Journal of Antimicrobial Chemotherapy (1975) 1, 439-442
Correspondence
The antimicrobial effect of combinations of dindamycin and metronidazole with spectinomycin
439
Downloaded from http://jac.oxfordjournals.org/ at UB Giessen on April 20, 2015
was defined as the absence of visible growth on the plates. The interpretation and quantification of antibiotic synergy in vitro is not straightSir, forward and different workers employ different The antimicrobial agent of choice for the methods. We have used the concept of the treatment of Bacteroides infections is probably fractionally inhibitory concentration (FIC) dindamycin. The recent implication of this for each drug (Bushby, 1973). The MIC drug in causing pseudomembranous colitis is of each drug at a known ratio can be expressed well known (Tedesco, Barton & Alpers, 1974) as the decimal fraction of the MIC of the and the role of this antibiotic in the patho- respective drug when acting alone (the FIC). genesis of this syndrome is, as yet, poorly When the sum of the FICs of the drugs in understood. The possible risk of using this combination is less than unity, then synergy agent has led to a search for other effective is said to occur. The lower the figure the drugs. Metronidazole has been used in the greater the synergy. treatment of infections caused by Bacteroides As would be expected dindamycin was the fragilis (Ingham et al., 1975) and spectinomost active drug with all but one isolate mycin has been reported as having activity in vitro against these organisms (Philips & being inhibited by 0.06 mg/1 or- less. The Warren, 1975; Ferguson & Smith, 1975; strains were all sensitive to 16 or 32 mg/1 of spectinomyrin and 0-25 or 0-5 mg/1 of Wise, Bedford & Andrews, in press). metronidazole. This study was designed to note if any bacteriostatic synergy could be demonstrated Table I. Bacteriostatic activity of combinations of against the Bacteroides sp. by combining spectinomycin and dindamycin against Bacteroides spectinomycin with either dindamycin or sp. metronidazole. We were interested to see if Sum of FICs at ratio's of any useful reduction in the dosage of spectinospectinomycin : dindamycin mycin or dindamycin could be anticipated Organism if the drugs were used in combination. no. 4096:12048:1 1024:1 512:1256:1128:1 Twenty recent clinical isolates of Bacteroides sp. were used. These were all penicillin resistant 1,2 0-75 but not speciated further. 3,8 1 An agar plate dilution chequer-board 4 0-75 procedure was used, incorporating the anti- 6,7, 11, biotics at differing dilutions in "Oxoid D.S.T." 12,18 0-75 0-75 0-63 0-75 agar plus 10% whole human lysed blood. 9 0-63 0-75 The range of dilutions of spectinomycin were 10 1 from 64 to 2 mg/1, dindamycin from 0-5 to 13 1 0-008 mg/1 and metronidazole from 2 to 15 0-63 0-06 mg/1. The organisms were applied by a 16, 19 1 Denley "Multipoint" inoculator, with an 20 5, 14, 17 Not possible to assess inoculum of 1 x 10* organisms/ml. The plates were incubated at 37 °C in "Gas-Pak" jars (B.B.L.) for 18 h. One jar was used for each The combination of dindamycin with pair of antimicrobial agents being investigated. spectinomyrin, as can be seen in Table I, The minimum inhibitory concentration (MIC) results in some bacteriostatic synergy. This
440
Correspondence
Table n . Bacteriostatic activity of combinations of spectinomycin and metronidazole against Bacteroides sp.
Organism no.
Sum of FICs at ratios of spectinomycin : metronidazole 256:1128:164:132:1
1, 5, 6, 8, 9, 11, 15, 18, 20 1 2,3 2 4, 16, 19 0-63 1 7, 13 10 0-63 12 0-75 14 0-75 1 17
2 0-75 0-75 1
The MICs of spectinomycin against the Bacteroides sp. have been reported as between > 100 mg/1 and 4 mg/1. As a 2 g dose of this drug can be expected to give a mean peak serum level of 138 mg/1 (Wagner, Novak, Leslie and Metzler, 1967), then it is possible that not all strains would be inhibited in vivo. Any drug in combination with spectinomycin which would increase its apparent activity would be an advantage. In this study wefoundthat very low amounts of clindamycin, as little as 0-12-0008 mg/1, will regularly halve the MIC of the spectinomycin. This cannot be claimed to be a highly significant increase in apparent activity of spectinomycin. The results from the study of a combination of metronidazole and spectinomycin are less encouraging. One could expect no useful bacteriostatic synergy from these two drugs. If spectinomycin is to be used for the treatment of Bacteroides infections more information will be needed as to the long term toxicity
of this dibasic aminoglycoside as large doses will necessarily have to be given, unless the drug can be combined with another agent which will enhance its activity. Clindamycin or metronidazole do not appear to fulfil adequately this role from this in vitro study. R. WISE J. M. ANDREWS K. A. BEDFORD Department of Medical Microbiology Dudley Road Hospital, Birmingham References Bushby, S. R. M. Trimethoprim-sulphamethoxazole in vitro microbiological aspects. Journal of Infectious Diseases 128 Supp: S442-462 (1973). Ferguson, L R. & Smith, L. L. Bacteroides fragills and spectinomycin. Journal of Antimicrobial Chemotherapy 1: 245-246 (1975). Ingham, H. R., Rich, G. E., Selkon, J. B., Hale, J. H., Roxby, C. M., Betty, M. J., Johnson, R. W. G. & Uldall, P. R., Journal of Antimicrobial Chemotherapy 1, 235-42 (1975). Philips, I. & Warren, C. Susceptibility of Bacteroides fragilis to spectinomycin. Journal of Antimicrobial Chemotherapy 1: 91-95 (1975). Wagner, J. G., Novak, E., Leslie, L. G. & Metzler, C. M. Absorption, distribution and elimination of spectinomycin dihydrochloride in man. International Zeitschrift fiir Klinische Pharmakologie, Therapie und Toxikologie 1: 261-285 (1968). Wise, R., Bedford, K. A. & Andrews, J. M. Proceedings of 9th International Congress of Chemotherapy (in Press). Sensitivity testing of anaerobes on solid media Sir, The recent increase in interest in the role of anaerobic bacteria in human disease and the treatment of infections due to these organisms has focussed attention on the need for standardized reproducible techniques for antibiotic sensitivity testing of obligate anaerobes. Although standardized methods for disc sensitivity testing of anaerobes have been described (e.g. Sutter, Kwok & Finegold, 1972) and although factors such as media and inoculum size are usually controlled in determinations of Minimum Inhibitory Concentrations (MICs), we suggest that at least one variable factor has not been adequately controlled in such tests. In studies on the in vitro activity of erythromycin against Gram-negative non-sporing anaerobes, we found poor correlation between the zones of inhibition produced by erythromycin discs on seeded plates and the MICs of the same test strains when determined by an
Downloaded from http://jac.oxfordjournals.org/ at UB Giessen on April 20, 2015
cannot be said to be marked at any ratio of the two drugs. The ratios are necessarily high due to the considerable differences in the MICs of the two drugs. In five of the strains (Nos 3, 8,13, 15 and 20) the effect of the drugs in combination can be said to be merely additive. The amount of clindamycin necessary to at least halve the spectinomycin MIC was 0-008 mg/1 in eleven strains, 0-05 mg/1 in two, 0-03 mg/1 in three, 012 mg/1 in one (strain No. 15) and it was impossible to assess in three. In Table II the effect of metronidazole and spectinomycin in combination is summarized. It can be seen that only minor bacteriostatic synergy occurred at some ratio of the two drugs in only six strains and that either an additive effect or indifference was the rule.
Correspondence
The antimicrobial effect of combinations of dindamycin and metronidazole with spectinomycin
439
Downloaded from http://jac.oxfordjournals.org/ at UB Giessen on April 20, 2015
was defined as the absence of visible growth on the plates. The interpretation and quantification of antibiotic synergy in vitro is not straightSir, forward and different workers employ different The antimicrobial agent of choice for the methods. We have used the concept of the treatment of Bacteroides infections is probably fractionally inhibitory concentration (FIC) dindamycin. The recent implication of this for each drug (Bushby, 1973). The MIC drug in causing pseudomembranous colitis is of each drug at a known ratio can be expressed well known (Tedesco, Barton & Alpers, 1974) as the decimal fraction of the MIC of the and the role of this antibiotic in the patho- respective drug when acting alone (the FIC). genesis of this syndrome is, as yet, poorly When the sum of the FICs of the drugs in understood. The possible risk of using this combination is less than unity, then synergy agent has led to a search for other effective is said to occur. The lower the figure the drugs. Metronidazole has been used in the greater the synergy. treatment of infections caused by Bacteroides As would be expected dindamycin was the fragilis (Ingham et al., 1975) and spectinomost active drug with all but one isolate mycin has been reported as having activity in vitro against these organisms (Philips & being inhibited by 0.06 mg/1 or- less. The Warren, 1975; Ferguson & Smith, 1975; strains were all sensitive to 16 or 32 mg/1 of spectinomyrin and 0-25 or 0-5 mg/1 of Wise, Bedford & Andrews, in press). metronidazole. This study was designed to note if any bacteriostatic synergy could be demonstrated Table I. Bacteriostatic activity of combinations of against the Bacteroides sp. by combining spectinomycin and dindamycin against Bacteroides spectinomycin with either dindamycin or sp. metronidazole. We were interested to see if Sum of FICs at ratio's of any useful reduction in the dosage of spectinospectinomycin : dindamycin mycin or dindamycin could be anticipated Organism if the drugs were used in combination. no. 4096:12048:1 1024:1 512:1256:1128:1 Twenty recent clinical isolates of Bacteroides sp. were used. These were all penicillin resistant 1,2 0-75 but not speciated further. 3,8 1 An agar plate dilution chequer-board 4 0-75 procedure was used, incorporating the anti- 6,7, 11, biotics at differing dilutions in "Oxoid D.S.T." 12,18 0-75 0-75 0-63 0-75 agar plus 10% whole human lysed blood. 9 0-63 0-75 The range of dilutions of spectinomycin were 10 1 from 64 to 2 mg/1, dindamycin from 0-5 to 13 1 0-008 mg/1 and metronidazole from 2 to 15 0-63 0-06 mg/1. The organisms were applied by a 16, 19 1 Denley "Multipoint" inoculator, with an 20 5, 14, 17 Not possible to assess inoculum of 1 x 10* organisms/ml. The plates were incubated at 37 °C in "Gas-Pak" jars (B.B.L.) for 18 h. One jar was used for each The combination of dindamycin with pair of antimicrobial agents being investigated. spectinomyrin, as can be seen in Table I, The minimum inhibitory concentration (MIC) results in some bacteriostatic synergy. This
440
Correspondence
Table n . Bacteriostatic activity of combinations of spectinomycin and metronidazole against Bacteroides sp.
Organism no.
Sum of FICs at ratios of spectinomycin : metronidazole 256:1128:164:132:1
1, 5, 6, 8, 9, 11, 15, 18, 20 1 2,3 2 4, 16, 19 0-63 1 7, 13 10 0-63 12 0-75 14 0-75 1 17
2 0-75 0-75 1
The MICs of spectinomycin against the Bacteroides sp. have been reported as between > 100 mg/1 and 4 mg/1. As a 2 g dose of this drug can be expected to give a mean peak serum level of 138 mg/1 (Wagner, Novak, Leslie and Metzler, 1967), then it is possible that not all strains would be inhibited in vivo. Any drug in combination with spectinomycin which would increase its apparent activity would be an advantage. In this study wefoundthat very low amounts of clindamycin, as little as 0-12-0008 mg/1, will regularly halve the MIC of the spectinomycin. This cannot be claimed to be a highly significant increase in apparent activity of spectinomycin. The results from the study of a combination of metronidazole and spectinomycin are less encouraging. One could expect no useful bacteriostatic synergy from these two drugs. If spectinomycin is to be used for the treatment of Bacteroides infections more information will be needed as to the long term toxicity
of this dibasic aminoglycoside as large doses will necessarily have to be given, unless the drug can be combined with another agent which will enhance its activity. Clindamycin or metronidazole do not appear to fulfil adequately this role from this in vitro study. R. WISE J. M. ANDREWS K. A. BEDFORD Department of Medical Microbiology Dudley Road Hospital, Birmingham References Bushby, S. R. M. Trimethoprim-sulphamethoxazole in vitro microbiological aspects. Journal of Infectious Diseases 128 Supp: S442-462 (1973). Ferguson, L R. & Smith, L. L. Bacteroides fragills and spectinomycin. Journal of Antimicrobial Chemotherapy 1: 245-246 (1975). Ingham, H. R., Rich, G. E., Selkon, J. B., Hale, J. H., Roxby, C. M., Betty, M. J., Johnson, R. W. G. & Uldall, P. R., Journal of Antimicrobial Chemotherapy 1, 235-42 (1975). Philips, I. & Warren, C. Susceptibility of Bacteroides fragilis to spectinomycin. Journal of Antimicrobial Chemotherapy 1: 91-95 (1975). Wagner, J. G., Novak, E., Leslie, L. G. & Metzler, C. M. Absorption, distribution and elimination of spectinomycin dihydrochloride in man. International Zeitschrift fiir Klinische Pharmakologie, Therapie und Toxikologie 1: 261-285 (1968). Wise, R., Bedford, K. A. & Andrews, J. M. Proceedings of 9th International Congress of Chemotherapy (in Press). Sensitivity testing of anaerobes on solid media Sir, The recent increase in interest in the role of anaerobic bacteria in human disease and the treatment of infections due to these organisms has focussed attention on the need for standardized reproducible techniques for antibiotic sensitivity testing of obligate anaerobes. Although standardized methods for disc sensitivity testing of anaerobes have been described (e.g. Sutter, Kwok & Finegold, 1972) and although factors such as media and inoculum size are usually controlled in determinations of Minimum Inhibitory Concentrations (MICs), we suggest that at least one variable factor has not been adequately controlled in such tests. In studies on the in vitro activity of erythromycin against Gram-negative non-sporing anaerobes, we found poor correlation between the zones of inhibition produced by erythromycin discs on seeded plates and the MICs of the same test strains when determined by an
Downloaded from http://jac.oxfordjournals.org/ at UB Giessen on April 20, 2015
cannot be said to be marked at any ratio of the two drugs. The ratios are necessarily high due to the considerable differences in the MICs of the two drugs. In five of the strains (Nos 3, 8,13, 15 and 20) the effect of the drugs in combination can be said to be merely additive. The amount of clindamycin necessary to at least halve the spectinomycin MIC was 0-008 mg/1 in eleven strains, 0-05 mg/1 in two, 0-03 mg/1 in three, 012 mg/1 in one (strain No. 15) and it was impossible to assess in three. In Table II the effect of metronidazole and spectinomycin in combination is summarized. It can be seen that only minor bacteriostatic synergy occurred at some ratio of the two drugs in only six strains and that either an additive effect or indifference was the rule.
