Enrichment and Isolation

Abstract

A section of this chapter focuses on the use of enrichment and cultivation procedures with molecular methods, such as wholecell fluorescent in situ hybridization (FISH) or denaturing gradient gel electrophoresis (DGGE), to monitor the progress of an enrichment, to evaluate the presence of contaminants, and to identify new isolates. Biophysical enrichments make use of such conditions as growth temperature, heat treatment, sonic oscillation, or UV irradiation to kill or inhibit the rest of the population. Biological enrichments may make use of specific hosts for selective growth of a particular organism, or they may take advantage of some pathogenic property, such as invasiveness, which the rest of the population does not possess. Bacteria are usually isolated from enrichment cultures by spatially separating the organisms in or on a solid medium and subsequently allowing them to grow into colonies. The chapter is designed to demonstrate the multiplicity and in many instances the considerable ingenuity of enrichment and isolation methods for bacteria by presenting specific selected examples. The buoyant density of bacteria in pure culture and in samples from natural aquatic environments has been studied by density gradient centrifugation in Percoll gradients, and the average density of a representative bacterium is 1.080 pg µm -3. Isolation of Legionella species, particularly those from the environment, can sometimes be facilitated by acidification of samples to pH 2.2, which kills contaminants more quickly than it does the Legionella species.