Enterobacteriaceae Dr Sabrina Moyo Department of Mircobiology and Immunology
The Enterobacteriaceae are a large, heterogeneous group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals.
The family includes many genera ( Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia, Proteus, and others).
Some enteric organisms, eg, Escherichia coli, are part of the normal flora and incidentally cause disease, while others, the salmonellae and shigellae, are regularly pathogenic for humans.
The Enterobacteriaceae are facultative anaerobes or aerobes, ferment a wide range of carbohydrates, possess a complex antigenic structure, and produce a variety of toxins and other virulence factors.
Thay are called Enterobacteriaceae, enteric gram-negative rods, and enteric bacteria but these bacteria may also be called coliforms.
Classification – more than15 different genera
Morphology and General Characteristics
Gram-negative, non-sporing, rod shaped bacteria
Ferment glucose and may or may not produce gas in the process (aerogenic vs anaerogenic)
Reduce nitrate to nitrite (there are a few exceptions)
Morphology and General Characteristics
Are facultative anaerobes
If motile, motility is by peritrichous flagella
Many are normal inhabitants of the intestinal tract of man and other animals
Some are enteric pathogens and others are urinary or respiratory tract pathogens
Differentiation is based on biochemical reactions and and differences in antigenic structure
O antigens are the most external part of the cell wall lipopolysaccharide and consist of repeating units of polysaccharide.
Some O-specific polysaccharides contain unique sugars.
O antigens are resistant to heat and alcohol and usually are detected by bacterial agglutination.
Antibodies to O antigens are predominantly IgM.
While each genus of Enterobacteriaceae is associated with specific O groups, a single organism may carry several O antigens.
Thus, most shigellae share one or more O antigens with E coli.
E coli may cross-react with some Providencia, Klebsiella, and Salmonella species.
Occasionally, O antigens may be associated with specific human diseases, eg, specific O types of E coli are found in diarrhea and in urinary tract infections.
K antigens are external to O antigens on some but not all Enterobacteriaceae.
Some are polysaccharides, including the K antigens of E coli; others are proteins.
K antigens may interfere with agglutination by O antisera, and they may be associated with virulence
eg, E coli strains producing K1 antigen are prominent in neonatal meningitis
K antigens of E coli cause attachment of the bacteria to epithelial cells prior to gastrointestinal or urinary tract invasion.
Klebsiellae form large capsules consisting of polysaccharides (K antigens) covering the somatic (O or H) antigens and can be identified by capsular swelling tests with specific antisera.
Human infections of the respiratory tract are caused particularly by capsular types 1 and 2; those of the urinary tract, by types 8, 9, 10, and 24.
Boiling for 15 minutes will destroy the K antigen and unmask O antigens.
K antigen is called the Vi (virulence) antigen in Salmonella
H antigens are located on flagella and are denatured or removed by heat or alcohol.
They are preserved by treating motile bacterial variants with formalin. Such H antigens agglutinate with anti-H antibodies, mainly IgG.
The determinants in H antigens are a function of the amino acid sequence in flagellar protein (flagellin).
Within a single serotype, flagellar antigens may be present in either or both of two forms,
phase 1 (conventionally designated by lower-case letters) and
phase 2 (conventionally designated by Arabic numerals).
The organism tends to change from one phase to the other; this is called phase variation.
H antigens on the bacterial surface may interfere with agglutination by anti-O antibody.
Antigenic Structure of Enterobacteriaceae
Carbohydrate fermentation patterns and the activity of amino acid decarboxylases and other enzymes are used in biochemical differentiation .
Some tests, eg, the production of indole from tryptophan, are commonly used in rapid identification systems, while others, eg, the Voges-Proskauer reaction (production of acetylmethylcarbinol from dextrose), are used less often.
Culture on "differential" media that contain special dyes and carbohydrates (eg, eosin-methylene blue [EMB], MacConkey's, or deoxycholate medium) distinguishes lactose-fermenting (colored) from non-lactose-fermenting colonies (nonpigmented) and may allow rapid presumptive identification of enteric bacteria.
Most grow well on a variety of lab media including a lot of selective and differential media originally developed for the the selective isolation of enteric pathogens.
Most of this media is selective by incorporation of dyes and bile salts that inhibit G+ organisms and may suppress the growth of nonpathogenic species of Enterobacteriaceae.
Many are differential on the basis of whether or not the organisms ferment lactose and/or produce H 2 S.
They may or may not be hemolytic.
The three most useful media for screening stool cultures for potential pathogens are TSI, and urea or phenylalanine agar.
The antigenic structure is used to differentiate organisms within a genus or species. Three major classes of antigens are found:
E coli and most of the other enteric bacteria form circular, convex, smooth colonies with distinct edges.
Enterobacter colonies are similar but somewhat more mucoid. Klebsiella colonies are large and very mucoid and tend to coalesce with prolonged incubation.
Salmonellae and Shigellae produce colonies similar to E coli but do not ferment lactose.
Some strains of E coli produce hemolysis on blood agar.