If expressed breastmilk is being used in the bottles, then washing them in warm, soapy water is fine. But if formula is being used in the bottles, the bottles will need to be washed in warm, soapy water and then sterilised. (If thrush is suspected, no matter what type of feeding, bottles should always be sterilised between feeds to prevent re-infection.) Bottles should always be put in the refrigerator after sterilising, as bacteria doesn't multiply as quickly in a cold environment (and sometimes it may be 12-24 hours before we use that bottle). And none of us want our babies to get sick!