S.typhoid And S.paratyphoid

Question 1

Classification

Answer 1

Family: Enterobacteriaceae
Genus: Salmonella
Species: typhi and paratyphi

Infective dose: 10^5 to 10^8

Question 2

Morphology

Answer 2

Gram negative rod
Motile (Petrichious flagella)
Sometime capsulated

Question 3

Growth characteristics

Answer 3

Found normally in the SI and LI

Facultatively anaerobic
Non-fastidious

Question 4

Antigenic structure

Answer 4

Somatic O antigens 
H antigen (flagellate)
Vi antigen (capsular) includes:
- F antigen (fimbrial)
- M antigen
- R antigen 
RFM antigen

Question 5

Virulence factors

Answer 5

> Endotoxin

> Capsule (for S.typhi and some strains of S.paratyphi)

> Adhesins- both fimbrial and nonfimbrial

> Type III secretion systems and effector molecules- 2 different systems may be found:

• one which promotes entry into intestinal epithelial cells
• one which promotes survival in macrophages

> Flagella- help bacteria to move through intestinal mucous

> Enterotoxins- may be involved in gastroenteritis

> Iron capturing ability

Question 6

Disease

Answer 6

Typhoid fever

Enteric fevers

Question 7

Microbiological diagnosis

Answer 7

Culture:
Blood agar plate: for specimens from normally sterile sites
Enrichment broth (GN, Selenite)
Differential medium (Mac Conkey’s, EMB agar): lactose-negative colonies
Selective medium (SS agar): colorless colonies with Black center

Biochemical tests
Motile
Do not ferment lactose
Ferment glucose 
Urease negative

Salmonella typhi is very hard to isolate from culture, mainly from serum, using serological tests

Serologic test for identification:
Rapid slide agglutination test

Serologic tests for diagnosis:
Tube dilution agglutination test (Widal test)
To > 1:100
Th > 1:200

Question 8

Treatment and prevention

Answer 8

Ampicillin
Trimethoprim-sulfamethoxazole
Third generation cephalosporin

Problem: drug resistance transmitted by plasmids

Antimicrobial susceptibility tests