A study on virulence markers as epidemiological tool for strains of Salmonella enterica subsp enterica serovar Paratyphi B from animal origin foods

A study on 22 Salmonella enterica serovar Paratyphi B (S. paratyphi B) var 'Java' (SPVJ) and 13 classical S. enterica serovar Paratyphi B (CSPB) strains isolated from poultry meat (12 SPVJ), fish (9 SPVJ, 11 CSPB), milk products (1 CSPB, 1 SPVJ) and pork (I CSPB) revealed that all strains except one CSPB strain of fish origin, were lethal to mice on intraperitoneal inoculation with 10(8) organisms from overnight grown culture. Only 5 strains revealed presence of a heavy plasmid (> 35.8 MDa) however, 35.8 MDa plasmid was common in all isolates. None of the isolates produced lecithinase, gelatinase, casienase, phospholipase A or C but many of the isolates produced DNase. Variable results were observed in salt aggregation assay with indices ranging from 0.6-2.2 M. All produced brick red colonies on Congo Red dye agar in > 24 h of incubation without any effect of 0.15% bile salt addition in the medium. All strains haemolysed washed erythrocytes of horse, guinea pig and human blood group B. The best haemolysis was evident on blood agar having horse or guinea pig erythrocytes. No correlation could be established between haemolysis of RBC of different origin and mouse lethality. Multiple drug resistance (MDR) pattern of S. Paratyphi B isolates revealed their resistance to dicloxacillin, cefazolin, colistin sulphate and ceftazidime and sensitiveness to augmentin, ciprofloxacin, nalidixic acid, enrofloxacin, tetracyclin, lomefloxacin, crystal violet, acriflavin and mercuric chloride. Other frequently resisted drugs were nitrofurantoin, streptomycin and cephalexin. Though different in vitro virulence markers have no or little correlation with mouse-lethality, but can be used for epidemiological typing during paratyphoid outbreaks of food origin, particularly haemolysin production and DNase test appears to be of immense value.