To analyze the pathogen distribution, antimicrobial resistance and antibiotic usage in adult patients with influenza pneumonia who developed secondary pulmonary bacterial infections in the intensive care unit (ICU), and to provide evidence for optimizing anti-infection treatment strategies and rational use of antibiotics in this population.

Clinical data of 117 adult patients with influenza pneumonia admitted to ICU of Beijing Ditan Hospital, Capital Medical University, from January 2009 to March 2025 were analyzed by a single-center retrospective study. Bacterial cultures were performed on deep respiratory tract specimens (sputum or bronchoalveolar lavage fluid). Species identification was carried out using the fully automated microbial mass spectrometry detection system VITEK-2 identification cards, and in vitro antimicrobial susceptibility testing was performed using the corresponding VITEK-2 susceptibility cards and the Kirby-Bauer (K-B) disc diffusion method. Toal of 117 patients were divided into infection group (53 cases) and non-infection group (64 cases) based on the presence of secondary pulmonary bacterial infection. The independent risk factors for secondary pulmonary bacterial infection were identified by Logistic regression analysis.

Among the 117 patients with influenza pneumonia, 53 (45.3%) were diagnosed with secondary pulmonary bacterial infection, from whom 90 bacterial strains were isolated, with an isolation rate of 76.9%. Gram-negative bacteria predominated (70 strains, 77.8%), with Acinetobacter baumannii (25 strains, 27.8%), Stenotrophomonas maltophilia (11 strains, 12.2%) and Pseudomonas aeruginosa (10 strains, 11.1%) being the top three. Total of 20 Gram-positive strains (22.2%) were isolated, mainly Staphylococcus aureus (6 strains, 6.7%), but no Streptococcus pneumoniae was detected. All patients received early empirical antibiotic therapy upon admission, with β-lactamase inhibitor combinations (30.8%, 36 cases), quinolones (28.2%, 33 cases) and carbapenems (17.9%, 21 cases) being the main choices. Compared with the non-infection group, patients in infection group had significantly higher rates of use of reserved-grade antibiotics (χ²=4.584, P=0.032), neutrophil percentage (NEUT%) (U=－2.207, P=0.027), C-reactive protein (CRP) level (U=－2.360, P=0.018), incidence of respiratory failure (χ²=11.069, P=0.001), acute kidney injury (χ²=6.612, P=0.010), shock (χ²=23.553, P=0.001) and rate of invasive mechanical ventilation (χ²=14.913, P=0.001), as well as longer durations of invasive mechanical ventilation (U=－2.925, P=0.003), ICU duration (U=－6.890, P=0.001) and total hospital duration (U=－6.747, P=0.001), all with significant differences. Multivariate Logistic regression analysis showed that invasive mechanical ventilation (OR=3.278, 95%CI: 1.275-8.430, P=0.014), shock (OR=5.372, 95%CI: 1.568-18.197, P=0.007) and neutrophil-to-lymphocyte ratio (NLR) (OR=1.075, 95%CI: 1.011-1.144, P=0.022) were all independent risk factors for secondary pulmonary bacterial infection.

Secondary bacterial infection in ICU adult patients with influenza pneumonia significantly increases the incidence of respiratory failure and prolongs total hospital and ICU duration. The predominant pathogens are Gram-negative bacilli. Clinically, early identification and monitoring of high-risk patients with invasive mechanical ventilation, shock and elevated NLR should be strengthened. In initial empirical anti-infection therapy, unnecessary use of reserved-grade antibiotics should be avoided, and individualized and rational antibiotic use should be pursued as much as possible.