The turnover of strains in intermittent and persistent nasal carriers of Staphylococcus aureus

Summary Objectives We aimed to examine the dynamics of Staphylococcus aureus nasal carriage in healthy adults. Method Selected S. aureus strains isolated from weekly nasal swabs obtained from 122 healthy young adults over a 13 week period were spa typed. Results The median duration of intermittent carriage was 4 weeks (IQR 2–6) and the median interval between episodes of carriage of different spa types was 3.5 weeks (IQR 2.25–4). 6/19 (32%) Persistent carriers were colonised with more than one spa type during the study, and in two persistent carriers a brief period of mixed colonisation with two spa types was observed. Even when the carriage strain changed, it was very rare for persistent carriers to have a period during which they were culture-negative (only 6/188 (3%) swabs submitted by persistent carriers failed to culture S. aureus ). Conclusions Our results imply that at least every eight weeks a healthy young adult is exposed to S. aureus sufficient to cause a new episode of carriage among intermittent carriers. Persistent carriers are almost always colonised with S. aureus and over the course of a year there will be at least one replacement of the dominant strain.