Shigella

Shigella is a genus of gram negative rods, which are non-spore forming, non-encapsulated and non-motile bacteria, responsible for causing enterocolitis and dysentery.

Pathogenesis

Route of Entry  
Route of entry is mainly by feco-oral route. Primary factors for transmission are the 4 F’s:
1.    Fingers
2.    Flies
3.    Food
4.    Feces

Only a small dose of bacteria is required (around 100 organisms) because they are resistant to hydrochloric acid present in stomach. Shigella causes disease exclusively in GIT. It produces dysentery by invading the cells  of the mucosa of the  distal ileum and colon. It is responsible for causing ulceration with local inflammation. It rarely penetrates through cell wall or enter blood stream.
Only ‘O’ antigen is present.

Virulence Factors
•    Endotoxin – lipopolysaccharide
•    Shiga-like Toxin (SLT), which is an enterotoxin
•    Enterotoxins

Enterotoxins inhibit host cell protein synthesis.

Predisposing factors

• Improper sewage disposal
• Poor hygiene

Diseases are more common among children and elderly. Also in mental hospitals.

Lab Diagnosis

Specimen
•    Fresh fecal specimen
•    If a delay is anticipated then suitable transport medium should be used.

Microscopy

Following features are seen on microscopy:
•    Gram negative rods
•    Non-spore forming
•    Non-encapsulated
•    Non- motile
•    Facultative

Culture
Incubation period is 1-4 days.
1.    XLD agar, red -pink colonies are formed, without black centers
2.    MacConkey’s agar, colorless colonies are formed
3.    Methylene blue stain when applied on fecal sample, stains shigella, salmonella or campylobacter

Biochemical Tests

Triple sugar iron (TSI) agar
On triple sugar iron, alkaline slant and acidic butt is produced. No gas or H2S is produced.

• Lactase        Negative
• H2S              Negative
• Urease         Negative
• Oxidase       Negative
• Citrate         Negative
• Lysine          Decarboxylose

Serological Tests
•    Slide agglutination test
•    PCR

Diseases

1.    Enterocolitis
2.    Dysentery

Clinical symptoms
•    Fever
•    Abdominal cramps
•    Diarrhea, which is watery at first, followed by stools with blood, mucus and pus cells

Treatment

• Fluid and electrolyte replacement
• In mild cases no antibiotics are required
• In severe cases Ciprofloxacin is used
• Antiperistaltic drugs are contraindicated