Disrupting `rcnB` gene in *Acinetobacter baumannii* enhances colistin-mediated killing

Acinetobacter baumannii is a critical multidrug-resistant (MDR) pathogen, posing significant therapeutic challenges, particularly with the rise of carbapenem resistance. Colistin, a polymyxin antibiotic, has been reintroduced as a last-resort treatment for these Gram-negative infections. However, the rapid emergence of colistin resistance severely limits its long-term clinical utility. Understanding novel mechanisms that modulate colistin susceptibility is crucial for developing strategies to overcome resistance and enhance the efficacy of this vital antibiotic.

Researchers employed a CRISPR-Cas9-based genome editing system to generate an A. baumannii AB5075 ΔrcnB mutant. This allowed for precise gene disruption to investigate the role of rcnB in colistin susceptibility. The study then assessed the impact of rcnB loss on colistin-mediated killing, membrane integrity, oxidative stress defenses, and efflux pump activity. Further mechanistic insights were gained through transcriptomic profiling to identify global stress-response network changes during colistin exposure in the mutant strain.

Loss of the rcnB gene markedly potentiated colistin-mediated killing in A. baumannii AB5075. This enhanced susceptibility was attributed to several associated changes within the bacterium. Specifically, the ΔrcnB mutant exhibited compromised membrane integrity, making it more vulnerable to colistin's action. Furthermore, the disruption of rcnB impaired the bacterium's oxidative stress defenses, reducing its ability to counteract colistin-induced damage. Reduced efflux pump activity was also observed, preventing the bacteria from expelling the antibiotic effectively. Transcriptomic profiling revealed that rcnB deletion profoundly reshaped global stress-response networks, including the suppression of fatty acid biosynthesis and reactive oxygen species (ROS)-detoxifying pathways. Alterations in metal ion and sulfur metabolism were also noted during colistin exposure, further contributing to the enhanced sensitivity.

The disruption of rcnB significantly potentiated colistin-mediated killing through a multifaceted mechanism involving membrane compromise, impaired oxidative stress defenses, and reduced efflux pump activity.