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<i>Legionella pneumophila</i> is responsible for causing Legionnaires’ disease and Pontiac fever, also known as legionellosis. The aim of this study was to investigate the mechanistic effect of a mixture of natural antimicrobials (Citrox BCL) in preventing <i>L. pneumophila</i> biofilm formation and reducing its in vitro virulence. The minimum inhibitory concentrations were detected at 0.06%, and the MBC was established at 0.125%. Based on the growth curve profile, the sub-inhibitory concentration of 0.02% was further used to study the mechanistic implications in the absence of a cytotoxic effect on A549 cells. At 24 h post-infection, Citrox BCL reduced (<i>p</i> = 0.005) the intracellular growth of <i>L. pneumophila</i> when the A549 cells or the bacteria were pre-treated with 0.02% Citrox BCL. This result was replicated when Citrox BCL was added during the 24 h infection assay leading to a reduction in intracellular growth (<i>p</i> = 0.003). Herein we show that at the sub-inhibitory concentration of 0.02%, Citrox CBL lowers the ROS levels in infected A549 cells and causes a 45% reduction in <i>L. pneumophila</i> EPS production, a reduction associated with the decline in biofilm formation. Overall, our results corroborate the low c-di-GMP production with the decrease in biofilm formation and low EPS levels. The low EPS levels seemed to be caused by the downregulation of the <i>tat</i>B and <i>tat</i>C gene expressions. Moreover, inhibition of <i>pvc</i>A and <i>pvc</i>B gene expressions, leading to lower siderophore levels, suggests that Citrox BCL reduces the ability of <i>L. pneumophila</i> to sequester iron and reduce biofilm formation through iron starvation.