EnterobacteriaceaeEnterobacteriaceae

Dr Sabrina MoyoDr Sabrina Moyo

Department of Mircobiology and Department of Mircobiology and ImmunologyImmunology

IntroductionIntroduction

The Enterobacteriaceae are a large, heterogeneous The Enterobacteriaceae are a large, heterogeneous group of gram-negative rods whose natural habitat is group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals. the intestinal tract of humans and animals.

The family includes many genera (The family includes many genera (Escherichia, Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia, Proteus,Serratia, Proteus, and others). and others).

Some enteric organisms, eg, Some enteric organisms, eg, Escherichia coli,Escherichia coli, are part are part of the normal flora and incidentally cause disease, of the normal flora and incidentally cause disease, while others, the salmonellae and shigellae, are while others, the salmonellae and shigellae, are regularly pathogenic for humans. regularly pathogenic for humans.

IntroductionIntroduction

The Enterobacteriaceae are facultative The Enterobacteriaceae are facultative anaerobes or aerobes, ferment a wide range of anaerobes or aerobes, ferment a wide range of carbohydrates, possess a complex antigenic carbohydrates, possess a complex antigenic structure, and produce a variety of toxins and structure, and produce a variety of toxins and other virulence factors. other virulence factors.

Thay are called Enterobacteriaceae, enteric Thay are called Enterobacteriaceae, enteric gram-negative rods, and enteric bacteria but gram-negative rods, and enteric bacteria but these bacteria may also be called coliforms. these bacteria may also be called coliforms.

EnterobacteriaceaeEnterobacteriaceae

Classification – more than15 different generaClassification – more than15 different genera EscherichiaEscherichia ShigellaShigella EdwardsiellaEdwardsiella SalmonellaSalmonella CitrobacterCitrobacter KlebsiellaKlebsiella EnterobacterEnterobacter HafniaHafnia SerratiaSerratia

EnterobacteriaceaeEnterobacteriaceae

ProteusProteus ProvidenciaProvidencia MorganellaMorganella YersiniaYersinia ErwiniaErwinia PectinobacteriumPectinobacterium

Morphology and General Morphology and General CharacteristicsCharacteristics

Gram-negative, non-sporing, rod shaped bacteriaGram-negative, non-sporing, rod shaped bacteria Oxidase –Oxidase – Ferment glucose and may or may not produce gas in Ferment glucose and may or may not produce gas in

the process (aerogenic vs anaerogenic)the process (aerogenic vs anaerogenic) Reduce nitrate to nitrite (there are a few exceptions)Reduce nitrate to nitrite (there are a few exceptions)

Morphology and General Morphology and General CharacteristicsCharacteristics

Are facultative anaerobesAre facultative anaerobes If motile, motility is by peritrichous flagellaIf motile, motility is by peritrichous flagella Many are normal inhabitants of the intestinal tract of Many are normal inhabitants of the intestinal tract of

man and other animalsman and other animals Some are enteric pathogens and others are urinary or Some are enteric pathogens and others are urinary or

respiratory tract pathogensrespiratory tract pathogens Differentiation is based on biochemical reactions and Differentiation is based on biochemical reactions and

and differences in antigenic structureand differences in antigenic structure

Antigenic structureAntigenic structure O antigensO antigens are the most external part of the cell wall are the most external part of the cell wall

lipopolysaccharide and consist of repeating units of lipopolysaccharide and consist of repeating units of polysaccharide. polysaccharide.

Some O-specific polysaccharides contain unique sugars. Some O-specific polysaccharides contain unique sugars. O antigens are resistant to heat and alcohol and usually are O antigens are resistant to heat and alcohol and usually are

detected by bacterial agglutination. detected by bacterial agglutination. Antibodies to O antigens are predominantly IgM.Antibodies to O antigens are predominantly IgM. While each genus of Enterobacteriaceae is associated with While each genus of Enterobacteriaceae is associated with

specific O groups, a single organism may carry several O specific O groups, a single organism may carry several O antigens. antigens.

Thus, most shigellae share one or more O antigens with Thus, most shigellae share one or more O antigens with E coli. E coli. E coliE coli may cross-react with some may cross-react with some Providencia, Klebsiella, Providencia, Klebsiella, and and

SalmonellaSalmonella species. species. Occasionally, O antigens may be associated with specific Occasionally, O antigens may be associated with specific

human diseases, eg, specific O types of human diseases, eg, specific O types of E coliE coli are found in are found in diarrhea and in urinary tract infections.diarrhea and in urinary tract infections.

Antigenic structureAntigenic structure K antigensK antigens are external to O antigens on some but not all are external to O antigens on some but not all

Enterobacteriaceae. Enterobacteriaceae. Some are polysaccharides, including the K antigens of Some are polysaccharides, including the K antigens of E coli;E coli; others are others are

proteins. proteins. K antigens may interfere with agglutination by O antisera, and they may be K antigens may interfere with agglutination by O antisera, and they may be

associated with virulence associated with virulence eg, eg, E coliE coli strains producing K1 antigen are prominent in neonatal meningitis strains producing K1 antigen are prominent in neonatal meningitis K antigens of K antigens of E coliE coli cause attachment of the bacteria to epithelial cells prior cause attachment of the bacteria to epithelial cells prior

to gastrointestinal or urinary tract invasion.to gastrointestinal or urinary tract invasion. Klebsiellae form large capsules consisting of polysaccharides (K antigens) Klebsiellae form large capsules consisting of polysaccharides (K antigens)

covering the somatic (O or H) antigens and can be identified by capsular covering the somatic (O or H) antigens and can be identified by capsular swelling tests with specific antisera. swelling tests with specific antisera.

Human infections of the respiratory tract are caused particularly by capsular 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.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.Boiling for 15 minutes will destroy the K antigen and unmask O antigens. K antigen is called the Vi (virulence) antigen in K antigen is called the Vi (virulence) antigen in SalmonellaSalmonella

Antigenic structureAntigenic structure

H antigensH antigens are located on flagella and are denatured or are located on flagella and are denatured or removed by heat or alcohol. removed by heat or alcohol.

They are preserved by treating motile bacterial variants with They are preserved by treating motile bacterial variants with formalin. Such H antigens agglutinate with anti-H antibodies, formalin. Such H antigens agglutinate with anti-H antibodies, mainly IgG. mainly IgG.

The determinants in H antigens are a function of the amino acid The determinants in H antigens are a function of the amino acid sequence in flagellar protein (flagellin). sequence in flagellar protein (flagellin).

Within a single serotype, flagellar antigens may be present in Within a single serotype, flagellar antigens may be present in either or both of two forms, either or both of two forms,

phase 1 (conventionally designated by lower-case letters) andphase 1 (conventionally designated by lower-case letters) and phase 2 (conventionally designated by Arabic numerals). phase 2 (conventionally designated by Arabic numerals). The organism tends to change from one phase to the other; this The organism tends to change from one phase to the other; this

is called phase variation.is called phase variation. H antigens on the bacterial surface may interfere with H antigens on the bacterial surface may interfere with

agglutination by anti-O antibody.agglutination by anti-O antibody.

Antigenic Structure of Antigenic Structure of EnterobacteriaceaeEnterobacteriaceae

Growth CharacteristicsGrowth Characteristics

Carbohydrate fermentation patterns and the activity of amino acid Carbohydrate fermentation patterns and the activity of amino acid decarboxylases and other enzymes are used in biochemical decarboxylases and other enzymes are used in biochemical differentiation . differentiation .

Some tests, eg, the production of indole from tryptophan, are commonly Some tests, eg, the production of indole from tryptophan, are commonly used in rapid identification systems, while others, eg, the Voges-used in rapid identification systems, while others, eg, the Voges-Proskauer reaction (production of acetylmethylcarbinol from dextrose), Proskauer reaction (production of acetylmethylcarbinol from dextrose), are used less often. are used less often.

Culture on "differential" media that contain special dyes and Culture on "differential" media that contain special dyes and carbohydrates (eg, eosin-methylene blue [EMB], MacConkey's, or carbohydrates (eg, eosin-methylene blue [EMB], MacConkey's, or deoxycholate medium) distinguishes lactose-fermenting (colored) from deoxycholate medium) distinguishes lactose-fermenting (colored) from non-lactose-fermenting colonies (nonpigmented) and may allow rapid non-lactose-fermenting colonies (nonpigmented) and may allow rapid presumptive identification of enteric bacteria.presumptive identification of enteric bacteria.

Growth CharacteristicsGrowth Characteristics

Most grow well on a variety of lab media including a lot of Most grow well on a variety of lab media including a lot of selective and differential media originally developed for the the selective and differential media originally developed for the the selective isolation of enteric pathogens. selective isolation of enteric pathogens.

• Most of this media is selective by incorporation of dyes and Most of this media is selective by incorporation of dyes and bile salts that inhibit G+ organisms and may suppress the bile salts that inhibit G+ organisms and may suppress the growth of nonpathogenic species of growth of nonpathogenic species of Enterobacteriaceae.Enterobacteriaceae.

• Many are differential on the basis of whether or not the Many are differential on the basis of whether or not the organisms ferment lactose and/or produce Horganisms ferment lactose and/or produce H22S.S.

Growth CharacteristicsGrowth Characteristics

They may or may not be hemolytic.They may or may not be hemolytic. The three most useful media for screening The three most useful media for screening

stool cultures for potential pathogens are TSI, stool cultures for potential pathogens are TSI, and urea or phenylalanine agar.and urea or phenylalanine agar.

The antigenic structure is used to differentiate The antigenic structure is used to differentiate organisms within a genus or species. Three organisms within a genus or species. Three major classes of antigens are found:major classes of antigens are found:

CultureCulture

E coliE coli and most of the other enteric bacteria form circular, and most of the other enteric bacteria form circular, convex, smooth colonies with distinct edges. convex, smooth colonies with distinct edges.

Enterobacter colonies are similar but somewhat more mucoid. Enterobacter colonies are similar but somewhat more mucoid. Klebsiella colonies are large and very mucoid and tend to Klebsiella colonies are large and very mucoid and tend to coalesce with prolonged incubation. coalesce with prolonged incubation.

Salmonellae and Shigellae produce colonies similar to Salmonellae and Shigellae produce colonies similar to E coliE coli but but do not ferment lactose. do not ferment lactose.

Some strains of Some strains of E coliE coli produce hemolysis on blood agar. produce hemolysis on blood agar.

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