Bactericidal activity and Inhibition of levofloxacin-induced resistance by antibacterial combination against hypervirulent Klebsiella pneumoniae.

Background: Hypervirulent Klebsiella pneumoniae (hvKP) infection is characterized by its potential severity and metastatic propensity, underlying the importance to employ bactericidal treatment promptly for infection source control. The aim of this study is to investigate the bactericidal activity of antibacterial combination against hvKP, with a particular focus on whether levofloxacin-based combinations can suppress levofloxacin-induced efflux pump related drug resistance.

Methods: We conducted in vitro antimicrobial susceptibility tests on 12 hvKP clinical isolates and the reference strain NTUH-K2044. Checkerboard assays were conducted to evaluate the combination activity of levofloxacin with amikacin or ceftazidime. Time-kill experiments were performed to determining the bactericidal activities of levofloxacin, amikacin, ceftazidime, and the levofloxacin-based combinations against NTUH-K2044. Additionally, the mechanisms underlying induced quinolone resistance were investigated.

Results: Twelve clinical hvKP isolates were highly susceptible to levofloxacin, amikacin, and ceftazidime. The levofloxacin-amikacin and levofloxacin-ceftazidime combinations both demonstrated antibacterial activities as indifference. Time-kill experiments showed that an eight-fold or lower minimal inhibitory concentration (≤ 8×MIC) of levofloxacin, ≤ 1×MIC of amikacin and of ceftazidime, exhibited bactericidal activity against NTUH-K2044 during the initial 1-4 h with various levels of bacterial counts decreasing followed by regrowth. The regrowth samples had high levofloxacin MICs indicating the occurrence of induced resistance. Real-time PCR and wild-type oqxR complementation demonstrated point mutations in oqxR of NTUH-K2044, resulting in an overexpression of the efflux pump encoding gene of oqxAB. Both the levofloxacin-amikacin and levofloxacin-ceftazidime combinations exhibited bactericidal synergy without bacterial regrowth when they were tested with 2×MIC of levofloxacin combined with either 1/2×MIC of amikacin or 1×MIC of ceftazidime.

Conclusions: These results demonstrated that levofloxacin combined with amikacin or ceftazidime exhibited superior bactericidal activities compared to levofloxacin against hvKP and might prevent levofloxacin-induced resistance related to oqxAB overexpression.