Enterobacteriaceae

▪Enterobacteriaceae is a large diverse family of bacteria commonly referred to as the fermentative, gram-negative, enteric bacilli.

▪ Indicating that they are gram-negative rods which can ferment sugars.

▪Many are normal flora of the intestinal tract of humans and animals.

Gram

negative

bacteria

under

microscope

▪ Components of the cell wall and surface, which are antigenic, have been extensively studied in some genera and form the basis of systems dividing species into serotypes.

▪ 1. O antigen: The outer membrane lipopolysaccharide (LPS). Its antigenic specificity is determined by the composition of the sugars that form the long terminal polysaccharide side chains linked to the core polysaccharide and lipid A.

2. K antigen: Cell surface polysaccharides may form a well-defined capsule or an amorphous slime layer and are termedthe K antigen .

3. H antigen: Motile strains have protein peritrichousflagella, which extend well beyond the cell wall, but it isabsent in nonmotile genera (Shigella and Klebsiella).

▪ Genus and species designations are based on phenotypiccharacteristics, such as patterns of carbohydratefermentation.

▪ The genera containing the species most virulent forhumans( true pathogens) are:

➢ Escherichia

➢Shigella

➢Salmonella

➢Klebsiella

➢Yersinia

▪ Other less common but medically important genera are :

➢Enterobacter

➢Serratia

➢Proteus

➢Morganella

➢Providencia.

1. They are gram-negative rods

2. If motile, they possess a peritrichousarrangement of flagella (Shigella and Klebsiella are non-motile)

3. They are facultative anaerobes

4. They are oxidase negative

5. All species ferment the sugar glucose but otherwise vary widely in their biochemical characteristics.

➢MacConkey agar Culture is the primary method ofdiagnosis; all Enterobacteriaceae are readily isolated onroutine media under almost any incubation conditions.Special indicator media such as MacConkey agar arecommonly used in primary isolation to speed separation ofthe many species

▪ Pink colored colonies of lactose fermenter and

▪ pale colored colonies of Non lactose fermenter

▪ Lactose fermenters are:

▪ Citrobacter

▪ Escherichia

▪ Enterobacter

▪ Klebsiella

▪ Non lactose fermenter are:

▪ Shigella

▪ Salmonella

▪ Yersinia

▪ Proteus

▪ IMViC test

▪ Triple Sugar Iron (TSI) Agar Test

▪ Urease Test

▪ Oxidase Test

▪ Catalase test

▪ Most strains of E coli ferment lactose rapidly

▪ and produce indole.

▪ These and other biochemical reactions aresufficient to separate it from the other species.

▪ They are motile.

E.Coli under the microscope

➢Clinical Syndromes:

▪ Enterobacteriaceae are by far the most common cause ofUTIs, and the most common species involved is E. coli

▪ 50% of primary nosocomial pneumonia in hospitals.

▪ Major cause of neonatal meningitis.

▪ Gastroenteritis

▪ Wound infections

▪ Peritonitis

▪ The most distinctive bacteriologic features of the genusKlebsiella are the absence of motility and the presence of apolysaccharide capsule. This gives colonies a glistening,mucoid character and forms the basis of a serotypingsystem.

▪ The most common species, is able to cause:

➢primary pneumonia

➢Urinary tract infection and

➢wound infections.

➢Non-lactose fermenting, non-spore forming, and usuallymotile rods.

➢Any infection caused by Salmonella is called asalmonellosis.

➢The majority of Salmonella cause diarrhea, but onespecies, S. typhi, may disseminate into the blood and causea severe form of salmonellosis called typhoid fever.

➢ Other species (S. enterica).

➢Clinical Syndromes:

▪ Salmonella is perhaps best known as a cause of bacterialfood poisoning.

▪ Enteritis (acute gastroenteritis)

▪ Enteric fever: prototype is typhoid fever and less severeparatyphoid fever

▪ Septicemia

1. Blood Agar: Non selective media for primary isolation Salmonella usually produce non-hemolytic smooth white colonies.

2. Selective or differential agar examples1. MacConkey Agar, and 2. Hektoen Enteric Agar; salmonellae produce green colonies with black centres)

3. Enrichment broths (e.g. Selenite broth)

4. Bismuth sulfite agar: produce black colonies.

5. Xylose Lysin Deoxycholate (XLD) Agar : Salmonella appearas pink (lactose non-fermenting colonies) with black center(due to H2S production; however, H2S negative salmonellahave recently emerged ), while Shigella and Pseudomonas:appear as red colonies without black centers

6. Deoxycholate Citrate Agar (DCA): Salmonella appear aspale colonies with or without black centers.

7. Salmonella-Shigella (SS) Agar: Used for selective recovery of Salmonella from stool culture Salmonellae usually produce lactose non-fermenting colonies with black centers.

8. Eosin Methylene Blue (EMB) Agar: Non lactose fermenters, including Salmonella, Shigella and Proteus forms colorless colonies.

▪ Any Shigella infection is called a shigellosis. Shigella mayproduce cytotoxins that cause abscesses and ulcers toappear in the large intestines resulting in dysentery(diarrhea with blood, mucous, and white blood cells in thestool).

▪ Shigella species are closely related to E coli

▪ Non-lactose fermenter.

➢Species:

Shigella dysenteriae: causes most serious form of bacillary

dysentery

Others: Shigella flexneri, Shigella sonnei, Shigella boydi

Shigella dysenteriae

under the microscope

▪ Shigella do not ferment lactose or produce hydrogen sulfide gas, so the resulting colonies will be colorless.

Shigella on SS agar Salmonella on SS agar +ve results

Black colonies

▪ Morphologically, Yersinia tend to be coccobacillary.

▪ The genus includes 11 species, of which Yersinia pestis,Yersinia pseudotuberculosis, and Yersinia enterocolitica arethe pathogens for humans.

▪ Both Y enterocolitica and Y pseudotuberculosis cause acutemesenteric lymphadenitis.

▪ Mucoid colonies, small (1-2 mm diameter) flat, colorless, orpale pink colonies.

▪ Proteus mirabilis, the most commonly isolated member of the group,

▪ P mirabilis and Proteus vulgaris share the ability to swarm over the surface of media, rather than remaining confined to discrete colonies. This characteristic makes them readily recognizable in the laboratory

Gram negative proteus with staphylococci

Proteus on chocolate agar Proteus on nutrient agar