The ENTEROBACTERIACEAE: Salmonella/Shigella

Genus salmonella is composed of biochemically and serologically diverse group of micro-organisms. Besides humans they also infect many animals. Capable of invading extra-intestinal tissue and causing enteric fever, most severe form of which is typhoid fever. Aerobic and facultatively anaerobic.After 24 hrs and incubation at 37Cf orm moderately large, grey-white, moist colonies on nutrient agar. Grows on differential agars like MacConkey bile-salt lactose agar, Brilliant green MacConkey Agar. Inhibitory to E.coli, proteus spp and other commensal enterobacteria. Excellent selective and differential medium for salmonella except Typhi. Low convex pale green translucent colonies. LF and rare strains of Salmonella serotypes produce blue-purple colonies. Antigenic structures are composed of H – Flegellar antigens, O – Somatic antigen, Vi – Surface antigen in some species only. Biochemical reactions : Glucose ,Mannitol ,Maltose produce A/G. Salmonella typhi do not produce gas. Lactose/Salicin/sucrose not fermented. Indole –.Methyl Red +.V P -Citrate +.Urease –.H2S – produced by Salmonella typhi. Paratyphi A do not produce H2S. Determinants of pathogenecity are: Adherence factors, Invasive factors and Factors evading host defense. Blood culture in case of enteric fever:1st week Positive in 90 %. 2nd week Positive in 75 %. 3rd week Positive in 60 %. > 3 weeks positive in 25 %.Salmonella enterocolitis does not need any antibiotic therapy. In severe diarrhea, fluid and electrolytes replacement is necessary. Antimicrobial therapy of invasive Salmonella infections is with ampicillin, SXT, or third generation cephalosporin and fluroquinolones.Not Peer Reviewe