The susceptibilities to arsenic and cadmium together with the detection of plasmid DNA were evaluated for use as epidemiological markers for the subtyping of Listeria monocytogenes. Plasmid DNA was detected in 34% of 322 apparently unrelated isolates of L. monocytogenes. The resistance to cadmium and arsenic differentiated 565 apparently unrelated cultures into four groups, the smallest being 5% of cultures resistant to both agents, and the largest (53%) being sensitive to cadmium and resistant to arsenic. The resistance patterns to these agents and the presence of plasmid DNA varied markedly between the serotypes of the cultures. The detection of plasmid DNA was strongly associated with cadmium resistance in serogroup 1/2 cultures, but not within those of serogroup 4. Arsenic resistance was not associated with plasmid DNA. All methods were sufficiently stable to be useful for epidemiological investigations. The techniques described here offer simple methods which can be easily utilized in laboratories without a specialized expertise for this bacterium.
Molecular analyses based on plasmid profile typing and pulsed-field gel electrophoresis have defined a strain of Salmonella enterica serotype Anatum associated with the consumption of a particular brand of formula-dried milk responsible for an outbreak in late 1996/early 1997 involving 15 infants and 2 relatives in the UK, and 2 infants in France. The study has demonstrated the value of laboratory-based surveillance involving identification of the outbreak strain at the molecular level coupled with food microbiology and targeted epidemiological investigations, and has highlighted the importance of rapid communication and subsequent international collaboration through the European Union-funded Salm-Net salmonella surveillance network.
Of over 2000 isolates of Salmonella typhimurium DT 193 from humans examined in the 2 year period 1991-92, 93% were antibiotic-resistant with the most common R-types being ASSuT (38%) and T (29%). Fourteen plasmid profiles were identified in DT 193 R-type ASSuT with the majority of isolates being characterized by a single plasmid of 80 MDa (pDEP 34) which in addition to coding for ASSuT, also hybridized with a spv gene probe prepared from the 50 MDa Salm. dublin serovar-specific plasmid. On the basis of restriction fragment length polymorphisms, two variant lines of pDEP 34-like plasmids were identified and a third line which had lost the genes coding for resistance to ampicillin, streptomycin and sulphonamides, was recognized. Although 18 plasmid profile types were identified in DT 193 R-type T, all isolates carried a high mol. wt plasmid which coded for tetracycline resistance only. Further discrimination was achieved on the basis of hybridization of tetracycline resistance plasmids with the spv gene probe and restriction enzyme fingerprinting. These results demonstrate that Salm. typhimurium DT 193 can be rapidly subdivided by antibiogram and that further subdivision can be achieved on the basis of plasmid profile, plasmid fingerprint and hybridization with a spv gene probe.
SUMMARYPlasmids were found in 1022 of 1089 (94%) of drug-sensitive strains of Salmonella enteritidis phage type 4 from humans (sporadic and outbreak cases), poultry (chickens) and eggs in England and Wales in the 5-year period 1988-92 and 25 plasmid profile patterns were identified. Strains characterized by a single plasmid of 38 MDa predominated (= plasmid profile type SE 38), comprising over 90% of isolates from humans, 70% from poultry and 92 % from eggs. Eleven profile types were identified in strains from humans, 21 in strains from poultry and 3 in strains from eggs. Eight of the 11 patterns identified in human isolates were found in strains from poultry and 2 in strains from eggs. In contrast 15 patterns seen in poultry were not found in strains from humans. Four percent of strains from humans and 13 % from poultry did not carry the 38 MDa plasmid but all strains from eggs were found to carry this plasmid. The second most common profile type in strains isolated between 1981 and 1988 was not identified in strains isolated from 1988-92. It is concluded that plasmid profile typing is a useful method for rapid differentiation within phage type 4 of S. enteritidis but that methods which can discriminate within the predominant profile type, SE 38, are now required.
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