Can bacteria form in an uncovered cup, indoors, and at room temperature? If so, how long would it take?

Question

Can bacteria form in an uncovered cup, indoors, and at room temperature? If so, how long would it take?

Answer 1

Bacteria will not "form" but the bacteria that are flying around on particles of dust will certainly get into the cup (assuming the cup was sterillized befoe hand, otherwise, it may not require any new bacteria from the air).  If there is water in the cup, some of the bacteria may find the environment to their liking and start reproducing.    How fast they reproduce depends on what kind of bacteria they are, temperature, pH, oxygen level, how much cholorine there is, amount of sunlight, things like that.  If you want a real active culture, get some water from an outdoor puddle, even a small drop may have thousands and thousands of single celled organisms.  These are much larger than bacteria but they will have bacteria with them.  Or use tap water and a small amount of rich black dirt, it probably has millions of bacteria of many many many different types in a single gram.