THE JOURNAL OF INFECTIOUS DISEASES. VOL. 135, SUPPLDfENT • l\IARCH 1977 iJ 1977 by the University of Chicago. All rights reserved.

In Vitro Susceptibility of Anaerobes: Comparison of Clindamycin and Other Antimicrobial Agents Vera L. Sutter

From the Research Service, Wadsworth Veterans Administration Hospital Center, and the Department of Medicine, University of California, Los Angeles, California

For a number of years, our laboratory has been actively engaged in testing anaerobic bacteria for in vitro susceptibilities to a variety of antimicrobial agents. Because clindamycin has been highly effective against anaerobes and has been in widespread use, we have continued to test and monitor its activity against anaerobic bacteria. The purpose of this report was to review our experience with clindamycin and to compare its activity against anaerobes with that of other antimicrobial agents. Most of the data presented are from results of agar dilution susceptibility tests [1] against >490 isolates of anaerobic bacteria from clinical specimens submitted to our laboratory from 1972 to 1975 [laJ. The activities of clindamycin and other antimicrobial agents against organisms in the Bacteroides fragilis group is shown in figure 1. Recently, it has been proposed that organisms recognized as subspecies of B. fragilis be given species status [2]. We are in agreement with this proposal, but because little or no difference has been observed in the susceptibility of these organisms to the antimicrobial agents in our studies and in those of others [3, 4], we will refer to these as the B. [ragilis group. Clindamycin inhibited all strains of B. fragilis at 8 fLg/ml, and 9670 of the strains were inhibited at 4 fLg/mL These results are in agreement with published reports [5-7], and, despite extensive usage of this drug at our institution, we have not observed resistance among the B. [ragilis group. However, resistant strains have been reported by other investigators [8]. As indicated in figure 1, clindamycin, metronidazole, and

chloramphenicol were the most active of the agents represented. Our recent experience wi th other penicillins has shown that ampicillin and amoxicillin are less active at achievable levels in blood than penicillin G, and that ticarcillin has activity similar to that of carbenicillin against B. tragilis [Ia]. The cephalosporins cephalothin and cephalexin are ineffective against most strains of B. iragilis. Cefazolin and cephaloridine have somewhat greater activity, and cefoxitin, a cephamycin, is the most effective of the cephalosporins tested thus far [9, 10]. In our initial studies of cefoxitin, in which> 100 isolates of the B. fragilis group were tested, all were susceptible to ~32 fLg/ml, whereas in our more recent studies, only 85% of the isolates were susceptible to similar concentrations. Strains resistant to tetracycline are now isolated with greater frequency than that reported previously [5, 11], and the analogues, doxycycline and minocycline, are somewhat more active than was tetracycline [la, 12, 13]. The activities of antimicrobial agents against Bacteroides melaninogenicus are illustrated in figure 2. With the exception of tetracycline, all of the agents were effective against the strains of B. melaninogenicus at achievable levels in blood. Doxycycline and minocycline were more effective than was tetracycline against these strains [l a, 13]. Cephalothin has been shown to be effective against 90% of strains at ~32 fLg/ml, whereas cefoxitin inhibited all strains at ~4 fLg/ml [9]. The susceptibilities of isolates of other Bacteroides species are shown in figure 3. The majority of these strains (94%) were inhibited by clindamycin at ~2 fLg/ml, and 96% of the strains were

Please address requests for reprints to Vera L. Sutter, Research Service, Wadsworth Veterans Administration Hospital Center, Los Angeles, California 90073.

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The in vitro activity of c1indamycin against anaerobic bacteria was compared with that of other antimicrobial agents. Clindamycin remained effective against most anaerobic bacteria found in infections, although resistance to the drug was found in certain groups, such as Peptococcus and Clostridium species.

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Figure 1. Susceptibilities of Bacteroides fragilis group to antimicrobial agents. Strains included Bacteroides distasonis (11), Bacteroides fragilis (30), Bacteroides ovatus (3), Bacteroides thetaiotaomicron (21), Bacteroides vulgatus (8), and three strains that belong in this group but did not conform to the description of the five species reported by Cato and Johnson [2].

Figure 2. Susceptibilities of 71 strains of Bacteroides melaninogenicus to antimicrobial agents.

inhibited at the level of 8 Itg/ml. Resistant strains included isolates of Bacteroides clostridiijormis subspecies clostridiiiormis, Bacteroides putredinis, and a Bacteroides species. Strains similar to the strain of B. clostridiiiormis observed in this study were found to have spores

and were described as Clostridium [14, 15], but spores have not yet been demonstrated in this particular strain. Erythromycin, metronidazole, and chloramphenicol showed activity similar to that of clindamycin against these strains. Penicillin G was effective against fewer strains at a

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Figure 3. Susceptibilities of 72 strains of other Bacteroides species to antimicrobial agents.

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level of ~32 p-gjml than was clindamycin at a concentration of 8 p.,gjml. Carbenicillin was more effective than was penicillin, and the efficacy of carbenicillin was equal to that of clindamycin at therapeutic levels. Ampicillin and amoxicillin appeared to be more effective than penicillin G agai nst these strains. Only 50% of the strains were susceptible to tetracycline at ~4 p-gjml. The cephalosporins and cefoxitin were generally less active against these organisms than was clindamycin. Clindamycin was very active against the strains of Fusobacterium species (figure 4). Previous reports have shown that the species Fusobacterium mortiierum and Fusobacterium varium were relatively resistant to clindamycin [16]. These species are found infrequently in human infection, and only two strains of F. mortiierum were obtained from 1972 to 1975; both strains were susceptible to clindamycin. With the exception of erythromycin, all of the antimicrobial agents were active against the fusobacteria. Figure 5 depicts the activi ties of the antimicrobial agents against anaerobic cocci. Clindamycin was active against all but nine strains of Peptococcus species. Resistance among Peptococcus was noted in earlier reports [5, 17]. Penicillin G, carbenicillin, and chloramphenicol in-

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hibited all of the cocci at achievable levels in blood. Erythromycin inhibited 87% and metronidazole inhibited 98% of the strains at achievable levels of each drug in serum. The cephalosporins and cefoxitin were generally active against anaerobic cocci [9, 10]. Resistance to tetracycline continued to emerge in this group of organisms, and

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[20]. Clostridium ramosum was also shown to be relatively resistant to clindamycin [20, 21]. None of the 10 recently obtained isolates of this organism was resistant. Erythromycin was more effective against these strains; 95% of the isolates were inhibited at ~2 flgjml. Penicillin G and carbenicillin as well as other penicillins were generally effective against Clostridium. The cephalosporins

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Clostridium species is illustrated in figure 7. Clindamycm Inhibited 91% of the strains at ~8 flgjml. Resistant strains included two of the eight Clostridium innocuum, one of two Clostridium difficile} and the single strain of Clostridium sporogenes included in the study. Resistance to clindamycin among these species, as well as others, was noted by Wilkins and Thiel

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Figure 6. Susceptibilities of 71 strains of anaerobic gram-positive asporogenous bacilli. Strains included species of Eubacterium (17), Arachnia (3), Propionibacterium (12), Actinomyces (16), Bifidobacterium (5), and Lactobacillus

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Figure 5. Susceptibilities of 115 strains of anaerobic cocci to antimicrobial agents. Strains included species of Peptococcus (59), Peptostreptococcus (29), Streptococcus (10), Veillonella (23), and Acidamino co ccus (3').

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cephaloridine and cefazolin have been shown to inhibit all clostridia at achievable levels in blood [9, 10]. Cephalothin was somewhat less effective and cephalexin relatively ineffective. The cephamycin cefoxitin was also relatively ineffective against these organisms. Metronidazole and chloramphenicol inhibited all of the clostridia at achievable levels in blood, but tetracycline inhibited only 55% of the strains at ~4 /Lgjml. In summary, clindamycin remains effective against most anaerobic bacteria found in infections. This is in contrast to the continuous emergence of resistance of anaerobes to tetracycline. With the widespread use of clindamycin in the past few years, we would expect to see a trend toward greater resistance. Resistant anaerobes were found in certain groups, such as PeptOCOCCllS and Clostridium species, but are no more frequent now than they were when clindamycin was originally introduced.

References

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I a. Sutter, V. L., Finegold, S. M. Susceptibility of anaerobic

2.

3.

4.

5.

6.

7.

8.

1. Sutter, V. L., Vargo. V. L., Finegold, S. M. [ed.]. Wads-

worth anaerobic bacteriology manual. 2nd ed. Wadsworth Veterans Administration Hospital Center, Los Angeles Anaerobic Bacteriology Laboratory, University of California, Los Angeles, Extension School of Medicine, 1975. 106 p.

1

9.

bacteria to 23 antimicrobial agents. Antimicrob. Agents Chemother. 10:736-752,1976. Cato, E. P., Johnson, J. L. Reinstatement of species rank for Bacteroides fragilis, B. ovatus, B. distasonis, B. thetaiotaomicron, and B. uulgatus: designation of neotype strains for Bacteroides thetaiotaomicron (Distaso) Castellani and Chalmers. Int. J. Syst. Bacteriol. 26:230-237, 1'976. Blazevic, D. J. Antibiotic susceptibility of the subspecies of Bacteroides [ragilis. Antimicrob. Agents Chemother. 9:481-484, 1976. Jones, R. N., Fuchs, P. C. Identification and antimicrobial susceptibility of 250 Bacteroides [ragilis subspecies tested by broth microdiIution methods. Antimicrob. Agents Chemother. 9:719-721, 1976. Martin, W. J., Gardner, M., Washington, J. A. II. In vitro antimicrobial susceptibility of anaerobic bacteria isolated from clinical specimens. Antimicrob. Agents Chemother. 1:148-158, 1972. Nastro, L. J., Finegold, S. M. Bactericidal activity of five antimicrobial agents against Bacteroides fragilis. J. Infect. Dis. 126:104-107, 1972. Sutter, V. L., Kwok, Y. Y., Finegold, S. M. Susceptibility of Bacteroides fragilis to six antibiotics determined by standardized antimicrobial disc susceptibility testing. Antimicrob Agents Chemother. 3:188193,1973. Dornbusch, K., Nord, C. E., Wadstrom, T. Biochemical characterization and in vitro determination of antibiotic susceptibility of clinical isolates of Bacteroides [tagilis. Scand. J. Infect. Dis. 6:253-258, 1974. Sutter, V. L., Finegold, S. M. Susceptibility of anaerobic bacteria to carbenicillin, cefoxitin, and related drugs. J. Infect. Dis. 131:417-422,1975.

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Clostridium perfringens (9), and Clostridium ramosurn (10). The re-

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21.

clostridiijormis subsp. girans (Prevot) Holdeman and Moore to the genus Clostridium as Clostridium clostridiijorme (Burri and Ankersmit) comb. nov.: emendation of description and designation of neotype strain. Int. J. Syst. Bactcriol, 26:205-211, 1976. Bartlett, J G., Sutter, V. L., Finegold, S. M. Treatment of anaerobic infections with lincomycin and clindamycin. N. Eng!. J. Med. 287:1006-1010,1972. Kwok, Y. Y., Tally, F. P., Sutter, V. L., Finegold, S. M. Disk susceptibility testing of slow-growing anaerobic bacteria. Antimicrob. Agents Chemother. 7:1-7. 1975. Lerner, P. I. Susceptibility of pathogenic actinornycetes to antimicrobial compounds. Antimicrob. Agents Chemother. 5:302-309,1974. Chow, A. W., Patten, V., Guze, L. B. Susceptibility of anaerobic bacteria to metronidazole': relative resistance of non-spore-forming gram-positive bacilli. J. Infect. Dis. 131:182-185,1975. Wilkins, T. D., Thiel, T. Resistance of some species of Clostridium to clindamycin, Antimicrob. Agents Chemother. 3:136-137, 1973. Tally, F. P., Armfield, A. Y., Dowell, V. R., Jr., Kwok, Y. Y., Sutter, V. L., Finegold, S. M. Susceptibility of Clostridium ramosum to antimicrobial agents. Antimicrob. Agents Chemother. 5:589-593, 1974.

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10. Tally, F. P., Jacobus, N. V., Bartlett, J. G., Gorbach, S. L. Susceptibility of anaerobes to cefoxitin and other cephalosporins. Antimicrob. Agents Chernether. 7:128-132, 1975. II. Sutter, V. L., Kwok, Y. Y., Finegold, S. M. Standardized antimicrobial disc susceptibility testing of anaerobic bacteria. I. Susceptibility of Bacteroides fragilis to tetracycline. Appl. Microbiol. 23:268-275, 1972. 12. Sutter, V. L., Tally, F. P., Kwok, Y. Y., Finegold, S. M. Activity of doxycycline and tetracycline versus anaerobic bacteria. Clin. Med. 80:31-38, 1973. 13. Chow, A. W., Patten, V., Guze, L. B. Comparative susceptibility of anaerobic bacteria to minocycline, doxycycline, and tetracycline. Antimicrob. Agents Chemother. 7:46-49, 1975. 14. Kaneuchi, C., Watanabe, J., Terada, A., Benno, Y., Mitsuoka, T. Taxonomic study of Bacteroides clostridiijormis subsp. clostridiijormis (Burri and Ankersmit) Holdeman and Moore and of related organisms: proposal of Clostridium clostridiijormis (Burri and Ankersmit) and Clostridium symbiosum (Stevens) comb. nov. Int. J. Syst. Bacteriol. 26:195-204,1976. 15. Cato, E. P., Salmon, C. W. Transfer of Bacteroides clostridiijormis subsp. clostridiijormis (Burri and Ankersmit) Holdeman and Moore and Bacteroides