Enteric gram-negative Rods: Morphology and Identification, Antigenic Structure, Toxins and Enzymes.

Question 1

Enteric Gram-negative rods

Answer 1

Enteric gram-negative rods are facultative anaerobes that are typically part of the normal flora of the human GI tract. They are gram-negative, non-spore-forming, rod-shaped bacteria. These include major pathogens such as Escherichia coli, Salmonella, Shigella, Klebsiella, Enterobacter, Proteus, and Serratia. They are characterized by their ability to ferment glucose and produce acid and gas. Identification is based on selective culture media (e.g., MacConkey agar), biochemical tests (e.g., IMViC tests), and growth patterns. Most enteric rods can ferment lactose, but some, like Salmonella and Shigella, do not.

Question 2

Antigenic structure of Enteric Gram-negative rods

Answer 2

Enteric gram-negative rods have several important antigens: 1) O antigen: the lipopolysaccharide (LPS) component of the outer membrane, a key virulence factor, used for serotyping. 2) H antigen: flagellar protein antigen that helps identify motile bacteria (e.g., E. coli). 3) K antigen: capsular polysaccharide antigen that inhibits phagocytosis. Some species, like Klebsiella, produce a large, prominent capsule. The Vi antigen is found in Salmonella typhi and contributes to its virulence by inhibiting phagocytosis.

Question 3

Toxins of Enteric Gram-negative rods

Answer 3

Enteric gram-negative rods can produce various toxins that contribute to their pathogenicity. 1) Enterotoxins: Exotoxins that cause diarrhea by affecting the intestinal mucosa. For example, E. coli strains (e.g., enterotoxigenic E. coli or ETEC) produce heat-labile (LT) and heat-stable (ST) enterotoxins. 2) Shiga toxin: Produced by Shigella dysenteriae and some strains of E. coli (e.g., O157:H7), this toxin inhibits protein synthesis, leading to cell death and bloody diarrhea. 3) Endotoxin: The LPS component of the cell wall, which, when released during bacterial lysis, can trigger systemic inflammatory responses, causing fever, shock, and disseminated intravascular coagulation (DIC).

Question 4

Enzymes of Enteric Gram-negative rods

Answer 4

Enteric gram-negative rods produce various enzymes that contribute to their virulence. 1) Urease: Produced by Proteus, Klebsiella, and H. pylori, it hydrolyzes urea into ammonia, increasing pH and facilitating colonization in acidic environments. 2) Hemolysins: Produced by E. coli, Salmonella, and Shigella, these enzymes lyse red blood cells and release nutrients. 3) Beta-lactamases: Enzymes produced by many enteric rods (e.g., E. coli, Klebsiella) that break down beta-lactam antibiotics, conferring resistance to penicillins and cephalosporins. 4) Collagenases and hyaluronidases: Secreted by Klebsiella and Enterobacter species, these enzymes break down host tissue, aiding in tissue invasion.