AGAR DISK DIFFUSION (BAUER-KIRBY) TESTS WITH VARIOUS FASTIDIOUS END NONFASTIDIOUS REFERENCE (ATCC) STRAINS - COMPARISON OF SEVERAL AGAR MEDIA

Several agar media (Mueller-Hinton agar, MHA; diagnostic sensitivity t est agar, DSTA; Schaedler agar, SchA; Todd-Hewitt agar with added yeas t extract, THYA; Wilkins-Chalgren agar, WCA) were compared using the B auer-Kirby agar disk diffusion test against six nonfastidious quality control strains: Staphylococcus aureus ATCC 25923 and ATCC 29913, Esch erichia coli ATCC 25922 and ATCC 35218, Pseudomonas aeruginosa ATCC 27 853, and Enteracoccus faecalis ATCC 29212. MHA, DSTA, and THYA yielded essentially comparable inhibition zones. However, WCA and SchA antago nized cotrimoxazole and aminoglycoside antibiotics; furthermore, SchA antagonized polymyxin B, and both WCA and SchA antagonized imipenem ag ainst the P. aeruginosa strain, but not against the E. coli strains. S heep blood-MHA (Bl-MHA), WCA, THYA, and DSTA were examined with Strept ococcus pyogenes ATCC 19615, Streptococcus agalactiae ATCC 13813, and Streprococcus pneumoniae ATCC 6306. In comparison with Bl-MHA, both WC A and THYA yielded comparable inhibition zones against S. pyogenes; DS TA afforded suboptimal growth. DSTA yielded larger inhibition zones wi th the majority of antimicrobial drugs against S. agalactiae, whereas WCA and THYA enhanced the activity of oxacillin and penicillin G again st this strain. S. pneumoniae strain ATCC 6306 grew well on Bl-MHA, yi elded suboptimal growth on WCA and faint growth on THYA, and failed to grow on DSTA; Chocolate-supplemented sheep blood-MHA (CHOC-MHA) was c ompared with Haemophilus test medium (HTM), WCA with added NAD, and TH YA with added hematin and NAD against Haemophilus influenzae strains A TCC 35056 and ATCC 49247. The activities of doxycycline and rifampin w ere enhanced against both strains by HTM, WCA + NAD, and THYA + hemati n + NAD. Only WCA + NAD antagonized cotrimoxazole against both H. infl uenzae strains, an effect due to thymidine; however, HTM antagonized c otrimoxazole against S. aureus ATCC 25923 and E. coli ATCC 25922. It w as concluded that Bl-MHA performed best for beta-hemolytic streptococc i quality control strains. Like-wise, CHOC-MHA was optimal for the two H. influenzae strains used in this comparative agar disk diffusion st udy.