FIG. 4.

Host lethality following intratracheal infection with K. pneumoniae. Seven-day survival experiments following various doses of wild-type and O-antigen-deficient strains were performed as described in Materials and Methods. Results were used to construct logistic models of the probability of survival on any given day at any given dose of either strain. Based on these models, curves of the bacterial dose versus the probability of survival were plotted for days 2 to 7 for each Klebsiella strain. Results indicate an overall reduction of lethality of 30-fold in the O-antigen-deficient mutant strain (P < 0.01). These results were used to establish the 72-hour LD₅₀ for the two strains (wild-type LD₅₀, 2 × 10³ CFU; mutant LD₅₀, 6 × 10⁴ CFU) that were used in subsequent experiments.