Activity of ten antimicrobial agents against anaerobic bacteria

Antibiotic susceptibility of human gut-derived facultative anaerobic bacteria is different under aerobic versus anaerobic test conditions

Facultative anaerobes are the most common cause of infections in anoxic parts of the human body, including deep wound, vagina, periodontal pockets, gastrointestinal tract, genitourinary tract and lungs. Generally, antibiotic susceptibility tests (AST) for facultative anaerobes are performed under aerobic conditions due to ease of handling and rapid growth. However, variation in susceptibility of facultative anaerobes to antibiotics under aerobic and anaerobic conditions can lead to failure of antibiotic treatment. Our study evaluated the susceptibility of facultative anaerobic microorganisms to antibiotics during growth under anaerobic or aerobic conditions. We compared the resistance patterns of representatives from 15 bacterial genera isolated from the human-gastrointestinal tract against 22 different antibiotics from six classes under aerobic and anaerobic conditions. Preliminary results obtained by a disc diffusion method were verified using minimum inhibitory concentration (MIC) testing. The results demonstrated that 7-strains had a similar pattern of drug resistance under both conditions, while the remaining ten strains had significant differences in resistance patterns between aerobic and anaerobic conditions for at least one antibiotic. We conclude that successful antibiotic therapy for host-associated pathogens requires proper assessment of the oxygen condition of the growth environment and MIC testing of each pathogen under anaerobic and aerobic conditions.

Antimicrobial Susceptibility of Enteric Gram Negative Facultative Anaerobe Bacilli in Aerobic versus Anaerobic Conditions

Antimicrobial treatments result in the host’s enteric bacteria being exposed to the antimicrobials. Pharmacodynamic models can describe how this exposure affects the enteric bacteria and their antimicrobial resistance. The models utilize measurements of bacterial antimicrobial susceptibility traditionally obtained in vitro in aerobic conditions. However, in vivo enteric bacteria are exposed to antimicrobials in anaerobic conditions of the lower intestine. Some of enteric bacteria of food animals are potential foodborne pathogens, e.g., Gram-negative bacilli Escherichia coli and Salmonella enterica. These are facultative anaerobes; their physiology and growth rates change in anaerobic conditions. We hypothesized that their antimicrobial susceptibility also changes, and evaluated differences in the susceptibility in aerobic vs. anaerobic conditions of generic E. coli and Salmonella enterica of diverse serovars isolated from cattle feces. Susceptibility of an isolate was evaluated as its minimum inhibitory concentration (MIC) measured by E-Test^® following 24 hours of adaptation to the conditions on Mueller-Hinton agar, and on a more complex tryptic soy agar with 5% sheep blood (BAP) media. We considered all major antimicrobial drug classes used in the U.S. to treat cattle: β-lactams (specifically, ampicillin and ceftriaxone E-Test^®), aminoglycosides (gentamicin and kanamycin), fluoroquinolones (enrofloxacin), classical macrolides (erythromycin), azalides (azithromycin), sulfanomides (sulfamethoxazole/trimethoprim), and tetracyclines (tetracycline). Statistical analyses were conducted for the isolates (n≥30) interpreted as susceptible to the antimicrobials based on the clinical breakpoint interpretation for human infection. Bacterial susceptibility to every antimicrobial tested was statistically significantly different in anaerobic vs. aerobic conditions on both media, except for no difference in susceptibility to ceftriaxone on BAP agar. A satellite experiment suggested that during first days in anaerobic conditions the susceptibility changes with time. The results demonstrate that assessing effects of antimicrobial treatments on resistance in the host’s enteric bacteria that are Gram negative facultative Anaerobe Bacilli requires data on the bacterial antimicrobial susceptibility in the conditions resembling those in the intestine.

Survey of anaerobic susceptibility patterns in Canada

The in vitro activity of penicillin, cefoxitin, moxalactam, ticarcillin, clindamycin, chloramphenicol, and metronidazole against 590 anaerobic isolates collected from five Canadian hospitals during 1984 was determined by an agar dilution technique. Cefoxitin, clindamycin, chloramphenicol, and metronidazole were very active against most of the isolates. No major regional differences in the susceptibility patterns were observed.

Antimicrobial susceptibilities of 1,292 isolates of the Bacteroides fragilis group in the United States: comparison of 1981 with 1982

A susceptibility survey of 537 strains of the Bacteroides fragilis group from eight centers in the United States was continued at the Tufts New England Medical Center in 1982. The results were compared with those of 755 organisms analyzed in 1981. Nine antimicrobial agents were tested by an agar dilution method. The respective percentages of resistance for 1981 and 1982 were as follows (%): cefoxitin, 8 and 10; moxalactam, 22 and 12; cefotaxime, 54 and 48; cefoperazone, 57 and 54; piperacillin, 12 and 7; clindamycin, 6 and 3; metronidazole, 0 and 0; chloramphenicol, 0 and 0; and tetracycline, 63 and 59. Regional differences in resistance rates were found. Declines in resistance to moxalactam, piperacillin, and clindamycin were noted at the participating hospitals. An outbreak of cefoxitin resistance was noted at the Tufts New England Medical Center, where the rate increased from 14 to 30%. The various species of the B. fragilis group had differing patterns of resistance; B. fragilis was the most susceptible species. Significant cross resistance among the beta-lactam agents was found. These data indicate the need to determine the susceptibility patterns of the B. fragilis group organisms within each hospital.

Effect of inoculum, pH, and medium on the activity of ciprofloxacin against anaerobic bacteria

The in vitro activity of ciprofloxacin against 30 Bacteroides fragilis group, 30 Clostridium, and 30 Peptococcaceae strains was determined by the agar dilution method in two different culture media at three pH and in three inoculum densities. In Wilkins-Chalgren agar the MICs for 90% of the strains were 0.12 micrograms/ml for B. fragilis, 0.5 micrograms/ml for Clostridium spp., and 2 micrograms/ml for Peptococcaceae. The pH and medium composition affected the MICs of ciprofloxacin.

Modified broth-disk surveillance of clindamycin-resistant Bacteroides fragilis group

The modified broth-disk susceptibility test was used to assess the incidence of clindamycin resistance in the Bacteroides fragilis group. Over a 34-month period, 544 isolates were tested, 21 (4%) of which were found to be resistant. There was no significant difference in resistance among the six species, and resistance did not increase with time. In a 12-disk assay, the concentrations of clindamycin achieved approximated the concentrations expected (expected breakpoint, 4.8 micrograms/ml).

Susceptibility of the Bacteroides fragilis group in the United States in 1981

The minimal inhibitory concentrations of nine antimicrobial agents was determined for over 750 clinical isolates of the Bacteroides fragilis group of anaerobic bacteria collected from nine centers in the United States during 1981. High resistance rates were documented for cefoperazone, cefotaxime, and tetracycline. Cefoxitin had the best activity of the beta-lactam antibiotics, whereas moxalactam and piperacillin had good activities. The resistance rate for clindamycin was 6%. There were no metronidazole- or chloramphenicol-resistant isolates encountered. There were significant differences in susceptibility among the various species of the B. fragilis group, particularly with moxalactam, cefoxitin, and clindamycin. Clustering of clindamycin-, piperacillin-, and cefoxitin-resistant isolates was observed at different hospitals. The variability of resistance rates with the beta-lactam antibiotics and clindamycin indicates that susceptibility testing of significant clinical isolates should be performed to define local resistance patterns.

Metronidazole (Flagyl IV, Searle)

Metronidazole is a narrow spectrum antibiotic with undoubted efficacy against common anaerobic bacteria; resistance is unusual. Therapeutic concentrations of the drug are attained throughout most body compartments after either oral or intravenous administration. The limited side effects of metronidazole are generally tolerable, transient, or reversible. Clinically, metronidazole is as effective as clindamycin and probably chloramphenicol against anaerobes. It has a definite advantage over clindamycin in CNS infections since clindamycin does not penetrate the CSF well. Metronidazole has no irreversible hematologic toxicities, nor has pseudomembranous colitis been definitely attributed to intravenous use of the drug. Metronidazole may replace chloramphenicol for use in anaerobic infections since it lacks the predictable hematologic toxicity of the latter drug. It should also be useful in patients who fail to respond to clindamycin or who develop pseudomembranous colitis while receiving clindamycin. Problems with metronidazole include a complicated preparation procedure, and the high cost of the drug. The single major drawback to the use of metronidazole is uncertainty about its carcinogenic potential in humans. Metronidazole is carcinogenic in animals and mutagenic in vitro, but has not increased the incidence of cancer in humans followed for relatively short periods. Thus, the risk appears to be small. Still, the question will not be resolved for years because of the long latency periods involved in carcinogenesis. Until that time, metronidazole should be used conservatively.

Tinidazole as preventive treatment in elective colonic surgery

Peroperative tinidazole and placebo treatment were compared with respect to incidence of postoperative infections in a double blind investigation on patients admitted for elective bowel surgery. Patients in the tinidazole group had a significantly reduced postoperative infection rate, compared with the placebo group (7.4% and 46%, respectively). Complications in general were fewer in the tinidazole group and costs for antibiotic treatment decreased. All tinidazole-treated patients had clinically effective blood concentrations of the drug at the time of the operation. In contrast with the placebo-treated subjects, the individuals who developed postoperative wound infections from the tinidazole group had no growth of anaerobic bacteria and no antibody response to Bacteroides sp. The use of tinidazole may therefore be recommended for the prevention of postoperative anaerobic infections in patients undergoing bowel surgery.

Susceptibility of anaerobic bacteria to nine antimicrobial agents and demonstration of decreased susceptibility of Clostridium perfringens to penicillin

The activity of moxalactam, cefoxitin, cephalothin, cefamandole, chloramphenicol, clindamycin, metronidazole, and ticarcillin was determined against 344 isolates of anaerobic bacteria. The activity of penicillin G was determined as well for 234 isolates not of the Bacteroides fragilis group. Moxalactam was more active than cephalothin and cefamandole and slightly less active than cefoxitin. Metronidazole was the most active antimicrobial agent against the B. fragilis group, whereas chloramphenicol was most active overall. Clostridium species were the most resistant group of organisms tested. Relatively high concentrations of penicillin were required to inhibit the C. perfringens strains: 80% at 0.5 U/ml and 100% at 16 U/ml. Our study demonstrates the need for periodic anaerobe susceptibility testing in order to better guide empiric antibiotic therapy.

Variation in susceptibility patterns of species within the Bacteroides fragilis group

Minimal inhibitory concentrations of eight antibiotics were determined for 159 clinical isolates of members of the Bacteroides fragilis group using the National Committee for Clinical Laboratory Standards proposed agar dilution reference method. Isolates were identified by standard techniques with deoxyribonucleic acid homolog confirmation. These closely related species demonstrated differences in susceptibility to commonly used drugs.

Anaerobic bacteria: evaluation of disc susceptibility to four cephalosporins

The disc diffusion technique was evaluated with 178 strains of anaerobes and four cephalosporins (cephalothin, cefamandole, cefazolin and cefoxitin). Good correlation in results was found in comparison with the agar dilution technique (p less than 0.001) with the exception of cefamandole and cefazolin against anaerobic cocci (p greater than 0.05). Choosing a breakpoint of 8 microgram/ml for distinguishing susceptible and resistant strains, we determined corresponding incubation, the rate of error is less than 1% for false susceptible and less than 5% for false resistant. However, some strains of anaerobic cocci required a 48 hour incubation period for allowing visible growth. Moreover, a great deal (60.5%) of overlapping zone diameters made interpretation of disc diffusion test difficult among Bacteroides fragilis strains classed as susceptible, intermediate and resistant occuring with cefoxitin. The results have shown that the cephalothin disk will not accurately predict susceptibility of B. fragilis to cefoxitin.