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.

go to top

 

Agents of Infectious Diseases

There are many agents of infectious diseases.

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.

go to top

 

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:

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.

 

go to top

 

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.

 Classification of Major Pathogens

 

Viruses

Bacteria

Fungi

Protozoa

Worms

Nucleic acids

DNA or RNA

DNA and RNA

DNA and RNA

DNA and RNA

DNA and RNA

Nuclear membrane

no

no

Yes

yes

yes

External cell wall

no

yes (usually) rigid peptidoglycan

Yes, Rigid chitin

no

no

Antibiotic sensitivity

no

yes

No

some

no

Replication/

Reproduction

within host cells

within and outside host cells by binary fission

within and outside host cells by binary fission and sexually

within and outside host cells by binary fission and sexually

outside host cells sexually

go to top

 

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.

go to top

return to the course syllabus

hits since March 19, 2002