Question

why bacteria mutation can be an adequate source of variation

Answer 1

So, bacteria reproduce asexually. This means that every time they reproduce, they are cloning themselves and passing on their exact traits to their "offspring". In this mode of reproduction, these new formed bacteria have no way of being genetically variant. To become genetically varied from other bacteria, they can do a couple of things. In this case, a mutation can allow for genetic variation. Mutations occur naturally, and can SOMETIMES give the bacteria an advantage.
For example, if you put a strain of bacteria in ampicillin, they will most likely die. If you keep exposing the same strain of bacteria to this antibiotic, one MAY eventually contract a mutation and have ampicillin resistance. This means that they can successfully survive and reproduce in the presence of ampicillin. This is why, in some cases, that scientists and doctors are becoming afraid of the bacteria, because somehow they are surviving antibiotics, meaning that medicines have to become stronger to kill them off. It is hypothesized that eventually, bacteria may become so resistant that we will not be able to kill them.