Molecular typing and antimicrobial susceptibility of oxacillin-susceptible, mecA-positive Staphylococcus aureus from pigs

Abstract:

[Objective] To investigate the prevalence, molecular characteristics and antibiotic resistance of oxacillin-susceptible, mecA-positive Staphylococcus aureus (OS-MRSA) isolates from pigs, we collected samples from 9 large-scale pig farms in four central and western provinces of China (Gansu, Shaanxi, Henan and Guangxi). [Methods] The OS-MRSA isolates were identified by amplifying nuc and mecA gene and oxacillin susceptibility tests. Twenty-six toxin-encoding genes, 16 kinds of commonly used antibiotic resistance and molecular typing (spa, MLST and SCCmec) of the isolates were detected.[Results] The results showed that 67 (7.6%, 67/884) of the 884 samples were contaminated with S. aureus. A total of 67 isolates were isolated, including 50 isolates of methicillin-susceptible Staphylococcus aureus (MSSA), 8 isolates of oxacillin-resistant mecA-positive (OR-MRSA) and 9 isolates of OS-MRSA. Among the 26 toxin-encoding genes, 9 toxin-encoding genes were detected, and the hla gene had the highest detection rate, followed by hld, hlb, hlg, sei, sem, seg, sen and seo. Sixteen isolates that carrying the enterotoxin-encoding genes, OR-MRSA and OS-MRSA isolates accounted for 37.5% (6/16) and 50.0% (8/16), respectively. The isolates carrying the enterotoxin-encoding genes were all ST9-t899 clone type. Among the 16 antibiotics tested, the isolates were resistant to 12 antibiotics, among which MSSA, OR-MRSA and OS-MRSA isolates were mainly resistant to 1-8, 10-12 and 7-11 antibacterial drugs, respectively. Four clone types (ST398-t571, ST9-t899, ST398-t034 and t11241) were identified, among which ST9-t899 was the only clone type of MRSA and ST398-t571 was the dominant clone type of MSSA. With the exception of four isolates where SCCmec typing was not detected, IV_b (76.5%, 13/17) was the only type in MRSA isolates. [Conclusion] To sum up, the sensitivity of pig MRSA isolates to oxacillin had changed, and they were generally sensitive to oxacillin. In addition, the dominant clone types of MSSA and MRSA strains were ST398-t571 and ST9-t899, respectively. The study also found that the clone type is significantly related to the toxin genes carrying status, and the clone type of the strain carrying the toxin genes was ST9-t899. Therefore, understanding the prevalence, molecular characteristics and drug resistance of MSSA and OS-MRSA from pigs in China can provide data support for the prevention and control of Staphylococcus aureus infection for pigs.

Key words:OS-MRSA;toxin-encoding genes;antimicrobial resistance;molecular typing