In HPC testing, which plating incubation duration applies for plate-count agar?

In HPC testing, which plating incubation duration applies for plate-count agar?

• 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)

HPC testing uses a broad-nutrient plate to capture heterotrophic bacteria, and those organisms can take longer to form visible colonies. Plate-count agar is incubated for 48-72 hours to ensure slow-growing cells are counted and to provide a reliable estimate of bacterial density in water. Incubation shorter than 48 hours can miss these slower colonies, leading to undercounts, while the eosin methylene blue plates are used for different purposes (such as selecting certain Gram-negative organisms) and have different typical timings. So the best match is plate-count agar incubated for 48-72 hours.

HPC testing uses a broad-nutrient plate to capture heterotrophic bacteria, and those organisms can take longer to form visible colonies. Plate-count agar is incubated for 48-72 hours to ensure slow-growing cells are counted and to provide a reliable estimate of bacterial density in water. Incubation shorter than 48 hours can miss these slower colonies, leading to undercounts, while the eosin methylene blue plates are used for different purposes (such as selecting certain Gram-negative organisms) and have different typical timings. So the best match is plate-count agar incubated for 48-72 hours.