Facts of LifeAre bacteria our enemy? [Archives:2005/845/Health]

May 26 2005

By: Dr. Khaled alNsour
For the Yemen Times

Hearing the word bacteria sets off an alarm bell to many people – including myself once before – thinking they are our enemy! Well, sometimes they are, but are all bacteria harmful? Will life be the same or better without them? Here is a look at those microorganisms.

Bacteria (singular: bacterium) are often the main causes for many animal and human diseases. These are the pathogenic bacteria which are notorious for diseases as cholera, tuberculosis, and gonorrhea. However, such disease-causing species are a comparatively tiny fraction of the bacteria as a whole. Certain types of bacteria, such as the actinomycetes, produce antibiotics such as streptomycin and nocardicin; others live symbiotically in the guts of animals (including humans) or elsewhere in their bodies, or on the roots of certain plants, converting nitrogen into a usable form.

The morphology of bacteria is simple. A single bacterium (shown in figure 1) lack nuclei that are found in other organisms. Moreover, their DNA forms a tangle known as a nucleoid, but there is no membrane around the nucleoid, and the DNA is not bound to proteins. This DNA forms loops. Bacteria contains plasmids, or small loops of DNA, that can be transmitted from one cell to another, either in the course of sex (yes, bacteria have sex) or by viruses. This ability to trade genes with all comers makes bacteria amazingly adaptable; beneficial genes, like those for antibiotic resistance, may be spread very rapidly through bacterial populations. It also makes bacteria favorites of molecular biologists and genetic engineers; new genes can be inserted into bacteria with ease.

Bacteria do not contain membrane-bound organelles such as mitochondria or chloroplasts, as in more complicated organisms or animal cells. However, photosynthetic bacteria, such as cyanobacteria, may be filled with tightly packed folds of their outer membrane. The effect of these membranes is to increase the potential surface area on which photosynthesis can take place.

The cell membrane is surrounded by a cell wall in all bacteria except one group, the Mollicutes, which includes pathogens such as the mycoplasmas. The composition of the cell wall varies among species and is an important character for identifying and classifying bacteria. In this figure, the bacterium has a fairly thick cell wall made of peptidoglycan (carbohydrate polymers cross-linked by proteins); such bacteria retain a purple color when stained with a dye known as crystal violet, and are known as Gram-positive (after the Danish bacteriologist who developed this staining procedure). Other bacteria have double cell walls, with a thin inner wall of peptidoglycan and an outer wall of carbohydrates, proteins, and lipids. Such bacteria do not stain purple with crystal violet and are known as Gram-negative.

(Figure 1; a single bacterium diagram. Courtesy of University of California, Berkeley)

Bacteria are so widespread that it is possible only to make the most general statements about their life history and ecology. They may be found on the tops of mountains, the bottom of the deepest oceans, in the guts of animals, and even in the frozen rocks and ice of Antarctica. One feature that has enabled them to spread so far, and last so long is their ability to go dormant for an extended period.

Most bacteria may be placed into one of three groups based on their response to gaseous oxygen. Aerobic bacteria (thrive in the presence of oxygen and require it for their continued growth and existence), anaerobic bacteria (cannot tolerate gaseous oxygen, such as those bacteria which live in deep underwater sediments) and facultative anaerobes (prefer growing in the presence of oxygen, but can continue to grow without it).

Classified by the source of their energy, bacteria can also fall into two categories: heterotrophs and autotrophs. Heterotrophs derive energy from breaking down complex organic compounds that they must take in from the environment. The other group, the autotrophs, fix carbon dioxide to make their own food source.

The ecosystem, both on land and in the water, depends heavily upon the activity of bacteria. The cycling of nutrients such as carbon, nitrogen, and sulfur is completed by their ceaseless labor.

To realize the importance of these microorganisms think about what would happen if dead and rotting organisms were not decomposed by bacteria? These dead organisms form the organic carbon which would quickly deplete the carbon dioxide in the atmosphere if not for the activity of decomposers. This may not sound too bad to you, but realize that without carbon dioxide, there would be no photosynthesis in plants, and no food. What bacteria do is a decomposition process, that is; the breakdown of these organisms, and the release of nutrients back into the environment.

The cycling of nitrogen is another important activity of bacteria. Plants rely on nitrogen from the soil for their health and growth, and cannot acquire it from the gaseous nitrogen in the atmosphere. The primary way in which nitrogen becomes available to them is through nitrogen fixation by bacteria such as Rhizobium, and by cyanobacteria. These bacteria convert gaseous nitrogen into nitrates or nitrites as part of their metabolism, and the resulting products are released into the environment. Some plants, such as liverworts, cycads, and legumes have taken special advantage of this process by modifying their structure to house the basteria in their own tissues.

So, after all, bacteria put the tang in the yogurt we eat and the sour in sourdough bread which represents a basic food element in our lives. Bacteria are of such immense importance that we should start giving them our respects rather than our fear!! Have a nice day