e.coli

Enterobacteriaceae
Small Gram-negative non-spore forming enteric bacilli
All Enterobacteriaciae:
1. ferment glucose with acid production
2. reduce nitrates (NO3 to NO2 or all the way to N2)
3. are oxidase negative
All are aerobic but can be facultatively anaerobic
Motile via peritrichous flagella except Shigella and Klebsiella which are non-motile
Capsule, slime layer, or neither
Possess fimbriae (pili)
Complex cell wall
Antigenic Structure: plays an important role for some species in epidemiology and classification
K (capsular) antigens: capsular polysaccharide, particularly heavy in Klebsiella
H (flagellar) antigens: flagellar proteins of motile genera and species; used for typing; absent in non-motile genera (Shigella and Klebsiella)
O (somatic) antigens: O-specific polysaccharide side chain of lipopolysaccharide; used for typing
Biochemically and metabolically diverse; ferment glucose by the mixed acid pathway; Klebsiella, Enterobacter and Serratia utilize the butanediol pathway

Introduction
E. coli and related bacteria constitute about 0.1% of gut flora and fecal–oral transmission is the major route through which pathogenic strains of the bacterium cause disease. Cells are able to survive outside the body for a limited amount of time, which makes them ideal indicator organisms to test environmental samples for fecal contamination. The bacterium can also be grown easily and inexpensively in a laboratory setting, E. coli is the most widely studied prokaryotic model organism, and an important species in the fields of biotechnology and microbiology
Pathogenic E.coli strains can be categorised based on elements that can elicit an immune response in animals, namely:
1. O antigen: part of lipopolysaccharide layer
2. K antigen: capsule
3. H antigen: flagellin
Pathogenesis of E. coli

Over 700 antigenic types (serotypes) of E. coli are recognized based on O, H, and K antigens. At one time serotyping was important in distinguishing the small number of strains that actually cause disease. Thus, the serotype O157:H7 (O refers to somatic antigen; H refers to flagellar antigen) is uniquely responsible for causing HUS (hemolytic uremic syndrome). Nowadays, particularly for diarrheagenic strains (those that cause diarrhea) pathogenic E. coli are classified based on their unique virulence factors and can only be identified by these traits. Hence, analysis for pathogenic E. coli usually requires that the isolates first be identified as E. coli before testing for virulence markers.

Pathogenic strains of E. coli are responsible for three types of infections in humans: urinary tract infections (UTI), neonatal meningitis, and intestinal diseases (gastroenteritis). The diseases caused (or not caused) by a particular strain of E. coli depend on distribution and expression of an array of virulence determinants, including adhesins, invasins, toxins, and abilities to withstand host defenses. These are summarized in Table 1 and applied to the discussion of pathogenic strains E. coli below.
Pathophysiology
Acute bacterial meningitis
The vast majority of neonatal meningitis cases are caused by E coli and group B streptococcal infections (28.5% and 34.1% overall, respectively). Pregnant women are at a higher risk of colonization with the K1 capsular antigen strain of E coli. This strain is also commonly observed in neonatal sepsis, which carries a mortality rate of 8%; most survivors have subsequent neurologic or developmental abnormalities. Low birth weight and a positive cerebrospinal fluid (CSF) culture result portend a poor outcome. In adults, E coli meningitis is rare but may occur following neurosurgical trauma .
Pneumonia
E coli respiratory tract infections are uncommon and are almost always associated with E coli UTI. No virulence factors have been implicated. E coli pneumonia may also result from micro aspiration of upper airway secretions that have been previously colonized with this organism in severely ill patients; hence, it is a cause of nosocomial pneumonia. However, E coli pneumonia may also be community-acquired in patients who have underlying disease such as diabetes mellitus, alcoholism, chronic obstructive pulmonary disease, and E coli UTI. E coli pneumonia usually manifests as a bronchopneumonia of the lower lobes and may be complicated by empyema. E coli bacteremia precedes pneumonia and is usually due to another focus of E coli infection in the urinary or GI tract.