TABLE 2.

Pathogenicity of E. coli strains in BALB/c mice^a

E. coli strain	No. of survivors/total no. of mice infected at the following bacterial challenge (CFU/animal):
	1 × 104	5 × 104	1 × 105	5 × 105	1 × 106	5 × 106	1 × 107	5 × 107
H16	5/5	9/10	5/10	0/10	0/5	0/5
H16(pBMS76)	5/5	7/10	7/10	1/10	0/5	0/5
M600 (msbB)			5/5	5/5	10/10	2/10	0/10	0/5
M600(pBMS76)	5/5	9/10	5/10	0/10	0/5	0/5

^aData are compiled from several experiments with groups of 5 or 10 mice 8 to 10 weeks old. Seed cultures of E. coli strains were inoculated from freezer stocks and grown overnight in TSB with antibiotic selection. Inoculum cultures were started with a 1/100 dilution of the seed cultures in 50 ml of TSB with antibiotics. Inoculum cultures were grown to mid-log phase (A₆₆₀, 0.6 to 0.8), chilled on ice for 10 min, and harvested by centrifugation. The cells were washed once in cold PBS and resuspended in immunogen diluent. Cell densities were adjusted based on A₆₆₀ values by use of conversion factors empirically derived from each strain during earlier growth studies. Cell densities were confirmed for each strain by plating dilutions targeted at 100 CFU/plate on TSA, and culture purity was checked by testing the highest-density dilution of each strain. Plate counts were within 10% of targeted CFU per milliliter for each of the strains, except for M600. Accurate density adjustment for strain M600 was hindered by the filamentation of the cells grown at 37°C. Actual CFU per animal for each of two experiments at inocula of 1 × 10⁶, 5 × 10⁶, and 1 × 10⁷ were 1.76 × 10⁶ and 1.24 × 10⁶, 8.8 × 10⁶ and 6.2 × 10⁶, 1.76 × 10⁷ and 1.24 × 10⁷, respectively. Mice were inoculated with a 0.2-ml volume administered by intraperitoneal injection. Animal lethality was monitored for 5 days, although the majority of deaths occurred within the first 48 h.