The Salmonella-Arizona Group

Pathogenic for humans or animals when acquired by the oral route.

Differ from the other enterics because it livess in the GI tracts of animals and transmitted from animals and animal products to humans (S.typhi is an exception as it is not zoonotic, carried only by humans)

Salmonella (like Shigella) is never considered part of the normal flora, it is always pathogenic

1. Antigenic Structure

Possess several O Ags (>60), different H Ags and Vi (just like K, capsular antigen, but salmonella named it Vi: for virulence) Ags

2. Classification ; complex.

3. Pathogenesis & Clinical Findings

Chiefly pathogenic in animals (reservoir for human infection)

Always enter via the oral route (contaminated food or drink); infective dose 10⁵-10⁸, most infections are acquired by eating contaminated food products

Host factors to resistance: gastric acidity, normal intestinal flora, local intestinal immunity.

A. The "Enteric Fevers" (Typhoid Fever):

S. typhi

Small intestine ≫ invade intestinal epithelium ≫ regional lymph node ≫ lymphatics ≫ bloodstream ≫ multiple organ (invasion bacteria are phagocytosed by monocytes and can survive intracellularly)

Incubation period (10-14 days) ≫ abdominal pain with constipation (adult) or mild diarrhea (infant) ≫ hepatosplenomegaly, Fever, malaise, headache, bradycardia, anorexia, loss of weight

: fever rises to a high plateau (41-41 degree C, 4-8 wks prolonged if not treated)

: Rose spots on the skin of the abdomen or chest may be developed in 1st week of infection

Chief complication : intestinal hemorrhage & perforation (mortality rate 10-15% -> less then 1% by treatment)

Some people recovering from typhoid fever become chronic carriers, harboring S. typhi in their gallbladders and excreting the bacteria constantly (about 3%)

B. Bacteremia With Focal Lesions:

S choleraesuis: usually cause systemic dissemination, does not involve the GI tract

Prolonged fever with blood culture positive

The risk is higher in pediatric and geriatric patient and the acquired immunodeficiency syndrome (AIDS)

The clinical presentation of bacteremia is like that of other gram (-) bacteremia but focal lesions in lungs, bones, meninges, etc can occur in as many as 10% of patients

C. Enterocolitis (Formerly "Gastroenteritis"):

Most common form of salmonellosis

Can be caused by any of hundreds of serotypes of Salmonella enteritidis

Incubation period (8-48 hours) ≫ nausea, abdominal pain, vomiting, low-grade fever ≫ spontaneous resolution

4. Diagnosis

Isolation from stool specimens requires use of selective media

Widal test: As least two serum specimens, obtained at intervals of 7-10 days

High titer of antibody to the O antigen >1:160 = active infection

High titer of antibody to the H antigen >1:160 = past immunization or past infection

High titer of antibody to the Vi antigen = carrier

5. Immunity

Infection with Salmonella usually confer certain degree of immunity ≫ reinfection may occur but is often milder than the first infection

6. Treatment

Enteric fever and bacteremia with focal lesions require antimicobial treatment

Enterocolitis does not require antimicrobial treatment

In most carriers, the organisms persist in the gall bladder and in the biliary tract (ampicillin alone or with cholecystectomy)

7. Prevention & Control

Most infections can be controlled by proper preparation of poultry and eggs (completely cooked) and avoidance of contamination of other foods with uncooked poultry products

Vaccination against S.typhi can reduce the risk of disease for travelers into endemic areas

Category: Microbiology Notes