Effect of efflux on telithromycin and macrolide susceptibility in Haemophilus influenzae

This study investigated the presence of telithromycin and azithromycin efflux in 58 clinical strains of Haemophilus influenzae with various susceptibilities to macrolides, azalides, and ketolides. Efflux pumps were studied by measuring accumulation of radioactive [3H]telithromycin and [N-methyl-3H]azithromycin in the presence and absence of carbonyl m-chlorophenylhydrazone (CCCP), a protonophore. In 17 strains for which the telithromycin MICs were 0.06 to 0.5 microg/ml (azithromycin MICs, < or = 0.06 to 0.125 microg/ml; clarithromycin MICs, < or = 0.06 to 2 microg/ml), telithromycin and azithromycin accumulations were high without CCCP and not affected by its addition, which indicates absence of efflux. In 22 strains for which the telithromycin MICs were 0.25 to 4 microg/ml (azithromycin MICs, 0.25 to 1 microg/ml; clarithromycin MICs, 1 to 8 microg/ml), initially low levels of telithromycin accumulation became higher after addition of CCCP, indicating a functioning efflux pump. Nineteen strains for which the telithromycin MICs were > or = 2 microg/ml had efflux as well as various mutations in ribosomal proteins L4, L22, and/or 23S rRNA (domains II and V). Of these 19 strains, the telithromycin MICs (> or = 8 microg/ml) for 17 of them were significantly raised (azithromycin, MICs 4 to >32 microg/ml; clarithromycin MICs, 8 to >32 microg/ml). From these results we conclude that telithromycin efflux with or without additional ribosomal alterations is present in all H. influenzae strains, except for those for which the telithromycin MICs were very low.