Preincubation of pneumococci with β-Lactams alone or combined with livofloxacin prevents quinolone-induced resistance without increasing intracellular levels of levofloxacin

Preincubation of pneumococci with sub-MIC concentrations of ceftriaxone (1/16 x MIC), cefotaxime (1/8 x MIC), and meropenem (1/4 x MIC) alone or combined with levofloxacin (1/8 x MIC) over 6 h Prevents the emergence of levofloxacin-resistant mutants after 96 h of incubation but does not affect the intracellular accumulation of levofloxacin in two penicillin-resistant pneumococcal strains, suggesting a link between the mechanism of action of beta-lactams and the emergence of quinolone-induced resistance in pneumococci.