Sterilize the drain piping with chlorine bleach in water 1/2 cup to 1 cup per gal of water. You could just pour this down the drain and let it sit for 20 min and then wash it out. This however might not come in contact with the upper part of the drain pipes.
You could try to fill the piping up to the air admittance pipe by temporarily removing the air admittance valve and inserting the right size of hose or a rag securely fastened to a rod or to a drain snake down the air admittance pipe to block the drain. Then pour the chlorine bleach solution down the drains so that the piping is filled with the diluted bleach. You might have to move the rag up and down at first to let the air out. The goal is to completely fill the piping with the bleach solution. Let sit for 20 min and then drain. After another 20 min rinse with water from the tap.