Genetic mechanisms of antimicrobial resistance identified in Salmonella enterica, Escherichia coli, and Enteroccocus spp. isolated from U.S. food animals

Jonathan G Frye,Charlene R Jackson

doi:10.3389/fmicb.2013.00135

Abstract

The prevalence of antimicrobial resistance (AR) in bacteria isolated from U.S. food animals has increased over the last several decades as have concerns of AR foodborne zoonotic human infections. Resistance mechanisms identified in U.S. animal isolates of Salmonella enterica included resistance to aminoglycosides (e.g., alleles of aacC, aadA, aadB, ant, aphA, and StrAB), β-lactams (e.g., blaCMY−2, TEM−1, PSE−1), chloramphenicol (e.g., floR, cmlA, cat1, cat2), folate pathway inhibitors (e.g., alleles of sul and dfr), and tetracycline [e.g., alleles of tet(A), (B), (C), (D), (G), and tetR]. In the U.S., multi-drug resistance (MDR) mechanisms in Salmonella animal isolates were associated with integrons, or mobile genetic elements (MGEs) such as IncA/C plasmids which can be transferred among bacteria. It is thought that AR Salmonella originates in food animals and is transmitted through food to humans. However, some AR Salmonella isolated from humans in the U.S. have different AR elements than those isolated from food animals, suggesting a different etiology for some AR human infections. The AR mechanisms identified in isolates from outside the U.S. are also predominantly different. For example the extended spectrum β-lactamases (ESBLs) are found in human and animal isolates globally; however, in the U.S., ESBLs thus far have only been found in human and not food animal isolates. Commensal bacteria in animals including Escherichia coli and Enterococcus spp. may be reservoirs for AR mechanisms. Many of the AR genes and MGEs found in E. coli isolated from U.S. animals are similar to those found in Salmonella. Enterococcus spp. isolated from animals frequently carry MGEs with AR genes, including resistances to aminoglycosides (e.g., alleles of aac, ant, and aph), macrolides [e.g., erm(A), erm(B), and msrC], and tetracyclines [e.g., tet(K), (L), (M), (O), (S)]. Continuing investigations are required to help understand and mitigate the impact of AR bacteria on human and animal health.

Highlights

ANTIMICROBIAL RESISTANCE (AR) IN BACTERIAAntimicrobial compounds have been used to treat bacterial infections since the middle of the twentieth century
Because of the genetic nature of resistance and the ability to select for resistant organisms through the use of antimicrobials, using these compounds in animals was considered a potential source of antimicrobial resistant bacteria which could be transmitted to humans (Aarestrup, 2005; Aarestrup et al, 2008; Shryock and Richwine, 2010)
GENETIC ELEMENTS ASSOCIATED WITH ANTIMICROBIAL RESISTANCE AND MULTI-DRUG RESISTANCE IN U.S Salmonella FOOD ANIMAL ISOLATES antimicrobial resistance (AR) genes are often arranged in cassette-like genetic elements, which may include multiple resistances and be associated with integrons or mobile genetic elements (MGEs) that can facilitate the expression of these genes as well as their movement both within a bacterium’s genome and horizontally between bacteria (Carattoli, 2001, 2003, 2008, 2009; Blake et al, 2003; White et al, 2003; Lindsey et al, 2011a; Douard et al, 2010; Folster et al, 2011; Frye et al, 2011; Glenn et al, 2011, 2012; Johnson et al, 2011b; Lindsey et al, 2009)

Summary

Introduction

ANTIMICROBIAL RESISTANCE (AR) IN BACTERIAAntimicrobial compounds have been used to treat bacterial infections since the middle of the twentieth century. Specific alleles of aminoglycoside resistance genes detected in several studies of NARMS U.S food animal isolates have included aac(3 ), aac(6 ), aadA, aadA1, aadA2, aadA12, aphAI, aph(3 )-Ii-iv, strA, and strB (Frye and Fedorka-Cray, 2007; Frye et al, 2008, 2011; Glenn et al, 2011; Lindsey et al, 2011a).

Results

Conclusion