Small molecules originating from microbes (SMOMs) play a significant role in bacterial-bacterial communication and in interactions between bacteria and their hosts. The aim of this study is a comparative analysis of the cytotoxic activity to human granulocytes and monocytes of two structurally close SMOMs: N-dodecanoyl-L-homoserine lactone (C₁₂-HSL) and 5-N-dodecyl resorcinol (C₁₂-AR). Cell viability was determined by Trypan blue dye exclusion. To distinguish cell death mechanisms, both necrosis and apoptosis tests were carried out. Cells undergoing apoptosis were identified by caspase-3 activity and the level of histone-associated DNA fragments. To evaluate cell lysis, the lactate dehydrogenase release test was used. In addition, the SMOM’s action on erythrocyte membrane stability was investigated.
The investigated SMOMs in micromolar concentrations showed more dose-dependent cytotoxicity against granulocytes than monocytes, but they used different mechanisms to impinge on the cell death pathway. C₁₂-HSL specifically induced apoptosis similar to the activity previously reported for 3-oxo-N-dodecanoyl-L-homoserine lactone that originated from Pseudomonas aeruginosa. In contrast, C₁₂-AR induced fast cytolytic effects (necrosis) as shown by the release of cytoplasmic lactate dehydrogenase, and presumably were defined by cellular membrane destabilisation.
Our data demonstrate that both C₁₂-HSL and C₁₂-AR can eliminate key defence cells, which would otherwise participate in the destruction of pathogenic bacteria. These results reinforce the SMOMs bifunctionality concept that such bacterial molecules not only regulate bacterial-bacterial interactions but also break immune defences as a new and important mechanism of host evasion.