Alteration of effectiveness of antibiotics by anaerobiosis.

In vitro antibiotic susceptibility tests of facultative organisms are routinely performed under aerobic conditions, despite the fact that many infections caused by these organisms occur in anaerobic areas, i.e., intra-abdominal absess. Experiments were performed aerobically and anaerobically to determine the susceptibility of E. coli, P. mirabilis, and K. pneumoniae to gentamicin, tobramycin, cephalothin, cefazolin, and cefamandole. Antibiotic sensitivities were determined by disc and agar dilution techniques in air and in anaerobic jars with CO2 absorbed. Tube dilution studies were performed in air and anaerobically and time-kill studies were done in aerobic and anaerobic broth. The amount of aminoglycoside required to inhibit bacterial growth was increased 4 to 20 times by anaerobiosis in 20 of 25 strains tested. Time-kill curves showed that bacterial killing by aminoglycosides was markedly impaired by anaerobiosis. Anaerobic conditions had no effect on the rate or extent of killing by cephalosporins. These data may have significance in determination of antibiotic susceptibility of facultative organisms under anaerobic tissue conditions. Antibiotic sensitivity testing done on these organisms in air may not reflect the actual state of antibiotic-bacterial interaction under conditions of the infection.