Bra hygiene is fascinating because practices involving washing and use vary widely. I have had patients that tell me they never wash their bras unless absolutely necessary. They worry bras are pricey and lose their shape once they are washed. And, I have other patients that wash them constantly because they feel like they sweat so much they have no choice!
The answer to how frequently to wash is not just somewhere in between these extremes but also technique of washing. It’s true that some textiles may stretch and lose their shape with repeat washing. Elastic in general breaks down with the heat of the dryer. When the point of a bra is support this isn’t great news. Washing your delicates separately and either hanging them to dry or drying with low heat can help. You can probably go about 3-4 uses before washing.
Textiles vary widely in how much bacteria they hold. Bacteria get caught up in the weave. There are many antimicrobial textiles these days. Often they integrate silver into the weave for its antimicrobial qualities. This can reduce odor and risks of recurrent folliculitis.(one of the most common conditions I treat is the staph carrier state where bacteria take up residence on our body and lead to recurrent infections - the breasts and underarms being one of most common locations). Remember odor is the result of bacteria acting in sweat. If there was no bacteria here there would be no of odor.