Microbio Lab 7

Question 1

coliforms

Answer 1

Gram-negative, facultative anaerobes that live in the intestines of animals; ferment lactose to produce acid and gas; non-endospore-forming`

Question 2

Why is E. Coli a good indicator bacterium?

Answer 2

1. It is present in the intestines of humans and several warm-blooded animals but is not routinely found in water or soil
2. the organism can be identified using specific microbiological tests
3. it is not as fastidious as the other pathogens and can survive longer in water samples

Question 3

presumptive test

Answer 3

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

Question 4

confirmed test

Answer 4

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

Question 5

completed test (IMViC tests)

Answer 5

Test used to determine if bacterium from presumptive test and confirmed test is either E. Coli or E. aerogenes

Question 6

Levine EMB agar

Answer 6

contains methylene blue, which inhibits gram-positive bacteria

coliforms produce large blue-black colonies with a green metallic sheen for E. Coli

Question 7

Endo agar

Answer 7

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

Question 8

membrane filter method

Answer 8

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

Question 9

TGEA

Answer 9

tryptone glucose extract agar (used in bacterial counts of food)

Question 10

Which type of food had the the highest bacterial count?

Answer 10

Ground meat due to it being fresh and not frozen.

Also food where bacteria is used in its production (yogurt)

Question 11

Which type of food had the lowest bacterial count?

Answer 11

Dried foods due to a lack of water that is available to the bacteria.

Question 12

Why might one expect to find coliforms in samples of meat?

Answer 12

Fecal matter from the animal’s intestines contaminates meat during butchering.

Question 13

High coliform counts in food indicate the potential for finding which pathogens?

Answer 13

Salmonella, shigella, vibrio cholera, intestinal viruses.

Question 14

In terms of food safety, why is it suggested to cook hamburgers medium-well to well-done, whereas steaks can be cooked rare?

Answer 14

Pathogens invade hamburger during grinding and processing– and the inside of steak is not processed.

Question 15

What considerations should be made to safely thaw frozen foods for later consumption?

Answer 15

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.

Question 16

Why is refrigeration not always an effective means for preventing food spoilage?

Answer 16

Some microbes are able to grow and reproduce at refrigerator temperatures.

Question 17

Why can milk become sour due to bacteria even though it has been pasteurized?

Answer 17

Some bacteria can survive pasteurization and grow at refrigerator temperatures (psychotrophs) to cause milk to sour

Question 18

Give two examples of foods that have high bacterial counts but are not spoiled.

Answer 18

yogurt and summer sausage

Question 19

Does a positive presumptive test mean that the water is absolutely unsafe to drink?

Answer 19

No– the bacteria could be harmless saprophytes or some other type of bacteria that does not cause disease in humans

Question 20

What might cause a false presumptive test?

Answer 20

There could be two or more non-coliform bacteria that are working together to produce sufficient gas in the tube.

Question 21

List three characteristics required of a good sewage indicator

Answer 21

Easy to identify, present only in sewage, lives slightly longer than pathogens in water

Question 22

The fermentation of what disaccharide is the basis for determining the presence of coliforms?

Answer 22

lactose

Question 23

describe the appearance of coliforms on Levine EMB agar.

Answer 23

Nucleated colonies with a dark center that may have a metallic green sheen.

Question 24

Why don’t health departments routinely test for pathogens instead of using a sewage indicator?

Answer 24

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)

Question 25

List five characteristics of coliform bacteria.

Answer 25

Gram-negative, facultative anaerobes, non-endospore forming, lactose-fermenting, acid & gas producing

Question 26

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

Answer 26

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)

Question 27

Once the completed test establishes the presence of coliforms in the water sample, why might you perform the IMViC tests on these isolates?

Answer 27

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.

Question 28

What gram-positive bacterium can give a positive presumptive test?

Answer 28

Clostridium perfrigens can grow and produce gas in the presumptive test.