Answer:
Water (and some liquids like milk), warmness/humidity
Some more factors include oxygen, acidity, and pH levels
Explanation:
As most of us can see with milk, (a liquid) placed in a warm environment will rot. That means bacteria has grown.
Another example of bacteria using oxygen is shown where some food companies put food in vacuum packaging. This prevents oxygen from entering (just like us, bacteria needs oxygen to grow). Puncturing a hole in vacuum packaged food will most likely cause it to go bad.
And finally, acidity and pH levels. High acidity will cause some bacteria to die. Very low acidity may cause some other bacteria to die. These acidity levels (or pH levels) must be neutral for most bacteria to grow. (Although some may need very high or very low levels). pH levels 6.5-7 are ideal for bacterial growth.
I hope this helps. (note I am not a scientist so some of this may be wrong)
I pulled this off of some science textbooks and workbooks.
