Cigarette smoke modulates virulence-related properties and host immune responses in Klebsiella pneumoniae

Abstract

Introduction Cigarette smoke is a major risk factor for respiratory infections and chronic inflammatory lung diseases, yet the mechanisms through which it alters host–pathogen interactions remain incompletely understood. Klebsiella pneumoniae is an opportunistic Gram-negative pathogen that causes severe infections, particularly in susceptible individuals. This study investigated the effects of cigarette smoke extract (CSE) on K. pneumoniae virulence-associated traits and host immune responses. Methods Three multidrug-resistant, extended-spectrum β-lactamase-producing clinical K. pneumoniae isolates were exposed to CSE. Bacterial growth, virulence gene expression ( mrkA, luxS , and AcrB ), biofilm formation, antimicrobial susceptibility, macrophage invasion, neutrophil phagocytosis, and cytokine responses in THP-1-derived macrophages were evaluated using microbiological, molecular, and cell culture assays. Results High concentrations of CSE (≥25%) suppressed bacterial growth, whereas 5–10% CSE had minimal effects. Exposure to 10% CSE significantly upregulated mrkA, luxS , and AcrB expression. Biofilm formation increased by 20.8–37.0%, and minimum inhibitory concentrations of ciprofloxacin and tigecycline increased two-fold following CSE exposure. CSE enhanced bacterial invasion of THP-1 macrophages and impaired neutrophil phagocytosis, reducing bacterial uptake by 17.8% and 29.9% after 30 and 60 min, respectively. Furthermore, CSE promoted a pro-inflammatory response characterized by increased TNF-α and IL-1β expression and reduced IL-10 expression across multiple time points. Discussion CSE enhances the virulence potential of K. pneumoniae while disrupting innate immune defenses and immune regulation. These findings demonstrate a harmful synergy between cigarette smoke and bacterial infection that may contribute to the increased susceptibility and severity of respiratory infections observed in smokers.