|Ph.D Student||Hadas Solnik-Isaac|
|Subject||Salmonella enterica Serotype Virchow: Molecular|
Characterization of Virulence and Resistance
|Department||Department of Biotechnology and Food Engineering||Supervisor||Full Professor Yaron Sima|
|Full Thesis text - in Hebrew|
Salmonella enterica serotype Virchow (S. virchow) is highly prevalent in humans and farm animals in Israel. The present study outlines the molecular epidemiology, invasive factors and resistance properties of S. virchow in Israel over a 9-year period.
Four different typing methods were used in order to differentiate between isolates for epidemiology research. The best discriminatory method for subserotype typing of S. virchow was pulsed-field gel electrophoresis (PFGE). By this method 22 closely related types (pulsotypes) were identified. On the other hand multilocus sequence typing (MLST) based on fimA, mdh and manB genes, phage typing (PT) and the plasmid profiles analysis have limited discriminatory capacity for S. virchow typing (only 4 to 6 types).
High similarity between the isolates using the different typing methods led us to the hypothesis that S. virchow in Israel emerged through clonal expansion. PFGE revealed two closely related clusters, each containing a predominant pulsotype. In addition PT analysis disclosed two major profiles, PT26 and PT31. These types differed from the dominant types in other countries.
Over 45% of the human and poultry isolates, were resistant to 4 or more different antimicrobial agents. A prominent six antibiotics pattern emerged which included the following: chloramphenicol, tetracycline, streptomycin, trimethoprim - sulphamethoxazole and nalidixic acid.
The molecular basis for S. virchow multi drug resistance (MDR) occurred through several genetic events. More than 43% of the isolates were positive to the presence of class 1 integrons. At least in some of the isolates the class 1 integrons contained genes confirming resistance to streptomycin, trimethoprim, sulfonamides and quaternary ammonium compounds. Thirteen ampicillin resistant isolates carried a 8227bp plasmid which contains the blaTEM gene specified for resistance to ampicillin and the tnpA, tnpR, lazA and rom genes. In addition a 3275bp plasmid was found in an ampicillin sensitive isolate.
Over 90% of S. virchow isolates were resistant to nalidixic-acid (MIC≥128 μg/ml), with reduced susceptibility to ciprofloxacin (MIC between 0.125 to 0.250 μg/ml). Investigation of the mechanisms of quinolone resistance revealed that this pathogen probably emerged from a parental clone that overproduced the AcrAB efflux pump and had a single point mutation in gyrA leading to the Asp87Tyr substitution.
The close resemblance between humans and poultry isolates with the different typing profiles, the similar resistance properties and the presence of genetic elements like point mutations, plasmids and the class 1 integrons point to poultry as a likely source of S. virchow in the food chain in Israel.