In VitroActivity of Polymyxin B in Combination with Various Antibiotics against Extensively Drug-Resistant Enterobacter cloacae with Decreased Susceptibility to Polymyxin B

Abstract

Against extensively drug-resistant (XDR) Enterobacter cloacae, combination antibiotic therapy may be the only option. We investigated the activity of various antibiotics in combination with polymyxin B using time-kill studies (TKS). TKS were conducted with four nonclonal XDR E. cloacae isolates with 5 log 10 CFU/ml bacteria against maximum, clinically achievable concentrations of polymyxin B alone and in two-drug combinations with 10 different antibiotics. A hollow-fiber infection model (HFIM) simulating clinically relevant polymyxin B and tigecycline dosing regimens was conducted for two isolates over 240 h. Emergence of resistance was quantified using antibiotic-containing (3؋ MIC) media. Biofitness and stability of resistant phenotypes were determined. All XDR E. cloacae isolates were resistant to all antibiotics except for polymyxin B (polymyxin B MIC, 1 to 4 mg/liter). All isolates harbored metallo-␤-lactamases (two with NDM-1, two with IMP-1). In single TKS, all antibiotics alone demonstrated regrowth at 24 h, except amikacin against two strains and polymyxin B and meropenem against one strain each. In combination TKS, only polymyxin B plus tigecycline was bactericidal against all four XDR E. cloacae isolates at 24 h. In HFIM, tigecycline and polymyxin B alone did not exhibit any killing activity. Bactericidal kill was observed at 24 h for both isolates for polymyxin B plus tigecycline; killing was sustained for one isolate but regrowth was observed for the second. Phenotypically stable resistant mutants with reduced in vitro growth rates were observed. Polymyxin B plus tigecycline is a promising combination against XDR E. cloacae. However, prolonged and indiscriminate use can result in resistance emergence.

Klebsiella pneumoniae in Singapore: Hypervirulent Infections and the Carbapenemase Threat

“…Synergy testing is typically not performed with the exception of research settings. The use of hollow-fiber-infection-model (HFIM) for synergy testing has been reported for testing antibiotic combinations for synergy (Lim et al, 2015 ; Cai et al, 2016b ). The HFIM is used to simulate the pharmacokinetics and pharmacodynamics of the tested drugs singly and in combination with time-kill studies.…”

The in vitro activity of polymyxin B and tigecycline alone and combination with other antibiotics against carbapenem-resistant Enterobacter cloacae complex isolates, including high-risk clones

“…However, due to the lack of new Ab development, PMBs continued to be used in clinical practice, even with the pharmacokinetic/pharmacodynamic limitations and increased risk of severe renal insufficiency. It was found that PMBs might promote membrane transport of other Abs, thereby enhancing bactericidal or bacteriostatic activities (2,3). The broad-spectrum antimicrobial agent, tigecycline (TGC), was also active against most Gram-negative isolates in vitro, including ESBL producers.…”

Antimicrobial Synergy Testing by the Inkjet Printer-assisted Automated Checkerboard Array and the Manual Time-kill Method

“…If the actual number of colonies in the undiluted drop in a series is lower than the number of colonies in subsequent dilutions, this suggests antibiotic carryover effect. Different options have been described for preventing this effect, including spreading a single aliquot over an entire plate 38 or spinning down the sample, removing the supernatant, and re-suspending in sterile saline prior to plating 39 . At each time point in the time-kill method, it is also critical for the investigator to efficiently but accurately remove an aliquot from each culture tube and perform serial dilutions.…”