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Coliform Test:

1) Two other types of media that can be used to help determine if an organism is a coliform are? (beside selective and differential)

2) Why is it that LTB is used to presumptively determine the presence or absence of coliforms, and BGLB is used to confim the presence of coliforms?

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Nestor Rutherford
Nestor RutherfordLv2
28 Sep 2019

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Related questions

Case Study:

Bacteria are everywhere—on our skin, in our food, suspended in our drinking water. Although the
microbial majority is benign, a few bacterial species can make us sick. To safeguard against food- and
water-borne disease, microbiologists routinely screen our drinking water for bacterial pathogens.
Animal feces contain coliform bacteria, microorganisms that inhabit the intestines of warm-blooded
animals. Many coliform bacteria are also found on plants and in soil and water. Coliform bacteria are
not pathogens themselves, but their presence indicates the possibility of finding pathogens. In contrast,
fecal coliform bacteria such as Escherichia coli are found in feces, and their presence in drinking water
indicates fecal contamination. E. coli can also be a pathogen itself, so if E. coli is found in drinking
water there is a good chance that other pathogens are present, too.1
To detect E. coli and other coliform bacteria, microbiologists filter water samples and place the filter in
a Petri dish containing growth medium such as Endo agar. Microorganisms from the water grow and
form colonies, giving an estimate of the number of bacteria in each milliliter of water.
While E. coli forms colored, shiny colonies on Endo agar, other coliforms grow as white or clear
colonies.

Dark colonies = E. coli
White/clear colonies = coliform bacteria
White + Dark colonies = total coliform
bacteria.

The guidelines of maximum allowable limits for total coliforms in drinking and recreational waters vary
by state. Representative limits appear in the table below.2, 3, 4

Table 1. Maximum allowable limits of fecal coliform bacteria
(cells/100 ml)
Total Coliforms E. Coli
Drinking Water 0 0
Recreational Waters 400 235

Questions
1. Why would we want to count the number of fecal coliforms in a water sample?
2. Do 400 coliforms per 100 milliliters strike you as very many?
3. How many other types of bacteria might be present in a normal water sample?
4. What problems might Mary encounter when she uses these techniques in Antarctica?

A. Indicate whether the following descriptions apply to nutrient, defined, selective,

differential, or enrichment media. If the statement applies to more than one, indicate which ones.

Answers may be used more than once.

____________________ if you want to take an environmental sample and grow a wide variety of

bacteria, you would use this medium

____________________ blood agar is an example, because you can see whether or not microbes

produce substances that lyse red blood cells

____________________ you might make this medium with a specially-formulated salt mixture,

some vitamins, and a refined protein source like peptone

____________________ the lauryl tryptose broth that we used for the presumptive coliform test

is this type of medium- we used it to increase the chance of actually seeing the coliforms

B. Indicate whether the following procedures are best performed with liquid or solid media. If

the statement applies to both, say “both.”

_______________________ you can count colonies of microbes grown with this medium

_______________________ you can grow facultative anaerobes with this medium

_______________________ you can use a spectrophotometer to measure growth in this medium

_______________________ you can prepare this medium by autoclaving it prior to use

Pre-Laboratory Questions

1. Why do bacterial cultures need to be incubated, usually for at least 24 hours, before we can see evidence of bacterial growth that can be observed with the unaided eye?

2. In what ways are the media used in this investigation selective?

3. In what ways are the media used in this investigation differential?

4. Consider a mixture of bacterial species from the same genus, such as Staphylococcus. Some members of this genus are harmful pathogens, while others are not. Would a selective medium alone be a good tool to separate the harmful species from the non-harmful species? Explain your answer.

Post-Lab Questions

1. Would simply adding a pH indicator like phenol red to a general-purpose medium, such as nutrient agar, result in a medium that is selective, differential, both, or neither? Why or why not?

2. Why were you instructed to inoculate each plate with three separate streaks, one of each bacterial species, instead of combining all three species and streaking them all onto the plate together?

3. Blood agar and chocolate agar are commonly-used media in many clinical and research laboratories. Why do you think they were not included in your kit?

4. Were there any discrepancies between what you saw in your experiment and what you expected to see based on the lab manual and your readings in this section?

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