M. 1989. Mechanisms of resistance in anaerobes and new developments in testing. Diagn. Microbiol. Infect. Dis. 12:117-1. 8. Finegold, S. M. 1992. Clinical relevance of antimicrobial susceptibility testing. Eur. J. Microbiol. Infect. Dis. 11:21-24. 9. Hedberg, M., C. Edlund, L. Lindquist, M. Rylander, and C. E. Nord. 1992. Purification and characterisation of an imipenem hydrolysing metallo-beta-lactamase from Bacteroides fragilis. J. Antimicrob. Chemother. 29:5-113.. Heimdahl, A., L. Konow, and C. E. Nord. 1981. Beta-lactamaseproducing Bacteroides spp in the oral cavity in relation to penicillin therapy. J. Antimicrob. Chemother. 8:225-229. 11. Heimdahl, A., and C. E. Nord. 1979. Effect of phenoxymethylpenicillin and clindamycin on the oral, throat and faecal microflora of man. Scand. J. Infect. Dis. 11:233-242. 12. Kling, P.-A., R. Ostensson, S. Granstrom, and L. G. Burman. 1989. A 7-year survey of drug resistance in aerobic and anaerobic fecal bacteria of surgical inpatients: clinical relevance and relation to local antibiotic consumption. Scand. J. Infect. Dis. 21:589-596. 13. Levy, S. B., B. Marshall, S. Schluederberg, D. Rowse, and J. Davis. 1988. High frequency of antimicrobial resistance in human fecal flora. Antimicrob. Agents Chemother. 32:1801-1806. 14. Murray, P. R., and D. M. Citron. 1991. General processing of specimens for anaerobic bacteria, p. 488-504. In A. Balows, W. J. Hausler, Jr., K. L. Herrmann, H. D. Isenberg, and H. J. Shadomy (ed.), Manual of clinical microbiology, 5th ed. American Society for Microbiology, Washington, D.C. 15. Nord, C. E., and M. Hedberg. 1990. Resistance to betalactam antibiotics in anaerobic bacteria. Rev. Infect. Dis. 12(Suppl. 2):231-234. 16. Phillips, I., A. King, C. E. Nord, and B. Hoffstedt. Antibiotic sensitivity of the Bacteroides fragilis group in Europe. Eur. J. Clin. Microbiol. Infect. Dis. 11:292-4. 17. Piddock, L. J. V., and R. Wise. 1987. Cefoxitin resistance in Bacteroides species: evidence indicating two mechanisms causing decreased susceptibility. J. Antimicrob. Chemother. 19:161-170. 18. Shaw, E. J., N. Datta, G. Jones, F. M. Marr, and W. J. B. Froud. 1973. Effect of stay in hospital and oral chemotherapy on the antibiotic sensitivity of bowel coliforms. J. Hyg. Camb. 71:529-534. 19. Sjostedt, S., P. Levin, L. Kager, A.-S. Malmborg, and U. Bergman. 1990. Hospital and catchment area antibiotic utilization and bacterial sensitivity in primary infections following gastric surgery in Huddinge, Sweden. Eur. J. Clin. Pharmacol. 39:211-216.. Tally, F. P., G. J. Cuchural, Jr., and M. H. Malamy. 1984. Mechanisms of resistance and resistance transfer in anaerobic bacteria: factors influencing antimicrobial therapy. Rev. Infect. Dis. 6(Suppl. 1):260-269. 21. Wexler, H. 1991. Susceptibility testing of anaerobic bacteria: myth, magic, or method? Clin. Microbiol. Rev. 4:470-484. Downloaded from http://aac.asm.org/ on November 9, 18 by guest