The transcriptional regulator SpxA1 influences the morphology and virulence of Listeria monocytogenes

ABSTRACT Listeria monocytogenes is a gram-positive facultative anaerobe and an excellent model pathogen for investigating regulatory changes that occur during infection of a mammalian host. SpxA1 is a widely conserved transcriptional regulator that induces expression of peroxide-detoxifying genes in L. monocytogenes and is thus required for aerobic growth. SpxA1 is also required for L. monocytogenes virulence, although the SpxA1-dependent genes important in this context remain to be identified. In this work, we sought to investigate the role of SpxA1 in a tissue culture model of infection and made the surprising discovery that Δ spxA1 cells are dramatically elongated during growth in the host cytosol. Quantitative microscopy revealed Δ spxA1 also forms elongated filaments extracellularly during early exponential phase in rich medium. Scanning and Transmission Electron Microscopy (STEM) analysis found the likely cause of this morphological phenotype is aberrantly placed division septa localized outside of cell midpoints. Quantitative mass spectrometry of whole cell lysates identified SpxA1-dependent changes in protein abundance, including a significant number of motility and flagellar proteins that were depleted in the Δ spxA1 mutant. Accordingly, we found that both the filamentation and the lack of motility contributed to decreased phagocytosis of Δ spxA1 by macrophages. Overall, we identify a novel role for SpxA1 in regulating cell elongation and motility, both of which impact L. monocytogenes virulence. IMPORTANCE Listeria monocytogenes is an environmental saprophyte and the causative agent of the serious foodborne disease listeriosis. L. monocytogenes uses a highly complex transcriptional network to rapidly respond to the transition from living in the environment to infecting a host. One of the regulators important for L. monocytogenes pathogenesis is SpxA1, although the SpxA1-regulated genes necessary for virulence remain undefined. This study set out to examine the role of SpxA1 during infection and discovered that SpxA1 regulates cell shape and motility during both extracellular and intracellular growth. Together, our analyses find that SpxA1 has a multifaceted role in virulence, as SpxA1 regulation of both cell morphology and motility are important for infection.