Question

Suppose you leave one agar plate uninoculated, and after the one week incubation period you observe bacterial colonies growing on it. What would you conclude? Think about the agar media, and assume that the lid was never removed.

Answer 1

Uninoculated sterile agar plate, should not show bacterial growth even after one week of incubation.

Obsevation of bacterial growth in unopened plate would mean that bacteria was present in the medium or the petriplates had initial load of bacteria or we have made some mistake in sterile handling while preparing plates.

To prepare agar plate, we subject medium to sterilization by wet heat generally for 15 min at 15 PSI. The method of sterilization, temperature and  time of sterilization depend on the heat sensitivity of media component. For media with heat sensitive components we filter sterilize these components and add it to sterile medium. Also if medium components have higher microbial load or for spore forming bacteria, it will require more time for sterilization to kill all bactria and spores.

Thus, observation of bacterial growth on un-opened agar plate will indicate reduced efficiency of medium sterilization