Some hypotheses on the aetiology of fatal infections in partially resistant hosts and their application to mice challenged with Salmonella paratyphi-B or Salmonella typhimurium by intraperitoneal injection

SUMMARY: Some hypotheses are considered which describe the aetiology of a fatal infection in a partially resistant host; i.e. a host which does not invariably die after inoculation with one bacterium. The hypothesis of independent action postulates that the mean probability per inoculated bacterium of multiplying to cause (or help to cause) a fatal infection is independent of the number of bacteria inoculated and, for a partially resistant host, is less than unity ( 1> p> 0). It predicts: (1) that the slope, b, of the probit-mortality/log-dose curve will be 2.0 or less a t the LD 50 point; (2) that, while hosts dying after inoculation with many LD 50 die as a result of the multiplica-tion of many of the inoculated bacteria, most of those dying from 1 LD 50 or less do so following the multiplication of only one of the inoculated bacteria, regardless of the total number of bacteria inoculated. When a mixture of several equally virulent, distinguishable variants of a given pathogen are inoculated, fatal infections caused by the growth of one bacterium should result in the predominance of only one variant a t post mortem. The hypotheses of maximum and of partial synergism postulate that inoculated bacteria co-operate so that the value of p increases as the size of the dos