ENTEROBACTERIACEAE

Most common group of gram negative rod which are culture in medical labortries.they have very complex taxonomy because of rapidly changing in nucleic acid hybridization and nucleic acid sequence. Enterobactericeae have 20-25 clinical significant species. (Monica 2000)

Enterobacteriaceae are naturally habitate of animals and humans intestine. It belongs to the large hetrogenous group of gram negative rods bacteria.this family consists of many genera’s like Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia, Proteus, and others. Some generas are normal flora of intestine and causes disease incidentally but other genera’s like shigellae and salmonellae are causing disease in humans.(Murray  et al., 2003)

Enterobacteriaceae are anaerobes or aerobes and ferment different kinds of carbohydrates, produce different kinds of toxins and other virulence factors. They are present in both forms motile and non motile. Clinical significant Enterobacteriaceae species are about 20-25 and others are infrequently incountered.on culture plates E.coli and other enteric bacteria expresses circular, convex, smooth colonies with distinct edges.E.coli is lactose fermenter while salmonellae and shigellae are non-lactose fermenters. Colonies of Klebsiella are very large and mucoid and have prolonged incubation time. For salmonellae and shigellae differentiation from other gram negative enteric bacteria present in stool Tri sugar iron (TSI) medium is used.(Murray  et al., 2003)

They have complex antigenic structure classified by more than 150 different heat-stable somatic O (lipopolysaccharide) antigens, more than 100 heat-labile K (capsular) antigens, and more than 50 H (flagellar) antigens.The most external part of the cell wall lipopolysaccharide are O antigens with repeating units of polysaccharides, some have unique sugars. K antigens are external to ‘’O’’ antigens in almost all Enterobacteriaceae not in all may interfere with agglutination by O antisera and also associated with virulence e.g., E coli strains producing K1 antigen are prominent in neonatal meningitis. H antigen is flagellar antigen present on flagella which are removed or denatured by heating and alcohol; it interferes with agglutination by anti-O antibody. Enterobacteriaceae are often present in urine, blood, pus, spinal fluid and sputum. (Murray et al., 2003)