Preliminary species-dependent growth responses of selected oral bacteria to e-cigarette Liquids and aerosol extracts: a single-species in vitro study.

BACKGROUND: The increased use of electronic cigarettes (e-cigarettes) has raised concern regarding their potential impact on oral health. E-liquids contain a complex mixture of chemical constituents that may influence the growth of selected oral bacterial species under controlled in vitro conditions. However, comparative data evaluating commercial flavour categories, labelled nicotine concentrations, aerosol extracts, and undiluted e-liquids across both cariogenic and commensal oral bacteria remain limited. This study aimed to provide a preliminary assessment of growth responses of selected oral bacteria representing cariogenic, acid-tolerant, early-colonising, and health-associated species to e-cigarette aerosol extracts and e-liquids with different commercial flavour labels and nicotine concentrations under controlled single-species conditions.

METHODS: In this in vitro study, aerosol extracts derived from 18 e-liquids with varying commercial flavour labels and nicotine concentrations (0, 14, and 20-25 mg/mL) were tested against selected oral bacterial species. Two independent bacterial cultures were evaluated, with three analytical replicates per condition within each biological repeat. Single-species planktonic growth was monitored over 48 h, and area under the curve (AUC) values were calculated. Growth-inhibitory activity of aerosol extracts and undiluted e-liquids was evaluated using an agar disk diffusion assay. The pH of undiluted e-liquids was also assessed as a semi-quantitative screening measure.

RESULTS: Descriptively, most AUC means at the product level were below the untreated growth control, with species-dependent magnitudes of change; the largest reduction was observed for Actinomyces naeslundii (A. naeslundii). No consistent pattern was observed across commercial flavour categories or labelled nicotine concentrations. In the separate Streptococcus mutans (S. mutans) direct-exposure experiment, mean AUCs were 72.5-82.5% lower than the untreated growth control, and undiluted e-liquids produced zones of growth inhibition in the agar diffusion assay, particularly in flavoured formulations. Approximate pH screening values ranged from 4.0 to 5.0.

CONCLUSIONS: These preliminary observations suggest species-dependent growth responses to e-cigarette aerosol extracts and marked growth inhibition by undiluted e-liquids under the tested conditions. Because biological replication was limited (n = 2) and the chemical composition and storage stability of the aerosol extracts were not analytically verified, the findings should be interpreted descriptively and require confirmation in larger, chemically characterised, clinically relevant biofilm models.