Loofahs are especially vulnerable to bacterial growth. These natural sponges have many nooks and crannies that -- especially when moist -- tend to invite bacteria. Additionally, dead skin cells commonly found on loofahs provide food for bacteria, giving them even more reason to move in. In fact, one study found that the amount of bacteria like P. aeruginosa, which causes a variety of infections, grew exponentially in 24 hours when exposed to a loofah sponge. The study's authors also found that soaking the sponge in a bleach solution on a regular basis killed off the bacteria, and thus helped to prevent infections. http://jcm.asm.org/content/32/2/469.full.pdf