Combination therapy provides a useful therapeutic approach to overcome resistance until new antibiotics become available. In this study, the pharmacodynamics, including the morphological effects, of polymyxin B (PMB) and meropenem alone and in combination against KPC-producing Klebsiella pneumoniae clinical isolates was examined. Ten clinical isolates were obtained from patients undergoing treatment for mediastinitis. KPCs were identified and MICs were measured using microbroth dilution. Time–kill studies were conducted over 24 h with PMB (0.5–16 mg/L) and meropenem (20–120 mg/L) alone or in combination against an initial inoculum of ca. 106 CFU/mL. Scanning electron microscopy (SEM) was employed to analyse changes in bacterial morphology after treatment, and the log change method was used to quantify the pharmacodynamic effect. All isolates harboured the blaKPC-2 gene and were resistant to meropenem (MICs ≥8 mg/L). Clinically relevant PMB concentrations (0.5, 1.0 and 2.0 mg/L) in combination with meropenem were synergistic against all isolates except BRKP28 (polymyxin- and meropenem-resistant, both MICs >128 mg/L). All PMB and meropenem concentrations in combination were bactericidal against polymyxin-susceptible isolates with meropenem MICs ≤16 mg/L. SEM revealed extensive morphological changes following treatment with PMB in combination with meropenem compared with the changes observed with each individual agent. Additionally, morphological changes decreased with increasing resistance profiles of the isolate, i.e. increasing meropenem MIC. These antimicrobial effects may not only be a summation of the effects due to each antibiotic but also a result of differential action that likely inhibits protective mechanisms in bacteria.

The patient is a 59-year-old woman who was presented to the emergency room in November 2019 with dyspnea. Her past medical history is notable for tobacco use, heavy alcohol use and chronic bronchitis. She had a temperature of 101.2°F, pulse rate of 123 beats per minute, respiratory rate of 22 breaths per minute and oxygen saturation of 83% on room air. Chest X-ray demonstrated a left perihilar infiltrate. Physical examination was notable for decreased breath sounds bilaterally, scattered inspiratory rhonchi and tachycardia. Leukocyte count was 17.1 with 89% neutrophils. Urine Pneumococcal antigen was positive. Urine Legionella antigen was negative. Influenza A/B PCR was negative. She was started empirically on ceftriaxone and azithromycin. Blood cultures were negative. Sputum culture was not collected until the second day of admission as she was unable to expectorate. Methods Approximately two milliliters of expectorated sputum were mixed thoroughly using a sterile swab and was then divided into two equal amounts. This was performed within 3 hours of expectoration. The first was sent to the microbiology laboratory in standard fashion (sterile plastic culture cup) and the second partition was sent in a novel transport/homogenization device. The proprietary device contains a fenestrated internal screen for mechanical homogenization, a chemical biofilm disruption component, and a non-animal based nutritive broth.