Occasionally, plates like the ones inoculated as part of this exercise are allowed to remain in the incubator for extended periods of time (two weeks or more). When the plates are examined, the agar is desiccated and Halobacterium salinarium is seen growing on plates labeled 5% NaCl and 10% NaCl, despite the fact that H. salinarium requires a minimum of 13% NaCl for growth. Explain why this happens.

Answer and Explanation:

This happened because when the agar was desiccated, all the water was released, but the solutes used in the culture medium remained in the petri dish, that is, all the nutrients remained available to H. salinarium, which survived by consuming these nutrients. Although the growth of H. salinarium requires a higher level of NaCl than the level left in the petri dish, we can consider that some colonies of H. salinarium adapted to the low level of NaCl and managed to survive, passing this characteristic to new colonies.