Nutritional Requirements of Pasteurella tularensis for Growth from Small Inocula

Abstract

The ability of P. tularensis to grow from small inocula (less than 105 organisms) was found to be critically dependent upon the supply of materials produced by this organism (growth-initiating substance, GIS). In contrast, large inocula showed no requirement for added GIS. The growth-promoting activity of GIS was enhanced in the presence of blood. The evidence favors the conclusion that the inoculum-dependent growth characteristics of P. tularensis are attributable to a nutritional heterogeneity of the bacterial population. According to this interpretation, GIS is produced by a relatively small number of bacteria in the population and is excreted by them into the surrounding culture fluid. The excreted material enables growth of the GIS-requiring organisms which constitute a major proportion of the total population. This commensal relationship appears to account for the characteristic all-or-none type of growth response elicited by GIS.