D-111. Cell Death of Macrophages Induced by Enterococci

S. Gröbner, E. Fritz, I. B. Autenrieth;
Univ. Hosp., Tübingen, GERMANY.

Enterococcus spp. are commensal microorganisms in the gastrointestinal tract. However, they can cause a variety of life-threatening infections, especially in immunocompromized patients and nosocomial settings. Cell death induction by enterococci might enable these bacteria to traverse the intestinal barrier and cause disseminated infections of their host. We show that enterococci of different species induce cell death in J774A.1 macrophages. Interestingly, only enterococci exposed to lysozyme were able to induce cell death in macrophages. Flow cytometric analyses of macrophages infected with enterococci revealed loss of cell membrane integrity indicated by propidium iodide uptake and decrease of the inner mitochondrial transmembrane potential Δψm. Microscopic analyses demonstrated morphologic characteristics of necrotic cell death. Inhibition of caspases, treatment of cells with cytochalasin D or rifampicin did not prevent cells from dying suggesting a caspase-independent mechanism of cell death independent of bacterial uptake or intracellular bacterial replication. Altogether, our results indicate that enterococci under specific conditions (exposure to lysozyme) can induce cell death in macrophages which is independent of caspases and exhibits features of necrosis. Moreover, we assume that a yet unknown enterococcal cell wall component, which is only exposed by treatment of enterococci with lysozyme and directly targets the host cell membrane, might be causative for cell death induction in J774A.1 macrophages. Cell death induction by enterococci in different cell types might be a general mechanism of these facultative pathogenic bacteria accounting for the transmutation of enterococci from commensals to infectious agents.