we never carry around milk in our bottles. always put the powder in a container and just carry around warm water in the bottles. that way you dont get clumps of milk sitting in the teat.
if you do happen to drop the teat on the ground and you are out, i would just get some boiling hot water and put the teat in it for 5 mins. any place that sells coffee should be able to provide this for you. also the bottles only stay sterile for a certain amount of time anyway so if you are worried about them being unsterile then you would be sterilising them constantly!