SHIGELLA Important Properties: @ Gram-negative, Lactose negative rods. @ Distinguished from Salmonella by: # No gas from glucose fermentation # Do not produce H2S. # Non-motile.

SPECIES: Shigellae are divided into the following: @ Group (A): S. dysenteriae, serotypes 1-10 @ Group (B): S. flexneri, serotypes 1-6. @ Group (C): S. boydii, serotypes 1-15. @ Group (D): S. sonnei, only one serotype.

Antigens: @ All shigellae have polysaccharide O antigens in their cell walls. @These antigens are used to divide the Shigella genus into 4 groups: A, B, C, & D Disease: Shigella causes enterocolitis (dysentery).  

Pathogenesis: @ Shigella has a very low ID50 @ ID50 (the 50 % infectious dose) is the number needed to cause infection in half the hosts. @ LD50 (the 50 % lethal dose) is the number needed to kill half the hosts. @ Ingestion of as few as 100 organisms causes disease

@ Shigellosis is a disease of humans only. @ It leads to bloody diarrhoea (dysentery) by invading the mucosa of the distal ileum and colon. @ This leads to inflammation and ulceration @ Shigella will not penetrate the intestinal wall or enter the bloodstream (unlike Salmonella)

@ Some Shigella strains produce an enterotoxin, but its role in pathogenesis is uncertain. @ The evidence for this: @ Shigella strains not producing enterotoxin can still cause dysentery. @ Non-invasive Shigella strains are non- pathogenic.

Clinical Findings: @ First fever, abdominal cramps, then watery diarrhea, which later contains blood and mucus. @ The severity of the disease depends on: # The species (S. dysenteriae is the most severe and S. sonnei is the mildest) # The age of the patient (children and elderly most affected). @ Antibodies are not protective

Laboratory Diagnosis: @ Shigellae form non-lactose-fermenting colonies on Mac Conkey & EMB agar. @ On TSI agar, they give an alkaline slant and an acid butt, without gas or H2S. @ Confirmation by slide agglutination

@ Methylene blue stain of feces detects polymorpho-nuclear cells (PMNs). # If present, an invasive organism is involved, e.g. Shigella, Salmonella, Campylobacter # If absent, a toxin-producing organism is involved, e.g. V cholerae, E coli, Clostridium perfringens.

Epidemiology: @ Shigella is transmitted from person to person by asymptomatic carriers. @ The four F's are the main sources of infection (Fingers, Food, Faeces, Flies) @ Portal of entry is the faecal-oral route. @ Food-borne outbreaks outnumber water- borne outbreaks by 2 to 1. @ Children under 10 yrs account for half of Shigella positive stool cultures.

Treatment: @ Main therapy is fluid & electrolyte replacement @ In mild cases, no antibiotics. In severe cases, ciprofloxacin is the best. @ Incidence of drug resistance is high, hence sensitivity tests are performed. @ Antiperistaltic drugs (Lomotil) are not used so as not to prolong the symptoms

Prevention: @ Interrupt the fecal-oral transmission by: # Proper sewage disposal. # Chlorination of water. # Hand washing by food handlers. # No vaccine, or antibiotic prophylaxis.

Salmonella and Shigella Important Features of Salmonella and Shigella Feature Shigella S. typhi Other Salmonellae Reservoir Humans Animals, Eggs, Poultry Infectious dose Low High Diarrhoea as a prominent feature Yes NO Invasion of blood stream Chronic carrier state Infrequent Lactose fermentation H2 S production Vaccine available