L12. Enteric Gram-Negative Bacilli: Salmonella, Shigella, Escherichia coli

What are Enterobacteriaceae?

A large family of gram-negative, facultatively anaerobic, rod-shaped bacteria commonly found in the intestines.

What are the biochemical characteristics of Enterobacteriaceae?

Ferment glucose, reduce nitrates to nitrites, oxidase-negative, and most are motile except for Shigella and Klebsiella.

How are Enterobacteriaceae classified based on lactose fermentation?

Lactose fermenters: E. coli, Klebsiella, Enterobacter.

O Antigen

Somatic antigen (heat-stable, used for serotyping)

H Antigen

Flagellar antigen (heat-labile, used for motile species).

K Antigen

Capsular antigen (heat-labile, helps evade phagocytosis).

Which Enterobacteriaceae genus is the most common cause of human infections?

Escherichia coli.

What special virulence factor do some Enterobacteriaceae produce that allows them to resist phagocytosis?

Capsules

What are the key structural differences between gram-negative and gram-positive bacteria?

Gram-negative bacteria have an outer membrane with lipopolysaccharides (LPS), a thin peptidoglycan layer, and porins.

What are the main characteristics of Salmonella species?

Gram-negative bacilli, facultative anaerobes, non-lactose fermenters, oxidase-negative, produce H₂S (except S. paratyphi A).

What are the two main species of Salmonella?

Salmonella enterica and Salmonella bongori.

How many serotypes of Salmonella exist?

More than 2,500 serotypes based on the Kauffmann-White classification.

What is the mode of transmission of Salmonella infections?

Fecal-oral route via contaminated food or water, direct contact with infected animals or people.

What food items are commonly associated with Salmonella outbreaks?

Poultry, eggs, dairy products, raw vegetables, and contaminated water.

What are the virulence factors of Salmonella?

Invasiveness, endotoxin (LPS), enterotoxins, surface antigens, intracellular survival, and type III secretion system.

What is the role of the Vi antigen in Salmonella typhi?

It prevents phagocytosis and is used to detect chronic carriers.

How does Salmonella invade the host?

It attaches to M cells in the intestine, invades Peyer’s patches, and spreads via the bloodstream.

What are the clinical syndromes caused by Salmonella?

Typhoid fever, gastroenteritis (non-typhoidal salmonellosis), bacteremia, and chronic carrier state.

What immune response is critical in clearing Salmonella infections?

Cell-mediated immunity (T-cells and macrophages).

What organism causes typhoid fever?

Salmonella typhi.

How is typhoid fever transmitted?

Fecal-oral route via contaminated food or water.

What is the incubation period of typhoid fever?

7-14 days.

What are the symptoms of typhoid fever?

Prolonged fever, rose spots, hepatosplenomegaly, leukopenia, intestinal perforation, and Peyer’s patch necrosis.

What serious complication can result from intestinal perforation in typhoid fever?

Peritonitis

What is the gold standard test for detecting typhoid fever?

Blood culture during the first week of illness.

What culture media are used for Salmonella?

MacConkey Agar, Salmonella-Shigella (SS) Agar, Hektoen Enteric (HE) Agar, XLD Agar

Results of culture media used for Salmonella (MacConkey Agar)

Colorless colonies (non-lactose fermenter).

Results of culture media used for Salmonella (Salmonella-Shigella (SS) Agar):

Black-centered colonies (H₂S production).

Results of culture media used for Salmonella (Hektoen Enteric (HE) Agar)

Green with black centers.

Results of culture media used for Salmonella (XLD Agar):

Red with black centers.

What test differentiates Salmonella from Shigella?

H₂S production test (Salmonella is H₂S-positive, Shigella is H₂S-negative).

What are the characteristics of Shigella species?

Gram-negative, non-motile, non-lactose fermenters, oxidase-negative.

What is the most virulent species of Shigella?

Shigella dysenteriae (produces Shiga toxin).

How does Shigella invade the intestine?

It invades M cells in Peyer’s patches, escapes phagolysosomes, and spreads intracellularly.

What is the function of Shiga toxin?

Inhibits protein synthesis by targeting the 60S ribosomal subunit, leading to cell death and hemorrhagic colitis.

What are the major diarrheagenic E. coli strains?

ETC, EIEC, EHEC, EPEC, EAEC

ETEC

Traveler’s diarrhea (heat-labile and heat-stable enterotoxins).

EIEC

Dysentery (invades colonic epithelium).

EHEC

O157:H7 (Shiga toxin, causes hemorrhagic colitis and HUS).

EPEC

Infantile diarrhea (adheres to intestinal epithelium).

EAEC

Persistent diarrhea (aggregative adherence).

What laboratory test confirms EHEC O157:H7?

• Sorbitol-MacConkey (SMAC) Agar: EHEC does not ferment sorbitol (colorless colonies).

What is the most common cause of urinary tract infections (UTIs)?

Escherichia coli