Effect of Several Components of Anaerobic Incubation on Antibiotic Susceptibility Test Results

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The factors influencing the in vitro activity of antibiotics during anaerobic incubation were studied by the disc method with a facultative organism, Escherichia coli. We observed the effects of incubation aerobically, anaerobically (Torbal jars), in a CO2 incubator, and aerobically and anaerobically with all CO2 removed. We also monitored pH changes during incubation and observed the effect of two different initial agar pH values (7.4 and 8.3). With aminoglycosides, zones were larger at pH 8.3 and, in each agar pH group, zones were decreased by incubation with increased CO2 (anaerobically and CO2 incubator). A fall in agar pH took place during the first 5 to 7 hr of incubation when increased CO2 was present. Decreased aminoglycoside zones in the presence of increased CO2 were due to fall in agar pH. Erythromycin showed the same zone size changes as the aminoglycosides. Chloramphenicol zones were somewhat smaller at the lower medium pH. Zones around tetracycline discs were largest after incubation anaerobically. Further aerobic (or CO2) incubation of plates after anaerobic incubation resulted in large “zones of relative inhibition” around the aminoglycoside discs. This suggests that these antibiotics had become more active after exposure to aerobic conditions. Our studies indicate that antibiotic susceptibility test results can be significantly altered by several components of anaerobic incubation including changes in agar pH and CO2 concentration as well as anaerobiosis per se.

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