demonstrate the presence of

an indicator bacterium

(eg. Escherichia coli)

found in humans, but not in the soil/water.

1. occurs primarily in the intestines of humans, not found in soil/water

2. can be easily identified by microbiological tests

3. not as fastidious as the intestinal pathogens => survives a little longer in water

- gram-negative

- facultative anaerobic

- non-endospore-forming

- rods

- ferment lactose => acid and gas in 48 hrs at 35 C

E. coli

Enterobacter aerogenes

1. Presumptive

2. Confirmed

3. Completed

What happens in the

presumptive test?

Tubes are inoculated with measured amounts of water.

(to see if the water contains any lactose-fermenting bacteria that produce gas)

If gas is seen, it is presumed that coliforms are present

to det the

most probable number (MPN)

of coliforms present per 100 mL water.

Levine EMB agar

Endo agar

Plates of Levine EMB agar and Endo agar

are inoculated from positive tubes

to see if

the organisms that are producing the gas are gram-negative.

1. small colonies

2. dark centers (nucleated)

1. nutrient agar slant

2. Durham tube of lactose broth

1. if gas is produced in the lactose tube

2. if a slide from the agar slant reveals that we have

a gram-negative, non-spore-forming rod

E coli are small and have a metallic sheen

E aerogenes are large and lack the sheen

1. Brilliant green bile lactose broth

2. Eijkman's medium

3. ED medium

What might cause a

false positive presumptive test?

1. easy to identify

2. present only in sewage

3. lives slightly longer than pathogens in water

1. gram-negative

2. facultative anaerobe

3. non-endospore forming

4. lactose-fermenting

5. acid and gas producing.

How does a

lactose broth with Durham tube detect coliforms?

Used to make

a slide for gram staining

to confirm morphology (gram-negative bacilli).