MRSA-ST398 was first reported associated to a skin lesion. Some microbiologic traits define isolates belonging to the MRSA-ST398 clone: chromosomal DNA cannot be restricted by SmaI enzyme and CC398 strains generally present resistance to tetracycline, which is commonly used in pig farming. The absence of certain important virulence factors such as PantonValentine Leukocidin and Toxic Shock Syndrome Toxin seems to be common in CC398 isolate. Nevertheless, this clone has been associated with both animal and human disease. The presence of genes coding for virulence factors was very poor in our collection. None of the studied isolates carried the PVL encoding genes lukF-PV and lukS-PV, contrasting to studies from Sweden and China that report PVL-positive isolates in patients who had no previous contact with animals. Generally, MRSA-ST398 lacks certain important virulence factors for humans. In our study genes involved in immune evasion were only detected in the single t899 isolate. These virulence factors are active only against the innate immune system in humans. The lack of these virulence factors may partially explain why these strains do not appear to be highly infectious for humans, and usually are associated with SST infections. However, a few severe infections by ST398 have been sporadically published in several countries, such as pneumonia or bacteraemia. Even though the majority of the patients in our study were colonized by MRSA-ST398, four SST infections and four respiratory tract infections were detected in our series. In addition, two invasive infections, bacteraemia and subdural empyema, were also detected, in patients showing previous nasal colonization. Death was related to the MRSA-ST398 infection in the bacteremic patient: an 84-year-old woman who was hospitalized in the HUB because of a thoracic aortic AbMole 12-O-Tiglylphorbol-13-isobutyrate aneurysm. One month after the surgery, the patient experienced a febrile episode and MRSA-ST398 was recovered from blood and from the central venous catheter tip. The isolate was resistant to AbMole Gemifloxacin mesylate tetracycline and beta-lactams. One week after treatment with vancomycin, the patient died. The isolates causing infection did not differ from the colonizing isolates, regarding genotype, virulence or antibiotic resistance profile. In our study, some data could not be collected properly such as the contact with animals, a risk factor for infection by LA-MRSA. However, a high percentage of the patients lived in or near to a rural environment. Another limitation was the selection of the isolates to be studied by the presence of tetracycline resistance. Although this is a common feature among MRSA-ST398, this approach could underestimate the number of isolates belonging to CC-398 in HUB. In conclusion, in the last two years the number of MRSA-ST398 isolates infecting or colonizing patients increased significantly in our setting, as well as the increase in tetracycline resistance. The emergence 
of this clonal lineage has also been reported in other countries in Europe. These studies showed a remarkable increase in the proportion of LA-MRSA isolates, including outpatients and primary health care patients, which were not covered in our hospital-based study. According to our results, the majority of studied isolates carried the genes encoding haemolysins and adhesion cellular factors, but other virulence factors usually found among S. aureus were not detected. Phenotypic expression of antibiotic resistance was variable among the MRSA-ST398 isolates and nearly half of the isolates were resistant to multiple antibiotics.