<span>These are:
1. <u>Number of microbes</u>: if there is a lot of microbes, then more time is needed to kill all of them.
2. <u>Characteristics of microbes</u>: some microbes have resistance genes, so they will not react to agents they are resistant to; some microbes have protective structures so treatment will be inhibited.
3. <u>Time of exposure</u>: different agents will need different time to kill microbes.
4. <u>Environment</u>: the effect of temperature, pH, organics both on microbes and agents.</span>
Because it is catalyst which speeds up or slows down the speed of the chemical reaction without being changed or used up