Epidemiological Significance of Viable but Nonculturable Microorganisms

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Although the term “viable but nonculturable” (VBNC) has been used in the literature during the past two decades to describe a survival strategy of microorganisms, notably human pathogens in the aquatic environment, there is an abundance of philosophical musings on this subject in the microbiological literature. For example, Radsimosky in 1930 (119) noted a significant difference in the number of autotrophic and organotrophic bacteria in water samples. Direct microscopic enumeration, used in water bacteriology at the time, yielded counts 200 to 5,000 times higher than culture counts on bacteriological plates (16, 17). The difference between results of bacterial enumeration by direct observation and subsequent measurement of oxygen demand led Butkevitch and Butkevitch (18) to conclude that a significant portion of a given bacterial population, which did not appear as colonies on plates, must be present in a resting stage. Knaysi (82) and Jennison (70) were able to demonstrate metabolic activity of organisms observed by direct microscopy to be present in a sample but failing to grow, hence not appearing as colonies on plates. ZoBell (156) reconfirmed earlier findings that plate culture counts of seawater, although the widely used method at the time (and to this day), yield only a small percentage of the bacteria actually present in a given sample.