Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) has emerged in the past 10 years as an important public health concern. Since first recognition in Europe, LA-MRSA has been identified in pigs in various countries on at least 4 continents, and significant concern has been raised about the potential impacts on pig farmers and veterinarians, rural residents and the broader population. Once on a farm, high rates of MRSA carriage can be encountered in pigs, but it is rarely an ‘all-or-nothing’ event, with MRSA-negative pigs being found on farms with high MRSA carriage rates. This raises questions about why some pigs are colonized while other remain MRSA negative. A variety of factors may be involved, with one possibility being differences in the nasal bacterial population (microbiota) and its overall genetic composition (microbiome). While not adequately studied, it has been assumed that the pig has a complex and abundant bacterial population in its nasal passages, and that may play a critical role in determining the fate of bacteria to which it becomes exposed. The objective of this study was to describe the nasal microbiome of slaughter-age pigs and to evaluate the influence of the microbiome on MRSA colonization.

 Nasal swabs were collected from farm- and age-matched pigs. Swabs were screened for MRSA, and positive and negative pigs were selected for microbiome study using a 2nd swab that was collected. Using molecular methods, the nasal bacterial microbiome was described and the two groups were compared. To further evaluate the influence of management on the nasal microbiome, swabs were collected from a set of age-matched pigs on a farm that fed a liquid diet.

 The nasal cavity of the pig contains a diverse and complex microbial population, with an average of close to 100 different bacterial species per pig. There was no significant difference in the number of different species, the bacterial diversity or the overall bacterial population composition in MRSA positive versus negative pigs, although there were significant differences in some bacterial groups.
Interestingly, there was a marked difference in the nasal microbiome between conventionally-fed and liquid-fed pigs. Liquid diet-fed pigs had a much different overall nasal microbial population, with significant differences at the Phylum level, consisting of greater proportions of Firmicutes and lesser proportions of Bacteroidetes and Proteobacteria.
This study has provided the most comprehensive understanding of the nasal microbiome of the pig, something that may be relevant for understanding of various bacterial (and perhaps viral) pathogens that can reside in the nasal cavity. It has shown that the nasal microbial population does not exert an apparent effect on the likelihood of MRSA shedding in pigs, but some minor population differences that were present deserve further study. The finding of a pronounced difference between pigs fed different diets indicates the potential for management practices to have a profound impact on the nasal microbiome, something that should be considered when studying a range of pig and zoonotic pathogens.
Researcher contact information: J Scott Weese,