Table 1. In vivo-adapted B. burgdorferi are highly infectious.

In vivo grown spirochetes		In vitro grown spirochetes
Spirochete dose	Number of mice infecteda/number of mice analyzed	Spirochete dose	Number of mice infecteda/number of mice analyzed
8×102	6/6	5×104	6/6
8×101	6/6	5×103	6/6
8×100	5/6	5×102	2/6
		5×101	1/6
		5×100	0/6
ID50 b	< 8 spirochetes	ID50 b	660 spirochetes

^aMouse infection was determined 3 weeks post inoculation by serological response to B. burgdorferi proteins and reisolation of spirochetes from ear, bladder and joint tissues.

^bThe ID₅₀ was calculated according to method of Reed and Muench [83].