Microorganisms and Infectious Agents

Microorganisms

• Microorganisms are essential for life on Earth, playing a vital role in the carbon and nitrogen cycles. Without them, decomposition of dead organisms and soil fertility would decrease.

• Probiotic bacteria, a type of "friendly" bacteria, compete with disease-causing species for resources and attachment sites, hindering harmful bacteria from establishing infections. They produce antimicrobial chemicals to further limit infection.

Types of Infectious Agents

• Infectious diseases can be caused by non-living agents, such as prions and viruses, as well as living microorganisms, like bacteria, fungi, and protozoa.

• Prions are abnormal protein molecules that affect normal proteins, causing structural changes in those proteins leading to fatal nerve-degenerative diseases (e.g., "mad cow disease," Creutzfeldt-Jakob disease). Prions lack nucleic acids.

• Viruses are parasites infecting various organisms (animals, plants, protozoa, bacteria). They consist of nucleic acid (DNA or RNA) and protein, but lack cellular structure. Viruses range significantly in size and structure. They are characterized by their narrow host range, requiring specific host cells for replication, making them difficult to cultivate in the laboratory; typically they are cultured in fertile chicken eggs or artificially cultured mammalian cells as hosts. Many only survive a short time outside host cells and are susceptible to heat and organic solvents. They can act as carriers (vectors) to carry genes to cells, such as in gene therapy. Though they may contain genes for enzymes produced by host cells, viruses generally possess few, if any, enzymes.

• Bacteria are the simplest living cells, easily grown in the laboratory. They are the most studied microorganisms for a much of the 19th and 20th centuries. Typically spherical or rod-shaped, and are 1-10mm in diameter. Bacteria have a cell wall that protects them against osmotic pressure changes and drying, allowing them to survive longer periods in dust environments; this cell wall's structure is an important target for antibiotics (e.g., penicillins). Ribosomes in bacteria are different structurally from those in eukaryotic cells which can also be used for targeting with certain antibiotics. Despite conforming to a general prokaryotic description, they vary significantly in terms of form, size, and complexity. Individual bacteria can double in as little as 20 minutes, and large bacterial colonies are visible to the naked eye.

• Fungi and protozoa are more complex than bacteria. Most fungi are not pathogenic (disease causing agents), but contaminants in manufactured medicines. Protozoa are mostly pathogenic; however, they are not usually spoilage organisms.

Additional Information

• The image data shows examples of microbial entities and the ability of electron microscopy to examine small entities.
• The largest common viruses, e.g. chicken pox, have a diameter of approximately 300 nm. Small viruses like the common cold virus have a diameter of approximately 20 nm. Elongated viruses like Ebola can have lengths up to 1400 nm with narrow widths..
• Bacteria are often unaffected by antibiotics used against fungal and bacterial infections.
• Bacteria can be resistant to various factors, such as UV light and toxic chemicals.