The Influence of Extracellular Products on the Behaviour of Mixed Microbial Populations in Magnesium-limited Chemostat Cultures

Abstract

SUMMARYMagnesium-limited mixed cultures, each containing two microbial species, were produced by exchange of small numbers of organisms between magnesium-limited chemostat cultures of Aerobacter aerogenes, Pseudomonas Jluorescens, Bacillus subtilis, B. megaterium, Staphylococcus epidermis and Torula utih. The fate of each species in these mixed cultures was determined following cross-inoculation. Gram-negative organisms invariably outgrew the Gram-positive ones, but the ability of B. subtilis and B. megaterium to outgrow each other, or to outgrow the yeast, depended on the inoculum size. This dependence resulted from the presence of specific extracellular products in the Bacillus cultures which stimulated their growth and uptake of magnesium. The concentration of these extracellular growth-promoting substance(s) in magnesium-limited cultures of B. subtilis varied with population density. Thus, when the magnesium concentration in the medium was decreased from 0.9 to 0.15 pg./ml. (thereby causing the steady-state population density to be decreased to one-eightieth the initial value), B. subtilis could no longer maintain itself against proportionately low concentrations of T. utilis. But addition of extracellular fluids from dense magnesium-limited B. subtilis cultures to magnesium-limited 2". utilis cultures enabled small numbers of the Bacillus now to outgrow the yeast. From these two observations it is concluded that magnesium assimilation by B. subtilis is more dependent on extracellular substance(s) than is magnesium assimilation by either T. utilis or the Gram-negative organisms. Modification of the basic theory of microbial growth in a chemostat culture, to take account of product-stimulated substrate-assimilation, is suggested.

Physiological Responses to Nutrient Limitation

“…There i.s evidence that magnesium-limited Bacilllus subtilis cells excrete compounds that bind magnesium, thereby improving their ability to compete with other organisms for growth-limiting amounts of magnesium (82). Similarly, cell walls of magnesium-limited cells of this organism showed an increased Mg 2+ binding affinity over magnesium-sufficient cells (83).…”

Complex dynamics of microbial competition in the gradostat

“…The main conclusion of these studies is that, in such a system, coexistence of the two species in the reactor is not practically possible, but one of them will eventually dominate and the other will be washed out. This has been demonstrated experimentally (Tempest et al, 1967;Jannasch, 1967Jannasch, , 1968Meers and Tempest, 1968;Harder and Veldkamp, 1971;Meers, 1971;Megee et al, 1972;Jost et al, 1973;Hansen and Hubbell, 1980;Takeya et al, 2004), while analysis of the model equations of the system shows that coexistence is theoretically possible at discrete values of the dilution rate, only when the specific growth rate curves of the two species cross at a positive value of the substrate concentration (Powell, 1958;Aris and Humphrey, 1977;Hsu et al, 1977;Hsu, 1978;Butler and Wolkowicz, 1985;Wolkowicz and Zhiqi, 1992;Lenas and Pavlou, 1994;Li, 1999). However, at these values of the dilution rate the system is structurally unstable (Powell, 1988) and random fluctuation in the operating conditions will eventually lead to extinction one of the two species .…”

Monoclonal antibodies as probes for detecting lipopolysaccharide expression on Escherichia coli from different growth conditions

“…Magnesium is important in maintaining the stability of the structural arrangement of larger molecules such as LPS within the outer membrane of Gram-negative bacteria (Costerton et al, 1974). Cell walls of magnesium-limited cells of Bacillus subtilis show an increased Mg2+-binding affinity over magnesiumsufficient cells (Meers & Tempest, 1968). Thus, certain organisms may respond to magnesium limitation by improving their ability to bind the ions by increasing negatively charged cell surface components such as LPS.…”