Microbio Lab 7 Flashcards
coliforms
Gram-negative, facultative anaerobes that live in the intestines of animals; ferment lactose to produce acid and gas; non-endospore-forming`
Why is E. Coli a good indicator bacterium?
- It is present in the intestines of humans and several warm-blooded animals but is not routinely found in water or soil
- the organism can be identified using specific microbiological tests
- it is not as fastidious as the other pathogens and can survive longer in water samples
presumptive test
looks for gas formation
15 tubes of lactose broth are inoculated with measured amounts of water to see if water contains any lactose-fermenting bacteria that produce gas; if gas is present it is presumed that coliforms are present in the water sample; also used to determine the most probable number
confirmed test
looks to see if organisms are gram-negative
Plates of Levine EMB agar or Endo agar are inoculated from positive tubes; both media inhibit growth of gram-positive bacteria; presence of colonies (EMB- large blue-black colonies with a green metallic sheen, Endo- pink to rose-red colonies) indicates gram-negative bacteria
completed test (IMViC tests)
Test used to determine if bacterium from presumptive test and confirmed test is either E. Coli or E. aerogenes
Levine EMB agar
contains methylene blue, which inhibits gram-positive bacteria
coliforms produce large blue-black colonies with a green metallic sheen for E. Coli
Endo agar
contains a fuchsin sulfite indicator that makes identification of lactose fermenters easy
presence of colonies suggest gram-negative bacteria and agar color change from light to dark suggests fermentation of lactose
membrane filter method
Reliable procedure for determining coliforms in water
water is passed through membrane which filters out bacteria; filter membrane is placed in petri dish with an absorbent pad saturated with Endo broth; coliform bacteria will have metallic sheen
TGEA
tryptone glucose extract agar (used in bacterial counts of food)
Which type of food had the the highest bacterial count?
Ground meat due to it being fresh and not frozen.
Also food where bacteria is used in its production (yogurt)
Which type of food had the lowest bacterial count?
Dried foods due to a lack of water that is available to the bacteria.
Why might one expect to find coliforms in samples of meat?
Fecal matter from the animal’s intestines contaminates meat during butchering.
High coliform counts in food indicate the potential for finding which pathogens?
Salmonella, shigella, vibrio cholera, intestinal viruses.
In terms of food safety, why is it suggested to cook hamburgers medium-well to well-done, whereas steaks can be cooked rare?
Pathogens invade hamburger during grinding and processing– and the inside of steak is not processed.
What considerations should be made to safely thaw frozen foods for later consumption?
When thawing frozen food, it should either be placed in a refrigerator overnight or in the microwave. It should not be thawed over long periods of time at room temperature or in warm water.
Why is refrigeration not always an effective means for preventing food spoilage?
Some microbes are able to grow and reproduce at refrigerator temperatures.
Why can milk become sour due to bacteria even though it has been pasteurized?
Some bacteria can survive pasteurization and grow at refrigerator temperatures (psychotrophs) to cause milk to sour
Give two examples of foods that have high bacterial counts but are not spoiled.
yogurt and summer sausage
Does a positive presumptive test mean that the water is absolutely unsafe to drink?
No– the bacteria could be harmless saprophytes or some other type of bacteria that does not cause disease in humans
What might cause a false presumptive test?
There could be two or more non-coliform bacteria that are working together to produce sufficient gas in the tube.
List three characteristics required of a good sewage indicator
Easy to identify, present only in sewage, lives slightly longer than pathogens in water
The fermentation of what disaccharide is the basis for determining the presence of coliforms?
lactose
describe the appearance of coliforms on Levine EMB agar.
Nucleated colonies with a dark center that may have a metallic green sheen.
Why don’t health departments routinely test for pathogens instead of using a sewage indicator?
It would be tedious and difficult to specifically test for each of the pathogens. Also, they are often fastidious and they might be overgrown by other bacteria in the water if we tried to culture and test for them. It is much easier to demonstrate the presence of some indicator bacterium (E. coli)
List five characteristics of coliform bacteria.
Gram-negative, facultative anaerobes, non-endospore forming, lactose-fermenting, acid & gas producing
How is each of the following media used for the detection of coliform bacteria?
a. lactose broth with Durham tube
b. Levine EMB agar
c. nutrient slant agar
a. detect gas production from lactose fermentation; b. Inhibition of gram-positive growth and lactose fermenters will produce “nucleated” colonies (black centers) and may have a metallic sheen; c. To make a slide for gram staining to confirm morphology (gram-negative bacilli)
Once the completed test establishes the presence of coliforms in the water sample, why might you perform the IMViC tests on these isolates?
Used to confirm the identity of the isolates as coliforms. These tests will differentiate E. Coli, which is associated with sewage, from E. aerogenes, which is not always associated with sewage.
What gram-positive bacterium can give a positive presumptive test?
Clostridium perfrigens can grow and produce gas in the presumptive test.
