TABLE 4.

Frequencies of spontaneous mutation to rifampin or spectinomycin resistance in Chlamydia spp.^a

Strain	Mutation frequency in presence of drug
	Rif (200 ng/ml)	Spc (300 μg/ml)
C. trachomatis L2
Wild type	1.55 × 10−7 ± 1.44 × 10−7	<3.60 × 10−8
L2R1	NAc	<2.40 × 10−8
C. psittaci 6BC
Wild type	7.34 × 10−7 ± 6.23 × 10−7	1.12 × 10−6 ± 1.70 × 10−6
Wild type after three passagesb	1.90 × 10−8 ± 0.7 × 10−8	4.48 × 10−5 ± 0.89 × 10−5
BCR1	NA	2.23 × 10−7 ± 1.59 × 10−7
BC0E1	7.54 × 10−8 ± 3.38 × 10−8	NA

^aThe frequency of spontaneously resistant mutants was calculated as described in Materials and Methods. Data are means ± SD.

^bInfectious particles from a single plaque were expanded during three developmental cycles to obtain a recent clonal population of C. psittaci 6BC.

^cNA, not applicable.