An investigation of the occurrence of antibiotic resistance genes among strains of Staphylococcus aureus isolated from animals and their relationship to human strains of Staphylococcus aureus

Robb, Andrew R. (2008). An investigation of the occurrence of antibiotic resistance genes among strains of Staphylococcus aureus isolated from animals and their relationship to human strains of Staphylococcus aureus. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000eb03

Abstract

Data has emerged which indicates that antimicrobial use in animals has created a reservoir of resistant bacteria and resistance genes that have spread to humans.

The aim of this study was to examine the influence of animal antimicrobial use on human strains of S. aureus. Phenotypic and genotypic methods assessed the genetic population structure, potential for host adaptation, frequency of antimicrobial resistance, presence and frequency of genes encoding tetracycline and macrolide resistance and structural variation in tetK genes in S. aureus from animals compared with human clinical strains. In addition, the transferability of tetK resistance plasmids from animal strains to S. aureus 8325-4 was investigated.

DNA based typing exhibited 100% typeability, high levels of discrimination and a high degree of concordance between methods. Multi-locus sequence typing (MLST) identified six sequence types (CC5, CC15, CC22, CC25, CC30 and CC45) common to both isolate collections that represented four of the five major human methicillin resistant Staphylococcus aureus (MRSA) clonal lineages. The relatedness of these clones was further supported by the analysis of 20 different virulence determinants.

Isolates of CC5 exhibited resistance to ciprofloxacin, clindamycin, erythromycin, penicillin, streptomycin, tobramycin and tylosin, CC 15 to penicillin and tetracycline, CC22 to penicillin and rifampicin and CC30 to penicillin. Isolates of CC45 were fully susceptible. Low level biocide resistance was detected but the significance of this was unclear.

The tetK and ermC genes were the predominant tetracycline and macrolide resistance genes. tetK was harboured by animal isolates of CC5 and CC15 and ermC by animal isolates of CC5. Restriction fragment length polymorphism (RFLP) of tetK amplicons produced indistinguishable restriction patterns. High frequency transfer of a tetK plasmid from a chicken S. aureus (CC5) to S. aureus 8325-4 was observed.

These data support the hypothesis that animals represent an important reservoir of antibiotic resistant S. aureus with the ability for strain and antibiotic resistance gene transfer to humans.