THE EFFECTS OF GROWTH DYNAMICS UPON PH GRADIENT FORMATION WITHIN AND AROUND SUBSURFACE COLONIES OF SALMONELLA-TYPHIMURIUM

pH measurements made in and around submerged colonies of Salmonella ty phimurium grown within a model gelatin gel system using pH-sensitive m icro- and macroelectrodes indicated some pH heterogeneity occurring in and around the bacterial colony. Inoculation density, initial pH and glucose concentration were all found to influence colony diameter and metabolism of Salmonella colonies. Colony growth in the presence of gl ucose, at pH 7.0 with an inoculation density of 1 cell ml(-1) led to a pH fall of 1-2 pH units after 2 d. At pH 5.0, with glucose, colony gr owth rates were much slower than at pH 7.0, and the pH change varied b y less than one pH unit often becoming alkaline. In the absence of glu cose, only small pH changes were observed within the medium, although growth rates were similar to those in glucose-containing media. At the higher inoculation density (ca 1000 cells ml(-1)), isolated pH change s were not observed. Morphological changes, such as the production of annular rings, were noted in stationary phase colonies as was alkali p roduction in colonies. These results are discussed in relation to obse rvations with surface colonies.