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  • Enterobacteriaceae (=enteric gram-negative rods, enteric bacteria, coliform bacteria): a large, heterogeneous group of gram-negative rods
  • Natural habitat is the intestinal tract of humans and animals
  • Some enteric organisms (E.coli) are part of the normal flora and incidentally cause disease, while others (salmonellae, shigellae) are regularly pathogenic for human


More than 25 genera and 110 species

Clinically signifiacnt Enterobacteriaceae: 20-25 species

1. Morphology & Identification

A. Culture & Gram staining

B. Growth characteristics

  • Culture on differential media, complex media: Table

2. Antigenic Structure

1) O antigens: The most external component of the lipopolysaccharide (LPS) of gram (-) bacteria. Differ from organism to organism, depending on different sugars and different side-chain substitutions. More than 150 different heat stable somatic O antigens.

2) K antigens: more than 100 heat-labile K (capsular, Kapsule) Ags

3) H antigens: more than 50 H (flagellar) Ags

* Vi antigens: the capsular Ags in Salmonella Typhi

2 type of diseases

1) Diarrhea

2) Various other infections: urinary tract infection (UTI), pneumonia, bacteremia, and sepsis, especially in debilitated hospitalized patients

1. Diarrhea

1) Background

- Intestinal tract facilitates huge fluxes of water: About 9 L of fluid enter the adult upper intestinal tract each day, however fecal water content is typically less than 200 mL/day

- The majority of this fluid is absorbed in the small intestine

- The absorptive and secretory functions of the intestines are also separated spatially

2) Mechanisms of bacteria cause disease in the intestinal tract

- Clinical manifestations depending on the "depth" of intestinal invasion

Depth of invasion

Involved site




or secretory diarrhea

No cell invasion

small intestine

production of enterotoxin ≫ absorptive capability of the villi,≫ hypersecretion of water

Vibrio cholerae , ETEC

Inflammatory diarrhea

the intestinal epithelial cells

frequently distal colon

Inflammation in invasion site, destroy mucosa ≫ dysentery

the dysentery by Shigella species

Enteric fever

the lymph nodes and blood stream

small intestine

The penetration of intact mucosa with bacterial multiplication in the lymphatics and then blood stream. Systemic symptoms such as fever and headache are prominent, diarrhea is often not present

Salmonella typhi

2. Various other infections

* E. coli, and other enteric bacteria (Proteus, Enterobacter, Klebsiella, Morganella, Providencia, Citrobacter, and Serratia species): normal flora of intestinal tract, upper respiratory tract and genital tract
* hospital or community-acquired infections: antibiotic resistant organism can cause disease in debilitated patients. When Foley catheters are in the urethra or when a patient aspirates vomitus.

Category: Microbiology Notes



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