For those subjects that experienced pulmonary exacerbation during the study period, titers are measured during exacerbation and at follow-up (1-3 months). For LukA, alpha toxin and LukS-PV, geometric mean titers (GMT) were significantly higher if S. aureus was detected at the time of exacerbation (p<0.01 for each comparison; Mann-Whitney U test). For LukA only, GMTs were significantly higher at exacerbation follow-up compared to titers during the exacerbation (p<0.05, Wilcoxon signed-rank test).