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Imagine the excitement felt by the Dutch biologist Anton van Leeuwenhoek in 1674 as he peered through his carefully ground microscopic lenses at a drop of water and discovered a world of millions of tiny "animalcules." Almost 100 years later the Danish biologist Otto Muller extended van Leeuwenhoek's studies and organized bacteria into genera and species according to the classification methods of Linnaeus. Other brilliant scientists went on to prove that a diverse collection of microbes was responsible for causing human disease. In 1946, the American microbiologist John Enders was the first to cultivate viruses in cell cultures, leading the way to the largescale production of virus cultures for vaccine development. The world that van Leeuwenhoek discovered was complex, consisting of protozoa and bacteria of all shapes and sizes. To understand this information and organize it in a useful manner, it is important to understand some of the basic aspects of medical microbiology. Some bacteria lack this cell wall structure and compensate by surviving only inside host cells or in a hypertonic environment.
The human body is inhabited by thousands of different bacterial species, some living transiently, others in a permanent parasitic relationship. One of the most important reasons for studying microbes is to understand the diseases they cause and the ways to control them. Most human diseases, however, are produced by organisms in the person's own microbial flora that spread to body sites where disease can ensue (endogenous infections). Although the human body is remarkably adapted to controlling exposure to pathogenic microbes, the physical barriers that prevent invasion by the organism and the immunologic response to infection are frequently inadequate.
The clinical microbiology laboratory plays an important role in the diagnosis and control of infectious diseases. In addition, the most sophisticated testing protocols are of little value if the collected specimen is not representative of the site of infection. The laboratory is also able to determine the antimicrobial activity of selected chemotherapeutic agents, although the value of these tests is limited.
Finally, the in vitro determination of an organism's susceptibility to a variety of antibiotics is only one aspect of a complex picture. It is important to realize that our knowledge of the microbial world is evolving continually. Bacteria are microscopic organisms whose single cells have neither a membrane-enclosed nucleus nor other membrane-enclosed organelles like mitochondria and chloroplasts. If flagella are present, they are made of a single filament of the protein flagellin; there are none of the "9+2" tubulin-containing microtubules of the eukaryotes. The plasma membrane (in Gram-positive bacteria) and both membranes in Gram-negative bacteria are phospholipid bilayers but contain no cholesterol or other steroids.
Gram-positive bacteria are encased in a plasma membrane covered with a thick wall of peptidoglycan.
Although the Gram stain might seem an arbitrary criterion to use in bacterial taxonomy, it does, in fact, distinguish between two fundamentally different kinds of bacterial cell walls and reflects a natural division among the bacteria. More recently, genome sequencing, especially of their 16S ribosomal RNA (rRNA), has provided additional insights into the evolutionary relationships among the bacteria.
Staphylococcus aureus is also a frequent inhabitant of the skin, nasal passages, and the gastrointestinal tract.
Some strains of Staphylococcus aureus secrete a toxin and can cause life-threatening toxic shock syndrome. Mycoplasmas are obligate parasites; that is, they can live only within the cells of other organisms.
Mycoplasma genitalium has 580,073 base pairs of DNA encoding 525 genes (485 for proteins; the rest for RNAs). Mycoplasma urealyticum has 751,719 base pairs of DNA encoding 651 genes (613 for proteins; 39 for RNAs). Workers at the JCVI have also succeeded in synthesizing the complete genome of one species of mycoplasma, inserted this into a second species, which converted the second species into the first. Actinobacteria dominate the microbial life in soil where they play a major role in the decay of dead organic matter.
These Gram-positive organisms are closely related to the actinobacteria and often classified with them. They are further subdivided into 5 clades: alpha-, beta-, gamma-, delta-, and epsilon proteobacteria. Certain colorless bacteria share the ability of chlorophyll-containing organisms to manufacture carbohydrates from inorganic raw materials, but they do not use light energy for this. They then use this energy to reduce carbon dioxide to carbohydrate (like the photosynthetic purple sulfur bacteria). These chemoautotrophs are responsible for the brownish scale that forms inside the tanks of flush toilets. This chemoautotroph oxidizes NH3 (produced from proteins by decay bacteria) to nitrites (NO2−). Causes meningococcal meningitis, an extremely serious infection of the meninges that occasionally occurs in very young children and in military camps.
Read about the 1991–92 cholera epidemic in Central and South America that made over 700,000 people ill and caused over 6,000 deaths.
The recurrent epidemics of the "black death" in Europe from 1347–1351, which killed off at least 30% of the population, was caused by this organism. Although no major epidemics have occurred in this century, the threat is not entirely over. Like green plants, these bacteria are photosynthetic, using the energy of sunlight to reduce carbon dioxide to carbohydrate.
Instead they use hydrogen sulfide to supply the electrons needed to synthesize NADPH and ATP. Photosynthetic bacteria contain special types of chlorophylls (called bacteriochlorophylls) incorporated into membranes. A diverse group that includes Bacteroides fragilis, one of the most abundant organisms in the human large intestine. These are thin, corkscrew-shaped, Gram-negative flexible organisms that range in length from a few to as many as 500 µm.
Treponema pallidum (right), the cause of syphilis, one of the most dangerous of the sexually transmitted diseases (STDs). Mothers can pass the infection on to their newborn babies causing serious eye disease and pneumonia. Several million people in the desert regions of Asia, Africa, and the Near East have been blinded by trachoma.

It is estimated that cyanobacteria are responsible for ~ 25% of the photosynthesis occurring on our planet. The micrograph is of Oscillatoria, a filamentous cyanobacterium (magnified about 800 times). There is now lots of evidence that both of these eukaryotic organelles evolved from once free-living bacteria. Why are the Gram-negative bacteria encased in two membranes while the Gram-positives have only one? In 1840, the German pathologist Friedrich Henle proposed criteria for proving that microorganisms were responsible for causing human disease (the "germ theory" of disease).
The era of chemotherapy was begun in 1910 when the German chemist Paul Ehrlich discovered the first antibacterial agent, a compound effective against the spirochete that causes syphilis. Thousands of scientists have followed these pioneers, each building on the foundation established by his or her predecessors, and each adding an observation that expanded our understanding of microbes and their role in disease.
However, the complexity of medical microbiology we know today rivals the limits of the imagination. To start, the microbes can be subdivided into four groups: viruses, bacteria, fungi, and parasites, each having its own level of complexity. They are prokaryotic organisms -- simple unicellular organisms with no nuclear membrane, mitochondria, Golgi bodies, or endoplasmic reticulum that reproduce by asexual division.
The size (1 to 20 Am or longer), shape (spheres, rods, spirals), and spatial arrangement (single cells, chains, clusters) of the cells are used for the preliminary classification of bacteria, and the phenotypic and genotypic properties of the bacteria form the basis for the definitive classification. Likewise, the environment that surrounds us, including the air we breathe, water we drink, and food we eat, is inhabited by bacteria, many of which are relatively avirulent and some of which arc capable of producing life-threatening disease.
The interaction can result in transient colonization, a long-term symbiotic relationship, or disease.
To improve the human body's ability to prevent infection, the immune system can be augmented either through the passive transfer of antibodies present in immune globulin preparations or through active immunization with microbial antigens.
Unfortunately, microbes can alter their antigenic complexion (antigenic variation) or develop resistance to even the most potent antibiotics. However, the ability of the laboratory to perform these functions is limited by the quality of the specimen collected froin the patient, the means by which it is transported from the patient to the laboratory, and the techniques used to demonstrate the microbe in the sample. This seems obvious, but many specimens sent to laboratories for analysis are contaminated during collection with the organisms that colonize the mucosal surfaces.
The laboratory must test only organisms capable of producing disease and only medically relevant antimicrobials. The virulence of the organism, site of infection, and patient's ability to respond to the infection influence the host-parasite interaction and must also be considered when planning treatment. Just as the early microbiologists built their discoveries on the foundations established by their predecessors, so, too, will we and future generations continue to discover new microbes, new diseases, and new therapies.
Another group of microbes, the archaea, meet these criteria but are so different from the bacteria in other ways that they must have had a long, independent evolutionary history since close to the dawn of life. The electron micrograph (courtesy of the Naval Dental Research Institute, Great Lakes, IL) shows Streptococcus mutans, a common inhabitant of the mouth.
They are probably the descendants of Gram-positive bacteria who have lost their peptidoglycan wall as well as much of their genome — now depending on the gene products of their host. Craig Venter Institute — JCVI) who determined the Mycoplasma genitalium sequence followed this work by systematically destroying its genes (by mutating them with insertions) to see which ones are essential to life and which are dispensable.
Many of them have turned out to be the source of valuable antibiotics, including streptomycin, erythromycin, and the tetracyclines.
As in tetanus, it isn't the growth of the organism (in the throat) that is dangerous but the toxin it liberates. Because of the similarities of their genomes, rickettsias may be the closest relatives to the ancestors of mitochondria. These bacteria live in a mutualistic relationship with the roots of legumes where they are able to "fix" nitrogen (N2) in the air into compounds that can be used by living things. These so-called chemoautotrophic bacteria secure the necessary energy by oxidizing some reduced substance present in their environment.
They complete the oxidation of partially oxidized iron compounds and are able to couple the energy produced to the synthesis of carbohydrate. There is a vaccine that is effective against several strains but unfortunately not the most dangerous one. Confined to the intestine, it is a frequent cause of human gastrointestinal upsets but is also found in many other animals (that are often the source of the human infection). DNA sequencing of samples retrieved from the bodies of plague victims of that era confirm this diagnosis. This is primarily a disease of small mammals, but about 100 people become infected each year in the United States. With this machinery, they can run photosystem I but not photosystem II (which explains their inability to use water as a source of electrons).
Thus they are restricted to such habitats as the surface of sediments at the bottom of shallow ponds and estuaries. They are found in vast numbers in soil and are major players in the decay of organic matter.
It has been estimated that these Gram-negative anaerobes make up >30% of the dry weight of human feces. To avoid this, pregnant women are usually tested for chlamydia and treated with antibiotics if they are infected. Their chloroplasts (in fact probably all chloroplasts) evolved from an endosymbiotic cyanobacterium. Evolutionary biologist James Lake has proposed that the Gram-negatives arose by one single-membrane bacterial ancestor engulfing another. Koch and Pasteur confirmed this theory in the 1870s and 1880s with a series of elegant experiments proving that microorganisms were responsible for causing anthrax, rabies, plague, cholera, and tuberculosis. This was followed by Alexander Fleming's discovery of penicillin in 1928, Gerhard Domagk's discovery of sulfanilamide in 1935, and Selman Waksman's discovery of streptomycin in 1943.
We now know that there are thousands of different types of microbes that live in, on, and around us, and hundreds that cause serious human diseases. Although the cell wall encircling bacteria is itself complex, there are two basic forms: a Gram positive cell wall with a thick peptidoglycan layer and a Gram negative cell wall with a thin peptidoglycan layer and an overlying outer membrane. These are known as exogenous infections, and examples include diseases caused by influenza virus, Neisseria gonorrhoeae, Coccidioides immitis, and Entamoeba histolytica.

The outcome of this interaction is determined by the virulence of the organism, the site of exposure, and the host's ability to respond to the organism. Thus, the battle for control between microbe and host continues, with neither side yet able to claim victon, (although the microbes have demonstrated remarkable ingenuity).
Because most diagnostic tests are based on the ability of the organism to grow, transport conditions must ensure the viability of the pathogen.
Because most infections are caused by endogenous organisms, it is virtually impossible to interpret the testing results with contaminated specimens. To test all isolated organisms or an indiscriminate selection of drugs can yield misleading results, with potentially dangerous consequences. This website is intended as a foundation of knowledge that can be used to enrich your understanding of microbes and their diseases. In fact, there is considerable evidence that you are more closely related to the archaea than they are to the bacteria! Spores are so resistant to adverse conditions of dryness and temperature that they may remain viable even after 50 years of dormancy. This interferes with the reciprocal inhibition of antagonistic pairs of skeletal muscles so the victim suffers violent muscle spasms. The toxin blocks the release of acetylcholine (also by destroying the SNAREs needed for exocytosis) from the terminals of motor neurons. Of the 485 protein-encoding genes, they have concluded that only 381 of them are essential to life. This means that they can only grow and reproduce while within the living cells of their host — certain arthropods (ticks, mites, lice, fleas) and mammals. The nitrites are then converted (by other nitrifying bacteria) into nitrates (NO3−), which supply the nitrogen needs of plants.
Its entire genome has been determined down to the last nucleotide: 4,639,221 base pairs of DNA encoding 4,377 genes. The bacteria liberate a toxin that causes massive diarrhea (10–15 liters per day) and loss of salts.
Here they must make do with whatever radiant energy gets through the green algae and aquatic plants growing above them. While they can be pathogenic elsewhere in the body, they live as commensals in the intestine digesting otherwise-indigestible polysaccharides and providing us with some fatty acids, sugars, and perhaps vitamin K.
The infection is usually spread by sexual intercourse making it the most common sexually-transmitted disease (STD). His analysis of many genes in the various bacterial groups indicate that the most probable ancestors of this possible endosymbiosis were a clostridium and an actinobacterium. Thus, the manifestations of disease can range from mild symptoms to organ failure and death. Not only can a patient be treated inappropriately with unnecessary antibiotics, but the true pathogenic organism may not be recognized among the plethora of organisms isolated and tested. In 1853, Cohn categorised bacteria as one of three types of microorganisms -- bacteria (short rods), bacilli (longer rods), and spirilla (spiral forms). Thus the victim shows signs of sympathetic nervous activity (dilation of the pupils, inhibition of urination) and skeletal muscle weakness.
It catalyzes the inactivation of a factor necessary for amino acids to be added to the polypeptide chain being synthesized on the ribosome.
In males, the bacterium invades the urethra causing a discharge of pus and often establishes itself in the prostate gland and epididymis.
Unless the water and salts are replaced quickly, the victim may die (of shock) in a few hours. As it spreads into the lymph nodes, it causes them to become greatly swollen, hence the name "bubonic" (bubo — swelling of a lymph node) plague.
However, the import of infected rabbits by game clubs has introduced the disease to Cape Cod and Martha's Vineyard in Massachusetts.
The complete genome of Haemophilus influenzae is known: 1,830,138 bp of DNA encoding 1,743 genes. However, the absorption spectrum of their bacteriochlorophylls lies mostly in the infrared region of the spectrum so they can trap energy missed by the green plants above them. It is easily cured if diagnosed, but many infections remain untreated and, in females, are a major cause of pelvic inflammatory disease. Clostridia are the only Gram-positive bacteria that have photosynthetic members and because the photosynthetic apparatus in all photosynthetic Gram-negative bacteria is in the inner membrane, perhaps the actinobacterium was the host and the clostridium the endosymbiont. The term bacteria was preceded in the 17th century by the microscopic animalcules described by Antony van Leeuwenhoek (1632-1723). Chemical alteration of the toxin produces a toxoid that still retains the epitopes of the toxin. This is probably because it has abandoned many of the genes needed for an independent existence choosing instead to depend on the genes of its host cell. Sensibly enough, the toxin has no such effect on the translation machinery of bacteria (or of chloroplasts and mitochondria). Like other intestinal diseases, cholera is contracted by ingestion of food or, more often, water that is contaminated with the bacteria. Once in the lungs, however, the bacteria can spread through the air causing the rapidly lethal (2–3 days) "pneumonic" plague. Incorporated in a vaccine, the toxoid provides a relatively long-lasting (~10 years) immunity against tetanus. The toxin is a protein and is quickly (10 minutes) denatured at 100 °C, so boiling home-canned products makes them safe to eat. If the infection is untreated (penicillin is usually effective although strains resistant to it are now being encountered), the resulting damage to the fallopian tubes may obstruct the passage of eggs and thus cause sterility. Untreated, ~30% of the cases of bubonic plague are fatal, and the figure for the pneumonic form reaches 100%. All seem to have acquired their infection as they used lawn mowers and brush cutters that presumably stirred up the organism from the carcasses of infected animals.

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27.08.2015, author: admin

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