Enterobacteriaceae: Escherichia, Klebsiella, Proteus Flashcards
Where are Enterobacteriaceae commonly found?
intestines of humans and animals; comprise normal gut flora
characteristics of Enterobacteriaceae
✔ Gram-negative bacilli
✔ Facultative anaerobes
✔ Oxidase negative (lack cytochrome oxidase)
✔ Catalase positive
✔ Non-capsulated except Klebsiella and some strains of E. coli
✔ Most are motile except Shigella and Klebsiella
✔ Ferment glucose with acid production
✔ Most reduce nitrate to nitrite via nitrate reductase
List 4 lactose fermenters under the family Enterobacteriaceae.
✔ Escherichia coli
✔ Klebsiella spp.
✔ Citrobacter spp.
✔ Enterobacter spp.
List 4 lactose non-fermenters under the family Enterobacteriaceae.
✔ Salmonella spp.
✔ Shigella spp.
✔ Proteus spp.
✔ Yersinia spp.
Briefly discuss important antigens of Enterobacteriaceae.
(1) O Antigen: This is part of the LPS on the outer membrane of the bacteria. It is heat stable.
(2) H antigen: These are flagellar antigens found on the flagella of motile Enterobacteriaceae. They are heat labile.
(3) K antigen: This refers to the capsular antigens, which are heat labile.
characteristics of Escherichia coli
✔ Gram-negative bacilli
✔ facultative anaerobe
✔ some strains are motile due to the presence of peritrichious flagella, but some strains are non-motile
Briefly discuss virulence factors of E. coli.
(1) Adhesins: These are surface proteins that help E. coli adhere to host cells.
(2) Surface antigens and toxins, such as the O antigen and K antigen. The O antigen is part of the lipopolysaccharide on the outer membrane of E. coli. The K antigen refers to the capsular polysaccharide; it protects against phagocytosis.
(3) Fimbriae enable attachment to uroepithelial cells/intestinal epithelium.
(4) Exotoxins e.g. haemolysins and enterotoxins. Haemolysins are toxins that can lyse red blood cells. Enterotoxins disrupt normal cellular functions in the intestines, leading to symptoms like diarrhea
(5) Lipid A component of LPS: can trigger strong immune responses, leading to inflammation and, in severe cases, septic shock.
(6) Aerobactin: This is a siderophore i.e. a molecule that binds and transports iron.
List 5 diarrhoeagenic strains of E. coli.
✔ Enterohemorrhagic E. coli (EHEC)
✔ Enterotoxigenic E. coli (ETEC)
✔ Enteropathogenic E. coli (EPEC)
✔ Enteroaggregative E. coli (EAEC)
✔ Enteroinvasive E. coli (EIEC)
3 examples of Extraintestinal pathogenic E. coli (ExPEC).
✔ Uropathogenic E. coli (UPEC)
✔ Neonatal meningitis E. coli (NMEC)
✔ Sepsis-associated E. coli (SEPEC)
Briefly discuss EPEC.
✔ It causes infantile diarrhoea in children and sporadic diarrhoea in adults.
✔ Attaches to epithelial cells of the small intestine.
✔ Formation of attaching and effacing (A/E) lesions in the intestinal epithelium.
✔ Resulting in destruction of microvilli.
Briefly discuss ETEC/STEC.
✔ It is a common cause of traveler’s diarrhea and infant diarrhea in developing countries.
✔ It colonizes the surface of the small bowel mucosa.
✔ It produces a heat stable toxin and a heat labile toxin.
✔ The heat labile toxin is functionally and structurally similar to the cholera toxin. It consists of one A subunit and five identical B subunits.
✔ The heat stable toxin binds to guanylate cyclase receptor leading to an increase in cyclic guanosine monophosphate (cGMP).
Briefly discuss EHEC.
◾ It produces potent cytotoxins that inhibit host cell protein synthesis.
◾ Associate with haemorrhagic colitis (HC) and haemolytic uremic syndrome (HUS).
◾ The serotype O157:H7 is one most commonly associated with human disease.
Briefly discuss EAEC.
✔ They cause acute and persistent diarrhoea in children and people who are immunocompromised.
✔ They adhere to epithelial cells of the intestinal mucosa in a very characteristic ‘stacked-brick’ pattern known as the aggregative adherence pattern.
Briefly discuss UPEC.
These E. coli serotypes commonly responsible for UTI are normally found in faeces.
UTI may be acquired through two routes:
(1) Ascending infection via the urethra
(2) Descending infection via the haematogenous route [This is a rare route where E. coli bacteria spread from the bloodstream to upper urinary tract organs, such as the kidneys.]
Briefly discuss NMEC.
◾ This strain of E. coli is often associated with E. coli K1 serotypes.
◾ The K1 serotype of E. coli has a specific capsular antigen that is antiphagocytic.
Discuss specimen collection when suspecting E. coli infection.
The appropriate specimen to collect depends on the site of infection. Common specimens include: stool samples, urine samples, blood samples, and wound swabs.
Briefly discuss culturing of E. coli. [Hints: conditions, BA, MacConkey, CLED, gram-staining]
◾ Culture in air: 35-37°C, 18-24 hours
◾ BA: generally non-haemolytic, save for some strains (EHEC, EPEC, EIEC)
◾ MacConkey: E. coli ferments lactose, producing acid that turns the colonies pink or red
◾ CLED agar: yellow colonies - lactose fermenters
◾ Gram stain of colonies: gram-negative rods
What is CLED agar?
Cystine-Lactose-Electrolyte-Deficient (CLED) agar is a type of culture medium used primarily for isolating and differentiating urinary pathogens.
✔ Cystine: Supports the growth of organisms like cystine-dependent coliforms.
✔ Lactose: Helps in differentiating lactose fermenters (e.g., E. coli, which produce yellow colonies) from non-lactose fermenters (e.g., Proteus, which produce blue-green colonies).
✔ Electrolyte-deficient: Reduces the swarming of Proteus species, which can otherwise spread across the entire plate, making it difficult to isolate individual colonies.
IMViC test for E. coli
Indole: positive, resulting in a red color upon the addition of Kovac’s reagent
Methyl red test: positive, red color when methyl red indicator is added
Voges-Proskauer test: negative, no color change upon addition of Barritt’s reagents
Citrate utilization test: negative [remains green]
Further notes:
Indole Test (I): This test determines the ability of bacteria to produce indole from the amino acid tryptophan. A positive result is indicated by a red ring at the top of the medium after adding Kovac’s reagent.
Methyl Red Test (M): This test detects the production of stable acid end-products from glucose fermentation. A positive result is indicated by a red color when methyl red indicator is added.
Voges-Proskauer Test (Vi): This test detects the production of acetoin (a neutral end-product) from glucose fermentation. A positive result is indicated by a red color after adding Barritt’s reagents.
Citrate Utilization Test (C): This test determines the ability of bacteria to use citrate as their sole carbon source. A positive result is indicated by a blue color change on Simmon’s citrate agar.
Key features of Klebsiella.
◾ Gram-negative bacilli
◾ Large polysaccharide capsule; colonies with mucoid appearance
◾ Non-motile
◾ Urease positive
5 clinically relevant species of Klebsiella. Categorize them into respiratory associated and sexually transmitted.
✔ K. pneumoniae [most commonly isolated]
✔ K. oxytoca
✔ K. ozaenae
✔ K. rhinosceleromatis
✔ K. granulomatis
The first four are respiratory associated.
K. granulomatis is sexually transmitted.
Common clinical manifestations of K. pneumoniae.
pneumonia [responsible for community-acquired pneumonias (CAPs) and hospital-acquired pneumonias (HAPs)], sepsis, UTI, bacteremia, meningitis, pyogenic liver abscesses
Briefly discuss culturing of Klebsiella. [Hints: conditions, BA, MacConkey, CLED, gram-staining]
◾ Culture in air: 35-37°C, 18-24 hours
◾ BA: large, moist, grayish-white colonies that are usually non-haemolytic
◾ MacConkey: pink colonies
◾ CLED: yellow colonies
◾ Gram-stain: gram-negative rods
IMViC test for Klebsiella
–++
Key characteristics of Proteus
◾ Non-lactose fermenters
◾ Urease positive
◾ Non-capsulated
◾ Highly motile; swarms on culture media
◾ Contain O (somatic antigen) and H (flagellar antigen)
Pathogenesis and clinical features of Proteus
✔ Highly motile, which may contribute to the bacteria’s ability to invade the urinary tract.
✔ Possess fimbriae that facilitate uroepithelial colonization.
✔ Urease enzyme hydrolyzes urea in the urine into ammonium hydroxide, increasing the urinary pH.
✔ This increase in pH can encourage calculus (stone) formation.
✔ The formed calculi can obstruct urine flow.
✔ Bacteria within the calculi are not easily reached by drugs, making treatment challenging.
Clinical features: UTI, wound infections, septicaemia, ear infections, brain abscesses
Briefly discuss culturing of Proteus. [Hints: conditions, BA, MacConkey, CLED, gram-staining]
◾ Culture in air: 35-37°C, 18-24 hours
◾ BA: large, moist, grayish colonies with characteristic swarming motility. They may show beta-haemolysis.
◾ MacConkey: pale/colorless colonies
◾ CLED: pale/colorless colonies, swarming motility is inhibited due to the electrolyte-deficient nature of the medium
◾ Gram-stain: gram-negative rods
