Question 1

Why is membrane filtration ideal for testing drinking water quality?

Accepted Answer

Membrane filtration works best for low turbidity water sources like drinking water, swimming pools, and lakes because the 0.45 micrometer pore size efficiently traps bacteria without clogging. High particulate matter can foul the filter and limit sample volume processing. This technique allows rapid bacterial enumeration and identification on selective growth media.

Question 2

What do total coliforms indicate about water contamination?

Accepted Answer

Total coliforms are non-pathogenic bacteria from warm-blooded animal intestines that indicate fecal release into water. Their presence signals that pathogenic microorganisms may also be present. Detection of coliforms means the water source has experienced fecal contamination and warrants further investigation for harmful pathogens.

Question 3

How are total coliform colonies identified after membrane filtration?

Accepted Answer

Total coliform colonies appear pink to dark red with a metallic surface sheen on selective media. Atypical colonies may be dark red, mucoid, or nucleated without sheen. Verification involves transferring colonies to lauryl tryptose broth with a Durham tube; turbidity plus gas production confirms total coliforms.

Question 4

What distinguishes fecal enterococci from other indicator bacteria?

Accepted Answer

Fecal enterococci colonies appear pink to dark red on selective media. They are confirmed by Gram staining as positive, ovoid bacteria grouped in pairs or short chains, combined with a negative catalase test showing no bubbling with hydrogen peroxide. This combination distinguishes them from catalase-positive bacteria like Citrobacter.

Question 5

What happens to water samples during the membrane filtration process?

Accepted Answer

Negative pressure draws water across a 0.45 micrometer filter, trapping bacteria and particles larger than the pore size. Smaller particles, viruses, and dissolved solids pass through into waste. After filtration, the membrane is rinsed three times with sterile water, then transferred to growth plates for culturing target microorganisms.

Question 6

Why are coliform indicator bacteria used instead of testing for all pathogens?

Accepted Answer

The sheer number and variety of fecal-oral pathogens makes individual testing impractical and expensive. Coliform indicator bacteria are consistently excreted in feces from warm-blooded animals, making them reliable markers of fecal contamination. Their presence suggests pathogenic organisms may also be present in the water source.

Question 7

When is a water source considered unfit for human consumption?

Accepted Answer

If more than 5% of water samples collected over a one-month period test positive for indicator bacteria, the source may be considered unfit for human consumption. This threshold indicates consistent fecal contamination and potential presence of disease-causing organisms that pose public health risks.

Isolation of Fecal Bacteria from Water Samples by Filtration - Assessment Quiz