Cefotaxime Acts Synergistically with Levofloxacin in Experimental Meningitis Due to Penicillin-Resistant Pneumococci and Prevents Selection of Levofloxacin-Resistant Mutants In Vitro

Abstract

Cefotaxime, given in two doses (each 100 mg/kg of body weight), produced a good bactericidal activity (؊0.47 ⌬log 10 CFU/ml ⅐ h) which was comparable to that of levofloxacin (؊0.49 ⌬log 10 CFU/ml ⅐ h) against a penicillin-resistant pneumococcal strain WB4 in experimental meningitis. Cefotaxime combined with levofloxacin acted synergistically (؊1.04 ⌬log 10 CFU/ml ⅐ h). Synergy between cefotaxime and levofloxacin was also demonstrated in vitro in time killing assays and with the checkerboard method for two penicillin-resistant strains (WB4 and KR4). Using in vitro cycling experiments, the addition of cefotaxime in sub-MIC concentrations (one-eighth of the MIC) drastically reduced levofloxacin-induced resistance in the same two strains (64-fold increase of the MIC of levofloxacin after 12 cycles versus 2-fold increase of the MIC of levofloxacin combined with cefotaxime). Mutations detected in the genes encoding topoisomerase IV (parC and parE) and gyrase (gyrA and gyrB) confirmed the levofloxacin-induced resistance in both strains. Addition of cefotaxime in low doses was able to suppress levofloxacin-induced resistance.One of the most challenging problems remains the permanent worldwide increase of penicillin-resistant pneumococcal strains. During the last years, resistance rates in the United States reached 51%, with 33% of the strains showing intermediate resistance (12). In some cases, additional resistance to cephalosporins has further reduced the therapeutic options for infections with penicillin-resistant strains. Furthermore, pneumococcal strains resistant to quinolones have already been isolated (2). Even more alarming is the report of treatment failure with quinolones due to the emergence of resistance during treatment (7). Until now, ␤-lactam antibiotics remain the drugs of choice for pneumococcal diseases, except when their penetration into infected tissues is limited, as is the case in meningitis. Based on actual guidelines, a combination of a cephalosporin with vancomycin is recommended for the empirical treatment of meningitis, especially when cephalosporinresistant strains are involved (13). However, the occurrence of recently isolated vancomycin-and cephalosporin-tolerant strains might lead to treatment failures and jeopardize the utility of this antibiotic regimen (16). A highly bactericidal antibiotic combination which does not lead to the emergence of resistance would represent a major advantage. For several years, cefotaxime and levofloxacin have been well-established monotherapies for pneumococcal diseases (20). In this work we have studied the potential synergy between cefotaxime and levofloxacin in vitro and in experimental meningitis and the effect of cefotaxime on levofloxacin-induced resistance in vitro. MATERIALS AND METHODSStrains and MIC determination. The two pneumococcal strains (Streptococcus pneumoniae WB4 and KR4) were originally isolated from two patients with pneumonia at the University Hospital of Bern, Bern, Switzerland. The MICs for WB4 were as follows: penicillin, 4 mg/liter...

Preincubation of Pneumococci with β-Lactams Alone or Combined with Levofloxacin Prevents Quinolone-Induced Resistance without Increasing Intracellular Levels of Levofloxacin

“…10 5 to 10 6 CFU/ml, as encountered in experimental meningitis). A pronounced synergy between the two antibiotic classes could be demonstrated in this experimental setting in vitro and in vivo (2,6,8). However, with higher bacterial inocula (ca.…”

“…At the end of the incubation period, the optical density, measured at 590 nm, ranged between 0.3 and 0.5. The antibiotic doses were identical to those used in previous works, preventing the emergence of levofloxacin-induced resistance (2,6,8). Control cultures did not contain any antibiotics.…”

“…A bactericidal antipneumococcal regimen that does not lead to the emergence of resistance would be a major progress. We have recently shown that cephalosporins (i.e., ceftriaxone and cefotaxime) and a carbapenem (i.e., meropenem) act synergistically with levofloxacin in vitro and in experimental meningitis against penicillin-resistant pneumococci (2,6,8). In addition, these combination regimens are able to prevent the emergence of levofloxacin-induced resistance in vitro in pneumococci.…”

“…In a last set of experiments, the potential synergy between levofloxacin and ␤-lactam antibiotics was investigated in vitro against the pneumococcal strain WB4, using high bacterial inocula (10 8 to 10 9 CFU/ml) similar to those used during previous experiments for the selection of quinolone-resistant mu- (2,6,8). In time-kill experiments over 12 h, the addition of ␤-lactam antibiotics in sub-MIC concentrations did not significantly increase the killing rates (less than 1 log 10 CFU/ml) compared to levofloxacin monotherapy (data not shown).…”

“…Antibiotics were added in sub-MIC concentrations, as used in previous cycling experiments in vitro (2,6,8) (1/16ϫ MIC for ceftriaxone, 1/8ϫ MIC for cefotaxime, and 1/4ϫ MIC for meropenem), and levofloxacin in concentrations corresponding to the MIC. Bacterial titers were determined at 0, 2, 4, 6, 8, 10, and 12 h by serial dilution of samples, plated on agar plates containing 5% sheep blood and incubated at 37°C for 24 h. Experiments were performed in triplicate, and results are expressed as means Ϯ standard deviations.…”