Klebsiella bacteria that lives asymptomatically in the human gut, but gains antibiotic resistance via genes from ingested bacteria.

Mycobacterium tuberculosis which lives in the human lungs, multiplying when the immune system is compromised.

Rhizobium bacteria that invade the roots of legume plants, form nodules and fix atmospheric nitrogen in exchange for carbohydrates.

Bacteria which live on the human skin and gain nutrients from secretions, but do not harm the host.