Background: Pseudomonas aeruginosa (Pa) infection is detrimental to people with cystic fibrosis (pwCF). Several clinical and genetic factors are known to cause early Pa infection, but previous infection with another microorganism has not been studied in this respect.

Methods: Using the Kaplan?Meier method, we computed cumulative incidences of bacterial and fungal initial acquisition (IA) and chronic colonisation (CC) in 1,231 pwCF under 18 years-old for methicillin susceptible and resistant Staphylococcus aureus (MSSA and MRSA), Stenotrophomonas maltophilia, Haemophilus influenzae, Achromobacter xylosoxidans and Aspergillus species. Previous infections were analysed as risk factors for Pa-IA and Pa-CC using Cox regression models.

Results: At 2 years-old, 65.5% pwCF had experienced at least one bacterial or fungal IA, and 27.9% at least one CC. Median age at Pa-IA was 5.1 years and Pa-CC was present in 25% pwCF by the age of 14.7 years. 50% acquired MSSA at 2.1 years, and 50% progressed to MSSA chronic colonisation at 8.4 years. At 7.9 and 9.7 years-old, 25% pwCF had been infected by S. maltophilia and Aspergillus spp. respectively. MRSA and A. xylosoxidans infections were rare events. The risk of Pa-IA and Pa-CC increased with all other species IAs, with hazard-ratios (HR) up to 2.19 (95% CI 1.18?4.07). The risk of Pa-IA increased with the number of previous bacterial/fungal IAs, primarily the first ever (HR=1.89, 95% CI 1.57?2.28), and then by 16% per additional pathogen; and likewise, for Pa-CC.

Conclusions: This study shows that previous bacterial/fungal infections are major risk factors for early Pa lung infection in pwCF.