Listeria

I have seen the presence of Listeria (non pathogenic species only) in a variety of samples during this first six months of our testing program. I am not surprised because several years ago a study was done in 1993 (approximate year) by Todd Pritchard of UVM where he documented the incidence of Listeria species in the environment of milk processing facilities in the state of Vermont and found it to be statistically higher in farmstead environments than in plant (factory) environments. I think that we have learned a great deal about Listeria and where it may get a niche for growth from this program. The most common place Listeria has been found is in floor drains. One can then imagine how it spreads through wet areas to reach drain tables and drain matting and cheese hoops in the cheese room or onto frames and racks and boards in the caves. Listeria is salt tolerant (up to 20% salt brine) so the testing of the brines has been a good development out of this project.

I read somewhere recently that Listeria is in 5% of raw milk tanker supplies in the USA. So, based on this information and Pritchards study, one can assume that it is in the farm environment.

Listeria can get in from soil, silage and manure. I suppose the bacteria migrate onto the cows udders, flanks, etc. These bacteria can also be brought in on boots that have been around barnyards. The Listeria monocytogenes bacteria is especially virulent towards elderly people, kids, and pregnant women. In addition Listeria is a psychrotroph so it can survive the cold and sometimes finds a niche in coolers if they are not kept clean and dry enough. Standing water can often be a source of Listeria. Milk processors have to be especially careful about keeping machinery very clean because creameries are usually pretty wet places. The temperature of the cleaning water really needs to be hot to make sure that Listeria is killed because it can survive injury by sanitizers and cleaning chemicals. In fact some research done in France recently has shown that Listeria will survive on wooden boards used for ripening cheese that were cleaned by chemicals but did not survive on boards cleaned by cold water. This is the concept of competition between species and shows that Listeria is able to repair itself better after cleaning and sanitizing than the beneficial microbes can. So, to prevent it, keep your place clean and dry and make everyone change footwear before they come into the creamery. Footbaths are only effective if they are cleaned and refilled every day, which is why I like to change footwear instead of using them in farmstead cheesemaking operations. That way everything has a chance to dry out between cheese makes. On the farm side just remember that it is going to be in the environment of the farm so your job is to prevent it from spreading from the cows, sheep, or goats into the milk. Clean and dry is the best defense.

In fact some research done in France recently has shown that Listeria will survive on wooden boards used for ripening cheese that were cleaned by chemicals but did not survive on boards cleaned by cold water. This is the concept of competition between species and shows that Listeria is able to repair itself better after cleaning and sanitizing than the beneficial microbes can.

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