Growth in Anaerobic Bottles is an Independent Predictor of Mortality in Pseudomonas aeruginosa Bloodstream Infections

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INTRODUCTION: Pseudomonas aeruginosa is a leading cause of bloodstream infections (BSI) in hospitalized patients. Although it is regarded as an aerobic microorganism, under certain conditions it can switch to anaerobic respiration using other terminal electron acceptors other than oxygen. The study aims to evaluate the clinical and microbiological factors associated with P. aeruginosa isolation in anaerobic blood culture bottles and determine whether anaerobic growth is an independent risk factor for septic shock and 30-day mortality in patients with P. aeruginosa BSI. METHODS: This was a retrospective unicentric study analyzing 734 episodes of P. aeruginosa BSI at a university hospital from 2010 to 2019. Clinical presentation, comorbidities, source of infection, microbiologic data, and outcomes were collected. Anaerobic growth was defined as the isolation of P. aeruginosa in at least one anaerobic blood culture bottle. Multivariate logistic regression models were used to identify factors associated with septic shock and 30-day mortality. RESULTS: P. aeruginosa was isolated in anaerobic bottles in 19.1% of cases, though it was never exclusively isolated in anaerobic bottles. While median time to positivity (TTP) in anaerobic bottles was significantly longer than in aerobic ones (16.5 h vs. 14.8 h, p < 0.01), TTP in aerobic bottles was shorter when P. aeruginosa was also isolated in anaerobic bottles (12.5 h vs. 15.5 h, p < 0.01). Factors significantly associated with anaerobic growth included chronic kidney disease, longer time of admission, active antibiotic treatment, and several sources of bacteremia (catheter-related, respiratory, and primary bacteremia). Anaerobic growth was independently associated with higher odds of septic shock (OR 2.8, p < 0.01) and increased 30-day mortality (OR 2.3, p < 0.01). Moreover, septic shock and mortality rates were higher when P. aeruginosa grew in both anaerobic bottles. CONCLUSIONS: Anaerobic growth of P. aeruginosa in blood cultures is an independent predictor of septic shock and 30-day mortality in patients with P. aeruginosa BSI. The potential relationship between higher bacterial load and biofilm formation in the source of infection with anaerobic growth of P. aeruginosa may contribute to the observed poorer outcomes.

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