system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why bacteria visibly grows on a glass plate and not on everything we touch?

Why in labs bacteria grows in different colors and we can see it but we can’t see it when we touch our phones, tables etc?
gpt
The bacterial colonies on an agar plate have millions of bacteria growing on top of one another. The agar contains lots of food, so the bacteria multiply very enthusiastically. 

The different colours are often caused by special genes that are put inside the bacteria. For example, there's a gene that makes a blue colour, which they use to see which bacteria *don't* have the gene the researchers want. 

In the environment, there's much less food, so the bacteria don't multiply as fast. It only takes one bacterium to form a colony though, so if you culture an environmental sample, then they'll also make those obvious blobs.