Imipenem-Induced Transcriptional Responses of Porin, Efflux Pumps, and Carbapenemase Genes in Clinical Carbapenem-Resistant Acinetobacter baumannii

<b>Background/Objectives</b>: Carbapenem-resistant <i>Acinetobacter baumannii</i> poses a critical threat due to its ability to acquire multiple resistance mechanisms and persist under antibiotic pressure. This study aimed to elucidate the molecular basis of imipenem resistance in clinical <i>A. baumannii</i> isolates by integrating phenotypic, molecular, transcriptional, and clonal analyses. <b>Methods</b>: Eleven <i>A. baumannii</i> isolates identified by MALDI-TOF MS (matrix-assisted laser desorption ionization time-of-flight mass spectrometry) were investigated. Antimicrobial susceptibility to imipenem and meropenem was assessed, followed by polymerase chain reaction (PCR) detection of Ade efflux pump, outer membrane porin, and OXA-type carbapenemase genes. Transcriptional responses to sub-inhibitory imipenem exposure were evaluated using quantitative real-time PCR, and clonal relatedness was assessed by arbitrarily primed PCR. <b>Results</b>: All isolates were carbapenem-resistant, with <i>bla_OXA-23</i> detected in all isolates and <i>bla_OXA-24</i> absent in one isolate. Transcriptional analysis revealed isolate-specific responses to imipenem exposure. Among Ade efflux pump components, only <i>adeR</i> exhibited expression changes, displaying either downregulation or upregulation depending on the isolate, whereas <i>adeA</i>, <i>adeB</i>, <i>adeC</i>, and <i>adeS</i> transcripts were not detected under the tested conditions. Outer membrane porin genes showed heterogeneous regulation, with <i>ompA</i> and <i>carO</i> downregulated, while some isolates showed increased expression. Expression of <i>oprD</i> varied among isolates, and <i>omp33-36</i> transcripts were detected in a single isolate and were reduced after exposure. Clonal analysis identified nine distinct genotypes, indicating genetic diversity and the absence of clonal dominance. <b>Conclusions</b>: These findings highlight the multifactorial and heterogeneous nature of carbapenem resistance in <i>A. baumannii</i>, emphasizing the interplay between regulatory efflux mechanisms, porin modulation, and carbapenemase carriage.