Physicochemical Factors in Growth

Abstract

Some physicochemical factors affecting microbial growth are controlled by the constituents of the culture medium (hydrogen ion activity, water activity, osmotic pressure, and viscosity). Others are controlled by the external environment (temperature, oxygen, light, hydrostatic pressure, and magnetic-field strength). This chapter addresses practical aspects of application and control of physicochemical factors. Continuous pH control may be achieved by the automatic addition of acid or base. The effects of hydrostatic pressure on the physiology and metabolism of bacteria are discussed. A number of Pseudomonas and Bacillus species also exhibit a microaerophilic phenotype when growing in low-substrate media. The oxidation-reduction (redox) potential (Eh ) provides a useful scale for measuring the degree of anaerobiosis. Some motile bacteria that possess magnetite- or greigite containing magnetosomes, e.g., Magnetospirillum (formerly Aquaspirillum) magnetotacticum, align with the Earth geomagnetic field, which has strength of about 1 G. As a consequence, they show a biased swimming behavior, and both north-seeking and south-seeking forms are known. This behavior has been termed magnetotaxis and is thought, in part, to help these bacteria (many of which are microaerophilic) orient toward aquatic sediments where dissolved-oxygen concentrations are lower than in surface waters.