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Background Indwelling pleural catheters (IPCs) offer symptom relief from pleural effusions. Usage is complicated by infection in ~5% of cases. 1 IPCs can be colonised when bacteria grow from pleural fluid without infective features.2 The causes of colonisation are undescribed and its relationship with infection unexplored.Aims Describe the incidence and microbiology of IPC colonisation.Identify clinical features associated with IPC colonisation.Compare asymptomatically colonising bacteria with those from infection.Methods All infected and non-infected IPCs were collected from four sites, along with associated metadata. Infected IPCs were identified clinically and non-infected IPCs categorised as ‘colonised’ if bacteria was cultured from them, and ‘clean’ if not. The genomes of all cultured organisms were sequenced, assembled and taxonomically classified. For Staphylococcus aureus isolates, a maximum likelihood phylogenetic tree was constructed from Snippy-core alignment using IQ-Tree, with S. aureus NCTC8325 as reference.Results Of 108 IPCs, 54 were ‘clean’, 34 colonised and 20 infected. Infected and colonised IPCs had longer mean insertion durations (197.6 and 181.3 days) compared with ‘clean’ ones (98 days).Of 34 colonised IPCs, 17 were colonised by multiple species. The most common causes were Coagulase-negative Staphylococci (CoNS n=18), Enterobacteriaceae (n=18), Corynebacteria spp (n=4) & S. aureus (n=4).Two of 20 infected IPCs were culture negative. We retrieved the same organism causing infection from 14/18 IPCs. The most common pathogens were S. aureus (n=8), Pseudomonas aeruginosa (n=3), Enterococcus spp (n=2) and Enterobacter cloacae (n=2). Figure 1 demonstrates clustering of S. aureus isolates from deep and superficial infection.Abstract S149 Figure 1Phylogenetic tree of Staphylococcus aureus isolates from colonised and infected IPCs. S. aureus NCTC8325 is the reference strainConclusion A significant proportion of IPCs are colonised. Colonisation and infection are associated with longer insertion duration. Enterobacteriaceae and CoNS are the most common causes of colonisation, whereas S. aureus and Pseudomonas aeruginosa commonly cause infection. S. aureus isolates causing deep infection show higher genetic similarity versus colonising strains. This, and our phylogenetic analysis, suggests colonisation does not cause infection, but rather there is a virulent subset of isolates causing infection. Ongoing work will identify genes and markers associated with infection.References Sundaralingam, et al. Chest 2022;161(5):1407–1425.Sethi, et al. Clinical & Translational Science 2023:1287–1288.