Molecular epidemiology of nitrofurantoin-resistant Escherichia coli from human and animals

Abstract

Nitrofurantoin is recommended as an empirical therapy for uncomplicated urinary tract infections (UTI). In Hong Kong, it is a preferred regimen for UTIs caused by ESBL-producing E. coli. Though resistance to nitrofurantoin has remained low since its advent in 1952, it is crucial to protect its efficacy through studying the resistance mechanism and epidemiology. The main resistance mechanism of nitrofurantoin involves sequential mutations in type I nitroreductases nfsA and nfsB which result in progressive loss of nitroreductase activities and thus increase the nitrofurantoin resistance. Later, a plasmid named pOLA52 (accession numberEU370913) was described as conferring resistance against nitrofurantoin. The plasmid harbored as multidrug efflux pump, OqxAB. However, its role on nitrofurantoin resistance has not been elucidated. The first objective of this study was to investigate the effect of oqxAB on nitrofurantoin susceptibility. Nitrofurantoin MICs and MPCs were compared between E. coli strains J53/pLOW2 and J53/pLOW2::oqxAB. J53/pLOW2::oqxAB had four-fold higher MIC (32 μg/ml) and two-fold higher MPC (256 μg/ml) than J53/pLOW2, suggesting oqxAB confers resistance to nitrofurantoin and facilitates the selection of strains with high-level resistance to nitrofurantoin. However, the interaction between the presence of oqxAB and mutations in nfsA and nfsB was inconclusive due to the experiment design. The second objective of the study was to examine the prevalence of nitrofurantoin resistance in E. coli from patients and animals in Hong Kong. A total of 644 fecal isolates were collected from food animals during 2012-2013 and 1,853 urinary isolates from patients collected in three periods (2004-2005, 2006-2008, 2013). Nitrofurantoin resistance rates of isolates were 5.6% (103/1,853) and 12% (77/644) in patients and food animals, respectively. Nitrofurantoin-resistant isolates were more likely to be multidrug resistant compared to nitrofurantoin-susceptible isolates (P< 0.001). Forty percent (72/180) of the nitrofurantoin-resistant isolates were positive for oqxAB, which was significantly higher than 0.6% (1/161) in the nitrofurantoin-susceptible isolates (P < 0.001). The higher prevalence of multidrug pump OqxAB in nitrofurantoin-resistant isolates may contribute to the higher prevalence of multidrug-resistant traits in nitrofurantoin-resistant E. coli. Next, we further investigated the genetic environment of oqxAB and plasmids carrying them in all oqxAB-carrying isolates. OqxAB in all but three isolates were flanked by IS26on both sides, suggesting that IS26mobilizesoqxABbetween plasmids of different sizes and replicon types. Eighteen of 73plasmids encoding OqxAB are conjugative. OqxAB plasmids with size ranging from 50-200kb, mainly belonged to Incompatibility group F. Prevalence of these oqxAB-carrying plasmids may pose a threat to human health since the substrate profile of the multidrug efflux pump encoded includes antibiotics from various drug classes that are important in combating infections by different pathogens. Selection pressure of an antibiotic from one drug class would result in a multidrug-resistant trait.