TABLE 1.

ΔcheD mutant cells are able to establish infection in C3H/HeN mice, but infectivity is considerably reduced^a

Strain	Dose (CFU)	No. of mice infected/no. tested	ID50
WT	7.5 × 102	0/7	1.8080 × 104
	7.5 × 103	1/7
	7.5 × 104	5/7
	9.3 × 105	7/7
ΔcheD	6.2 × 102	0/7	1.5694 × 105
	6.2 × 103	1/7
	6.2 × 104	1/7
	1.00 × 106	6/7
	1.00 × 107	7/7
cheDcom	5.6 × 102	0/7	1.9483 × 104
	5.6 × 103	2/7
	5.6 × 104	5/7
	8.2 × 105	7/7

^aMice were injected with the indicated B. burgdorferi clones and doses. Infectivity was determined by reisolation of B. burgdorferi from the tissues of euthanized mice (ear skin, ankle joint, and urinary bladder). ID₅₀ and statistical analyses were performed using probit regression, as described in Materials and Methods. The ΔcheD mutant spirochetes were significantly attenuated compared to the ID₅₀ of the wild type (P = 0.0125) or the complemented B. burgdorferi (P = 0.0005) (significance was defined as a P value of ≤0.05).