ANTIBIOTIC SUSCEPTIBILITY TESTS WITH FASTIDIOUS AND NONFASTIDIOUS BACTERIAL REFERENCE STRAINS - EFFECTS OF AEROBIC VERSUS HYPERCAPNIC INCUBATION

Representative antimicrobial drugs were examined under aerobic and hyp ercapnic (3 and 5% v/v CO2) incubation with the Bauer-Kirby agar disk diffusion, a broth microdilution method, and the agar dilution procedu re against nonfastidious, standard ATCC quality control strains and ag ainst beta-hemolytic streptococcal, two pneumococcal, and Haemophilus influenzae ATCC strains. It was found that an atmosphere of 3-5% C0(2) merely antagonized amikacin, gentamicin, and netilmicin; the activity of penicillin G was antagonized only against Staphylococcus aureus AT CC 29213 in broth media, but not against any of the other strains. The activity of teicoplanin, and less so that of vancomycin, was enhanced only against S. aureus strain ATCC 25923, but not against the other s trains. It was concluded that susceptibility tests, excluding aminogly coside antibiotics, of beta-hemolytic streptococci, pneumococci, and H . influenzae and H. parainfluenzae should be incubated under 3% (candl e jar or incubator) or 5% CO2 (incubator) so as to ensure optimal grow th of capnephilic strains and thus avoid potentially misleading large inhibition zones or deceptively low minimal inhibitory concentrations.