4 - Enrichment, isolation and counting of soil microorganisms

Abstract

The chapter discusses enrichment, isolation, and counting of soil microorganisms. A great number of morphological and physiological types of microorganisms can be found in soils. Enrichment and isolation of soil microorganisms are based on the cultivation of these microorganisms in liquid or agar media. To obtain anaerobic growth conditions for facultative anaerobes, it is sufficient to prevent the oxygen diffusion in the growth medium. This can be achieved by slowly passing a high-purity gas over a hot column. If necessary, clean the gas further by passing it over a cold catalyst or Oxisorb. The isolation of strictly anaerobic bacteria by plating methods poses special problems. Cellulose is mainly degraded by fungi in soils. In contrast, most bacteria can only be cultivated on partially hydrolyzed cellulose. Pseudomonaceae are gram-negative bacteria that are found in soil, water, and air. Oligotrophic bacteria include taxonomically and physiologically different microbial groups. Until the early 1970s, the classical concept of aerobic diazotrophs—that is, bacteria able to use molecular nitrogen (N2) as their sole nitrogen source for growth—referred only to bacteria able to grow under atmospheric oxygen concentrations. Soils and water may contain up to 108–1012 bacterial cells per gram of soil or milliliter of water, if determined by microscopic techniques. Since 1970, epifluorescence microscopy has become the major technique for direct enumeration of bacteria and fungi in soil. Cyanobacteria are prokaryotic microorganisms containing chlorophyll a and producing oxygen as a by-product of their photosynthetic activity. Cultures of cyanobacteria can be preserved after growth and should not be maintained in refrigerator-like other prokaryotes.