Antimicrobial resistance and genotyping of Staphylococcus aureus obtained from food animals in Sichuan Province, China

Abstract

Background: Staphylococcus aureus (S. aureus), especially methicillin-resistant Staphylococcus aureus (MRSA), is considered a common zoonotic pathogen, causing severe infections. The objective of this study was to investigate the antimicrobial susceptibility, resistance genes and molecular epidemiology among MRSA and methicillin-susceptible Staphylococcus aureus (MSSA) isolated from food animals in Sichuan Province, China.

Methods: This study was conducted on 236 S. aureus isolates. All isolates were subjected to antimicrobial susceptibility testing by using a standard microbroth dilution method. The Polymerase Chain Reaction (PCR) was performed to identify genes encoding the β-lactams resistance (blaZ, mecA), macrolides (ermA, ermB, ermC) and aminoglycosides (aacA-aphD). The molecular structures and genomic relatedness of MRSA isolates were determined by staphylococcal chromosome cassette mec (SCCmec) typing and pulsed-field gel electrophoresis (PFGE), respectively.

Results: Among 236 isolates, 24 (10.17 %) were recognized as MRSA. MRSA isolates showed different resistance rates to 11 antimicrobials ranging from 33.33 to 100 %, while for MSSA isolates the rates varied from 8.02 to 91.51 %. Multi-drug resistance phenotype was found in all MRSA isolates. The ermC gene encoding macrolides-lincosamides-streptogramin B was the most prevalent gene detected in 87.29 % of the S. aureus isolates, followed by ermB (83.05 %), blaZ (63.98 %), aacA-aphD (44.07 %), ermA (11.44 %) and mecA (11.02 %) genes. The prevalence of resistance genes in MRSA isolates was significantly higher than that of MSSA. Regarding the molecular morphology, SCCmec III (12/24, 50 %) was the most common SCCmec type. Furthermore, the PFGE typing showed that 24 MRSA were divided into 15 cluster groups (A to O), the major pulsotype J encompassed 25 % of MRSA isolates.

Conclusions: The S. aureus isolates from food animals in Sichuan province of China have severe antimicrobials resistance with various resistance genes, especially MRSA isolates. Additionally, the genetic pool of MRSA isolates is diverse and complex, and further investigation is necessary.

Keywords: Antimicrobial resistance gene; MRSA; MSSA; Molecular typing.

Fig. 1

Comparison of resistance of MRSA and MSSA isolates to antimicrobials. PEN, penicillin; AMP, ampicillin; OXA, oxacillin; GEN, gentamicin; KAN, kanamycin; AMK, amikacin; TET, tetracycline; ERY, erythromycin; AZM, azithromycin; CIP, ciprofloxacin; SIZ, sulfafurazole. * indicates significant difference (P<0.05), ** indicates extremely significant difference (P<0.01) of resistance rate between MRSA and MSSA

Fig. 2

Distribution of multi-drug resistance in MRSA and MSSA isolates. MRSA, methicillin-resistant Staphylococcus aureus; MSSA, methicillin-susceptible Staphylococcus aureus

Fig. 3

The gels images of MRSA isolates by PFGE typing. a: 2, BMD18; 3, BMD24; 4, XCCow3; 5, YAD4; 6, YAD2; 7, GLD51; 8, GLD54; 9, YAC4; 10, BMD41; 11, BMD74; 12, BMD30; 13, BMD17; 14, YAD3; 1 and 15, Xba-digested DNA of Salmonella Braenderup H9812 used as DNA molecular size marker; b: 2, BMD42; 3, GLD59; 5, YAC1; 6, BMD68; 7, BMD23; 8, GLD83; 9, YAC2; 10, GLD93; 11, BMD9; 12, BMD20; 13, BMD69; 4, error (The forth lane was made by mistake.); 1 and 14, same as the 1 and 15 in a

Fig. 4

The clusters diagram of 24 MRSA isolates generated by PFGE typing. XCCow3, BMD42, YAD4, YAD3, BMD20, BMD69, BMD23, BMD9, GLD59, GLD83, BMD68, BMD74, BMD17, BMD30, BMD18, BMD24, BMD41, YAC4, GLD51, GLD54, GLD93, YAC1, YAC2, YAD2: strain numbers

Fig. 5

Map of Sichuan showing location of the thirteen areas where S. aureusisolates were collected