Question

In a petri dish, there are 50+ colonies and each colony is small in size. In another petri dish, there are only 10 colonies but they are bigger in size. Why does this occur?

Answer 1

The observed change can either be due to the nature of the microorganism plated on the two plates, or due to the plates themselves which may be different in their composition.

For example if equal the same microbial culture was used to inoculate both plates, then we can assume that the antibiotic/antimicrobial concentration is higher in the second plate compared to the first plate. If we were dealing with bacteria for example, the first plate may contain antibiotics below the resistance threshold of the bacteria, but still close to the minimal inhibitory concentration (MIC), and hence multiple bacteria may be able to replicate and form colonies albeit slowly, forming many small colonies (these are called colony forming units or CFUs). The second plate may contain a higher concentration of antibiotics, and hence only those bacteria in the inoculum that possess a higher resistance threshold can survive. On this plate, most of the bacteria won't be able to replicate, but the ones that are tolerant to the high antibiotic stress can grow and flourish, since they don't have to compete with many others for resources. A similar phenotype can be observed if the two plates differ in the concentration of some essential amino acids which the bacteria cannot synthesise by themselves. In this case, the second plate may be deficient in that amino acid,making growth and replication impossible for individuals who cannot synthesise that amino acid de novo. Here too individuals that harbour mutations that help them to synthesise that amino acid can survive and replicate on the plate, forming large colonies. This is because the number of mutants with a particular mutation in any given population is very small. Such results can be obtained from replica plating experiments where the same colonies of bacteria that are obtained after mutagenesis are plated on selection plates deficient in one or more essential nutrients, to screen for specific mutants.

The alternate explanation is that the plates are identical, but the microorganisms plated on the two plates are different. Here too one can expect the individual characteristics of the microorganism to influence its growth and survival on the plate which may be why one plate has a larger number small colonies (for example bacteria) while the other has smaller number of large colonies (for example fungus).

I hope this helps :)