Going braless is de rigueur these days.
I wouldn’t refer to them as “germs.” The bacteria present on the bra reflects the normal flora of the skin that the bra is in contact with. Outside the gastrointestinal tract the moist armpits (axillae) are areas of or body that are most heavily colonised with our normal flora. This diverse zoo of microorganisms known as the microbiota are generally harmless and even beneficial for human health.They keep us healthy and protecting us from disease. They include Staphylococcus epidermidis and other coagulase negative Staphylococci, Corynebacteria (diphtheroids), Micrococci and less frequently occasionally Gram negative organisms. In healthy individuals these are generally not harmful.
With the build-up of human sweat and skin oils the amount of bacteria may increase and impact the skin microbiome. Some of the less protective fast growing bacteria such as gram negative organisms may have a growth advantage and predominate. Lots different of factors may contribute to this build up at different rates – [such as] presence of skin conditions. My daughter suggested boob size! She is well endowed. Others include the presence of skin conditions such as eczema, hormones, and antibiotics.
For reference, here’s a pic of a 24 hour culture from a bra worn for three days, showing part of the normal flora – a coagulase negative Staphylococcus (Staphylococcus epidermis). No other organisms were cultured on this occasion:
Generally speaking they would not be harmful unless broken skin was present.
Level of hygiene is just one factor that can affect the skin microbiome. Climate is another. Processing of sweat by staphylococci and corynebacteria result in the malodour [stinky smell] characteristically associated with sweat in humans and for me this would determine how often the bra should be washed – hot humid conditions much more frequently than cool dry environments.