Livestock-Associated Methicillin Resistant Staphylococcus aureus (LA-MRSA) Clonal Complex (CC) 398 Isolated from UK Animals belong to European Lineages.

The research investigates the origins of a strain of drug-resistant bacteria (LA-MRSA CC398) found in UK animals, using genome sequencing and phylogenetic reconstruction. The results suggest multiple independent incursions into the UK from other European countries, evidenced by different genetic nodes of the UK isolates. Additionally, the presence of certain genes in the bacteria strains may indicate a role of heavy metal exposure in antibiotic resistance.

Study Background and Approach

• The study was focused on understanding the possible origins of Livestock-Associated Methicillin Resistant Staphylococcus aureus (LA-MRSA) Clonal Complex (CC) 398. This strain of bacteria has seen a rise in prevalence in UK animals.
• Scientists applied whole genome sequencing (WGS) to 89 different isolates, including 12 samples from the UK isolated between 2013 and 2015, and others from a variety of animal hosts across Europe and other regions.
• Phylogenetic reconstruction, which uses genetics to infer the relationships between different groups, was applied to the genome sequence data.

Key Findings

• The 12 UK isolates of LA-MRSA CC398 were found to be closely related to European sub-lineages of the bacteria, suggesting multiple separate introductions into the UK, rather than a single introduction followed by local proliferation.
• These isolates were found in different animals: three from healthy pigs, one from turkey, and three from horses, and others mostly from pigs.
• The research uncovered the presence of antimicrobial, heavy metal, and disinfectant resistance genes in most of the isolates tested. This means that the bacteria demonstrated resistance to more than three types of antimicrobial drugs, in addition to showing resistances to substances like zinc and cadmium.

Implications

• The findings hint at the bacterial isolates in UK animals being the result of independent incursions from other European countries, rather than a single localised outbreak.
• The presence of heavy metal resistance, particularly to zinc and cadmium, was detected in all samples from food-producing animals (pigs and poultry), but not from horses, suggesting that heavy metal exposure may play a role in selecting for certain antibiotic-resistant strains of bacteria.

Limitations

• The absence of some major genes associated with toxicity and virulence in human-associated MRSA was expected in these animal strains, indicating potential genetic dissimilarities.
• The study also notes that the single turkey isolate from the UK did not carry a specific set of genes found in US strains, suggesting potential differences in adaptation or selection pressures in different locales.