And previous experience.And another creatard that does not know the difference between an assumption and a deduction.
We can be very sure that a known bacterial growth agent will work because it has worked millions of times before. The same goes for other materials and times used. Usually an experiment has an unknown in it and that is tested compared to known substances. Conclusions are based upon deductions from the data supplied by the experiment. No need for guessing, presumptions, assumptions etc..
Of course creatards try to foist off their flaws upon others.