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.

FeatureShigellaS. typhiOther SalmonellaeReservoirHumansHumansAnimals, Eggs,

PoultryInfectious doseLowHighHighDiarrhoea as a prominent feature

YesNOYes

Invasion of blood stream

NOYesYes

Chronic carrier stateNOYesInfrequentLactose fermentationNONOH2 S productionNOYesYesVaccine availableNOYesNO

Important Features of Salmonella and Shigella