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Salmonella typhi of Bacteriology

The genus Salmonella consists of bacilli that parasites the intestines of vertebrates and human beings. It causes Enteric fever, which includes Typhoid and Paratyphoid fever. The most important species of the genus is Salmonella typhi which causes typhoid fever.

Morphology

Salmonellae are Gram – negative rods (1 – 3µm × 0.5 µm in size). They are motile with peritrichous flagella, non – capsulated and non – sporulated (Figure 7.14).
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Cultural Characteristics

They are aerobic and facultative anaerobe, optimum temperature – 37°C and pH is 7 – 7.5. They grow on the following media and show the following characteristic colony morphology (Table 7.12).

Table 7.12: Colony morphology of Salmonella typhi

Media

Colony Morphology
Nutrient Agar Colonies are large, circular, smooth, translucent
MacConkey Agar Colourless colonies (non – lactose fermenters)
SS – Agar Colourless colonies with black centered.

Pathogenicity

Salmonella typhi causes typhoid fever and its pathogenesis is discussed in flowchart 7.6.

Source of infection – food, feces, fingers, flies
Route of entry – faecal oralroute (ingestion)
Incubation period – 7 – 14 days
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Clinical Manifestations

  • The illness is usually gradual, with headache, malaise (feeling of discomfort), an2orexia (loss of appetite), coated tongue, abdominal discomfort with either constipation or diarrhea.
  • Hepatosplenomegaly (enlargement of liver and spleen), step ladder pyrexia (continuous fever) and rose – spots (during 22nd or 3rd week).

Laboratory Diagnosis

Specimens:

Blood, stool and urine are the clinical samples collected from typhoid patients. The selection of relevant specimen depends upon duration of illness, which is very important for diagnosis (Table 7.13 & Figure 7.15).

Table 7.13: Specimen collection for typhoid

Duration of disease

Specimen examination

% Positivity
1st Week Blood culture 90
22nd Week Blood culture
Faeces culture
Widal test		 75
50
Low titer
3rd Week Widal test
Blood culture
Faeces culture		 80 – 100
60
80

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The bacteriological diagnosis of enteric fever consists of the following methods, which are:

  • Isolation of the bacilli
  • Demonstration of antibodies

Isolation of the bacilli

The typhoid bacilli are isolatedfrom the following clinical specimens which are tabulated (Table 7.14).

Table 7.14: Isolation method of typhoid bacilli from various clinical speciments.

Demonstration of Antibodies:

Slide – agglutination: The isolate is identified by slide agglutination with ‘O’ and ‘H’ antisera.

Widal Test:

It is an agglutination test for detection of agglutinins ‘H’ and ‘O’ in patients with enteric fever. Salmonella antibodies start appearing in the serum at the end of 1st week and rise sharply during the 3rd week of enteric fever.

Prophylaxis

Various types of vaccine and their doses are given in Table 7.15.

Table 7.15: Various types of vaccine and their doses.

Vaccine

Doses
TAB – Vaccine 2 doses of 0.5 ml at an interval of 4-6 weeks
Typhoral 3 doses on alternate days. It gives 65-96% protection for 3-5 years and is safe
Typhin – Vi A single dose of 25µg

Treatment and Control Measures

  • Antibacterial therapy has been very effective in the treatment of patients.
  • Ampicillin, amoxicillin and cotrimoxazole are useful in the treatment of typhoid fever.
  • At present, ciprofloxacin is the drug of choice.
  • Typhoid fever can be effectively controlled by sanitary measures for disposal of sewage, clean water supply and supervision of food processing and handling.