Yes, bacteria can grow in tea that has been brewed for a long time. This is especially true if you sweeten your tea with sugar or another sweetener. If tea leaves aren't stored right, bacteria can also grow on them. But this doesn't happen very often. I have kept some teas for years, and none of them have ever grown bacteria that I could see. Keep in mind that bacteria are not caused by cloudy tea. This has to do with the way the compounds in the tea react when they get cold. Our article about why tea gets cloudy goes into more detail about this.