Bacterial Examination of Water (Exercise 45) Flashcards
Does a positive presumptive test mean that the water is absolutely unsafe to drink?
No, water may be unsafe but requires further testing. E. aerogenes can be of nonfecal origin.
What might cause a false positive presumptive test?
The positive may result from gram-negative (non-coliform) bacteria.
List three characteristics required of a good sewage indicator.
1) Occurs in the intestines of humans and other warm-blooded animals
2) Easily identified
3) Survives longer than pathogens in water supplies
The fermentation of what disaccharide is the basis for determining the presence of coliforms?
Lactose
Describe the appearance of coliforms on Levine EMB agar.
Coliforms produce dark nucleated colonies, E. coli produces a metallic green sheen.
Why don’t health departments routinely test for pathogens instead of using a sewage indicator?
It would be difficult to test for specific pathogens, and they may be masked by bacterial growth.
List five characteristics of coliform bacteria.
1) Lactose fermentation- produces gas and acid in 48 hours at 35°C
2) Facultative anaerobe
3) Gram-negative
4) Rod-shaped
5) Do not form endospores
How is lactose broth with a durham tube used to detect coliforms?
Tests for lactose fermentation, the presence of gas and a color change indicates a positive result.
How is Levine EMB agar used to detect coliforms?
EMB agar is both selective and differential. It is selective because it slightly inhibits growth of gram -positive bacteria, and promotes growth of gram-negative bacteria. It is differential because it distinguishes between E. coli (metallic sheen), and other gram-negative rod-shaped bacteria such as E. aerogenes.
How is a nutrient agar slant used to detect coliforms?
A sample is prepared using the NA slant, the sample is fixed to a slide and used to visually confirm the presence of gram-negative nonspore-forming bacteria.
Once the completed test establishes the presence of coliforms in the water sample, why might you perform the IMViC tests on these isolates?
In order to conclusively differentiate E. coli from E. aerogenes.
