Evaluating Cell Viability with LDH Assay

Cell viability was assessed using the lactate dehydrogenase (LDH) detection kit (Sigma-Aldrich). To avoid potential interference of bacteria with LDH activity, the intracellular LDH assay was applied (32 (link)). Briefly, 3D A549 cells were washed twice with Hank’s Balanced Salt Solution (HBSS, Life Technologies) and lysed with 1% Triton X-100 (Sigma-Aldrich) by vigorous pipetting. LDH activity was further determined as described in the manufacturer’s protocol. Afterwards, cell viability was expressed as a percentage of the uninfected control. When viability was lower than 80% and a statistically significant difference was obtained compared to the uninfected control (p< 0.05), the commensal bacterium was considered as cytotoxic (33 (link), 34 (link)).