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Methicillin-susceptible Staphylococcus (S.) aureus (MSSA) and methicillin-resistant S. aureus (MRSA) are colonizers of skin and mucosa and may cause severe infections both, in humans and animals. In humans, some studies have shown that MSSA and MRSA co-colonization of the anterior nares is common and one clone can be found rather than differing types of MSSA and MRSA. In the last decade, MRSA were frequently detected in healthy farm animals and livestock professionals. In this context MSSA carriage in pig farmers was postulated as a possible protective effect against the colonization with MRSA. This might also be interest for possible interventions in pigs. The objective of our study was to investigate the colonization dynamics and clonality of both, MSSA and MRSA in pigs over a longer time period. Eighteen sows were nasally sampled three times every ten weeks. Additionally, 50 environmental samples from walls, toys, brushes, watering places, and troughs were taken, together with boot swab samples and a pooled dust sample. Samples were investigated for MSSA and MRSA, respectively.
Spa-typing was done with up to five MRSA and MSSA isolates found per sample and time point (total: 133 MSSA and 120 MRSA isolates, respectively); 59 selected isolates were further investigated by microarray. 16.7 % of sows were MSSA/MRSA non-carriers and permanent arriers of MSSA, respectively at all sampling times; 38.9 % of sows infrequently carried both, MSSA and MRSA; 66.7 % of sows showed a changing carriage status over time. CC398 (t034, t011) and CC9 (t337, t1430, and t13816) associated spa-types were exclusively found among MRSA and MSSA, respectively. In 44.4 % of sows up to two different types of MSSA were present at the same time and sample. Strains of the same clonal lineage showed a high genetic identity despite their origin. Highly identic clones were present in sows and their environment. As conclusion, MSSA/MRSA may not exclude each other in the anterior nares of pigs. Pigs may also carry different clones at the same time.
This calls for a critical review of the number of isolates that need to be analyzed, in particular in the course of in-depth epidemiological investigations.