Chloramphenicol resistance genes in Salmonella enterica subsp. enterica serovar Typhimurium isolated from human and animal sources in Hungary

The presence of the chloramphenicol resistance genes cat, cmlA, flo, and the role of plasmids and class 1 integrons in the spread and persistence of chloramphenicol resistance were investigated on a collection of 40 Salmonella enterica serovar Typhimurium strains isolated from animals and humans in Hungary, by PCR and conjugation. Three groups of chloramphenicol resistant strains were identified. Eleven animal and 13 human isolates harboured the flo gene, encoding resistance to chloramphenicol and florfenicol, and possessed integrons of 1.0 Kb and 1.2 Kb typically found on the multidrug resistance island of S. Typhimurium DT104. Fifteen human strains had two different chloramphenicol resistance genes: the catB3 gene, identified as a gene cassette within a 1.45 Kb integron, and the catA gene, both of which were located on and transferred by a 140 Kb plasmid from a representative strain to the E. coli recipient via conjugation. A single animal strain had the catA gene alone, which was also transferred by a 35 Kb plasmid via conjugation. These three groups of strains belonged to three distinct genetic clusters, as it was revealed by macrorestriction analysis of 18 selected strains. This study provides information on the versatile genetic background of the chloramphenicol and florfenicol resistances in S. Typhimurium in Hungary and points to the significance of mobile genetic elements such as conjugative R-plasmids and integrons in the spread and persistence of chloramphenicol resistance genes. The results also indicate the predominance of the flo gene among animal strains and its appearance among human strains in Hungary.