As stated above you do have to prove your competence to perform the analyses in your internal lab. However (and i state this as a Quality Manager in an ISO 17025 accredited lab) you don't want to get involved with the process of accrediting your methods or performance tests. They are time and budget consuming.
So the easy and acceptable way would be for you to have a schedule of proving your continuous competence by testing samples for each parameter you want to test in your internal lab against an external accredited lab.
A small trick there is to make sure that the accredited lab is actually accredited for the specific parameters you want to be testing (eg it could be accredited for E. coli in water and not in food). This way you will have more trust in your methods and also it could put an auditor "in his place" if he went as far as asking the external lab's scope of accreditation to cross-check it.
Since microbiological test methods are pretty stable through the years (as compared to chemical), you could perform a light annual schedule to validate your methods through a rotation. eg If you have 6 total parameters you could do them all once in 2021 to set up a basis and then rotate them like 3 in 2022 and the other 3 in 2023 and stick to this unless some mind-blowing change takes place!!!
Have in mind that you only need to prove that your method is working or even simpler that you get the same result as the external lab in each specific parameter. The method itself that you and the external lab use is kind of irrelevant. You only need to show the same end results.
Hope it helps,
Edited by Evans X., 19 February 2021 - 12:36 PM.