Experimental colonization of pigs with methicillin-resistant Staphylococcus aureus (MRSA): insights into the colonization and transmission of livestock-associated MRSA.

Two models were used for colonizing pigs under experimental conditions. In the first model, six 5-week old piglets were challenged by nasal and gastrointestinal inoculation with a mixture of four strains representing the most prevalent methicillin-resistant Staphylococcus aureus (MRSA) sequence types (ST398, ST9) and spa types (t08, t011, t034, t899) associated with pig farming. In the second model, the vagina of a pregnant sow was inoculated with the same MRSA mixture shortly before farrowing. While MRSA carriage was unstable following nasal-gastrointestinal inoculation of piglets, vaginal inoculation of the sow resulted in persistent carriage of t011-ST398 and t899-ST9 in all newborn piglets. The results from the two models provide evidence that livestock-associated MRSA can efficiently spread by vertical perinatal transmission and that direct colonization of weaned piglets is hampered by unknown host, bacterial or environmental factors. The vaginal inoculation model described in this study represents a useful tool for studying MRSA-host interactions in pigs having the same genetic background.