Carriage of Staphylococcus aureus among Portuguese nursing students: A longitudinal cohort study over four years of education

BACKGROUND Staphylococcus aureus is a major human pathogen that can colonize healthy people mainly in the anterior nares. The aim of the present study was to evaluate S. aureus nasal colonization over time among Portuguese nursing students, including methicillin-resistant S. aureus (MRSA). METHODS AND FINDINGS In this longitudinal cohort study, we collected 280 nasal swabs from nursing students at 14 time points over four years of schooling (2012-2016). The isolates were characterized by pulsed-field gel electrophoresis (PFGE), spa typing, multilocus sequence typing (MLST), and SCCmec typing for MRSA. Among 47 students, 20 (43%) carried methicillin-susceptible S. aureus (MSSA) at admission, but none was colonized with MRSA. A total of 19 students (40%) became colonized after exposure during the nursing training, out of which five carried MRSA. Overall, 39 students (83%) had S. aureus detected at least once during the study period. Among the 97 MSSA isolates, most (65%) belonged to four clones: PFGE A-ST30 (21%), B-ST72 (20%), C-ST508 (13%), and D-ST398 (11%). Three of the five MRSA carriers were colonized with the predominant clone circulating in Portuguese hospitals (ST22-IVh) and two with ST3162-II. Colonization of nursing students was highly dynamic with continuous appearance of strains with distinct PFGE types in the same individual. CONCLUSIONS A considerable proportion of students became colonized by S. aureus, including MRSA, during the nursing education, evidencing this population represents an important reservoir of S. aureus. Therefore, education on infection control measures in nursing schools is of major importance.