CRISPR/Cas9-targeted smpB mutation revealing roles in biofilm formation, motility, and antibiotic susceptibility in Acinetobacter baumannii.

BACKGROUND: Acinetobacter baumannii is a multidrug-resistant pathogen and a major cause of hospital-acquired infections worldwide. Its ability to survive in harsh environments and evade antibiotic treatments underscores the urgent need for new therapeutic targets. Emerging evidence suggests that the small protein B (SmpB) may also play broader roles in bacterial virulence, including regulation of biofilm formation, motility, and stress adaptation. However, the specific contributions of SmpB to these pathogenic traits in A. baumannii remain poorly defined. Addressing this knowledge gap is essential for evaluating SmpB as a potential antimicrobial target and developing new strategies to combat multidrug-resistant infections.

METHODS: CRISPR/Cas9-mediated gene editing was used to generate a targeted smpB mutant in A. baumannii. The smpB mutant was assessed for growth, biofilm formation, motility, antibiotic susceptibility, and virulence. Biofilm was quantified via crystal violet staining and microscopy, while motility was examined using swimming, swarming, and twitching assays. Antibiotic susceptibility was evaluated using disk diffusion. Virulence was tested in the Galleria mellonella infection model. Proteomic analysis was performed to identify changes in protein expression associated with smpB disruption.

RESULTS: CRISPR/Cas9-mediated editing successfully introduced a C212T nucleotide substitution in the smpB gene, resulting in an A89G amino acid change. Growth curve analysis showed no significant difference between the wild-type and smpB mutant strains under nutrient-rich conditions. However, the mutant exhibited a significant reduction in biofilm formation (p = 0.0079) and impaired twitching motility, while swimming and swarming motility remained unaffected. Antibiotic susceptibility testing revealed increased sensitivity to ceftizoxime, piperacillin/tazobactam, and gentamicin, alongside decreased susceptibility to cefepime, tetracycline, and spectinomycin. In the G. mellonella infection model, the smpB mutant showed reduced virulence, with 84% larval survival compared to 72% in the wild type (p = 0.4183). Proteomic analysis revealed downregulation of key stress response and virulence-associated proteins, including GroEL, DnaK, RecA, and PirA, while proteins involved in ribosome maturation and transcription, such as RimP and RpoA, were upregulated. STRING network analysis supported the broad regulatory role of SmpB in biofilm formation, motility, stress adaptation, and pathogenesis.

CONCLUSION: This study demonstrates that SmpB is a key regulator of biofilm formation, twitching motility, antibiotic response, and virulence in A. baumannii. While not essential for growth under optimal conditions, smpB disruption impairs multiple pathogenic traits and alters stress-related proteomic pathways. These findings highlight the potential of SmpB as a novel antimicrobial target, offering a promising strategy to weaken bacterial virulence without promoting resistance. Targeting the trans-translation system may pave the way for innovative therapies against multidrug-resistant A. baumannii.