Introduction: Listeria monocytogenes is a foodborne pathogen characterized by its capability to invade and replicate in multiple human cell types and to spread from cell to cell. In addition to known virulence factors, also the absence of the late cell division protein DivIVA leads to severe virulence defects (1, 2). Based on the characteristic cell-chaining phenotype of the L. monocytogenes divIVA mutant, the question arose how the cell shape effects in vitro virulence. Therefore, we here compare the virulence properties of coccoid and filamentous mreB (involved in lateral cell growth) and ezrA (involved in Z-ring formation) mutants to the cell-chaining phenotype of the divIVA mutant.

Material and Methods: IPTG-dependent mreB and ezrA depletion strains were constructed and the presence of the expected coccoid and filamentous phenotypes was verified during growth under standard laboratory conditions and in in vitro infection experiments. All cell morphotypes were tested for replication, invasion and cell-to-cell spread in various eukaryotic cell lines. Additionally, the expression of actin tails, required for movement and spreading during infection, was analysed by phalloidin staining.

Results: The coccoid and filamentous growth of the mreB and ezrA depletion strains did neither impair the multiplication in infected macrophages nor did it influence the invasion in human hepatocytes, as opposed to the replication and invasion deficient divIVA mutant. On the other hand, filamentous growth of the ezrA depletion strain led to reduced plaque sizes compared to the wild type. No cell-to-cell spread could be observed for the coccoid growing mreB depletion strain as well as the divIVA mutant, which coincided with a reduced abundance or absence of actin filaments.

Summary: Our findings indicate that the cell shape plays a minor role for efficient in vitro infection. The severe virulence defect of the divIVA mutant therefore seems to be associated with a DivIVA-dependent influence on other virulence factors or their release via the SecA2 secretion route (1).

1 – Halbedel et al. (2012), Mol Micro, 83: 821-839.

2 – Kaval et al. (2014), Mol Micro, 94: 637-654.