**12.** *Staphylococcus aureus*

Amongst the earliest pathogens to become clinically prominent in the life of patients with CF is *Staphylococcus Aureus* (SA) [58]. Its prevalence globally is 44%–56% (2008) in its methicillin sensitive form (MSSA) and 8%–23% for methicillin resistant strains (MRSA). The difficulty in accurately establishing the relevance of SA as a pathogen in CF may lie in the complexity involved in characterising the specific virulence of the strains involved. MSSA, *Small colony variant MSSA* (SCV-MSSA) and MRSA (both community acquired-MRSA and healthcare associated MRSA) all differ in resistance patterns and apparent virulence. A study by Hoffman et al. in 2006 [59] suggested that co-colonisation with MSSA and PA favoured the formation of SCV-MSSA. Subsequent work by Besier et al. [60] associated colonisation with SCV-MSSA with worse lung function, more PA co-infection and more antibiotic resistance than those with MSSA. This is especially telling when colonisation with PA and SA is not uncommon. During a 1990s US-based clinical trial [61] an analysis of baseline sputum found 43% of participants to culture both PA and SA. Current evidence suggest that patients culturing MRSA have worse lung function and require more antibiotics than those with MSSA [62] and that PA/MRSA cocolonisation is associated with more pronounced decline in lung function than PA/MSSA cocolonisation [63]. Controversy still exists surrounding the long established practice of prescribing prophylactic antimicrobials with activity against MSSA. Although studies have suggested reduced cough and less culture positivity in the setting of prophylaxis, no clear benefits in terms of outcomes have been observed. Furthermore, several studies have pointed to increased and earlier incidence of PA culture in patients undergoing MSSA suppression therapy [64]. Whilst guidelines published by international bodies vary on their recommenda‐ tion regarding chronic anti-staphylococcal use, a recent Cochrane review found no convincing evidence for treatment [65] and reiterated concerns around the increase in PA colonisation.
