Characterization of methicillin-resistant Staphylococcus aureus CC398 obtained from humans and animals on dairy farms.

The study investigates the genetic relationships and antimicrobial resistance of a specific type of methicillin-resistant Staphylococcus aureus (MRSA) found in humans and animals in dairy farms. It suggests that the MRSA strains across different species and environmental samples on the same farm may either have originated from diverse sources or evolved variations from a single strain.

Research Methodology

• The researchers collected 125 MRSA isolates from 26 different dairy farms. The isolates were taken from various sources like milk samples, dairy cattle, calves, dust samples from pig and veal calf sheds, dogs, a horse, a sheep, and humans.
• The MRSA isolates were analysed using CC398-specific PCRs, spa typing, SCCmec typing and ApaI macrorestriction. These tests can articulate the genetic framework of the MRSA strains and identify their specific subtypes. CC398 is a particular type of MRSA that has adapted to live in livestock and is especially common in pigs.
• Antimicrobial susceptibility of the MRSA isolates was tested using broth microdilution. This method reveals whether the MRSA strains are resistant to different types of antimicrobial drugs.

Results

• All 125 isolates belonged to the MRSA CC398 type.
• Eight different subtypes were identified (t011, t108, t034, t567, t1184, t1451, t2287 and t3934).
• Both types IV (48 instances) and V (67 instances) SCCmec elements were found among the isolates. SCCmec elements are mobile genetic elements that make MRSA strains resistant to certain antibiotics. 10 isolates were not typeable.
• Six main macrorestriction patterns were found with up to 23 different sub-patterns. Macrorestriction is a method which uses enzymes to cut DNA into large pieces, whose patterns can then be analyzed.
• Twelve different resistance patterns were found, and 68 isolates showed resistance to more than one type of antibiotic (multiresistant).

Interpretation and Conclusion

• Analysis of the results suggest that the same MRSA CC398 subtype might be interchanging between different species in some farms, including humans.
• In other cases, different MRSA CC398 subtypes were detected in various species, suggesting either separate importation of MRSA into the farm, or that one strain of MRSA has diversified during colonization. These diversifications might be reflected in changes in certain genetic and resistance patterns.

The study’s findings underscore the importance of biosecurity and appropriate antibiotic usage in farms, in order to mitigate the risk of MRSA transmission, and reduce the potential for the emergence of highly resistant strains.