Abstract
A symposium on viable but nonculturable (VBNC or VNC) microorganisms in natural environments was held under the joint auspices of the Japanese Society of Microbial Ecology and the Japanese Society of Soil Microbiology and its invited papers were reviewed here. In response to unsuitable environmental conditions (starvation, salt stress, etc.), bacteria change their metabolic activity and cellular components to maintain viability for extended survival, and in some cases, enter a non-growing stage. VBNC bacteria clearly display vital physiological signs (respiration and/or protein synthesis) and harbor vital cellular components (enzymes and/or nucleic acids) but do not multiply in ordinary culture media without certain resuscitation processes. It is important to set up criteria for estimating the risk of VBNC human pathogens such as Escherichia coli O157 and Vibrio cholerae in environment to prevent infectious diseases from unknown sources. Most heterotrophic bacteria in river water, seawater, activated sludge, and soil are also considered VBNC and oligotrophism may be related to their behavior as VBNC bacteria. Obligate symbionts such as VA mycorrhizal fungi and obligate parasitic fungi such as Plasmodiophora brassicae are also nonculturable though they may not be considered VBNC microorganisms. Their artificial cultivation is desired due to their agricultural significance. Developing new culture technologies for both VBNC bacteria and nonculturable fungi should be promoted as well as molecular and fluorescent techniques. A multidisciplinary approach involving many microbiologists specialized in aquatic, soil, medical and physiological fields may enable to clarify these issues in future.
