Jatin,

Store your TBST stock solution in freezer. Adding Sodium azide to the solution when making the primary antibody dilution can rectify your problem(Blocking solution is rich source of food for microbes).  *Wash blots properly after primary antibody to remove sodium azide before incubating with HRP-secondary antibodies.

Hi, 

If its precipitation reaction than it has to be appear in TBST buffer itself but if you are observing it during prepartion of primary antibody with blocking solution than its kind of contamination.It would be better option to store your TBST solution in 4 degree always and add freshly prepared 3 % BSA  ( instead of readymade available blocking solution).It is best way to escape such problem.

Thanks to both of you. I reckon it's the casein I am using for blocking that could be causing microbial contamination.   Is the membrane still good to go for the addition of the secondary antibodies despite the contamination, you think?