What prevents contamination during primary fermentation?

Adding a Sterilock (an antibacterial airlock) to your standard bubbler airlock will ensure no bacteria or wild yeasts gain access to the fermenting beverage during the critical first stages of fermentation

Every batch of homebrew is contaminated. You can't hope to have zero airborne contaminants in your wort especially with it just sitting there mostly open during the chill process. (depending on how you chill).

That said brewers yeast loves wort. Brewers yeast starts to create an undesirable environment for wild yeasts and bacteria. Primarily through lowering the pH, then buy using up nutrients and lastly creating alcohols. These things do not necessarily kill wild contaminants, but they create a static growth environ for them. The use of hops also has a bacteria-static force on the contaminants too.

So your wort is contaminated. And what little contaminants are in there, they do start to multiply early on, just as the yeast are trying to as well. But my pitching the proper amount of yeast that lag is short and the good things listed above start to happen. Brewers yeast simply wins out.

(Wort is acidic also which tends to help as well)

• Plastic primary fermenters are made of a food-grade plastic that resists contamination. The plastic has few surface imperfections that would allow bacteria and unwanted yeast to camp out for the next big lunch.
• The cleaning agents used in preparing brewing equipment are brutal. PBW, for example, will take out any organic substance upon which the "bad bugs" will feed, and kill most of them as well. Combine that with an Oxygen sanitizer, and you've effectively wiped them all out.
• Pitching yeast is like a brute force attack. Even when just using a dry yeast packet, you are outnumbering contaminants by a factor of around a billion to one. The yeast will multiply and use up all the sugar before the bad bugs have a chance.

Well, typically it's a little more than a garbage pail with a lid! It's food grade plastic with smooth surfaces and no major scratches so that it can be well cleaned and sanitized. The lid is airtight and the airlock keeps microorganisms out. The real key is good cleaning and sanitization of all your equipment.

However, even with good sanitation, often something can slip in. Even if something undesirable makes it into the wort, it would have a tough time getting a foothold in the environment. Alpha acids from the hops have antibacterial properties against Gram-positive bacteria, but have less effect on yeast, promoting yeast development over other organisms. Within a day or two, alcohol content is high enough to discourage infection as well. Most undesirables that make it into the container will be outcompeted by the yeast or killed off by the environment.

Basically the process of fermentation is what prevents undesirable organisms from taking hold and multiplying in your primary fermenter. As the yeast reproduce they in effect take over the environment and dominate any undesirable organisms present. The production of CO2 also drives the impurities to the top where it creates a blanket over the beer which combats most airborne bugs. Also as the level of alcohol increases it makes it more difficult for foreign organisms to survive let alone multiply.

There are many professional brewers that use open fermenters for primary so a closed system is not necessary to keep the bugs out. A little more care is needed in secondary since there is alcohol present and exposure to air will cause oxidation. This is why a closed system for secondary is more important.