not just an issue of bacteria, the system uses a refrigerant gas and there is also an oil in the system which lubricates seals and o-rings between components, if you dont run the A/C the gar doesnt get circulated nor does the oil, this means the seals etc are more likely to dry up shrink back or perish and leak gas therefore meaning the system will not work and need regassing, i use mine at the very least once a week. and remember its not only good for lowering the temp, the system takes moisture out of the air so will help clearing steamed up windows in the winder/rain.