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Microbio Lab #61
Lab #61 – Bacteriological Examination of Water: Qualitative Tests
| Question | Answer |
|---|---|
| Does a positive-presumptive test mean that the water is absolutely unsafe to drink? Explain. No, a positive presumptive test indicates that the water might be unsafe to drink but further testing is warranted. | No, a positive presumptive test indicates that the water might be unsafe to drink but further testing is warranted. |
| What might cause a false positive presumptive test? | A false positive presumptive test may be due to two or more non-coliform bacteria working synergistically to produce sufficient gas in the tube. |
| List three characteristics required of a good sewage indicator. | Easy to identify. Present only in sewage, not in soil. Lives slightly longer than pathogens in water. |
| What enteric bacterial diseases are transmitted in polluted water? | Typhoid fever, bacillary dysentery, and cholera |
| Name one or more protozoan diseases transmitted by polluted water. | Amoebiasis caused by E. histolytica, cryptosporidiosis caused by C. parvum, and giardiasis caused by G. lamblia. |
| Why don’t health departments routinely test for pathogens instead of using a sewage indicator? | |
| List five characteristics of coliform bacteria. | Coliform bacteria are gram-negative, facultative anaerobic, non-endospore forming bacilli that ferment lactose to produce acid and gas. |
| How is Lactose broth with Durham tube used for the detection of coliforms? | Detect gas production from lactose fermentation. |
| How is Levine EMB agar used for the detection of coliforms? | a. of gram-positive growth and lactose fermentors will produce “nucleated” colonies (black centers) and may have a metallic sheen. |
| How is Nutrient agar slant used for the detection of coliforms? | 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? | The IMViC tests (indole, methyl red, Voges-Proskauer, citrate) are used to confirm the identity of the isolates as coliforms. These tests will differentiate E. coli, which is associated w/ sewage, from E. aerogenes, which isn't always assoc. w/ sewage. |
Created by:
michellevu