Evolution of Bacteria Shapes

Bacteria colony. Muntasir du

In the classification of living things, the Bacteria domain consists of prokaryotic single celled organisms that usually have a cell wall surrounding them made of peptidoglycans. These bacteria can live individually, or in a colony. Since these prokaryotes are living things, they go through evolution just as larger organisms do. Their traits are controlled through natural selection and can adapt over generations to form new species of bacteria.

One of the main features of bacteria are their shapes. Just as cells of different types vary in shape based on their functions, bacteria also vary in shape. The bacillus, or rod shaped bacteria, are thought to be the most ancient of the bacterial shapes. The oldest known fossils that have been discovered in Canada are these bacillus shaped bacteria. Over time, various environmental pressures acted upon the bacteria and other shapes evolved. Coccus bacteria are round shaped and spirilla bacteria are spiral shaped. How, and why, did these shapes evolve? There are several hypotheses as to how these changes occurred.

Nutrient Ingestion

One of the most important of life activities for all living things, and even bacteria, is acquiring energy. Some bacteria are photosynthetic or chemosynthetic and can create their own energy either from sunlight or chemicals. However, many types of bacteria require a source from which to get their nutrients.

Most heterotrophic prokaryotes surround and engulf nutrients and bring them inside of the cell to digest and use as energy. It is thought that round shapes were able to perform this sort of nutrient uptake more efficiently than the ancient rod shaped bacillus bacteria.

Movement and Locomotion

One life process that requires a large amount of energy is movement or locomotion.

This is especially energy consuming for a single celled organism. Many unicellular prokaryotes move by changing shape. The ability to move can be very important to some types of organisms. This would contribute to the evolution of different shapes of bacterial cells. Over time, bacteria that have adapted to being able to change their morphology using the least amount of energy have been selected for and energy wasting shapes would not be beneficial and were selected against over time. Of course, not all types of bacteria necessarily need to move by changing shape and instead have appendages like flagella that can control locomotion. The evolution of different shapes would not be as imperative in these groups of bacteria.

The ability to move can also help bacteria avoid being eaten themselves. Some larger types of bacteria are preyed upon by protozoans. This is a classic predator-prey relationship that leads to the evolutionary arms race known as coevolution. Changing morphology to create locomotion can help the bacteria escape the predation by the protozoans.

Adhesion

Many bacteria, including infectious types of prokaryotes, often rely on sticking to other cells throughout their lives in order to survive and reproduce.

These single celled organisms usually have an outer layer called a capsule that is sticky to help them band together in a colony or to a host organism's cells. The shape of a bacteria's capsule has evolved depending on its purpose. Coccus bacteria, the ones that are round shaped, can form colonies in the shape of a single strand (called strepto), diplo (two individual bacteria stuck together), or a configuration that looks like a bunch of grapes (staphylo). Other shaped bacteria can also take some of these colony shapes, or are better off as individual bacteria.

Antibody Resistance

Another evolutionary adaptation for bacteria is the ability to resist antibody attacks. Scientists hypothesize that small changes in morphology may contribute to this resistance. Clearly this would be a favorable adaptation since infectious bacteria are killed off by antibiotics.

If only the antibiotic resistant bacteria remain in a colony, they will survive and reproduce and thrive. That means they will continue to evolve and adapt over time.

Microevolution of Bacteria

More research is being conducted on the evolution of bacteria shape. Some of this research is focused on the microevolution of the bacteria and changes in DNA. The sequence of nitrogenous bases on each nucleotide subunit determines the traits of the bacteria. It has been found that the shape is also dependent upon the ratio of adenine and thymine to guanine and cytosine. As the DNA mutates, either by chance or by evolutionary design, the morphology of the bacteria will continue to change and evolve along with it.