Heteroresistance to carbapenems in invasive Pseudomonas aeruginosa infections

Highlights

• Significant increase in carbapenem heteroresistance (CHR) in Pseudomonas aeruginosa between 2011 and 2015 was observed.

• Mechanistic analysis revealed that efflux system overexpression and decreased OprD may have contributed to CHR.

• First study suggesting that biofilm formation may be associated with CHR in P. aeruginosa.

• Previous carbapenem exposure was identified as a common independent risk factor for IPM and MEM heteroresistance.

• Increase in carbapenem prescription during the study period correlated with increasing IPM and MEM heteroresistance.

Abstract

Heteroresistance is common in a variety of microbes, however carbapenem heteroresistance among invasive Pseudomonas aeruginosa infections has not been thoroughly characterised to date. The objective of this study was to investigate the mechanisms, molecular epidemiology and risk factors for invasive carbapenem-heteroresistant P. aeruginosa (CHPA) infections between 2011 and 2015 in Chongqing, China. A significant increase in the rates of heteroresistance to imipenem and meropenem was observed during the study period. Mechanistic analysis revealed that efflux system overexpression and decreased OprD could have contributed to carbapenem heteroresistance in P. aeruginosa. It was also observed that all of the subpopulations produced enhanced levels of biofilm compared with their native strains. Moreover, previous carbapenem exposure was identified as a common independent risk factor for imipenem-heteroresistant (IPM-HR) and meropenem-heteroresistant (MEM-HR) isolates, but patients infected with MEM-HR isolates were at higher risk of poor outcomes than those with IPM-HR isolates. Most importantly, there was a remarkable increase in the prescription of carbapenems during the study period, which was demonstrated to correlate significantly with the quarterly increasing prevalence of IPM-HR and MEM-HR isolates, respectively. These findings show the necessity of routine detection of carbapenem-heteroresistant strains and that strict control of carbapenem use is critical to reduce CHPA infections in hospitalised patients.

Introduction

Pseudomonas aeruginosa, a Gram-negative non-fermenting bacillus, is regarded as one of the major causes of nosocomial infections, with severe morbidity and mortality [1], [2]. Treatment options for P. aeruginosa infection have been challenging because this pathogen is prone to acquire resistance to multiple antibiotics [3]. Carbapenems, primarily imipenem (IPM) and meropenem (MEM), have been widely used to treat serious infections and display strong activity against P. aeruginosa, especially in patients with infections caused by multidrug-resistant P. aeruginosa [4], [5]. However, the emergence of carbapenem heteroresistance may reduce the effectiveness of carbapenem treatment in these patients [6].

Heteroresistance can generally be defined as the presence of subpopulations of bacterial cells with higher levels of antibiotic resistance than those of the rest of the population in the same culture [7]. Heteroresistant isolates are usually identified as susceptible by automated susceptibility assays, which can be explained by the very small proportion of resistant colonies that usually remain undetected, ultimately leading to failure of conventional therapeutics [6]. This phenomenon is different from tolerance, which corresponds to the presence of colonies capable of surviving lethal doses of antimicrobial agents but with susceptibilities are similar to that of the native isolate [8], [9]. Heteroresistance has been identified in almost all Gram-positive and Gram-negative bacteria since it was first proposed in 1947. Currently, heteroresistance to carbapenems has been well documented in clinical isolates of Escherichia coli, Klebsiella pneumoniae and Acinetobacter baumannii [10], [11]. Only two previous studies [12], [13] have reported mechanisms that could be associated with carbapenem-heteroresistant P. aeruginosa (CHPA), namely low expression of the OprD porin or overexpression of efflux systems. However, these studies have not evaluated other mechanisms such as metallo-β-lactamase (MBL) carbapenemases, AmpC cephalosporinase and biofilm formation, that may be involved in carbapenem heteroresistance in P. aeruginosa. Moreover, little is known regarding the incidence of newly acquired CHPA isolates that may more accurately present dynamic changes in the heteroresistance burden in clinical settings. In addition, no studies have investigated the risk factors and clinical outcomes of patients with nosocomial CHPA infections. Therefore, a comprehensive understanding of the mechanisms, molecular epidemiology and risk factors for carbapenem heteroresistance in P. aeruginosa will help to fight against resistant pathogens and develop guidelines to improve patient outcomes.

Accordingly, the objectives of this study were: (i) to describe the prevalence of carbapenem heteroresistance among successive invasive P. aeruginosa isolates collected over 5 years; (ii) to identify the mechanisms and clonal relatedness among these CHPA strains; (iii) to characterise the risk factors and clinical outcomes associated with acquisition of invasive CHPA infections; and (iv) to reveal the probable correlation between the incidence of CHPA isolates and carbapenem consumption during the research period.