Which of the following methods is best for counting fecal bacteria from a stream to determine the safety of the water for drinking?
a. Dry weight
b. Turbidity
c. Viable plate counts
d. Membrane filtration

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Understand the goal: We want to count fecal bacteria in a water sample to assess its safety for drinking. This requires a method that accurately detects and quantifies bacteria, especially those indicating fecal contamination.

Evaluate each method: Dry weight measures the total biomass of microorganisms but does not differentiate fecal bacteria specifically, so it is not ideal for this purpose.

Turbidity measures cloudiness of water caused by particles, including bacteria, but it is not specific and cannot distinguish fecal bacteria from other particles or microbes.

Viable plate counts involve spreading a diluted water sample on agar plates and counting colonies that grow, which gives a count of live bacteria but can be time-consuming and may not be selective for fecal bacteria.

Membrane filtration involves filtering a known volume of water through a membrane that traps bacteria, then placing the membrane on selective media to grow and count fecal bacteria colonies. This method is sensitive, specific, and widely used for water safety testing.

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Fecal Bacteria as Water Contaminants

Fecal bacteria, such as Escherichia coli, indicate contamination by fecal matter and potential presence of pathogens. Monitoring their levels in water helps assess safety for drinking, as high counts suggest health risks from waterborne diseases.

Viable Plate Counts

Viable plate counts involve spreading a water sample on nutrient agar to grow and count live bacterial colonies. This method estimates the number of living bacteria but can be time-consuming and may underestimate bacteria that do not grow well on plates.

Membrane Filtration Technique

Membrane filtration captures bacteria on a filter, which is then placed on selective media to grow colonies. It is highly effective for detecting and quantifying fecal bacteria in water, allowing for accurate and rapid assessment of water safety.

Which of the following methods is best for counting fecal bacteria from a stream to determine the safety of the water for drinking?a. Dry weightb. Turbidityc. Viable plate countsd. Membrane filtration