22 Questions
What is the primary purpose of performing triple sugar iron agar and urea agar tests in E. coli identification?
To screen for E. coli prior to definitive identification procedures
What is the temperature at which E. coli can grow, which is an important characteristic in its identification?
44.5°C
What is the significance of an E. coli colony count above 4/dL in municipal drinking water?
It indicates unacceptable fecal contamination
Why is disinfection of the public water supply important?
To prevent the spread of enteric pathogens
What is the role of bile salts or bacteriostatic dyes in water quality testing?
They exert selective pressure on the growth of certain bacteria
Why is the ability to ferment lactose with the production of acid and gas an important characteristic of E. coli?
It is a metabolic criterion for its identification
What is the function of phenol red in Urea Agar?
To detect the production of urease
What is the purpose of TSI agar?
To screen for organisms based on their metabolic criteria
Which organisms are urease-positive?
Proteus species and K. pneumoniae
What happens to the pH of the medium when an organism produces urease?
It becomes more alkaline
What is the significance of gas production in TSI agar?
It indicates the fermentation of lactose
What happens to the slant in TSI agar if lactose is not fermented but glucose is?
It turns red
What is the purpose of inoculating the organism into the butt and across the surface of the slant?
To provide different oxygen levels for growth
What is the significance of a yellow color in the butt and on the slant?
It indicates the production of acid
What is the primary purpose of using selective differential media such as MacConkey's agar or EMB agar in the laboratory diagnosis of Enterobacteriaceae?
To identify lactose fermenters and non-lactose fermenters
What is the significance of the presence of coliforms in the public water supply system?
It indicates fecal contamination of the water supply
Which of the following organisms is exclusively a large intestine organism?
E.coli
What is the characteristic of E.coli colonies on EMB agar?
They have a characteristic green sheen
What is the purpose of using an agglutination test in the laboratory diagnosis of Salmonella and Shigella?
To identify the genus and group of the organism
What is the term used to describe the group of organisms that includes not only E.coli but also other inhabitants of the colon such as Enterobacter and Klebsiella?
Coliforms
What is the metabolic criterion used in the identification of Enterobacteriaceae?
Lactose fermentation
What type of media is normally inoculated with specimens suspected of containing members of the Enterobacteriaceae and related organisms?
Blood agar plate and EMB agar
Study Notes
E. coli Identification
- E. coli is identified by its ability to ferment lactose with the production of acid and gas, its ability to grow at 44.5°C, and its characteristic colony type on EMB agar.
Screening Tests
- An additional set of screening tests, consisting of triple sugar iron (TSI) agar and urea agar, is performed prior to the definitive identification procedures.
Triple Sugar Iron (TSI) Agar
- TSI agar is used to detect lactose fermentation, which is the most important metabolic criterion used in the identification of Enterobacteriaceae and related organisms.
- The reactions of several important organisms are presented in Table 18–6.
- If lactose is fermented, a large amount of acid is produced, which turns the phenol red indicator yellow both in the butt and on the slant.
- If lactose is not fermented but the small amount of glucose is, the oxygen-deficient butt will be yellow, but on the slant, the acid will be oxidized to CO2 and H2O by the organism and the slant will be red (neutral or alkaline).
- If neither lactose nor glucose is fermented, the butt and slant will be red (neutral or alkaline).
Urea Agar
- Urea agar is used to detect urease production.
- The important components of this medium are urea and the pH indicator phenol red.
- If the organism produces urease, the urea is hydrolyzed to NH3 and CO2, and the color of the phenol red changes from light orange to reddish purple.
- The important organisms that are urease-positive are Proteus species and K. pneumoniae.
Laboratory Diagnosis
- Laboratory diagnosis involves inoculating specimens onto two media, a blood agar plate and a selective differential medium such as MacConkey's agar or EMB agar.
- Agglutination tests can be used to identify the genus of the organism and to determine whether it is a member of group A, B, C, or D.
Coliforms and Public Health
- Contamination of the public water supply system by sewage is detected by the presence of coliforms in the water.
- E. coli is used as an indicator of fecal contamination because it is exclusively a large intestine organism.
- A coliform count above 4/dL in municipal drinking water is indicative of unacceptable fecal contamination.
This quiz covers the identification of E. coli in water quality testing, including its ability to ferment lactose, grow at 44.5°C, and its characteristic colony type on EMB agar.
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