Bacteria are unicellular (single cell) organisms that do not require living tissue to survive. They vary in size and shape and are classified and named accordingly. Its characteristics may assist in the rapid identification of microbes.
The major groups of bacteria are:
- Bacilli, or rod-shaped organisms, for example, Clostridium tetani, the microbe causing tetanus or “lockjaw”. The microbe survives as a spore in the soil and contaminates puncture wounds. A toxin from the bacterium causes seizures and muscle spasms and, eventually, respiratory failure;
- Spirals, which include spirochetes and vibrios, for example, Treponema Pallidum, the cause of syphilis, and Borrelia Burgdorferi, the agent causing Lyme disease, common in the northeastern United States, spread from field mice by deer ticks. This infection can be quite serious, affecting the brain, heart, and joints;
- Cocci, or spherical forms. Cocci are further categorized by their characteristic groupings:
a. Diplococci are pairs of spherical bacteria, for example, pneumococcus, a common cause of
pneumonia.
b. Streptococci are chains, frequently causing respiratory infections, and
c. Staphylococci are clusters. An example is Staphylococcus Aureus, a cause of skin
infections.
The basic structure of bacteria includes the following:
- An outer rigid cell wall protects the microbe and provides a specific shape. A bacterium has one of two types of cell walls, gram-positive or gram-negative, which differ in their chemical composition. This difference can be quickly determined in the laboratory using a Gram stain and provides a means of identification and classification for bacteria. This classification is useful in selecting appropriate antimicrobial therapy. For instance, penicillin acts on the cell wall of gram-positive bacteria. This cell wall is important also because human cells do not have cell walls. A drug such as penicillin does not damage human cells but is effective against bacteria.
- A cell membrane is located inside the bacterial cell wall. This semipermeable membrane selectively controls the movement of nutrients and other materials in and out of the organism. Metabolic processes also take place in the cell membrane.
- An external capsule or slime layer is found on some, but not all, bacteria. This layer is found outside the cell wall and offers additional protection to the organism. It also interferes with the human defense, phagocytosis.
- One or more rotating flagella attached to the cell wall provide motility for some species.
- Pili or fimbriae are tiny hair-like structures found on some bacteria, usually in the gram-negative class. Pili assist in attachment of the bacterium to tissue and also in the transfer of genetic material (DNA) to another bacterium, thus leading to a mutation.
- Bacteria contain cytoplasm, within which are contained the chromosomes (one long strand of DNA) ribosomes (RNA), and plasmids (DNA) fragments that are important in drug resistance). These constituents provide for the metabolism, growth, reproduction, and unique characteristics of the bacterium.
- Some bacteria secrete toxic substances, toxins, and enzymes. Toxins consist of two types, exotoxins, and endotoxins.
a. Exotoxins are usually produced by gram-positive bacteria and diffuse through body fluids.
They have a variety of effects, often interfering with nerve conduction, such as the neurotoxin from the tetanus bacillus. Other toxins (enterotoxins) may stimulate the vomiting center. Exotoxins stimulate antibody or antitoxin production and after being processed to reduce the toxic effect, can be used as toxoids to induce an immune response.
b. Endotoxins are present in the cell wall of gram-negative organisms and are released after the organism dies. Endotoxins may cause fever and general weakness, or they may have serious effects on the circulatory system, causing increased capillary permeability, loss of vascular fluid, and endotoxic shock.
c. Enzymes are produced by some bacteria and a source of damage to the host tissues or cells. For example, hemolysin is produced by bacteria called “hemolytic streptococcus.” This enzyme destroys red blood cells as seen on a culture medium containing red blood cells. Other enzymes assist the bacteria to invade tissue by breaking down components. For example, collagenase breaks down collagen, and streptokinase dissolves blood clots.
8. Several species can form spores (endospores), a latent form of the bacterium with a coat that
is highly resistant to heat and other adverse conditions. These bacteria can survive long
periods in the spore state, but they cannot reproduce when in spore form. Later, when
conditions improve, the bacteria resume a vegetative state and then can reproduce. Tetanus
and botulism are two examples of dangerous infections caused by spores entering the body,
where they return to the vegetative state and replicate.
Bacteria duplicate by a simple process called binary fission, a division of the cell to produce two daughter cells identical to the parent bacterium. The rate of replication (generation time) varies from a few minutes to many hours, depending on the particular microbe. If binary fission occurs rapidly, a large colony of bacteria can develop very quickly, and this leads to the rapid onset of infection. The limiting factors to bacterial growth include insufficient nutrients and oxygen, the effects of increased metabolic wastes in the area, and changes in PH or temperature, all of which cause microbes to die faster than they can divide. Thus the colony eventually self-destructs.
