In HPC testing, where are the diluted water samples placed and how long are they incubated?

In HPC testing, where are the diluted water samples placed and how long are they incubated?

• A. On eosin methylene blue agar plate, 24-48 hr
• B. On eosin methylene blue agar plate, 48-72 hr
• C. On plate-count agar, 48-72 hr
• D. On plate-count agar, 24-48 hr

Answer: (C)

In heterotrophic plate count testing, you’re trying to estimate the number of viable heterotrophic bacteria in water, so you use a nonselective, general-purpose medium that will support a wide range of organisms. Plate-count agar is the medium of choice because it provides a broad nutrient base without selecting for specific groups, giving a representative count of viable bacteria. The diluted water sample is spread onto plate-count agar plates, and these plates are incubated for about 48 to 72 hours. This incubation window allows most heterotrophic bacteria present to form visible colonies so they can be counted and used to calculate the concentration of organisms in the original water sample. EMB agar, which is selective for many Gram-negative, lactose-fermenting bacteria, is not used for HPC because it biases the count toward specific organisms rather than reflecting the total heterotrophic population.

In heterotrophic plate count testing, you’re trying to estimate the number of viable heterotrophic bacteria in water, so you use a nonselective, general-purpose medium that will support a wide range of organisms. Plate-count agar is the medium of choice because it provides a broad nutrient base without selecting for specific groups, giving a representative count of viable bacteria.

The diluted water sample is spread onto plate-count agar plates, and these plates are incubated for about 48 to 72 hours. This incubation window allows most heterotrophic bacteria present to form visible colonies so they can be counted and used to calculate the concentration of organisms in the original water sample.

EMB agar, which is selective for many Gram-negative, lactose-fermenting bacteria, is not used for HPC because it biases the count toward specific organisms rather than reflecting the total heterotrophic population.