L-ascorbic acid exerts anti-microbial and anti-virulence effects against multi-drug resistant Burkholderia cepacia complex.

Burkholderia cepacia complex (Bcc) encompasses a group of multi-drug resistant, Gram-negative bacilli that poses a significant threat to individuals with cystic fibrosis, and the immunocompromised population. Due to limitations with available anti-microbial therapeutics, alternative strategies using synthetic chemical compounds are widely being explored. Here, we evaluated the anti-microbial, anti-biofilm and anti-virulence effects of L-ascorbic acid against a clinical isolate of the B. cepacia complex (henceforth designated as B. cepacia for the sake of simplicity). Antibiotic susceptibility was assessed by the Kirby-Bauer's disk diffusion method, and β-lactamase production was confirmed by the iodometric assay. Broth microdilution revealed a minimum inhibitory concentration (MIC) of 5 mg/mL for L-ascorbic acid against B. cepacia. At sub-MIC levels, L-ascorbic acid delayed bacterial growth and prolonged the lag phase. Furthermore, when combined with a sub-inhibitory concentration of penicillin, L-ascorbic acid exhibited significant growth inhibition, indicating a strong additive effect. At a concentration of 1.25 mg/mL, L-ascorbic acid significantly reduced biofilm (51.30%) and exopolysaccharide production (57.83%). It also impaired bacterial swarming besides suppressing the hemolytic and urease functions of B. cepacia. Silver staining of protein electrophoresis by SDS-PAGE revealed a pronounced reduction in protein band intensity in the isolate treated with L-ascorbic acid (1.25 mg/mL). Docking studies suggest that L-ascorbic acid may interact with biofilm-associated proteins in B. cepacia by binding to key regulatory proteins, including the acyl-homoserine lactone synthase (CepI), the CepR receptor, the BceF tyrosine kinase domain, and PhzA/B. Altogether, L-ascorbic acid demonstrated anti-microbial and anti-biofilm effects, complemented by in silico evidence suggesting possible interactions with quorum sensing-associated proteins, supporting its likely role as an adjunct anti-infective under in vitro conditions.