Abstract
Porcine strains of livestock-associated methicillin resistant Staphylococcus aureus (LA-MRSA) have been recognized in many countries and have been shown to be able to cause human infection. Resistance to non-beta lactam antibiotics has been reported but non-susceptibility to vancomycin, which is known to occur in human MRSA, has so far not been observed in LA-MRSA. Such resistance is typically fairly low level involving changes in the cell wall thickness. The development of resistance is usually preceded by presence of a sub-population having an increased MIC, which is selected for by exposure to vancomycin. This study investigated vancomycin susceptibility of one hundred porcine MRSA isolates using three MIC methods including spiral gradient endpoint (SGE) technique which allows visualization of more resistant sub-populations. SGE revealed 16 strains with an MIC above 2.0 mg/L, of which 14 were determined to have MIC 4 mg/L by agar dilution (AD). SGE revealed a further two isolates with MIC < 2 mg/L had a sub-population >2 mg/L. In addition, trailing endpoints not reaching resistance were present in 26 isolates with MIC < 2 mg/L. Sequencing of the genes of the VraSR/GraSR two component systems of ten of the resistant strains for comparison with susceptible strains revealed changes, including the presence of stop codons, in vraS and graR, but these were not consistent in all isolates. Other genetic changes may contribute to vancomycin non-susceptibility and require investigation. As failure to respond to treatment has been reported in clinical isolates with MIC > 1.5 mg/L, the presence of vancomycin non-susceptibility in porcine isolates is of concern and further monitoring of LA-MRSA is essential.
Highlights
Methicillin resistant Staphylococcus aureus (MRSA) has long been an important hospital-associated pathogen, but its emergence in the community and, more recently, in livestock has led to increasing concern
Whilst vancomycin non-susceptibility has been reported in several countries and there have been a number of reports of treatment failure for human clinical isolates, this study appears to be the first to describe reduced susceptibility in MRSA of porcine origin
The overall average MIC was close to the breakpoint by all three MIC methods used
Summary
Methicillin resistant Staphylococcus aureus (MRSA) has long been an important hospital-associated pathogen, but its emergence in the community and, more recently, in livestock has led to increasing concern. Asian isolates are more likely to be multi-drug resistant, displaying resistance to up to eight agents (Ho et al, 2012) To date it appears that LA-MRSA strains are less virulent than human isolates and, these strains do colonize and occasionally cause infections in man, their human-to human transmission risk is much lower than that of typical MRSA (Bootsma et al, 2011). Their ability to cause infection does increase concern, as their antibiotic resistance profile is extensive leading to limited choices of therapy. To date there have not been any reports of resistance to vancomycin in LA-MRSA, though non-susceptibility to this drug has frequently been reported in typical human MRSA isolates (Howden et al, 2010)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
