Effects of Different Test Conditions on MICs of Food Animal Growth-Promoting Antibacterial Agents for Enterococci

The influence of the addition of sheep blood to Mueller-Hinton II agar and the effects of aerobic incubation with or without CO₂ and of anaerobic incubation were tested with bacitracin, tylosin, avoparcin, virginiamycin, avilamycin, narasin, and flavomycin on enterococci. The antibacterial activity of bambermycin (Flavomycin) was strongly inhibited by the addition of blood, except with the species Enterococcus faecium, Enterococcus mundtii, Enterococcus hirae, Enterococcus casseliflavus, and Enterococcus gallinarum, which were not susceptible to this antibiotic on blood-free medium. With all other antimicrobials except avoparcin and tylosin, the presence of blood resulted in MIC increases of 1 to 3 log₂ differences. Incubation in aerobic or anaerobic atmospheres enriched with CO₂ lowered the susceptibility of enterococci to tylosin and increased their susceptibility to avilamycin, narasin, and avoparcin. This effect was most pronounced in tests on blood-free media. Results of susceptibility tests incubated under anaerobiosis and in a CO₂-enriched atmosphere did not differ. For all enterococcal species, the preferred conditions for testing the susceptibility are Mueller-Hinton II medium supplemented with blood and incubation in a CO₂-enriched atmosphere. However, when only E. faecium and Enterococcus faecalis are being tested, Mueller-Hinton II medium without blood incubated aerobically gives satisfactory results.