GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

14. TYPES OF FERMENTATION

14.5. FORMIC ACID FERMENTATION AND THE FAMILY ENTEROBACTERIACEAE

14.5.1. Characteristics of Enterobacteria

Certain acid-producing microorganisms are grouped together physiologically because formic acid is a characteristic, albeit minor, product of their Fermentation. Since some typical representatives of this group inhabit the intestine, the entire family is named Enterobacteriaceae. These are Gram-negative, actively motile, non-spore-forming rods with peritrichous flagellation. They are facultative anaerobes capable of generating energy through both Respiration (aerobic conditions) and fermentation (anaerobic conditions). They have very modest nutritional requirements, growing readily on simple mineral media.

Escherichia coli is an intestinal inhabitant, although not the most abundant one (Bacteroides and Bifidobacterium predominate in the gut microflora). This bacterium retains its viability for a long time outside the intestine, a property utilized to detect fecal contamination in drinking Water.

The normal intestinal microflora also includes Proteus vulgaris and Enterobacter aerogenes. The family Enterobacteriaceae encompasses Bacteria of the genus Erwinia, which are plant pathogens affecting stems, leaves, and roots. Klebsiella pneumoniae is the CAUSATIVE AGENT OF Pneumonia. Salmonella typhimurium is the most common bacterium causing gastroenteritis (irritation of the digestive tract mucosa caused by lipopolysaccharide toxins synthesized by this bacterium). Vibrio cholerae is the causative agent of cholera; it does not belong to the Enterobacteriaceae family, but is metabolically closely related to its members.

Drinking water analysis. The primary goal of drinking water analysis is to detect Escherichia coli. While this bacterium is a normal and harmless resident of the gut, and its presence in water is not dangerous in itself, pathogenic bacteria may inhabit the intestine alongside Escherichia coli. Together with Escherichia coli, they are shed in the feces of infected individuals and may enter drinking water. Rather than employing specialized Methods to detect every group of potential pathogens, a reliable indicator of contamination is used—namely, the resident gut bacterium Escherichia coli. The detection of this species in a water sample indicates that the water has been contaminated with excrement and intestinal bacteria, among which pathogenic forms may also be present. The standard for drinking water stipulates that the total bacterial Cell count per 1 ml must not exceed 100, while 100 ml of water must be completely free of Escherichia coli Cells.

Lactose-containing media are used to detect Escherichia coli. Besides Escherichia coli, other coliform bacteria (such as Enterobacter aerogenes) and lactic acid bacteria can also grow on lactose. However, on media containing lactose, peptone, eosin, and methylene blue, Escherichia coli forms dark blue colonies with a metallic sheen (whereas Enterobacter aerogenes forms pink colonies without a metallic sheen).