Colistin Resistance in Monophasic Isolates of Salmonella enterica ST34 Collected From Meat-Derived Products in Spain, With or Without CMY-2 Co-production.

Xenia Vázquez,Vanesa García,Javier Fernández,Víctor Ladero,Rosaura Rodicio,M Rosario Rodicio,Margarita Bances,María De Toro

doi:10.3389/fmicb.2021.735364

Abstract

Colistin is a last-resort antibiotic in fighting severe infections caused by multidrug resistant Gram negative pathogens in hospitals. Zoonotic bacteria acquire colistin resistance in animal reservoirs and mediate its spread along the food chain. This is the case of non-typhoid serovars of Salmonella enterica. Colistin-resistant S. enterica in foods represents a threat to human health. Here, we assessed the prevalence of colistin-resistance in food-borne isolates of S. enterica (2014–2019; Asturias, Spain), and established the genetic basis and transferability of this resistance. Five out of 231 isolates tested (2.2%) were resistant to colistin. Four of them, belonging to the European monophasic ST34 clone of S. Typhimurium, were characterized in the present study. They were collected from pork or pork and beef meat-derived products, either in 2015 (three isolates) or 2019 (one isolate). Molecular typing with XbaI-PFGE and plasmid profiling revealed distinct patterns for each isolate, even though two of the 2015 isolates derived from the same sample. The MICs of colistin ranged from 8 to 16 mg/L. All isolates carried the mcr-1.1 gene located on conjugative plasmids of the incompatibility groups IncX4 (2015 isolates) or IncHI2 (2019 isolate). Apart from colistin resistance, the four isolates carried chromosomal genes conferring resistance to ampicillin, streptomycin, sulfonamides and tetracycline [blaTEM–1, strA-strB, sul2, and tet(B)] and heavy metals, including copper and silver (silESRCFBAGP and pcoGE1ABCDRSE2), arsenic (arsRSD2A2BCA1D1) ± mercury (merEDACPTR), which are characteristically associated with the European ST34 monophasic clone. The 2019 isolate was also resistant to other antibiotics, comprising third generation cephalosporins and cephamycins. The latter phenotype was conferred by the blaCMY–2 gene located on an IncI1-I(α)-ST2 plasmid. Results in the present study identified meat-derived products as a reservoir of a highly successful clone harboring transferable plasmids which confer resistance to colistin and other clinically important antibiotics. An important reduction in the number of food-borne S. enterica detected during the period of the study, together with the low frequency of colistin resistance, underlines the success of One Health initiatives, such as those implemented at the UE, to control zoonotic bacteria along the food chain and to halt the spread of antimicrobial resistance.

Highlights

Non-typhoidal serovars of Salmonella enterica subsp. enterica are one of the leading causes of bacterial food-borne infections in humans and animals worldwide (Majowicz et al, 2010)
During the 2014 to 2019 period, 231 isolates of S. enterica were recovered from food samples in Asturias
The number of isolates decreased considerably along time, from nearly 96 in 2014 down to 4– 10 since 2017, a favorable trend that probably correlates with implementation in Spain of the integrated European Union (EU) legislation aimed to monitor and control Salmonella along the food chain (European Commission, 2005; Hugas and Beloeil, 2014)

Summary

Introduction

Non-typhoidal serovars of Salmonella enterica subsp. enterica are one of the leading causes of bacterial food-borne infections in humans and animals worldwide (Majowicz et al, 2010). Since around 2005, a monophasic variant of this serovar with the antigenic formula 4,[5],12:i:- and sequence type (ST) 34 has emerged in Europe, originally in pigs and later in other domestic animals (EFSA, 2010; Hauser et al, 2010; Hopkins et al, 2010; Sun et al, 2020) This “European monophasic clone” has experienced a global expansion, being responsible for multiple outbreaks and sporadic cases of human salmonellosis in America, Asia, and Australia (Soyer et al, 2009; Mulvey et al, 2011, 2013; Arai et al, 2018; Elnekave et al, 2018; Sun et al, 2020). The European clone harbors two chromosomal genetic elements, namely the RR region and SGI-4, both involved in resistance Isolates of this clone are typically resistant to ampicillin, streptomycin, sulfonamides and tetracycline (tetraresistance pattern ASSuT) with the responsible genes, blaTEM−1, strA-strB, sul, and tet(B), located on RR that contains a mercury resistance locus. It has been proposed that the use of heavy metals as growth promoters in pork production, after the ban of antibiotics with such an aim in European Union (EU), could have contributed to the epidemiological success of the clone (Mastrorilli et al, 2018; Clark et al, 2020)

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