Given listeria causes approximately 1,600 illnesses and 260 deaths in the USA annually, news that "researchers at the University of Southampton are trialling a device designed to detect the most common cause of listeriosis directly on food preparation surfaces," is very very encouraging:
If the trials of this device prove its effectiveness, it would a huge benefit for everyone, not just the immune compromised, if the use of such technology became mandatory in commercial food preparation.
Mayo Clinic on Listeria Infections:
Note: "Symptoms may begin a few days after you've eaten contaminated food, but it may take as long as two months (emphasis mine) before the first signs and symptoms of infection begin."
I was placed on a 'Low Listeria diet" during a recent hospital stay due to my CLL caused neutropenia. It does tend to restrict your diet choices somewhat.