Isolation of L-form variants after antibiotic treatment in Staphylococcus aureus bovine mastitis

An unstable L-form of Staphylococcus aureus was identified in milk samples from 3 quarters of 2 cows after treatment with cloxacillin. Milk samples incubated on standard 5% blood agar plates were culture-negative for 7 to 30 days after treatment, but S aureus was reisolated in 80% of 66 samples by additional culturing on enriched L-form media when incubated in 10% CO2 at 37 C. The organism was identified at various phases of reversion of L-form agar and was confirmed on transfer to blood agar plates.