Microbiology is a specialized area of biology dealing with organisms too small to be seen without sufficient magnification. Microbiologists study bacteria, fungi, parasites, and viruses, including their interactions with humans, animals, plants, and the environment.
While viruses are not a living organisms like bacteria, fungi, and parasites, they are studied by microbiologists. In addition, there is an infectious agent responsible for numerous neurodegenerative diseases found in animals and humans known as “prions”
Bacteria are small, single-celled organisms that occur in almost any natural environment. Common bacteria are too small to be seen individually without the aid of a microscope. Bacteria can multiply to form groups or colonies on a food source. After a sufficient number of replication cycles a colony of bacteria can be seen with the naked eye.
Viewed under a microscope, different kinds of bacteria will have different shapes or forms. Many bacteria have either a spherical shape or an elongated rod shape. A spherical shaped bacterium (singular) is called a coccus, and a group of spherical-shaped bacteria (plural) are called cocci. A rod shaped bacterium is called a bacillus, and a group of rod-shaped bacteria are called bacilli.
Some species of bacteria appear as individual cells microscopically. Other bacterial species may combine to form pairs (e.g., diplococci), groups of four (tetrads), grape-like clusters (e.g., staphylococci), and chains (e.g., streptococci or streptobacilli).
The fungi consist of two major groups of microbes, molds and yeasts. Molds are multicellular organisms. Yeasts are single-celled organisms. Molds and yeasts tend to be significantly larger than bacteria. Both molds and yeasts are widely distributed in nature, both in the soil and in dust carried by air.
Molds have a branching filamentous structure, and can develop into colonies visible as a colorful, furry or downy coating on food or surfaces. They reproduce by producing small spores, which are not related to bacterial spores mentioned above. Mold spores can be picked up and spread by air currents. If mold spores settle on suitable surfaces, they will begin to germinate and produce new mold growth.
Yeasts are usually egg-shaped, and tend to be smaller than molds. Like molds, yeasts can be spread via air currents. They reproduce by a process known as budding. Visible colonies of yeast are generally slimy in appearance and creamy white.
Parasites are living organisms that derive nourishment and protection from other living organisms called hosts. These organisms live and reproduce within the tissues and organs of infected human and animal hosts. There are different types of parasites, and they range in size from single-celled protozoa to multi-cellular worms.
Protozoan parasites are visible only through a microscope. Many adult parasitic worms are visible without a microscope; however, a microscope is necessary for detecting eggs and preadult forms of some worms. Identification of the adult forms of certain parasitic worms can also require microscopy.
Viruses are much smaller than bacteria. They are too small to be seen with a standard light microscope. An electron microscope is necessary to see viruses. These microbes are not true living organisms. They are composed of genetic material—either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)—enclosed in a protein coat.
A virus must invade a living host cell in order to replicate. Once inside the host cell, the viral genetic material directs the host cell’s “machinery” to make more virus particles, which interferes with normal host cell function and may result in destruction of the host cell.
Infectious Agent – Prions
Prions are not true living organisms; they are proteinaceous infectious particle with no nucleic acid genome. The prion protein exists in every mammal in a healthy state and is called cellular prion protein (PrPc). When misfolded they become a proteinaceous infected particle (PrPSc).
Protein molecules fold into a three-dimensional (3-D) form. The 3-D form of a particular protein is related to its particular biological function. Folding into its normal form, then, allows a protein to be functionally active. Failure of a protein molecule to fold into its 3-D normal form renders it dysfunctional and harmful to tissue or organs.
Prions are the prevailing theory for what causes a group of neurodegenerative diseases called transmissible spongiform encephalopathies (diseases of the brain).
Bovine spongiform encephalopathy (BSE), also referred to as mad cow disease, is the transmissible spongiform encephalopathy that affects cattle. There are several transmissible spongiform encephalopathies that affect humans, such as Creutzfeldt– Jakob disease (CJD), Gerstmann-Straussler Scheiker Syndrome, Fatal Familial Insomnia and Kuru, but only CJD has been associated with meat consumption. The prion responsible for BSE appears to be infective to humans, and the resulting disease is called variant Creutzfeldt–Jakob disease (vCJD).