Because of 12 mins reproductive cycle
bacterias reproduce after every 12 mins . due to human factors such as inventions of new antibiotics, major pollutions in the environment , dirty uncovered water spaces etc. has led to their formation. by the introduction of new medicines for killing bacterial diseases, bacterias have found out new biochemical pathways to survive (which is done due to the change in the DNA protien source). the manufacturing of new chemical compounds have led the pathway for their growth . therefore different species of bacterias formed by the introduction of these acivities multiply at a faster rate which has thus led to their evolution.