Physiological and transcriptional responses of lactic acid bacteria to sub-inhibitory cadmium exposure

Cadmium is a widespread environmental contaminant that can adversely impact microbial growth and metabolism. While the ability of Lactic acid bacteria (LAB) to bind and sequester cadmium has been extensively investigated, less is known about their physiological adaptation to sub-inhibitory cadmium exposure.

Following the initial determination of the minimum inhibitory concentration (MIC), four selected LAB strains were exposed to sub-inhibitory cadmium concentrations (1/8, 1/4 and 1/2 × their respective MIC values) and investigated using a combined physiological and transcriptional approach. Growth performance, culturability, viability, and acidification activity were assessed alongside the expression of cadA, cadD, mntH, and czcA, genes involved in metal transport, homeostasis, and detoxification, respectively.

The results revealed marked strain-dependent differences in cadmium tolerance and adaptation. Even at sub-inhibitory concentrations, cadmium affected bacterial physiology by reducing growth and acidification activity. Limosilactobacillus reuteri showed the strongest reduction in relative growth, which decreased to 18.8% of the untreated control at $\frac{1}{2}$ × MIC, while Nile Red fluorescence increased up to 5.61-fold, indicating pronounced alterations in the membrane lipid environment. LIVE/DEAD analyses demonstrated a concentration-dependent increase in the population of membrane-compromised cells, although proportional losses in cell viability did not always accompany reductions in culturability. Nile Red fluorescence analyses further revealed significant alterations in the cellular lipid environment, suggesting membrane remodeling as an important component of the adaptive response to cadmium exposure. Transcriptional analyses further showed heterogeneous expression patterns among strains, suggesting that adaptation to cadmium is regulated through strain-specific responses rather than a shared regulatory strategy.

Overall, these findings indicate that LAB responses to cadmium involve strain-dependent physiological and transcriptional changes. The study highlights the importance of considering strain-specific traits when evaluating bacterial responses to metal stress and provides a basis for future investigations into the molecular mechanisms underlying adaptation to sub-lethal cadmium exposure.