Microbes, November 2001

Introduction

Agents of Infectious Diseases

Viruses

Bacteria

Scale of Sizes

Introduction

A host is any organism capable of supporting the nutritional and physical requirements of another. A microbe is a microscopic organism. The presence and multiplication of an organism on or within a host is called colonization or infection. We refer to the colonization of one organism by another as symbiosis. If the symbiotic relationship benefits both organisms, it is called mutualism. Commensalism is a symbiotic relation in which one organism benefits and the other is not harmed. Parasitism occurs when the infecting organism benefits and the host is harmed. If the host sustains injury or pathological changes in response to the parasite, the process is an infectious disease. Anything causing a disease is said to be a pathogen.

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Agents of Infectious Diseases

There are many agents of infectious diseases.

• Viroids are small, circular RNA molecules and are the smallest known pathogens and cause some diseases of agricultural plants, but no known human diseases.
• Prions are proteins that can force conformational changes in natural proteins and seem to cause disease (kuru, new variant Creutzfeldt-Jakob disease, and fatal familial insomnia).
• A virus is an obligate intracellular pathogen (meaning that it must live within the cells of its host in order to reproduce). They cause chicken pox, common cold, encephalitis, Epstein-Barr syndrome, hepatitis, herpes, influenza, HIV disease, HTLV, Kaposi's sarcoma, measles, monkey pox, mumps, rabies, rubella, ST-warts, various hemorrhagic fevers, yellow fever, etc.
• Bacteria (singular bacterium) are autonomously replicating unicellular organisms lacking an organized nucleus (prokaryotes) and organized intracellular organelles. They have only a single chromosome and most have a cell wall of the polymer peptidoglycan. They cause anthrax, boils, diphtheria, bacillary dysentery, cholera, diarrhea, endocarditis, food poisoning, gas gangrene, gonorrhea, haemophilus influenza, Legionnaire's disease, meningitis, pneumonia, septicemia, scarlet fever, urethritis, whooping cough, etc.
• Spirochetes are bacteria that are helical-shaped and have filaments wound about the cell wall along the entire length of the cell. The best known disease caused by spirochetes is syphilis.
• Mycoplasmas are unicellular prokaryotes without cell walls that are capable of independent replication, and they cause tuberculosis.
• Rickettsiae and chlamydiae are obligate intracellular pathogens with a rigid peptidoglycan cell wall, which reproduce asexually by cell division, and can contain both DNA and RNA. Rickettsiae cause typhus and Rocky Mountain spotted fever. Chlamydiae are responsible for some bronchitis, sinusitis, pneumonia, lymphogranuloma venereum (an STD), and trachoma.
• Algae are eukaryotes that carry out oxygenic photosynthesis.
• Fungi are free-living eukaryotic saprophytes (organisms which live on dead things) consisting of yeasts and molds (diseases: athlete's foot, histoplasmosis, coccidioidmycosis, cryptococcosis, pneumocycstis carinii pneumonia, ringworm, thrush, etc.). The picture below shows a relatively mild case of the characteristic "cottage cheese" plaques caused by the fungus Candida albicans.

• Protozoa are unicellular eukaryotes that lack cell walls, e.g., Chagas' disease, cryptosporidiosis, giardiasis, malaria, oral candidiasis, sleeping sickness, toxoplasmosis, trichomoniasis, etc.
• Helminths are worms, that cause filariasis, hookworm, pinworm, schistosomiasis, tapeworm, trichinosis, etc. A typical filariasis infection is a chronic infestation with threadlike filaria nematodes. They congregate in the lymph nodes and surrounding tissue and eventually block the flow of lymph. This results in pain and significant swelling in the area involved.

The claim that prions cause new variant Creutsfeldt-Jakob disease has been disputed in a 2001 paper by Venters. His arguments are very cogent. He claims that the original research published in the Lancet was based on insufficient data.

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Viruses

Viruses have no organized cellular structures but simply a protein coat, called the capsid, surrounding a nucleic acid core, called a genome, of either RNA or DNA, but never both. The capsid together with the genome is called the nucleocapsid. The nucleocapsid may be surrounded by an envelope that is composed of a lipid bilayer containing protein spikes. An entire virus particle is called a virion. Viruses are classified as: DNA or RNA, single strand or double strand, helical or linear, envelope or no envelope, by their symmetry, then family and species.

Viruses may engage in the process of lysogeny:

• Attachment to a cell,
• Fusion with the cell membrane,
• Penetration of the cell membrane,
• Uncoating of the virus,
• Replication of viral nucleic acid within the cell,
• Migration to the nucleus of the cell,
• Integration of the nucleic acids to form a provirus,
• RNA nuclear transport,
• Synthesis of proteins used in the virus coat,
• Assembly of structural subunits,
• Reencapsidation,
• Release of the mature virions. Frequently the virions bud off the cell surface. Sometimes they are released during cell lysis, wherein the cell bursts and disperses the reproduced virions.

In order to attach to a cell, a virion must possess protein molecules, called receptors, that can fit receptors on the cells they infect. Viruses are specific with regard to the types of cells to which they can attach/infect. For instance, the influenza virus is specific to respiratory epithelial cells. Epstein-Barr virion can only bind to receptors in the oral or nasal mucosa. Herpes virus infects cells of the nervous system.

 

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Bacteria

Gram positive bacteria (which stain purple under Gram stain) have a thick bilayer wall of the polymer peptidoglycan. Gram negative bacteria (which stain red) have a thin layer of this polymer and an additional lipopolysaccharide outer layer, LPS, which is often endotoxic (capable of initiating inflammation and cell-mediated immune responses and can lead to septicemia), e.g., Salmonella, Shigella, and Escherichia.

Bacteria are further classified: by shape: a bacillus is rod-shaped, a coccus is ball-shaped, a spirilium is spiral-shaped, a vibrio is comma-shaped, a cocco-bacillus is ovoid-shaped, and other combinations; whether they need oxygen (aerobic) to survive or not (anaerobic); their form of reproduction; genus; and species.

Some bacteria are motile (capable of motion) because of the presence of a flagellum. There can be one or more flagella on each bacterium and they can be attached to one or both ends singly or in tufts (lophotrichous) or attached at many places on the cell surface (peritrichous).

 The following table is a summary listing of the characteristics of the major pathogens.

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Scale of Sizes

Sizes: viruses 28-200 nm and 5-160K base pairs, bacteria 1000-2000 nm and 1000-9000K base pairs. If a small virion (30 nm) were one inch long, a large bacterium would be 10 inches long, and a five-foot tall person would be about 800 miles high. The following picture (from the University of Queensland website) shows the relative scale of sizes.

 

Recently scientists have found examples of so-called nanobacteria which are smaller than viruses. The study of these unusual creatures is ongoing and controversial.

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