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Antimicrobial resistance of non-typhoidal Salmonella isolates from egg layer flocks and egg shells

https://doi.org/10.1016/j.ijfoodmicro.2015.02.025Get rights and content

Highlights

  • Study showed a low frequency of AMR among Salmonella enterica isolates.

  • A low frequency of Salmonella isolates harboured AMR genes and a class 1 integron.

  • The most commonly detected AMR genes were blaTEM, tet A and dhfrV.

  • The low frequency of AMR among Non Typhoidal Salmonella isolates represents minimal public health risk.

Abstract

This study was conducted to examine the antimicrobial resistance (AMR) of Salmonella spp. isolated from commercial caged layer flocks in New South Wales and South Australia. All Salmonella isolates (n = 145) were subjected to phenotypic and genotypic characterisation of AMR and carriage of integrons. The majority of Salmonella isolates (91.72%) were susceptible to all antimicrobials tested in this study. Limited resistance was observed to amoxicillin and ampicillin (5.51%), tetracycline (4.13%), cephalothin (2.06%) and trimethoprim (0.68%). None of the isolates were resistant to cefotaxime, ceftiofur, ciprofloxacin, chloramphenicol, gentamycin, neomycin or streptomycin. A low frequency of Salmonella isolates (4.83%) harboured antimicrobial resistance genes and a class 1 integron. The most commonly detected AMR genes among the Salmonella isolates were blaTEM (2.07%), tet A (1.38%) and dhfrV (0.69%). Overall, Salmonella enterica isolates exhibited a low frequency of AMR and represent a minimal public health risk associated with the emergence of multidrug resistant Salmonella spp. from the Australian layer industry.

Introduction

Salmonella spp. is a major cause of foodborne illness worldwide. Consumption of contaminated food products such as pork, meat, egg and egg related products are among the most common sources of Salmonella infection (Hur et al., 2012). In Australia, outbreaks of human salmonellosis are associated with the consumption of contaminated food products containing chicken meat or egg products (OzFoodNet Working Group, 2012). In Australia, a total of 11,992 Salmonella cases were reported in 2010, representing 53.7 cases per 100,000 people which is higher compared to the previous 5 years with an average infection rate of 41.8 cases per 100,000 people (OzFoodNet Working Group, 2012).

Typically, infection with Salmonella is self-limiting producing mild gastroenteritis, however, severe infection occurs common in elderly and immunocompromised individuals (Parry and Threlfall, 2008). Severe, systemic salmonellosis may require treatment with antimicrobials such as fluoroquinolones and extended-spectrum cephalosporins (Parry and Threlfall, 2008). The use of antimicrobial agents in the prevention and treatment of many infectious diseases and as a growth promoter is well known both in veterinary and human medicine (Hur et al., 2012). Indiscriminate use of antibiotics in both animal and human populations has, however, led to an emergence of multidrug resistant Salmonella strains (Anjum et al., 2011). The emergence and dissemination of antibiotic resistance to Salmonella is of significant global concern for both animal and public health. Moreover, the transfer of multidrug resistant Salmonella spp. to humans through food producing animals can compromise the treatment options (Hur et al., 2012).

Compared with many other countries, Australia currently has a very conservative approach for antibiotic usage in commercial egg layer flocks. Antimicrobials, such as fluoroquinolones, are prohibited and ceftiofur is not approved for mass administration in food producing animals (Cheng et al., 2012, Obeng et al., 2012). To date, there is little information available characterising antimicrobial resistance (AMR) in Salmonella isolated from commercial Australian egg layer flocks. In this study, the phenotypic and genotypic AMR was characterised for multiple Salmonella isolates recovered from layer flocks and egg shells.

Section snippets

Bacterial strains and serotyping

A total 145 Salmonella isolates were used in this study. Samples were isolated from 33 commercial caged layer flocks sourced from a total of 13 farms from New South Wales (10 farms) and South Australia (3 farms). All Salmonella isolates used in this study were previously isolated in our laboratory during epidemiological studies (Chousalkar and Roberts, 2012, Gole et al., 2014a, Gole et al., 2014b). Details of Salmonella isolates, sources and their distribution are presented in Table 1. All

Antimicrobial susceptibility screening of Salmonella isolates

The Salmonella isolates selected for this study displayed a low but wide spectrum of antibiotic resistance (Table 2). A total of 91.72% (133/145) of the Salmonella isolates were susceptible to all tested antimicrobials. Overall, resistance was observed to amoxicillin and ampicillin (5.51%), tetracycline (4.13%), cephalothin (2.06%) and trimethoprim (0.68%) (Table 2). Resistance to cefotaxime, ceftiofur, ciprofloxacin, chloramphenicol, gentamycin, neomycin, or streptomycin was not observed for

Discussion

The widespread use of antibiotics both in both human and veterinary medicine is well known, but their imprudent use may give rise to multidrug resistant strains of Salmonella (Anjum et al., 2011). This is the first Australian-based descriptive study characterising the occurrence of phenotypic and genotypic antimicrobial resistance of Salmonella isolates recovered from commercial layer flocks. It is noteworthy that the majority of Salmonella isolates (91.72%) in this study were susceptible to

Competing interests

All authors declare no competing interest.

Acknowledgements

This research was conducted within the Poultry CRC (Grant no 3.3.2), established and supported under the Australian Government's Cooperative Research Centres Program. Mr. Vivek Pande is an International Postgraduate Research Scholarship recipient at University of Adelaide. We would like to acknowledge Nikita Nevrekar for technical help during this research work. We would also like to thank all egg producers who participated in this study.

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      Citation Excerpt :

      Such data on AMR among Salmonella isolates from egg laying flocks are limited. Pande et al. (2015) undertook a localised antimicrobial susceptibility study of Salmonella sp. isolated from commercial layer flocks and eggshells from commercial caged systems and reported 91.7% of isolates to be susceptible to all tested antimicrobials with low levels of resistance observed to ampicillin (5.5%), tetracycline (4.1%), cephalothin (2.1%) and trimethoprim (0.8%). However, this study only investigated commercial layer flocks in caged systems from South Australia and New South Wales, and further investigation is required to understand the prevalence of AMR in all commercial layer systems Australia-wide.

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