(569) Use of anti-P. aeruginosa antibodies and characterization of P. aeruginosa isolates for early antibiotic treatment monitoring

Background: P. aeruginosa (Pa) lung infection causes a lung function decline and a systemic increase of anti-Pa serum antibodies. In the initial phases of Pa infection early antibiotic treatment (EAT) can clear the bacterium but many patients can be recolonized with the same or different Pa strains. The aim of this study was to evaluate the anti-Pa immune response as marker of eradication treatment in the early stages of Pa infection in non-chronically infected patients and to analyze characteristics of Pa isolates from these patients.

Methods: During 2008-2021, sera from patients with non-chronic infection undergoing EAT were tested using ELISAs for the presence of antibodies to Pa antigens (St-Ag) (serogroups 1-17) (positive cut off >= 2.96 EU) and Pa alkaline protease (AP), elastase (Ela), and exotoxin A (ExoA) (borderline cut off between 500 and 1250 and positive cut off >1250). Sera were collected at the time of Pa T (0) detection and after 6-12 months of EAT T (1). Airway samples were collected and antimicrobial susceptibility testing (AST) was performed. Whole genome sequencing (WGS) was conducted on Pa isolates and sequence type (ST) was analyzed. Eradication was defined as 3 successive negative Pa cultures in 6 months.

Results: A total of 180 sera from 96 non-chronically infected patients (median age 8.62 years, range 0.86-32.9) were collected. At T (0) we analyzed 103 sera, the mean antibody titer was below the lower cut-off level for three (St-AG, AP, Ela) out of four antigens used. At T (1), we analyzed 77 sera, the antibody titer showed a significant decrease (P< 0.05) in titer against St-AG, AP, Ela and ExoA in patients in whom antibiotic therapy was successfully performed (53, 68.8%), whereas titers for all four antigen used increased in patients in whom antibiotic therapy failed to eradicate the organism (24, 31.2%). St-AG showed higher specificity, whereas Exo A showed higher sensitivity to detect Pa-positive subjects using both cut-offs. In successfully eradicated Pa patients, the correlation between microbiological data and serologic result was 96%, 94%, 94%, and 89% using St-Ag, AP, ELA and Exo A, respectively, with a lower cut-off. At T (0), Pa strains had low resistance. However, a higher percentage of resistance to aminoglycosides, quinolones, imipenem, and ceftalozane/tazobactam was observed at T (1) in the group of patients who keep harboring Pa lung infection. In this group, 16 Pa strains from 8 patients (8 isolated at T (0) and 8 at T (1)) were further analyzed, identical Pa genotypes after therapy were detected in 6 (75%) cases. No cross-infection between patients or epidemic clones were detected.

Conclusions: Antibody titers were found useful to monitor early Pa infection and as a marker of the efficacy of eradication treatment. ST analysis of Pa could be useful to monitor outcome and follow-up of eradication therapy in case of regrowth or colonization with a new strain. Increased antibiotic resistance may be associated with failure of EAT.

Acknowledgements: This work was supported by the Tuscany Region [grant number K38].

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