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The Ecology of Chloramphenicol‐Resistance in Salmonella typhimurium and Escherichia coli in Calves with Endemic Salmonella Infection

1981; Wiley; Volume: 50; Issue: 1 Linguagem: Inglês

10.1111/j.1365-2672.1981.tb00876.x

2056-5232

A. H. Linton, John F. Timoney, J. A. Hinton,

Bacteriophages and microbial interactions

A batch of 40 calves intensively reared on a milk replacer diet was monitored over 34 d for salmonella and Escherichia coli excretion patterns. A total of 1055 Esch. coli was isolated on unsupplemented bile lactose agar (BLA), and 537 on BLA supplemented with chloramphenicol, from 10 selected calves. The antibiotic resistance pattern and O‐serogroup were determined for each isolate. Salmonella typhimurium phage type DT 193 carrying six R determinants was detected in five calves on intake; after 14 d 28 were excreting the organism but by the time the calves left the farm (34 d) only a small number were found to be excreting the organism indicating that the infection was self limiting by about six weeks of age. In contrast, multiply resistant Esch. coli were being excreted in increasing numbers with time. Of the isolates on BLA 95.5% were resistant to at least one antibacterial agent and many were resistant to up to six agents. An unusually high proportion (49%) was resistant to chloramphenicol and this R determinant was frequently associated with R determinants for at least streptomycin, sulphonamide and tetracycline, a combination regularly present in the salmonellas. Chloramphenicol resistance is carried on an H 2 plasmid in Salmonella typhimurium phage type 193 and it was found that 11% of 146 randomly selected isolates of Esch. coli , carrying the same range of R determinants, were infected with an H 2 plasmid. H 2 plasmids were found in a number of O‐sero‐groups of Esch. coli and chloramphenicol resistance was present in 37 O‐serogroups. It is suggested that transfer from the salmonellas to Esch. coli possibly took place before the calves entered the farm and the transconjugants were selected by the antibiotics used on the farm. Subsequent transfer between Esch. coli could explain the wide distribution of chloramphenicol‐resistance in many serogroups. The transfer of H 2 plasmids is thermosensitive and no transfer could be demonstrated in vivo but occurred readily in voided faeces held at 28°C. The importance of Esch. coli as a reservoir of chloramphenicol resistance and the public health implications are discussed.