Good Bacteria versus Bad

Last week at our Local Cheeses, Dairy Farms, and Cheese Lovers presentation at Gainesville’s The Atrium senior community, I was asked a question to which I responded inadequately. It was a very good question, but I had not anticipated it and was unable to give it the full explanation it deserved. So I’ll use this blog post as an opportunity to try to do it right.
 
The question went something like this: “You said that the good bacteria used in cheesemaking overwhelm and outcompete the bad bacteria. My question is how do they do that?”
 
So here is what I hope will be a better answer.
 
1)    Oxygen is in short supply in milk. Bacteria in the starter culture (Lactococcus lactis or similar) thrive in the absence of oxygen, a condition that many other bacteria can't tolerate.
2)    Starter bacteria are added in overwhelming numbers so they will be far more abundant than other bacteria in the milk.
3)    The milk is at a temperature favorable for growth and reproduction of the starter bacteria, but less so for some less desirable bacteria.
4)    In growing and reproducing explosively, the huge numbers of starter bacteria rapidly deplete the supply of the sugar lactose, the most available food, converting it to lactic acid.
5)    The accumulation of lactic acid makes the milk more acidic, increasing its acidity as much as 100 fold, and  making conditions unsuitable for many other bacteria.
6)    The good bacteria are helped in their competition by cheesemakers who alter the selective environment by adding salt to the curd, creating conditions that inhibit many bacteria.

There may be even more to it if one were to delve more fully into the microbial environment, but if I could have given the above answer to the questioner, I would have felt much better about having provided a worthy response.