Scanning electron micrograph (SEM) of Listeria monocytogenes. L. monocytogenes is an aerobic, non-sporing gram-positive bacteria. This bacterium is also a soil saprophyte that may become pathogenic in man, especially in cases of immunosuppression from drugs or disease. The most common manifestation in adults is meningitis. The disease listeriosis is much more severe in newbons, who may have contracted the disease from their mother through transplacental infection.