Printed in Great Britain Nutritional Requirements of Pasteurella tularensis

The ability of Pasteurella tularensis to grow from small inocula (less than 1 0 ~ 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 favours 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