Microbiology: Escherichia coli Overview

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Questions and Answers

What indicates a positive result for indole production after adding Kovac’s reagent?

Green slant formation

No ring formation

Red ring formation (correct)

Yellow ring formation

Which organisms produce a positive urease test?

Escherichia coli

Salmonella enterica

Proteus vulgaris (correct)

Klebsiella pneumoniae

What result is indicative of no green slant formation in the FeCl3 test?

Negative result indicated by yellow color (correct)

Presence of hydrogen sulfide production

Positive result for Proteus species

Positive result for Morganella species

How much inoculum is required for a urease test?

5 loopfuls of the test organism Signup and view all the answers

What does the addition of 10% FeCl3 help to identify in the test results?

Indole production Signup and view all the answers

What is the primary virulence factor associated with E. coli O157:NM?

K1 antigen Signup and view all the answers

Which of the following outbreaks is NOT associated with E. coli O157:NM?

Fast food chain in Australia Signup and view all the answers

What reaction does E. coli O157:NM exhibit in Triple Sugar Iron (TSI) testing?

Acid over acid gas Signup and view all the answers

Which toxin is produced by E. coli O157:NM that affects vascular endothelial cells?

Verotoxin Signup and view all the answers

Which characteristic is true about E. coli O157:NM regarding lactose fermentation?

Lactose fermenter Signup and view all the answers

What is the mode of transmission for pathogenic E. coli?

Fecal-oral route by contaminated food and water Signup and view all the answers

What colony characteristic is associated with E. coli on MacConkey agar?

Flat, dry, pink colonies Signup and view all the answers

What does a yellow slant and yellow butt indicate in a TSI test for E. coli?

Fermentation of glucose and lactose Signup and view all the answers

What type of toxins are produced by Enterotoxigenic E. coli (ETEC)?

Heat-labile and/or heat-stable enterotoxins Signup and view all the answers

What is the significance of indole production in E. coli testing?

Demonstrates ability to produce a red ring in tryptone broth Signup and view all the answers

In which agar medium might E. coli show β-hemolytic activity?

Blood agar Signup and view all the answers

What does aerogenic mean in the context of a TSI test?

Gas produced throughout the medium indicating anaerobic fermentation Signup and view all the answers

What effect do enterotoxins from ETEC have on the intestines?

Induce water and electrolytes to enter the lumen, causing diarrhea Signup and view all the answers

Which of the following describes the motility of marcescens?

Motile and variable Signup and view all the answers

What characteristic distinguishes coliforms from other Gram-negative rods?

They are lactose fermenters Signup and view all the answers

Which statement about the urease test results of Proteus is accurate?

Some strains produce rapid and abundant urease Signup and view all the answers

What is a common description of the colonies formed by the genus Proteus?

Foul odor and resemble burnt chocolate Signup and view all the answers

In the IMVIC reaction, what result indicates the detection of tryptophanase?

(+, -, +, -) Signup and view all the answers

Which of the following properly describes ONPG for marcescens?

Positive with beta-galactosidase Signup and view all the answers

How do marcescens strains generally perform on lactose fermentation?

Slow lactose fermenters Signup and view all the answers

Which of the following genera is associated with urinary tract infections next to E. coli?

Morganella Signup and view all the answers

What is the primary purpose of using differential moderately selective media in stool cultures?

To isolate enteropathogens like Salmonella and Shigella Signup and view all the answers

Which media is specifically used for isolating S. typhi and Shigella species?

Selenite F Broth Signup and view all the answers

What does the presence of black centers on colonies in Salmonella Shigella Agar (SSA) indicate?

Production of hydrogen sulfide (H2S) Signup and view all the answers

Which media is NOT used to isolate coliforms in stool cultures?

Salmonella Shigella Agar (SSA) Signup and view all the answers

In which type of media would you isolate Salmonella species specifically?

Tetrathionate Broth Signup and view all the answers

What is the role of pH indicators in differential moderately selective media?

To indicate changes in acidity during fermentation Signup and view all the answers

What characteristic of Shigella spp. colonies is typically observed on Salmonella Shigella Agar?

Colorless colonies with black centers Signup and view all the answers

Which of the following best describes the purpose of enrichment media in stool culture?

To enhance the growth of enteropathogens before subculturing Signup and view all the answers

Study Notes

Escherichia coli (E. coli)

E. coli is a facultative anaerobe, gram-negative rod that is part of the normal flora of the human gut.
It is a lactose fermenter, with greenish, metallic sheen colonies on EMB agar.
It can ferment lactose producing yellow slant and yellow butt, and gas, on TSI agar.
Indole positive: Tryptone broth with a red ring when Kovac’s reagent is added.

Pathogenic Strains of E. coli

There are five pathogenic strains of E. coli: enterotoxigenic (ETEC), enteroinvasive (EIEC), enterohemorrhagic (EHEC), enteropathogenic (EPEC), and enteroaggregative (EAEC).

Enterotoxigenic E. coli (ETEC)

ETEC produces two enterotoxins: heat-labile toxin (LT) and heat-stable toxin (ST).
These toxins cause watery diarrhea by inducing water and electrolytes into the lumen of the small intestines.

Laboratory Diagnosis of Pathogenic E. coli

Identification of pathogenic E. coli strains relies on culture methods, biochemical tests, and molecular detection.

Enrichment Media & Techniques

Specialized media such as Salmonella Shigella Agar (SSA), Selenite F Broth, Tetrathionate Broth, and Gram-negative Broth (GN broth) are utilized to isolate enteric pathogens from stool samples.
These media inhibit the growth of coliforms while promoting the growth of enteropathogens.

Salmonella and Shigella Isolation

Stool or rectal swabs are inoculated into enrichment media like Selenite F Broth or Tetrathionate Broth followed by subculturing on selective media like SSA.
Salmonella is isolated from Tetrathionate Broth, while Shigella is isolated from Selenite F Broth.

Differentiation of E. coli Strains

Identifying the specific strain of pathogenic E. coli is important for appropriate treatment and infection control.
The differentiation of the strains can be achieved using serological tests and molecular methods.

Coli O157: H7

This is a non-motile strain that synthesizes the K1 antigen and Verotoxin or Shiga-like toxin.
Verotoxin damages vascular endothelial cells and can cause hemorrhagic colitis.
This strain has been linked to foodborne outbreaks, particularly in meat products like hamburgers.

Other Important Microorganisms

Other notable Gram-negative rods that may be encountered in the laboratory include Proteus, Providencia, and Morganella.
They are non-lactose fermenters and rapid urease producers. These organisms are commonly associated with urinary tract infections.

Proteus

Proteus is a motile bacterium with a distinctive foul odor and "chocolate cake" or "burnt chocolate" colonies on agar plates.
It produces the enzyme tryptophanase and can be identified via the IMVIC reaction (Indole, Methyl Red, Voges-Proskauer, Citrate).
It is commonly found in the gut and is associated with urinary tract infections.

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Description

This quiz covers essential information about Escherichia coli, including its characteristics, pathogenic strains, and the laboratory diagnosis methods used for identification. Test your knowledge on the role of E. coli in the human gut and the implications of its pathogenic variants.