ENTERIC GRAM-NEGATIVE RODS
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27.7.07
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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
Classification
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 spatially2) Mechanisms of bacteria cause disease in the intestinal tract
- Clinical manifestations depending on the "depth" of intestinal invasion
| | Depth of invasion | Involved site | mechanism | Example |
| Non-inflammatory 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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