Order Enterobacterales
The Enterobacterales are a diverse group of Gram-negative bacilli that commonly inhabit the gastrointestinal tracts of humans and animals. While many are part of the normal flora, some become opportunistic or outright pathogens.
- They are facultative anaerobes, meaning they can grow in both aerobic and anaerobic environments.
- Oxidase-negative — distinguishes them from non-fermenting Gram-negative rods like Pseudomonas.
- Ferment glucose and reduce nitrates to nitrites.
- Most are motile with peritrichous flagella, but exceptions exist (e.g., Klebsiella, Shigella).
- Several members are lactose fermenters, a key feature for lab identification.
They are major causes of urinary tract infections, sepsis, pneumonia, and gastroenteritis, especially in hospitalized and immunocompromised patients.
Genus Escherichia — Species: E. coli
General Characteristics
Escherichia coli is one of the best-studied bacteria in medicine. It exists normally in the colon, but certain strains can cause serious infections when they acquire additional virulence genes.
- Gram-negative, straight rods that stain pink and appear singly or in pairs
- Motile with peritrichous flagella (flagella distributed all over the surface)
- Facultative anaerobe – grows in both aerobic and anaerobic conditions
- Grows well on blood agar, MacConkey agar (produces pink colonies), and EMB agar (green metallic sheen)
- Biochemically:
- Lactose fermenter
- Indole positive
- Methyl red positive
- Voges-Proskauer negative
- Citrate negative (IMViC profile: ++– –)
- Oxidase negative, catalase positive
These features help identify E. coli in mixed cultures and differentiate it from other Enterobacterales in the lab.