Evolution of chloramphenicol resistance, with emergence of cross-resistance to florfenicol, in bovine Salmonella Typhimurium strains implicates definitive phage type (DT) 104

The prevalence of resistance to florfenicol, a phenicol drug newly introduc ed in veterinary therapy, was determined in 86 chloramphenicol-resistant Sa lmonella Typhimurium isolates from cattle collected during 1985-1995, All w ere highly resistant to chloramphenicol (MICs greater than or equal to 128 mg/L) and 38 were simultaneously resistant to florfenicol (MICs >16 mg/L) a nd to beta-lactam agents, spectinomycin, streptomycin, sulphonamides and te tracyclines. The isolates susceptible to florfenicol harboured the chloramp henicol acetyl transferase gene, cat of type I. All the forfenicol-resistan t isolates harboured the floR resistance gene and the characteristic multip le resistance genetic locus, previously characterised in a S. Typhimurium D T104 strain and identified by a multiplex PCR, Plasmid profiles and ribotyp e patterns were determined for all the isolates. The florfenicol-resistant isolates were grouped into the same ribotyping pattern and presented simila r plasmid profiles, whereas the florfenicol-susceptible isolates showed a w ider genetic diversity that is usual for S, Typhimurium. Thus, the florfeni col-resistant isolates could represent a clonal cluster, closely related to , if not of DT104 phage type, which appeared in 1989 and is now predominant within chloramphenicol-resistant S, Typhimurium. The multiplex PCR provide d a useful tool to survey further evolution of multiresistant S, Typhimuriu m strains.