Introduction: What are microbes?
They are invisible to us, but each day we are exposed to a multitude of different microorganisms. For the most part they do us no harm, but occasionally we are challenged by disease-causing (pathogenic) microbes that can be detrimental to our health.
All the microorganisms that live on us and in our environment are at the lowest segment of the trunk of the evolutionary tree. Yet what they lack in sophistication and superior development, they overcome by tremendous versatility ability to multiply rapidly, and, for some microbes, an incredible potential to mutate.
A Wealth of Knowledge
Prior to 1675, nothing was known of microorganisms. It was the invention of the microscope by the Dutchman Antonie van Leeuwenhoek that allowed us to see these previously invisible "animalcules" as he called them. With the light microscope he was able to see bacteria and larger microorganisms. Although there was evidence that smaller microorganisms existed and caused disease in humans and other animals, it was not until the development of the electron microscope in 1939 that viruses could be seen. Since then, our understanding of the classification diversity and pathogenic potential of the viruses, bacteria, fungi, and protozoa that infect and coexist with humans has increased exponentially Alongside this knowledge, it is important to stress that new pathogens continue to emerge and old pathogens reemerge or evolve to become even more pathogenic.
VIRUSES
Viruses are the most primitive of the microbes. They can have no existence independent of their host. To reproduce themselves, they must first attach to the host cell. This they do by recognizing the particular shape of a structure on the cell, which acts as a receptor. If this receptor is not present, the cell cannot be infected.
Once the virus has attached to the host cell it enters it and "uncoats." By various mechanisms, it then shuts off the host cell's normal biosynthetic pathways and subverts them into making new copies of the virus. These assemble either as large clusters inside the cell or at the cell membrane and are released by the cell bursting open, or by the virus "budding" from the cell surface. Each infected cell can release thousands of new virus particles, which then go on to infect other cells. This makes viruses "obligate intracellular parasites."
Finally, the host cell dies. If this process affects lots of cells, tissue damage which can be permanent or fatal.
However, the infected cells are not passive, but cry for help by releasing small molecules called cytokines and chemokines to recruit our immune system to the site of the infection. This contributes to the symptoms of infection, such as fever, and, in the respiratory tract for example, partial blockage of the airways and the production of sputum.
There are several consequences resulting from the virus only being able to reproduce itself inside a host cell. Firstly once it has been excreted from the host, virus numbers fall (this is not the case for most bacteria which are free-living). Secondly because the virus uses our synthetic machinery to reproduce itself, designing effective drugs to inhibit viruses is very difficult and this is why we have so few antiviral drugs.
Virus Characteristics
Viruses are very small, ranging in size from 25 to 350 nanometers (nm) in diameter. Some just have a protein coat, a "capsid," but others have an envelope of "lipid" surrounding the capsid. This envelope is usually acquired as the virus buds from the cell it has infected and is thus derived from the host cell membrane. In general, the viruses that have envelopes survive less well in the outside world (the environment) and are easily destroyed.
Viruses are unusual in that for some, the genetic code is of ribonucleic acid (RNA) and for others it is deoxyribonucleic acid (DNA), but never both, unlike humans. The shape of the capsid for some viruses is helical (spiral) and for others, it is a structure with 20 "facets." These characteristics make it possible to classify viruses into different families, genera, and species to provide a universally accepted scheme for naming and describing new viruses.
BACTERIA
Although some bacteria are obligate intracellular parasites like viruses, most are free-living and, providing they can scavenge enough food, will keep on reproducing. They reproduce by binary fission, each bacterium dividing to produce two identical offspring (progeny). Under optimal conditions, they can divide every 20 minutes so that after 18 -- 24 hours, one bacterial cell can divide to produce up to 10 million descendants. To put this in context, the world's human population is around 5,000 million.
Bacteria Characteristic
Bacteria range in size from 0.5 to 1 micron (µ) in diameter to 3 -- 15 µ long. Their classification is based upon their shape, staining characteristics, growth characteristics, biochemical properties and, increasingly on their genomic make up. Bacteria can be rod shaped (bacilli), spherical (cocci), or spiral (spirochaetes). The bacilli can be straight or bent. Cocci can be arranged in pairs (diplococci), in clusters (staphylococci) or in long chains (streptococci). Some bacteria, such as Bacillus anthracis (which causes anthrax), produce survival packages called spores. These allow the bacteria to survive under harsh conditions such as drying, heating, and irradiation, where non-spore producing bacteria are killed. When conditions improve, the spores germinate and the bacterium reproduces itself.
In 1895, Dr. Christian Gram, a Danish physician, devised a staining technique that is still used today to classify bacteria. Gram-positive bacteria, which stain blue, have one cell membrane and a thick cell wall. Gram-negative bacteria, which stain pink or red, have two cell membranes and a thinner cell wall.
Some bacteria are killed in the presence of oxygen. These are called obligate anaerobes. Other bacteria (facultative anaerobes) can grow whether oxygen is present or not.
FUNGI
The fungi form a large kingdom, but only a small number are pathogenic for humans. Fungi differ from bacteria in several ways. They are "eukaryotes," meaning that they have chromosomes in a nucleus and have internal structures called organelles. They are usually bigger than bacteria, more likely to be branching, and reproduce both sexually and by binary fission. Some, such as Candida albicans, remain as single cells, whereas others form large branching networks called mycelia.
Fungi can be subdivided into those that cause superficial infections (such as dermatophytes which cause an infection on the skin), those that cause infections in the tissues beneath the skin (subcutaneous), and those that disseminate around the body (systemic). They range from single-celled yeasts, such as Candida albicans (mentioned above) to large multicellular organisms with a sex life, such as those causing ringworm and athlete's foot. They have a cell wall and range in size from 5 µ to a few millimeters (mm) long.
PROTOZOA
The protozoa form a large kingdom of eukaryotes consisting of single-celled organisms. They have nuclei and intracellular organelles but no cell wall. These are single-celled organisms and begin to resemble human cells. Some, such as Entamoeba histolytica, reproduce by binary fission but others, such as Cryptosporidium parvum or Plasmodium falciparum, have a sexual phase of reproduction. They are quite fragile and those that are shed into the external environment usually produce hardy thick-walled cysts. Only a minority of the protozoa are pathogenic for humans. They range in size from 5 to 20 µ.
MULTICELLULAR PARASITES
Nematodes (round worms), trematodes (flukes), and cestodes (tapeworms) consist of large numbers
