Occurrence of chloramphenicol acetyltransferase and Tn9 among chloramphenicol-resistant enteric bacteria from humans and animals.

The occurrence of chloramphenicol acetyltransferase (CAT) among chloramphenicol resistant enteric bacteria from humans, animals (cats, dogs) and sewage was examined. The enzyme appears to be the basis of resistance in 83 and 84 per cent of bacteria of humans and sewage origin, and in 50 per cent of bacteria from animals. In order to identify type I CAT among chloramphenicol resistant isolates, total cellular DNA was probed with a 32P-labelled fragment of the CAT structural gene from the transposable element Tn9. Nineteen per cent of chloramphenicol resistant enteric bacteria of clinical origin, 11% of sewage isolates, and 11% of veterinary isolates gave positive hybridization results. The difference between bacteria of clinical and veterinary origin in respect of both parameters tested is significant and is interpreted as indicating genetic dis similarity between the two pools in regard to chloramphenicol resistance. This may reflect a lack of contact between the two pools, or host bacterial factors with select against CAT-mediated (and type I CAT more specifically) resistance to chloramphenicol.