SUSCEPTIBILITIES OF BOVINE SUMMER MASTITIS BACTERIA TO ANTIMICROBIAL AGENTS

The susceptibility to 9 antimicrobial agents of 32 aerobic bacterial i solates and to 10 antimicrobial agents of 37 anaerobic bacterial isola tes from 23 cases of bovine summer mastitis (16 Actinomyces pyogenes i solates, 8 Streptococcus dysgalactiae isolates, 3 S. uberis isolates, 3 S. acidominimus isolates, 2 Streptococcus spp,, 15 Peptostreptococcu s indolicus isolates, 10 Fusobacterium necrophorum isolates, and 12 is olates of anaerobic gramnegative rods) was determined by the agar dilu tion method, All isolates except one Bacteroides fragilis isolate (p-l actamase producer) were susceptible to penicillin G, amoxicillin, amox icillin-clavulanate, cefoxitin, clin damycin, and chloramphenicol (the B. fragilis strain was susceptible to the last four), which had MICs at which 90% of isolates were inhibited (MIC(90)s) of less than or equ al to 0.06, less than or equal to 0.06, less than or equal to 0.06, 0. 25, less than or equal to 0.06, and 4.0 mu g/ml, respectively, Spiramy cin was active against the gram-positive aerobes (MIC(90), 1.0 mu g/ml ) but not against the anaerobes (MIC(90), 16.0 mu g/ml). Similar trend s were noted for susceptibilities of aerobic and anaerobic bacteria to ofloxacin (MIC(90)s, 2.0 and 8 mu g/ml, respectively), Occasional str ains of aerobic streptococci were resistant to oxytetracycline, but al l anaerobes were susceptible, Tinidazole was active against all anaero bes (MIC(90), 2.0 mu g/ml). beta-Lactamase was produced only by the B. fragilis isolate.