Identification and in Situ Detection of Intracellular Bacteria in the Environment

Abstract

Today it is generally accepted that our knowledge of bacterial diversity in the environment has been severely limited by the need to obtain pure cultures prior to characterization by testing for multiple physiological and biochemical properties. In addition, the morphology of microorganisms is in general too simple to serve as a basis for a reliable and proper classification; only in rare cases does it allow the in situ identification of individual population members by microscopy (Woese, 1987). Viable plate count or most probable-number techniques have been used for quantification of active cells in different environments but are always selective and can therefore not yield sufficient documentation of the true community structure (Table 1). For aquatic habitats as well as soils and sediments it has been frequently reported that direct microscopic counts exceed viable-cell counts by several orders of magnitude (Torsvik et al.,1990; Ferguson et al.,1984; Jones, 1977). This phenomenon is known as the “great plate count anomaly” described by Staley and Konopka (1985). Any estimation of the numbers of bacteria in the environment, whether they are pathogens, indicator organisms or genetically modified microorganisms, must allow for the fact that a proportion of the target organisms have entered the non-culturable but viable fraction of the microbial population. This accounts especially for bacterial endosymbionts colonizing free-living and parasitic protozoa although the roles such endosymbionts play in host survial, infectivity, and invasiveness are unclear (Fritsche et al., 1993).