Minor pathogens

Enterobacter

Colonies are mucoid and lactose fermenting like Klebsiella colonies. It is mostly a nosocomial pathogen associated with UTIs, pneumonia and sepsis following instrumentation.

Serratia

Produces non diffusible red pigment in culture. It is a slow lactose fermenter. Associated with hospital acquired UTIs, pneumonia, sepsis, wound infections and endocarditis in intravenous drug abusers.

Citrobacter

Another minor pathogen causing UTIs, meningitis and otitis media. May show false positive tests for Salmonella due to antigen sharing.

Shigella

It is the causative organism of dysentery (passage of blood and mucus in loose stools).

Human pathology

Some strains produce exotoxins. Most dangerous is the verocytotoxin which is the same as EHEC verotoxin (as discussed above under EHEC). Even as little as 10 bacilli can cause infection. Patients present with abdominal cramps and the passage of blood and mucus in stools accompanied by tenesmus. Fever may be present. Intestinal ulcers may develop. Secondary complications can develop such as arthritis, conjunctivitis, intussusception, parotitis and HUS.

Laboratory diagnosis of shigellosis

Stool samples will show the presence of pus cells and RBCs. Culture on Mac Conkey’s agar shows non lactose fermenting colonies. No H2S is produced meaning no blackening of TSI, SS agar (Salmonella Shigella agar) and Wilson and Blair medium. Biochemical tests help to differentiate the strains. Mannitol is fermented by all strains except Shigella dysenteriae. Shigella sonnei ferments sucrose and lactose late.