Measurement of the Number of Viable Microbes on Solid Growth Media

This approach consists of measuring the number of viable cells capable of forming colonies on a suitable growth medium. Plate count is determined by using the pour plate method (0.1–1 mL of microbial suspension is mixed with molten agar medium in a petri dish), or the spread plate method (0.1 mL of bacterial suspension is spread on the surface of an agar plate). The results of plate counts are expressed as colony forming units (CFU). The number of CFU per plate should be between 30 and 300. Membrane filters can also be used to determine microbial numbers in dilute samples. The sample is filtered and the filter is placed directly on a suitable growth medium.

                                     

Culture-based methods have been routinely used in soil, aquatic, and wastewater microbiology, but they reveal only about 0.1–10 percent of the total bacterial counts in most environments (Pickup, 1991). Indeed, some microorganisms (e.g., E. coli, Salmonella typhimurium, Vibrio spp.) can enter into the viable but nonculturable (VBNC) state and are not detected by plate counts, especially when using selective growth media (Koch, 2002; Roszak and Colwell, 1987). The VBNC state can be triggered by factors such as nutrient deprivation or exposure to toxic chemicals. This phenomenon is particularly important for pathogens that may remain viable in the VBNC state for longer periods of time than previously thought. The VBNC pathogens may remain virulent and cause disease in humans and animals.