Organic acid-associated antimicrobial and antibiofilm activity of Lactobacillus rhamnosus ATCC 7469 against Acinetobacter baumannii and Staphylococcus aureus

Abstract Considering the relevance of biofilms in bacterial resistance and the need for innovative therapies against pathogenic microorganisms, alternative strategies using probiotics as antimicrobial agents have emerged. This study aimed to evaluate the antimicrobial and antibiofilm activities of the cell-free supernatant (CFS) of Lactobacillus rhamnosus ATCC 7469 against biofilm-forming strains of Acinetobacter baumannii and Staphylococcus aureus . The activity of CFS was evaluated against A. baumannii ATCC 19606, A. baumannii 301, S. aureus ATCC 12598, and S. aureus ATCC 29213. Antimicrobial activity against planktonic cells, inhibition of biofilm formation, and destruction of mature biofilms were assessed at different CFS dilutions. The corresponding concentrations of lactic and acetic acids were determined by high-performance liquid chromatography (HPLC). CFS showed high antimicrobial activity, with growth inhibition ranging from 88.26% to 100% at the two highest concentrations tested (197.8/108.7 and 107.0/54.5 mM lactic/acetic acid). At 57.8/28.3 mM, growth inhibition ranged from 60.05% to 81.65%, whereas at 27.2/13.5 mM it ranged from 21.12% to 41.03%. CFS also strongly inhibited biofilm formation, with inhibition rates ranging from 66.17% to 100%, whereas destruction of mature biofilms ranged from 24.75% to 88.11%. HPLC analysis demonstrated the presence of lactic and acetic acids in the CFS, with lactic acid being the predominant organic acid detected. Neutralization of the CFS to pH 7.0 markedly reduced its antimicrobial and antibiofilm activities, supporting an important contribution of acidity and acid-associated components to the observed effects. Overall, L. rhamnosus ATCC 7469 CFS exhibited substantial antimicrobial and antibiofilm activity against A. baumannii and S. aureus , supporting further investigation of Lactobacillus -derived CFS as a potential local antimicrobial strategy.