TABLE 2.

Effect of rifampin and spectinomycin on chlamydial plaque formation

Antibiotic	Drug concn (ng/ml [Rif] or μg/ml [Spc])	C. psittaci 6BC		C. trachomatis L2
		Input PFUa	No. of plaquesb	Input PFUa	No. of plaquesb
Rif	2	NT	NA	2.4 × 103	Too many to count
	5	NT	NA	2.4 × 105	0
	10	4.8 × 104	0	2.2 × 105	0
	25	4.8 × 105	0	2.2 × 106	0
	50	2.1 × 107	42	2.8 × 107	2
	100	108	50	2.2 × 107	1
	200	108	23	6.4 × 107	2*
Spc	20	1.1 × 105	0	2.4 × 103	Too many to count
	40	2.9 × 105	0	2.4 × 105	0
	100	2.0 × 106	0	1.9 × 106	0
	200	2.1 × 107	13	2.3 × 107	0
	300	108	81	4.6 × 107	0*

^aThe largest inoculum size tested that was not cytotoxic.

^bThe average number of plaques (minimum of three independent experiments). NT, not tested; NA, not applicable; *, cell monolayer was unhealthy due to the large concentration of infectious particles.