Usage patterns
〈物〉に菌がつくfor germs to get on something
If you don't wash your hands well, germs will get on them.
菌が繁殖するfor bacteria/fungi to multiply
Bacteria tend to multiply easily in damp places.
菌に感染するto become infected with a bacterium/fungus
You can get infected with bacteria through a wound.
菌を培養するto culture bacteria/fungi
They are culturing that bacterium in the lab.
菌を殺すto kill germs