As already stated here, Listeria specifically as a gram-positive bacterium does not survive in beer as long as the pH is properly adjusted, and sufficient hops is present. It would be incorrect to say that all gram-positive bacteria cannot survive since some can survive for up to 30 days in low alcohol beers and non-alcoholic beers. Lower storage temperatures can increase this time.
Listeria is commonly found in meat and food processing plants but rarely in breweries. The main source of contamination here is touch, but in a brewery, I see this only as a potential problem when NA beers are produced. And even there, sufficient hops will kill this particular bacterium quickly.
I find here in the US that some food inspectors are not familiar with the unique properties of beer and its unique defense mechanisms against pathogens. Again, this only applies to regular beers and low alcohol beers and non-alcoholic beers certainly require either pasteurization or preservatives to protect the consumer from potential harm.
There are quite a few papers about Listeria and beer and a quick google search will produce abstracts that you can show your inspector and likely take this subject off his or her mind.
Cheers,
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Dirk Loeffler
Loeffler Hygiene & Safety Solutions
(404) 229-7583
loeffler.dirk@loefflerhygiene.com------------------------------