Reduced susceptibility to tetracyclines is associated in vitro with the presence of 16S rRNA mutations in Mycoplasma hominis and Mycoplasma pneumoniae

Extract

Sir,

Mycoplasma pneumoniae and Mycoplasma hominis are aetiological agents of respiratory and genitourinary tract infections, respectively, for which tetracyclines present potential for empirical treatment.¹ As mycoplasmas possess a small number of rrn operons, one for M. pneumoniae and two for M. hominis, the target-related mechanism of resistance to tetracyclines caused by 16S rRNA mutations could be expected as previously described for Helicobacter pylori.² The purpose of this study was to identify such a mechanism in the reference strains M. hominis PG21 and M. pneumoniae FH by selecting in vitro for tetracycline-resistant mutants and sequencing the 16S rRNA genes of the obtained mutants.

Growth conditions and antibiotic susceptibility testing of the mycoplasma strains have been described previously.³ Two selection methods, with either broth or agar medium, were used for M. hominis PG21 (ATCC 23114), although only the broth-based selection was done for M. pneumoniae FH (ATCC 15531).⁴ Broth-selected mutants were obtained by serial transfers of M. hominis PG21 and M. pneumoniae FH in appropriate Hayflick-modified broth medium containing increasing subinhibitory concentrations of doxycycline. Stepwise selection of doxycycline-resistant mutants was performed onto Hayflick-modified agar medium containing increasing inhibitory concentrations of doxycycline, as described previously.⁴ Two steps were performed with doxycycline concentrations at 2× and 8× MIC for the respective parent strain.