Bacterial morphology: why have different shapes?

Abstract

The fact that bacteria have different shapes is not surprising; after all, we teach the concept early and often and use it in identification and classification. However, why bacteria should have a particular shape is a question that receives much less attention. The answer is that morphology is just another way microorganisms cope with their environment, another tool for gaining a competitive advantage. Recent work has established that bacterial morphology has an evolutionary history and has highlighted the survival value of different shapes for accessing nutrients, moving from one place to another, and escaping predators. Shape may be so important in some of these endeavors that an organism may change its morphology to fit the circumstances. In short, if a bacterium needs to eat, divide or survive, or if it needs to attach, move or differentiate, then it can benefit from adopting an appropriate shape.

Figure 1

Defenses against bacterivory. Protists can ingest only those bacteria that are “just right” as far as size and shape are concerned (the “Goldilocks effect”) [1]. Pictured are some of the morphological ways bacteria can protect themselves by becoming “not right.”

Figure 2

Example of simple shape adaptations triggered by selective pressures. The upper two rows of “slider bars” represent: 1) the quantity of available nutrients (from Low to High), and 2) the numbers of nearby predators (from Low to High). As these two environmental conditions change, bacteria may respond with morphological adaptations, two of which are illustrated beneath the sliders. As described in the text, one cell (dark blue) elongates or becomes smaller, while the other (light blue) modifies the length of its prostheca. Intermediate conditions may evoke intermediate responses.