HPC method, and how long are samples incubated?

HPC method, and how long are samples incubated?

• A. A 10:1 dilution of water to sample inoculated to an eosin methylene blue agar plate, 24-48 hr
• B. A 10:1 dilution of water to sample inoculated to an eosin methylene blue agar plate, 48-72 hr
• C. Diluted water samples are placed on a plate-count agar, 24-48 hr
• D. Diluted water samples are placed on a plate-count agar, 48-72 hr

Answer: (D)

Heterotrophic plate count looks at the total viable bacterial population using a non-selective growth medium, typically plate-count agar. To get a representative count, the water sample is diluted and spread on PCA, then incubated long enough for slower environmental bacteria to form colonies. The standard incubation is 48–72 hours, which allows a more accurate reflection of the heterotrophic bacteria present. Eosin methylene blue agar is used for estimating coliforms/E. coli, not HPC, so using that medium with a 24–48 hour incubation would measure a different bacterial group. Therefore, diluted samples plated on plate-count agar and incubated for 48–72 hours is the correct approach.

Heterotrophic plate count looks at the total viable bacterial population using a non-selective growth medium, typically plate-count agar. To get a representative count, the water sample is diluted and spread on PCA, then incubated long enough for slower environmental bacteria to form colonies. The standard incubation is 48–72 hours, which allows a more accurate reflection of the heterotrophic bacteria present. Eosin methylene blue agar is used for estimating coliforms/E. coli, not HPC, so using that medium with a 24–48 hour incubation would measure a different bacterial group. Therefore, diluted samples plated on plate-count agar and incubated for 48–72 hours is the correct approach.