Effect of sub-inhibitory tigecycline (½-MIC) on AcrAB-TolC and mar/ram/sox regulatory genes in Enterobacter cloacae complex isolates

TL;DR

This study examines how sub-inhibitory tigecycline affects gene expression in Enterobacter cloacae, revealing regulatory responses linked to antibiotic resistance.

Contribution

The study identifies specific regulatory gene activation patterns in response to sub-inhibitory tigecycline in Enterobacter cloacae isolates.

Findings

Tigecycline exposure increased tolC, acrA, marA, and ramA gene expression in resistant isolates.

TGC-S isolates showed marA and marB induction without efflux pump activation.

Resistant strains displayed greater transcriptional shifts compared to susceptible isolates.

Abstract

The global rise of antimicrobial resistance (AMR) in Enterobacterales, including the Enterobacter cloacae complex, is narrowing treatment options. Tigecycline, a last-resort antibiotic for the treatment of multidrug-resistant (MDR) Gram-negative pathogens, is increasingly compromised by emerging resistance mechanisms, notably efflux pump overexpression and regulatory network adaptation. In this study, sixty clinical isolates of Enterobacter cloacae (thirty-eight tigecycline-resistant [TGC-R], twenty-two tigecycline-susceptible [TGC-S]) were analyzed to investigate gene expression changes in efflux pumps and regulatory genes under tigecycline pressure (1/2 minimum inhibitory concentration [MIC]) and standard conditions. Tigecycline exposure markedly increased tolC and acrA together with the regulators marA and ramA, while acrB increased only modestly. This indicates a strong regulatory component to the tigecycline response. In contrast, TGC-S isolates exhibited significant induction of marA, marB without corresponding activation of efflux pumps. Δlog2FC analysis highlighted distinct transcriptional shifts between exposed and unexposed groups, with resistant strains displaying greater divergence. Heatmaps and boxplot visualizations, supported by Wilcoxon test statistics, underscored the regulatory responses associated with tigecycline pressure. These findings indicate that, alongside AcrAB-TolC upregulation, stress-responsive regulators (marA, ramA) are strongly induced by sub-inhibitory tigecycline, underscoring the multifactorial regulation of tigecycline response in the E. cloacae complex.

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