It's relatively low. The fermentation process tends to increase acidity and lactobacilli then out-compete other bacteria in that environment. That said, the advantage of fermentation in terms of food safety is that in general if you have a batch turn on you, you will know it. Anything that would make you sick you would not want to put in your mouth in the first place. (Unlike, say, meat, where you can have a perfectly tasty piece of meat that will still make you violently ill.)
