April 18, 2008

Describing Unicellular Morphology

Posted in Microbiology tagged , , , at 10:25 pm by D. Borst

Bacteria, like other organisms, come in a variety of shapes and sizes. Unfortunately, the way scientists have described these unicellular entities does is not necessarily related to how we describe shapes in everyday life. Below, I will discuss some of the basic descriptors used to discuss the form of these cells.

Cocci is a term that is used to describe cells that are spherical. These cells are small and compact, and appear to be small round shapes under a microscope. Coccus bacteria can form many different types of colonies, including pairs, or diplococci; chains, or streptococci; tetrads, a square arrangement of four cells in a plane; sarcina, an arrangement of eight cells with the center of each cell at the vertex of a cube; and in grape like clusters known as staphylococci. You the names of these structures probably remind you of various pathogens such as Staphylococcus aureus and Streptococcus pneumoniae, which are organisms that take the forms relating to their names.

Rod shaped bacteria are extended cells that are reminiscent of sticks or dowels. They are also known as bacilli. As they are not perfectly symmetrical, they can form different types of colonies based on the orientation of how they attach. Diplobacilli are rod shaped bacterial colonies that join one another end to end, and streptobacilli are colonies of a large number of bacilli that attach end to end. When bacilli are attached to one another so that their long sides touch, they are referred to as palisades. As may have been guessed, the Bacillus family of bacteria are rod shaped, and form numerous types of colonies.

Spirillia are types of bacteria that are similar to the rod bacteria but having a rigid helical shape. Examples of this morphology include the aptly named genus Spirillium.

Spirochetes are like spirillia in that they have a helically coiled cell, but instead of being rigid, the shape is more flexible. The coils on spirochetes are generally tighter than those on spirillia.

Filamentous colonies of bacteria are long flexible strands of cells, and are a common organizational type. Some bacteria form simple filaments such as cyanobacteria, while others form more complex branched networks.

One consequence of the different morphologies of cells is the variable metabolic rate of these organisms. A sphere is the most compact shape in nature, having the largest internal volume per unit surface area. The surface area to volume ratio is very important, as the cell membrane is the site of many important cell functions. As an organism decreases in volume or as its shape deviates from spherical its surface are to volume ratio will increase.

This increase in relative surface are is extremely important. It means both that 1) the organism has to expend more energy maintaining its outer membrane and 2) it has greater contact with the outside world, meaning that it has greater opportunity to absorb nutrients, signals, etc… For example, lets say that there are two equal-volume cells, one shaped as a spirochete and one as a coccus, in the same medium (i.e. same nutrient concentration). The spirochete has a much more extended form, leading it to have a greater surface area. All considerations other than basic morphology being equal (density of proteins on the membrane, capacity to create ATP etc…) the spirochete cell would be able to bring in more nutrients per unit time than the coccus cell, simply since it has a greater surface area, and thus greater number of transporters.

The comparative biologists in the audience will know that beyond the pure capacities that I have been talking about, metabolic rate also scales with surface area to volume ratio. A possible rationale is apparent for this in the case of multicellular endothermic organisms (Endothermic organisms regulate their body temperature by producing their own heat, and the greater the surface area of an organism, the faster it will loose that heat. Thus as your surface area to volume ratio increases, your metabolic rate will also increase), however it is unclear why metabolic rate scales in the same manner for unicellular organisms.

For students however, it is sufficient to know that metabolic rate does scale with an organisms surface area to volume ratio. A consequence is that prokaryotic organisms will have higher metabolic rates than eukaryotic organisms, since prokaryotes are generally smaller than eukaryotes.

Those who are paying attention may also have realized that colonial organization will also have an effect on the useable surface area of a unicellular organism. Arranging oneself in a colonial fashion has consequences in that it disrupts access of an organism to the medium that it is growing in. A result of this is that the metabolic rate of microorganisms in colonies will often be lower than the metabolic rate of freely floating organisms.

The information for this post was attained during the April 4th Microbiology Class Lecture by Dr. Popa, and online at various Wikipedia articles.

Starting slow, but hopefully Ill sustain.

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