Final answer:
Listeria monocytogenes is a gram-positive psychrophile that can grow under refrigerated conditions, making it a concern for food safety and leading to listeriosis.
Step-by-step explanation:
The characteristics of Listeria monocytogenes indicate that Option 3: Psychrophilic nature and Option 4: Ability to grow in refrigerated conditions are correct descriptions for this bacterium. L. monocytogenes is a gram-positive, facultative anaerobe that prefers cold environments, which is characteristic of psychrophiles. Unlike Option 2, it does not have a gram-negative cell wall structure. This organism's ability to reproduce at low temperatures found in refrigerated environments makes it a notable concern for food safety, as it can lead to listeriosis, a serious infection particularly dangerous for pregnant women, neonates, the elderly, and the immunocompromised. The growth of L. monocytogenes at refrigerated temperatures and its tolerance to high salt concentrations facilitate its survival and proliferation in contaminated food products such as deli meats and soft cheeses.