Abstract
BackgroundIntegrons are mobile DNA elements and they have an important role in acquisition and dissemination of antimicrobial resistance genes. However, there are limited data available on integrons of Staphylococcus aureus (S. aureus) from bovine mastitis, especially from Chinese dairy cows. To address this knowledge gap, bovine mastitis-inducing S. aureus isolates were investigated for the presence of integrons as well as characterization of gene cassettes. Integrons were detected using PCR reactions and then further characterized by a restriction fragment-length polymorphism analysis and amplicon sequencing.ResultsAll 121 S. aureus isolates carried the class 1 integrase gene intI1, with no intI2 and intI3 genes detected. One hundred and three isolates were positive for the presence of 12 resistance genes, either alone or in combination with other gene cassettes. These resistance genes encoded resistance to trimethoprim (dhfrV, dfrA1, dfrA12), aminoglycosides (aadA1, aadA5, aadA4, aadA24, aacA4, aadA2, aadB), chloramphenicol (cmlA6) and quaternary ammonium compound (qacH) and were organized into 11 different gene cassettes arrangements (A-K). The gene cassette arrays dfrA1-aadA1 (D, 44.6%), aadA2 (K, 31.4%), dfrA12-orfX2-aadA2 (G, 27.3%) and aadA1 (A, 25.6%) were most prevalent. Furthermore, 74 isolates contained combinations of 2 to 4 gene cassette arrays. Finally, all of the integron/cassettes-positive isolates were resistant to aminoglycoside antibiotics.ConclusionsThis is the first study on the integrons and gene cassette arrays in S. aureus isolates from milk of mastitic cows from Northwestern China and provide the evidence for class 1 integron as possible antibiotic resistance determinants on dairy farms.
Highlights
Integrons are mobile DNA elements and they have an important role in acquisition and dissemination of antimicrobial resistance genes
The aims of this work were (i) to determine the class 1 integron-associated genes in S. aureus isolates obtained from dairy farms in Shaanxi province in Northwestern China and (ii) to amplify and characterize their gene cassette contents and antibiotic resistance gene cassette arrays by sequencing of S. aureus isolates
Antimicrobial resistance profiles High levels of resistance to antibiotics had been found in the bovine milk-associated S. aureus isolates, including to penicillin G (97/121, 80.2%), ampicillin (97/121, 80.2%), kanamycin (83/121, 68.6%), gentamicin (82/121, 67.8%), tetracycline (52/121, 43%), erythromycin (40/121, 33.1%; 38 isolates exhibited the inducible macrolide-lincosamide-streptogramin resistance phenotype and 2 isolates expressed the constitutive macrolide-lincosamide-streptogramin resistance phenotype and chloramphenicol (36/121, 29.8%) in our previous reports using the same isolates [10, 13]
Summary
Integrons are mobile DNA elements and they have an important role in acquisition and dissemination of antimicrobial resistance genes. There are limited data available on integrons of Staphylococcus aureus (S. aureus) from bovine mastitis, especially from Chinese dairy cows. To address this knowledge gap, bovine mastitis-inducing S. aureus isolates were investigated for the presence of integrons as well as characterization of gene cassettes. The evolution of multidrug resistance is relatively fast due to horizontal or lateral gene transfer, Integrons are mobile DNA elements that are composed of three DNA segments, including a 5′ conserved segment (5’CS), an internal variable region with one or more resistance gene cassettes of different lengths and sequences, and a 3′ conserved segment (3’CS). The 5’CS encodes a recombinase (integrase) which allows for sitespecific insertion of resistance-gene cassettes between two highly conserved adjacent nucleotide sequences. Mobile integrons have been a major driver in the spread of antibiotic resistance and ongoing use of antibiotics has swelled their numbers in recent years [2]
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