Absence of Antibacterial, Anti‑candida, and Anti-dengue Activities of a Surfactin Isolated from Bacillus subtilis

Abstract

Background: Biosurfactants are biological compounds that possess many pharmacological proprieties. Among them, surfactin is one of the most
active against pathogens of medical interest, such as fungi, bacteria, and enveloped viruses. Objectives: In this study, we aim to evaluate the
antibacterial anti‑Candida, and anti-dengue potential of a surfactin biosynthesized by Bacillus subtilis ATCC 6633. In addition, the fractional
inhibitory concentration (FIC) was calculated to determine the behavior of this biosurfactant in association with antibacterial and antifungal drugs
of clinical use. Materials and Methods: B. subtilis ATCC 6633 culture was maintained on nutrient agar plates, and biosurfactant production
was carried out in mineral salt medium with 2% glucose. The isolated and purified surfactin was used for determination of antibacterial and
antifungal effects by the broth microdilution method. A checkerboard assay was performed to determine the potential synergic effect of
surfactin with gentamicin, penicillin, and nystatin. Finally, the activity against Dengue virus (DENV) was evaluated through quantification of
cell viability after viral infection. Results: Although it had low cytotoxicity (CC50 >400 µg/mL), surfactin was inactive against Gram‑negative
and Gram‑positive bacteria, Candida species, and DENV at the highest concentration tested (500 µg/mL). According to FIC values, none of
the antimicrobials tested showed a synergistic association with surfactin. Conclusions: Surfactin produced by B. subtilis ATCC 6633 is not a
promising antimicrobial agent, and its combination with clinically available antibiotics does not lead to a synergic effect.