Exercise 15: Bacterial Examination of Water

Day 1 of examination is called:

presumptive test

Coliforms are:

members of the family Enterobacteriaceae

all coliforms are gram-positive or gram-negative? Can ferment or cannot ferment lactose? Gas production?

-gram-negative

-CAN ferment lactose

-will have gas production

Coliform bacteria are often found in:

feces

What is a common coliform?

E. coli

Presence of coliforms in water indicate what?

fecal contamination of the water

What test is done on Day 1? What is this looking for? Why is this test used?

multiple-tube lactose fermentation; looking for lactose fermentation WITH gas production; used as initial screening for presence of coliform bacteria

Day 1 testing will conclusively indicate fecal contamination, T or F?

false; will not conclusively indicate fecal contamination but it is suggestive (hence the same "presumptive!!")

What method is used for the presumptive test?

Most Probable Number (MPN) method

MPN=

Most Probable Number

What does the MPN method estimate?

estimates # of microbes present in water samples

For the MPN method, samples are diluted into what broth? What strength?

lactose broth; at double or single strength

How many tubes are inoculated for MPN and how many mLs of sample per tube?

-3 tubes inoculated with 10 mL of sample

-3 tubes inoculated with 1 mL of sample

-3 tubes inoculated with 0.1 mL of sample

What is placed inside the tubes to detect gas production from lactose fermentation?

Durham tubes

Which tubes are double strength? Single strength?

-DS= 3 tubes of 10 mL

-SS= 3 tubes of 1 mL and 3 tubes of 0.1 mL

Day 2 of examination is called:

confirmed test

What test is done on day 2?

a 3 zone streak from the positive lactose fermentation tube that was done on day 1

What medium is used for day 2 testing?

EMB and an Endo Agar plate

Day 3 of examination is called:

Completed Test

EMB plate is selective for:

gram negative bacteria

EMB plate is differential for:

lactose fermenters

Like EMB, what medium is also selective for gram negative bacteria and differential for lactose fermenters?

Endo agar

E. coli specimen is placed on a Endo plate. What will the results yield?

E. coli will produce red/pink colonies with a metallic sheen

What is the membrane filter technique?

a method of examining water for bacterial contamination by passing the water through a filter

What are the size of the pores on the filter for the membrane filter technique?

0.45 um

What does the filter catch when doing the membrane filter technique?

all the bacteria in the water; allows the water to pass through

For the membrane filter technique, what are the two mediums the filter can be placed in to identify if coliforms are present? What color do they turn?

-mENDO Agar--->pink; shows total coliforms by a golden sheen

-mFc Agar--->light blue; shows fecal coliforms as darker blue colonies on the agar