Bacteria evolve quickly because they divide at such a rapid rate. If bacterial division wasn’t kept in check by competing bacteria, the “offspring” of a single cell would cover the whole planet in just a few days! The huge mass of bacteria means that there is lots of opportunities for mutation. Once a mutation happens, and it happens to be a successful mutation (like antibiotics resistance), it gets passed on quickly because the bacteria divide so fast. Bacteria can also transfer resistance genes between each other when they reproduce, which means that a successful genetic feature will very quickly become very common.
One important thing, as Vera says, is that bacteria can transfer genes from one to another. This can be very bad, because if a harmless type of bacteria evolves antibiotic resistance, it could pass its resistance gene on to a more dangerous type of bacteria!
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