Strengthen the Barrier ...
It is virtually impossible to prevent bacteria such as Listeria in dairy and food because there are multiple strains and new strains continuing to form. Listeria is a natural bacteria present in many foods such as deli meats, raw dairy products and even raw sprouts. Numerous dairies have closed because they could not prevent all Listeria monocytes. The best prevention is to strengthen our stomach acid so that should our bodies encounter Listeria, it will be quickly neutralized preventing any symptoms from developing.
The first line of defense in our body is our physical barrier. This is an actual sealed structure which prevents what is not "us" from reaching what is "us". Your skin provides a waterproof envelope that separates you from everything outside of you. That way what wants to enter the body has to penetrate your skin or enter through an orifice such as your mouth.
Mucous membranes line each of your orifices (think of your nose) and when something irritating or foreign enters that area, mucous is produced to stop it from progressing and to give your immune cells time to identify it for further action. Chronic sinusitis, allergies, mucous colitis are all conditions where there are foreign particles that have become lodged in the membranes or where molecules that mimic dangerous substances are mistakenly identified by the immune system.
Other defensive secretions include tears, saliva and stomach acid. Long-term use of antacids actually open up the stomach lining to bacteria such as H. Pylori because stomach acid has a strong antibacterial action to protect the gut from pathogenic bacteria in the food you eat or water you drink.