Susceptibilities of bovine summer mastitis bacteria to antimicrobial agents

Abstract

The susceptibility to 9 antimicrobial agents of 32 aerobic bacterial isolates and to 10 antimicrobial agents of 37 anaerobic bacterial isolates from 23 cases of bovine summer mastitis (16 Actinomyces pyogenes isolates, 8 Streptococcus dysgalactiae isolates, 3 S. uberis isolates, 3 S. acidominimus isolates, 2 Streptococcus spp., 15 Peptostreptococcus indolicus isolates, 10 Fusobacterium necrophorum isolates, and 12 isolates of anaerobic gram- negative rods) was determined by the agar dilution method. All isolates except one Bacteroides fragilis isolate (β-lactamase producer) were susceptible to penicillin G. amoxicillin, amoxicillin-clavulanate, cefoxitin, clindamycin, and chloramphenicol (the B. fragilis strain was susceptible to the last four), which had MICs at which 90% of isolates were inhibited (MIC90s) of ≤0.06, ≤0.06, ≤0.06, 0.25, ≤0.06, and 4.0 μg/ml, respectively. Spiramycin was active against the gram-positive aerobes (MIC90, 1.0 μg/ml) but not against the anaerobes (MIC90, 16.0 μg/ml). Similar trends were noted for susceptibilities of aerobic and anaerobic bacteria to ofloxacin (MIC90s, 2.0 and 8 μg/ml, respectively). Occasional strains of aerobic streptococci were resistant to oxytetracycline, but all anaerobes were susceptible. Tinidazole was active against all anaerobes (MIC90, 2.0 μg/ml). β-Lactamase was produced only by the B. fragilis isolate.