Bladder is sterile in males and females Altered conditions (change in pH) can cause potential pathogens in the urethra (E.coli and P.mirabilis) to multiply and become pathogenic --usually cause UTIs in women Vagina is weakly acidic and contains glycogen -Lactobacillus acidophilus, organism in the vagina, ferments glycogen and produces lactic acid -Lactic acid maintains a local acidic environment