Intraperitoneal infections of mice. Survival curves were determined for mice infected with parental YPIII/pIB102, mutants YPIII/pIB10215 (V12 [Δ2-4]) and YPIII/pIB10217 (V14 [Δ8-10]), or the virulence plasmid-cured strain YPIII. Mice were infected intraperitoneally with successive 10-fold dilutions (104 to 107 bacteria/ml) of a bacterial suspension and monitored at least twice daily for 13 days. The exact doses for each strain were determined by viable counts, and the dose for YPIII was 2.1 × 107, that for YPIII/pIB102 was 3.1 × 107 to 3.1 × 104, that for YPIII/pIB10215 was 3.25 × 107 to 3.25 × 105, and that for YPIII/pIB10217 was 2.4 × 106 to 2.4 × 104.