What can be concluded about a bacterium that varies in shape when cultured in different media?

When a bacterium shows variation in shape depending on the culture media it is grown in, this phenomenon is known as pleomorphism. Pleomorphism refers to the ability of some bacteria to alter their shape and size in response to environmental conditions, including the composition of the growth medium. This adaptability is significant because it can affect the bacterium's metabolic activities, virulence, and survival strategies.

In contrast, the other options do not accurately describe the observed change in shape. While genetic mutations can lead to changes in some bacteria, they do not typically cause variations based on the media they are grown in. Spore-forming bacteria may have distinctive shapes related to their spore forms, but this characteristic does not directly correlate with changes based on growth media. Similarly, while some pathogenic bacteria can be pleomorphic, not all pleomorphic bacteria are pathogenic, making it insufficient to conclude that a bacterium's shape variability is indicative of pathogenicity. Therefore, exhibiting pleomorphism is the most fitting conclusion for a bacterium that varies in shape under different culture conditions.