[Analysis of the plasmid profile of various Salmonella serotypes]

Every year foodborne infections cause millions of illnesses but many of them go undiagnosed and unreported. The epidemiology of these illnesses is changing, new pathogens have emerged (Escherichia coli O157:H7, Cyclospora cayetanensis, Vibrio vulnificus). Salmonella spp. is the most common bacterial cause of acute enterocolitis with us. All over the world, as well as in our country the most often isolated serotype is Salmonella Enteritidis. A great problem in many countries is the multiresistant Salmonella Typhimurium, as well as other serotypes resistant to a great number of antimicrobial drugs (S. Hadar, S. Typhi). Clinical microbiologists are often asked to determine the relatedness of bacterial isolates. Recently, traditional methods of strain typing such as bacteriophage typing, resistotyping and serotyping, have been supplemented or replaced in many laboratories with newer molecular methods such as plasmid fingerprinting, ribotyping. PCR-based methods, etc. The goal of strain typing is to provide evidence that epidemiologically related isolates collected during an outbreak are also genetically related and thus represent the same strain.

In the laboratory for Enterobacteriaceae of the Institute of Public Health Novi Sad in the four year period (1995-1998) 3659 primary isolates of Salmonella spp. were isolated using standard bacteriological methods (cultural, biochemical and serological examination). For certain strain of Salmonella Enteritidis, S. Typhimurium and S. Hadar susceptibility to antimicrobial agents was tested by disc-diffusion test (Kirby-Bauer) and plasmid profiles were analyzed. Plasmid DNA was extracted by Birnboim and Doly alkaline lysis method and plasmid bands were separated by electrophoresis in agarose gel.

In the period of 1995-1998 the most common serotype isolated was Salmonella Enteritidis with 3017 (82.5%) of the total number of isolated Salmonellas; S. Typhimurium 203 (5.5%), S. Hadar 118 (3.2%). Plasmid profiles were tested in 10 S. Enteritidis isolates that originated from patients with sporadic cases of diarrhea. All investigated strains had one plasmid band with molecular weight of 38 MDa. All isolates were susceptible to all antibiotics tested. Ten isolates of S. Hadar originated from one outbreak from food samples and stools of patients with diarrheal disease and from the worker in the restaurant. All isolates were resistant to ampicillin and streptomycin, and plasmid profile analysis showed 5 plasmid bands with molecular weights of 13, 5.4, 4.2, 2.0 and 1.7 MDa. Chosen strains of S. Typhimurim were not epidemically related. Strains number 1, 3, 4, 5 were susceptible to all antibiotics tested, and had only one plasmid of 50 MDa, strain number 7 was resistant to streptomycin and had 2 plasmid bands of 50 and 1.7 MDa, other strains were multiresistant and had different plasmid profiles with 4-7 plasmid bands with molecular weights ranging from 50-1.4 MDa.

Plasmid profile analysis is not a sufficient method for examination of Salmonella Enteritidis which is the most common cause of enterocolitis with us. It is, however, a helpful method for proving epidemiological and clonal relatedness of Salmonella isolates that are resistant to antimicrobial agents and have a great number of plasmids (such as some strains of S. Typhimurim and S. Hadar).