CAT-III CHLORAMPHENICOL RESISTANCE IN PASTEURELLA-HAEMOLYTICA AND PASTEURELLA-MULTOCIDA ISOLATED FROM CALVES

Chloramphenicol, which had been used extensively for antimicrobial vet erinary therapy, was prohibited in Europe in 1994. Soon after it becam e available, resistance to this drug was detected, generally conferred by plasmids encoding inactivating enzymes, the chloramphenicol acetyl transferases (CAT), in Gram-negative as well as in Gram-positive bacte ria. In the last few years, resistance to antibiotics emerged in Paste urella strains from breeding herds and this evolution was followed by a national surveillance network. Chloramphenicol-resistance was more r ecently detected in multiresistant strains. We studied 25 strains of P asteurella, selected for their resistance to chloramphenicol. Producti on of a CAT was demonstrated in all these strains. PCR amplification i ndicated that the CAT produced was of type III for 23 of them. In thes e strains, chloramphenicol-resistance was mediated by plasmids of abou t 5.1 kb. Southern blots on restriction fragments suggested a high deg ree of homology between these 5.1 kb plasmids. In the two other strain s, production of a CAT type I was demonstrated, and the corresponding genes were either shown on a plasmid of 17 or 5.5 kb.