I wouldn't think mold would form if the bottles are sealed tightly with the liquid in them...but if they are let to sit too long...then yes mold will form. Most plastic bottles do not form an air tight seal...therefore allowing mold growing bacteria to enter.
Also when washing your bottles make sure they are washed in hot soapy water...and dried completely to lessen the chance of mold.
Anything will grow mold or spoil eventually no matter what you put it in. Simply wash as instructed by angela5271, and be sure to refrigerate whatever it is you are putting in the bottle when you are not drinking it or it has sat out for a while (say after about 30 minute-1 hour) This should help keep your beverage fresh and unspoiled for a longer amount of time.