Bacteria are capable of reproducing very rapidly. If any possess a mutation that confers an advantage, it can be passed down during asexual reproduction to future generations. Antibiotics can act as "selecting agents" in the environment. Those with a resistance gene can survive and reproduce and those without this gene, will die off. Bacteria can perform conjugation, whereby two different bacteria can exchange genetic material between them, cell to cell. Usually a transposon or plasmid is needed for this process, which allows a beneficial gene to spread rapidly through the bacterial population. This is the reason that there is always a need for new and stronger antibiotics--bacteria are capable of rapid reproduction and can exchange genes between them.