Inactivation of chloramphenicol by Staphylococcus aureus biotype C from humans & animals.

During an investigation, 55 biotype C (bovine and caprine biotype) Staphylococcus aureus isolated from 43 cows suffering from mastitis, and 20 biotype C Staph. aureus strains from the nares and the side of nail-tips of the right thumbs of 20 farm workers (milkers and animal attendants) on six small dairy farms in Assam and Meghalaya were isolated. Three strains from the former and six strains from the latter from among the isolates on two of the farms were found resistant to chloramphenicol, when tested with a routine susceptibility test. Test of the organisms by the agar dilution method indicated that the resistant strains had a minimal inhibitory concentration for chloramphenicol of 32 micrograms/ml or more, while, the chloramphenicol sensitive strains and two reference strains, Staph. aureus ATCC 25923 and Micrococcus luteus ATCC 9341, had < or = 8 micrograms/ml. Two bovine and five human chloramphenicol resistant strains showed positive results when tested by the Gots test. When these strains were tested by a standard method in broth, containing 30 micrograms chloramphenicol per ml, all showed evidence of inactivating chloramphenicol up to a non-detectable level within 36 h. Inactivation of chloramphenicol by Staph. aureus has clinical significance.