Microorganisms

views updated

Microorganisms

Introduction

Microorganisms are life forms that are too small to be seen with the naked eye, but that play an important role in human health and disease. The main types of microorganisms are bacteria, fungi, protozoa, and viruses. The first three are single-celled organisms, of which only bacteria have a nucleus. Viruses, however, need to be inside a host cell in order to survive.

Microbes occupy a wide range of ecological niches. Some live inside the human intestine or on the skin, others are found in soil, on the ocean floor, or even in the Arctic ice cap. Microbes have potential for both benefit and harm. They help keep the digestive system healthy and play an important role in decomposing dead plants and animals. However, microbes also cause a wide range of diseases, from colds and flu, to tuberculosis, AIDS, and cholera.

History and Scientific Foundations

The first microbes were observed by the Dutch biologist Anton van Leeuwenhoek (1632–1723) in 1674 using a primitive microscope he had invented. He observed what he called “animalcules” of all shapes and sizes in samples from many sources. The origin and function of these life forms was widely discussed over the next two centuries, but it was not until the nineteenth century that the scientific foundations of microbiology were laid down by Louis Pasteur (1822–1895) and Robert Koch (1843–1910).

Pasteur's work in the 1870s and 1880s showed that putrefaction depended upon the action of microbes. He went on to develop pasteurization, a technique of gentle heating that stops food and drink from spoiling by decreasing their levels of microbial contamination. Meanwhile, Koch isolated the anthrax bacillus in 1876 and the tuberculosis bacillus in 1882. By the end of the century, the microbes responsible for plague, meningitis, gonorrhea, typhoid, tetanus, diphtheria, dysentery, and pneumonia had been discovered and characterized.

Koch pushed forward the germ theory of disease, which rested on his four postulates. First, the responsible microorganism had to be isolated from infected animals, and then cultivated and identified in the laboratory. Finally, on re-injection to other lab animals, the disease had to be reproduced.

Microbes are classified according to the hierarchical system adopted for plants and animals, with related species being grouped together in the same genus. For example, Staphylococcus aureus and Staphylococcus epidermidis are two species belonging to the Staphylococcus genus. This name comes from the Greek word “staphyl,” meaning bunch of grapes and “coccus,” which means grain or berry, and refers to the appearance of the bacteria under the microscope.

Bacteria are single-celled organisms of average length two micrometers and average diameter of 0.5 micrometers. They occur in a range of characteristic shapes from which their names are sometimes derived—for instance, rods (bacilli), spheres or ovals (cocci), and spirals (spirochaetes). Bacteria do not have a nucleus and their genetic material (DNA) lies free in the cell or on tiny circular structures called plasmids. They cause a range of infections, including sore throats, pneumonia, and food poisoning.

Fungi and protozoa have a cell structure that is more like that of a human cell, with a nucleus carrying their DNA. Protozoa are single-celled microbes, including algae and trypanosomes, which often have complex lifecycles and interactions with their human hosts. They cause some serious tropical diseases, including malaria. The fungi group includes yeasts, molds and mushrooms. They play an important part in decomposing biological material, such as dead plants, and in the food and drink industry. Some fungi cause diseases of the skin and hair, such as athelete's foot. Fungal infections can also cause health problems in people with reduced immunity, such as those with HIV/AIDS.

Viruses were first discovered in the late 1880s when it was realized that some disease agents could pass through filters that would usually hold back the smallest bacteria. These filtrates were implicated in diseases such as yellow fever and foot and mouth disease (a disease of animals). Unlike other microbes, viruses need a living cell in which to replicate themselves. Without a host, they die. Viruses are responsible for a range of human diseases, including hepatitis, AIDS, colds, influenza and some forms of pneumonia and meningitis.

WORDS TO KNOW

COLONIZATION: Colonization is the process of occupation and increase in number of microorganisms at a specific site.

ERADICATION: The process of destroying or eliminating a microorganism or disease.

PATHOGEN: A disease causing agent, such as a bacteria, virus, fungus, etc.

RESISTANT ORGANISM: Resistant organisms are bacteria, viruses, parasites, or other disease-causing agents that have stopped responding to drugs that once killed them.

Applications and Research

Understanding microbiology leads to a better appreciation of many diseases, which can allow for more accurate diagnosis and effective treatment. Sometimes one microorganism can cause many different diseases, such as S. aureus which causes strep throat, scarlet fever, and toxic shock syndrome. On the other hand, one disease can often be caused by many different microorganism. For instance, pneumonia can be caused by adenovirus, respiratory syncytial virus, influenza virus, parainfluenza virus, and cytomegalovirus. Among the bacteria that can cause pneumonia are Streptococcus pneumoniae, Chlamydia pneumoniae, Haemophilus influenzae, Klebsiella pneumoniae and Pseudomonas aeruginosa. The disease can also be caused by mycoplasma, which are organisms that have some of the characteristics of both bacteria and viruses.

Most microorganisms are actually harmless to human health. Pathogens—microbes that cause disease—are of two kinds, strict pathogens and opportunistic pathogens. A strict pathogen is always associated with disease; these include Mycobacterium tuberculosis, which causes tuberculosis (TB), and rabies virus. However, most human infections are caused by opportunistic pathogens, which colonize, or normally live on the skin or in the nose or mouth, or in the surroundings without causing any problems. However, if they enter unprotected sites like the blood, they may cause disease. Among the most common of the opportunistic pathogens are Pneumocystis carinii, Candida albicans, and cytomegalovirus, which often cause complications among those with HIV/AIDS.

Impacts and Issues

Infectious diseases including HIV/AIDS, malaria, and tuberculosis, continue to claim millions of lives each year. There are also emerging infections such as SARS and bird flu, which have the potential to cause pandemics. However, science has shown that infections can be defeated—smallpox has been eradicated and polio and measles could be eradicated in the near future.

Antibiotics—which are active against bacteria, fungi, but not viruses—have been the major weapon against infection. Starting with the introduction of penicillin in the 1940s, doctors now have a wide range of drugs against infections like TB. Anti-viral drugs are also making AIDS a chronic disease rather than a death sentence. However, microorganisms are developing resistance against these chemical weapons so it is vital that scientists continue to extend their understanding of microbial physiology so that new drugs against infection can be developed.

Vaccines are the other major tool against microbial disease. They generally contain either a killed or weakened version of the microbe or a part of the organism, such as a protein borne on its surface, which can elicit an immune response. There is an urgent need for the development of vaccines against malaria, AIDS, and hepatitis C.

A new and more detailed level of understanding of microbiology may come from genetics. The genomes of several medically significant microbes have now been solved. Their genes have been recorded and will now, hopefully, be identified. These include Hemophilus influenzae, which can cause meningitis or pneumonia, Nesseria meningitidis, another meningitis organism, and Streptococcus pneumoniae which causes meningitis and pneumonia. The genomics approach means a better understanding of how microorganisms cause human illnesses and new opportunities for developing more effective antibiotics, antivirals, and vaccines.