Although most bacteria cannot colonize in the high acidity and low oxygen conditions in brewing beer, the bacteria that can cause a lot of problems for beer makers. In addition to contaminating a supply, growing bacteria compete for nutrients with the yeast that brewers use for fermentation.
Probrewer.com lists types of lactic acid bacteria that are most often found to be contaminants of beer. These microorganisms can grow even in conditions of low oxygen and high acidity. They drastically disrupt the intended taste of the beer.
To prevent contamination, brewers must commit to an extremely strict practice of keeping beer-making materials sanitized. Check out this video sponsored by the American Society for Microbiology to learn more about this phenomenon.