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[[Image:E coli at 10000x, original.jpg|thumb|250px|right|A cluster of ''[[Escherichia coli]]'' [[Bacterium|bacteria]] magnified 10,000 times.]]
[[Image:E coli at 10000x, original.jpg|thumb|250px|right|A cluster of ''[[Escherichia coli]]'' [[Bacterium|bacteria]] magnified 10,000 times.]]


Revision as of 21:56, 15 November 2006

A cluster of Escherichia coli bacteria magnified 10,000 times.

A microorganism or microbe is an organism that is microscopic (too small to be visible to the naked eye). Micro-organisms are often described as single-celled, or unicellular organisms; however, some unicellular protists are visible to the naked eye, and some multicellular species are microscopic. The study of microorganisms is called microbiology.

History

Evolution

Single-celled microorganisms were the first forms of life to develop on earth, approximately 4 billion years ago and for about 3 billion years, all organisms were microscopic.[1] Therefore, for the majority of the history of life on earth the only form of life were microorganisms.[2]

Most microorganisms reproduce rapidly and in great number. Microbes such as bacteria can also freely exchange genes by conjugation, transformation and transduction between widely-divergent species.[3] This horizontal gene transfer, coupled with a high mutation rate and many other means of genetic variation, allows microorganisms to swiftly evolve (via natural selection) to survive in new environments and respond to environmental stresses. This rapid evolution has led to the recent development of 'super-bugs' - pathogenic bacteria that are resistant to modern antibiotics.[4]

Discovery

Prior to Anton van Leeuwenhoek's discovery of microorganisms in 1676, it had been a mystery as to why grapes could be turned into wine, milk into cheese, or why food would spoil. Leeuwenhoek did not make the connection between these processes and microorganisms, but he did establish that there were forms of life that were not visible to the naked eye. Leeuwenhoek's discovery, along with subsequent observations by Lazzaro Spallanzani and Louis Pasteur, ended the long-held belief that life spontaneously appeared from non-living substances during the process of spoilage.

Lazzarro Spallanzani found that microorganisms could only settle in a broth if the broth was exposed to the air. He also found that boiling the broth would sterilise it and kill the microorganisms.

Louis Pasteur expanded upon Spallanzani's findings by exposing boiled broths to the air, in vessels that contained a filter to prevent all particles from passing through to the growth medium, and also in vessels with no filter at all, with air being admitted via a curved tube that would not allow dust particles to come in contact with the broth. By boiling the broth beforehand, Pasteur ensured that no microorganisms survived within the broths at the beginning of his experiment. Nothing grew in the broths in the course of Pasteur's experiment. This meant that the living organisms that grew in such broths came from outside, as spores on dust, rather than spontaneously generated within the broth. Thus, Pasteur dealt the death blow to the theory of spontaneous generation and supported germ theory.

In 1876, Robert Koch established that microbes can cause disease. He did this by finding that the blood of cattle who were infected with anthrax always had large numbers of Bacillus anthracis. Koch also found that he could transmit anthrax from one animal to another by taking a small sample of blood from the infected animal and injecting it into a healthy one, causing the healthy animal to become sick. He also found that he could grow the bacteria in a nutrient broth, inject it into a healthy animal, and cause illness. Based upon these experiments, he devised criteria for establishing a causal link between a microbe and a disease in what are now known as Koch's postulates. Though these postulates are no longer entirely accurate, they do retain historical importance in the development of scientific thought.

Classification

A phylogenetic tree of life based on differences in rRNA, showing the separation of bacteria, archaea, and eukaryotes.

Microorganisms can be found almost anywhere in the taxonomic organization of life on the planet. Bacteria and archaea are almost always microscopic, whilst a number of eukaryotes are also microscopic, including most protists and a number of fungi. Viruses are generally regarded as not living and therefore are not microbes, although the field of microbiology also encompasses the study of viruses.

Bacteria

Main article: Bacteria
Staphylococcus aureus bacteria magnified about 10,000x

Bacteria are the simplest and the most diverse and widespread group of organisms on Earth. Bacteria inhabit practically all environments where some liquid water is available and the temperature is below +140 °C. They are found in sea water, soil, human gut, hot springs and in food. Practically all surfaces which have not been specially sterilised are covered in bacteria. The number of bacteria in the world is estimated to be around five million trillion trillion , or 5 × 1030.[5]

Bacteria are practically all invisible to the naked eye, with few extremely rare exceptions, such as Thiomargarita namibiensis.[6] They are unicellular organisms and lack organelles. Their genome is a single string of DNA, although they can also harbour small pieces of DNA called plasmids. Bacteria are surrounded by a cell wall. They reproduce by binary fission. Some species form spores, but for bacteria this is a mechanism for survival, not reproduction. under optimal conditions bacteria can grow extremely rapidly and can double as quickly as every 10 minutes.[7]

Archaea

Main article: Archaea

Archaea are single-celled organisms lacking nuclei and are therefore prokaryotes, classified as Monera in the alternative five-kingdom taxonomy. They were originally described in extreme environments, but have since been found in all types of habitats.[8]

A single organism from this domain has been called an "archaean." Furthermore, this biologic term is also used as an adjective.

Eukaryotes

Main article: Eukaryote
File:Chaos diffluens.jpg
An amoeba, a typical eukaryotic microorganism

All living things which are individually visible to the naked eye are eukaryotes (with few exceptions, such as Thiomargarita namibiensis), including humans. However, a large number of eukaryotes are also microorganisms. Eukaryotes are characterised by the presence of organelles such as in the cells. These structures are absent in bacteria and archaea. The nucleus is an organelle which houses the DNA. DNA itself is arranged in complex chromosomes.[9] mitochondria are organelles that are vital in metabolism as they are the site of cellular respiration. mitochondria evolved from symbiotic bacteria and retain a remnant genome.[10] Plant cells also have cell walls and chloroplasts in addition to other organelles. Chloroplasts produce energy from light by photosynthesis. They were also originally symbiotic bacteria.[10]

Unicellular eukaryotes are those whose members consist of a single cell throughout their life cycle. This qualification is significant since most multicellular eukaryotes consist of a single cell at the beginning of their life cycles. Microbial eukaryotes can be either haploid or diploid, and some organisms have multiple cell nuclei (see coenocyte). However, not all microorganisms are unicellular. Microbial eukaryotes can have multiple cells.

Of the eukaryotic groups the protists are always unicellular, and thus microorganisms. This is a diverse group of organisms which do not fit into other groups of eukaryotes. Several algae species are unicellular plants. The fungi also have several unicellular species, such as baker's yeast (Saccharomyces cerevisiae). Animals are always multicellular, although they may not be visible to the naked eye.

Habitats and ecology

Microorganisms are found in virtually every habitat present in nature. Even in hostile environments such as the poles, deserts, geysers, rocks, and the deep sea, some types of microorganisms have adapted to the extreme conditions and sustained colonies; these organisms are known as extremophiles. Extremophiles have been isolated from rocks as much as 7 kilometres below the earth's surface,[11] and it has been suggested that the amount of living organisms below the earth's surface may be comparable with the amount of life on or above the surface.[12] Extremophiles have been known to survive for a prolonged time in a vacuum, and can be highly resistant to radiation, which may even allow them to survive in space.[13] Many types of microorganisms have intimate symbiotic relationships with other larger organisms; some of which are mutually beneficial (mutualism), while others can be damaging to the host organism (parasitism). If microorganisms can cause disease in a host they are known as pathogens.

Extremophiles

Main article: Extremophile

Certain microbes have adapted so that they can survive and even thrive in conditions that are normally fatal to most lifeforms. Microorganisms have been found around underwater black smokers and in geothermal hot springs, as well as in extremely salty bodies of water.

Soil microbes

Symbiotic microbes

Importance

Microorganisms are vital to humans and the environment, as they participate in the Earth's element cycles such as the carbon cycle and nitrogen cycle, as well as fulfilling other vital roles in virtually all ecosystems, such as recycling other organisms' dead remains and waste products through decomposition. Microbes also have an important place in most higher-order multicellular organisms as symbionts. Many blame the failure of Biosphere 2 on an improper balance of microbes.

Use in food

Microorganisms are used in brewing, baking and other food-making processes.

The lactobacilli and yeasts in sourdough bread are especially useful. To make bread, one uses a small amount (20-25%) of "starter" dough which has the yeast culture, and mixes it with flour and water. Some of this resulting dough is then saved to be used as the starter for subsequent batches. The culture can be kept at room temperature and continue yielding bread for years as long as it remains supplied with new flour and water. This technique was often used when "on the trail" in the American Old West.

Microorganisms are also used to control the fermentation process in the production of cultured dairy products such as yogurt and cheese. The cultures also provide flavour and aroma, and to inhibit undesirable organisms.[14]

Use in science

Microbes are also essential tools in biotechnology and the study of biochemistry, genetics and molecular biology.

Use in warfare

Main article: Biological warfare

Microorganisms and human health

Microbes in human digestion

Microorganisms can form an endosymbiotic relationship with other, larger, organisms. For example, the human digestive system depends on bacteria that live inside the intestines to help break down food.

Diseases and immunology

Microorganisms are the cause of many infectious diseases. The organisms involved include bacteria, causing diseases such as plague, tuberculosis and anthrax: protozoa, causing diseases such as malaria, sleeping sickness and toxoplasmosis; and also fungi causing diseases such as ringworm, candidiasis or histoplasmosis. However, other diseases such as influenza, yellow fever or AIDS are caused by viruses, which are not living organisms and are not therefore microorganisms.

Hygiene

Main article: Hygiene

Hygiene is the avoidance of infection or food spoiling by eliminating microorganisms from the surroundings. As microorganisms, particularly bacteria, are found practically everywhere, this means in most cases the reduction of harmful microorganisms to acceptable levels. However, in some cases it is required that an object or substance is completely sterile, i.e. devoid of all living entities and viruses. A good example of this is a hypodermic needle.

In food preparation microorganisms are reduced by preservation methods (such as the addition of vinegar), clean utensils used in preparation, short storage periods or by cool temperatures. If complete sterility is needed, the two most common methods are irradiation and the use of an autoclave, which resembles a pressure cooker.

There are several methods for investigating the level of hygiene in a sample of food, drinking water, equipment etc. Water samples can be filtrated through an extremely fine filter. This filter is then placed in a nutrient medium. Microorganisms on the filter then grow to form a visible colony. Harmful microorganisms can be detected in food by placing a sample in a nutrient broth designed to enrich the organisms in question. Various methods, such as selective media or PCR, can then be used for detection. The hygiene of hard surfaces, such as cooking pots, can be tested by touching them with a solid piece of nutrient medium and then allowing the microorganisms to grow on it.

There are no conditions where all microorganisms would grow, and therefore often several different methods are needed. For example, a food sample might be analysed on three different nutrient mediums designed to indicate the presence of "total" bacteria (conditions where many, but not all, bacteria grow), molds (conditions where the growth of bacteria is prevented by e.g. antibiotics) and coliform bacteria (these indicate a sewage contamination).

Microorganisms in fiction

Microorganisms have frequently played an important part in science fiction, both as agents of disease, and as entities in their own right.

Some notable uses of microorganisms in fiction include:

See also

References

  1. ^ Schopf J (1994). "Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic". Proc Natl Acad Sci U S A. 91 (15): 6735–42. PMID 8041691.
  2. ^ DeLong E, Pace N (2001). "Environmental diversity of bacteria and archaea". Syst Biol. 50 (4): 470–8. PMID 12116647.
  3. ^ Wolska K (2003). "Horizontal DNA transfer between bacteria in the environment". Acta Microbiol Pol. 52 (3): 233–43. PMID 14743976.
  4. ^ Enright M, Robinson D, Randle G, Feil E, Grundmann H, Spratt B (2002). "The evolutionary history of methicillin-resistant Staphylococcus aureus (MRSA)". Proc Natl Acad Sci U S A. 99 (11): 7687–92. PMID 12032344.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ University of Georgia Campus News
  6. ^ Schulz H, Jorgensen B. "Big bacteria". Annu Rev Microbiol. 55: 105–37. PMID 11544351.
  7. ^ Eagon R. "Pseudomonas natriegens, a marine bacterium with a generation time of less than 10 minutes". J Bacteriol. 83: 736–7. PMID 13888946.
  8. ^ Robertson C, Harris J, Spear J, Pace N (2005). "Phylogenetic diversity and ecology of environmental Archaea". Curr Opin Microbiol. 8 (6): 638–42. PMID 16236543.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  9. ^ "Eukaryota: More on Morphology." [1] (Accessed 10 October 2006)
  10. ^ a b Dyall S, Brown M, Johnson P (2004). "Ancient invasions: from endosymbionts to organelles". Science. 304 (5668): 253–7. PMID 15073369.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ Szewzyk U, Szewzyk R, Stenström T (1994). "Thermophilic, anaerobic bacteria isolated from a deep borehole in granite in Sweden". Proc Natl Acad Sci U S A. 91 (5): 1810–3. PMID 11607462.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. ^ Gold T (1992). "The deep, hot biosphere". Proc Natl Acad Sci U S A. 89 (13): 6045–9. PMID 1631089.
  13. ^ Horneck G (1981). "Survival of microorganisms in space: a review". Adv Space Res. 1 (14): 39–48. PMID 11541716.
  14. ^ "Dairy Microbiology". University of Guelph. Retrieved 2006-10-09.
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