Batch growth of Salmonella typhimurium LT2: stoichiometry and factors leading to cessation of growth.

Salmonella typhimurium LT2 was grown in batch culture (trypticase soy broth, with 0.3%(w/v) yeast extract, 1% (w/v) glucose and 0.5% (w/v/) NaCl, 20 degrees C) at a range of initial pH (4.4, 4.8, 5.0 and 7.0). The consumption of oxygen and glucose was found to be independent of initial pH, and stoichiometric with growth. Mean yield coefficients of 6.9 x 10(-15) and 15.5 x 10(-15) mol oxygen/cell were estimated. Calculation of the instantaneous state of carbon during the cultivation showed stoichiometric conversion of glucose into biomass, carbon dioxide and organic acids. The concentration of the undissociated form of the primary acidic product (acetic acid) was shown to be the factor limiting growth.

Growth of food-borne pathogenic bacteria in oil-in-water emulsions: I--Methods for investigating the form of growth.

Methods are presented for investigating the site and form of growth of bacteria in model oil-in-water emulsions and in dairy cream. Following growth of the bacteria, the continuous aqueous phase is gelled using agarose and the oil phase removed using a mixture of chloroform and methanol. Using this method, the authors have found that Listeria monocytogenes, Salmonella typhimurium and Yersinia enterocolitica grow in the form of colonies in concentrated oil-in-water emulsions. Colonies of L. monocytogenes and Y. enterocolitica also form in artificially-inoculated fresh and tinned dairy cream. If information about the precise site of growth is not required, the authors have discovered that intact colonies can be liberated from the model emulsions by dissolving away the oil phase with chloroform:methanol.

Growth of food-borne pathogenic bacteria in oil-in-water emulsions: II--Effect of emulsion structure on growth parameters and form of growth.

The growth rates and yields of Listeria monocytogenes and Yersinia enterocolitica were determined in liquid culture media, and in model oil-in-water emulsions that contained 30, 70 or 83% (v/v) hexadecane. In emulsions with a mean droplet size of 2 microns containing 83% (v/v) hexadecane, the growth of both organisms resulted in decreased yields. Additionally, in these emulsions adjusted to pH 5.0 or 4.4 the growth rate of L. monocytogenes was significantly less than in other model systems which had an aqueous phase of equivalent chemical composition. Microscopic examination of the 83% (v/v) emulsion showed that its microstructure immobilized the bacteria, which were constrained to grow as colonies. Bacteria behaved similarly in model emulsions of either hexadecane or sunflower oil. Manipulation of the droplet size distribution of the emulsions changed the form and rate of growth of bacteria within them.

Yeast flocculation: a dynamic equilibrium.

The steady state in yeast flocculation is a dynamic equilibrium between flocculated and dispersed yeast cells. The free cell concentration is directly proportional to the total cell concentration and may be expressed as an equilibrium constant. Increased agitation decreases floc size and equilibrium constant whilst increasing floc-surface area and free-cell concentration. Values of equilibrium constant are influenced by agitation in a complex relationship probably involving the floc-surface area and floc momentum. Inhibition of flocculation by mannose and low pH is reversible and becomes greater with increased agitation. Both these inhibitions appear consistent with a weakening of flocculent bond strength by these inhibitors.