How are microbes classified
Biology for Medicines: Fundamentals of Medical Microbiology
Table of Contents
All living things are divided into three domains:
- Eukaryotes (Living beings with a cell nucleus): people, animals, plants, fungi
- Prokaryotes (Living organisms without a nucleus): bacteria
- Archaea: "Primordial Bacteria"
Protists summarize the groups of unicellular pathogens with a cell nucleus (i.e. eukaryotes). The protists include:
- Protozoa (important pathogens: malaria, sleeping sickness)
- unicellular algae
- certain mushrooms
Morphology and structure of the bacteria
Bacteria are prokaryotes: they do not have a delimited nucleus. The DNA is therefore compressed without a membrane in the core equivalent, the Nucleoid, in the cytoplasm. There are also no other internal compartment systems as in eukaryotes (no ER, no mitochondria, etc.): The entire metabolism of a bacterium takes place in the cytoplasm.
The two most important characteristics with which bacteria can be classified microscopically are the external appearance of the bacterium, which Gram stain and the type of flagellation (if any).
The following table gives an overview of the morphological classification of bacteria.
|Cocci||spherical, immobile, singly or in pairs (Diplococci), thread-like to each other (Streptococci) or in heaps together (Staphylococci)|
|Bacilli||elongated shape (chopsticks)|
|Spirils||turned shape (screws)|
|Spirochetes||helical, extra-long, flagellate filaments, locomotion corkscrew-like, subgroup are Treponemes|
|Vibrios||Gram-negative rod-shaped bacteria, curved comma-shaped, flagellated|
Image: “Different arrangements of cocci: (a) streptococci (cell division in one plane), (b) diplococci (remain as two cocci after cell division), (c) tetrads (cell division in two planes), (d) packet cocci or sarcines (Cell division in all three levels), (e) Staphylococci (cell division occurs irregularly in several levels). " by Y tambe. License: CC BY-SA 3.0
The cell wall of the bacteria
The cell wall connects to the outside of the cell membrane. It is used for mechanical stability and mass transfer. The peptidoglycan is a component of almost every bacterial wall Murein. Murein consists of N-acetylglucosamine and N-acetylmuramic acid. Longitudinal and cross-linking through oligopeptides creates a Murein sacculusthat protects the cell and acts contrary to the osmotic pressure.
Classification of bacteria according to Gram stain
In order to type bacteria, the method of Gram stain used after the Danish bacteriologist Hans Christian Gram (1853–1938). It is an important diagnostic tool because Gram-negative bacteria are less sensitive to penicillin thanks to an additional membrane in the form of a lipid bilayer.
Steps of the Gram staining:
- Violet staining of cells
- Wash out the stain with alcohol
- Red staining of the cells
Gram positive bacteria: purple, Gram negative bacteria: red
Image: “Scheme of a cell wall of gram-negative bacteria” by Matthias M. License: CC BY-SA 3.0
Gram-positive bacteria have a thick layer of murein. Gram-negative bacteria have a comparatively thinner murein layer, but a second membrane in the form of a lipid bilayer.
Infection with Gram negative bacteria is more critical to the host. The lipopolysaccharides of the second membrane layer can when decomposed by the host body as Endotoxins be released. Endotoxins act as pyogens and cause high fever and chills.
Overview of gram-positive and gram-negative bacteria and diseases
|Clostridium perfringens||Gas fire|
|Staphylococcus aureus||nosocomial infections|
|Streptococcus pneumoniae||Pneumonia, other infections|
|Haemophilus influenzae||Sinusitis, bronchitis, meningitis|
|Escheria coli||Urinary tract infections, neonatal meningitis|
|Mycoplasma pneumoniae||atypical pneumonia|
Flagella and pili in bacteria
About 50% of prokaryotes can use Scourges(Flagella) move around. Can scourge monotrich (singular) or polytrich occur (in plural) on the bacterium. The bacterial flagella consist of the protein flagellin and are not covered with protuberances of the cell membrane. The flagella are arranged monopolar (Flagella at one end of the cell), bipolar (at both cell ends) or peritrich (distributed over the whole bacterium).
Some of the flagella proteins are species-specific and thus act as antigens that can be identified in serology (e.g. in Salmonella and E. Coli).
Pili resemble the flagella in structure, but are much smaller. Through the Pili (also fimbria) the bacterium can attach better to surfaces and other bacteria. The adherence of a bacterium to a potential host is made possible by fimbriae: They thus represent a virulence factor.
The metabolism of bacteria
Almost all bacteria need organic substances to survive and are therefore classified as heterotrophic organisms. Bacteria are further classified according to their relationship to oxygen.
|Obligatory aerobic (aerophilic)||Oxygen is needed to maintain metabolism|
|obligatory anaerobic||There are no enzymes in the respiratory chain, energy is obtained from anaerobic glycolysis, and oxygen is toxic|
|microaerophilic||Oxygen is required for growth, but concentrations that are too high cause growth to stop|
|optional anaerobic||Growth is possible with or without oxygen|
Bacteria are able to transfer genetic material to one another. Because bacteria are able to integrate foreign DNA into their genome, they can recombine their existing gene stock. Different bacterial strains can transfer genetic properties to one another in this way.
The three mechanisms of gene transfer
- conjugation: Parasexual transmission through contact via F-Pili
- Transduction: Gene transfer by bacteriophages (viruses that infect bacteria)
- transformation: Uptake of free, isolated foreign DNA into the bacterial genome
The cause of the pathogenic effect of bacteria
Various factors influence the virulence (strength of the pathogenicity) of bacteria.
- Number of bacteria that have entered
- Possession of adhesins
- Invasion Factors
- Multiplication rate
- formation of Endo- and exotoxins
- Ability to evade the immune system
Endotoxins: Endotoxins are produced when components of the bacterial cell wall break down (see above) when bacteria die. Endotoxins activate the complement cascade and the coagulation cascade in the host through cytokines, which can develop into septic shock. Symptoms that are not specific to the disease and trigger endotoxins are: fever, pain, shock, fatigue and malaise.
Exotoxins: Some bacteria can produce and secrete toxins themselves. If these are directed against a host, it will suffer from very severe disease-specific symptoms. Examples are that cholera-Toxin, Botulinus-Toxin, Diphtheria-Toxin and that tetanus-Toxin.
The antibacterial substances: antibiotics and chemotherapy drugs
Bactericidal Substances kill bacteria, bacteriostatic only inhibit growth. Antibiotics are substances synthesized from bacteria and fungi that act against other microorganisms. Antibiotics and chemotherapy drugs intervene in certain steps in bacterial cell metabolism. They inhibit replication, transcription and translation or damage the bacterial cell wall or membrane. For example, penicillins inhibit the synthesis of murein.
Fungi belong to the group of eukaryotes and, like plants, have cell walls, vacuoles, a plasma flow and are immobile. In almost all mushrooms, however, these consist of chitin, not cellulose. Fungi do not carry out photosynthesis, but receive their substrates for metabolism through their property as Saprophytes(Digester recycler).
The forms of growth and reproduction of mushrooms
Fungi usually consist of cell threads, the hyphae. The organism of a fungus consists of tube systems, with the hyphae forming a highly branched network, the Mycelium. Yeasts are exceptions that do not form hyphae or mycelium.
Fungi can reproduce sexually and asexually. Asexual reproduction occurs through division, disintegration of the hyphae, sprouting (with yeasts) or the formation of conidia (contains asexual, mitotically formed spores). Sexual reproduction results from the fusion of two physiologically different cells. The resulting diploid The cell can now germinate and form diploid cells. If the reduction division only takes place in the zygote stage, then arise haploid Spurs.
The toxic synthesis products of mushrooms
Some mushrooms can produce toxic substances that are dangerous to humans. The following overview summarizes the most important producing mushrooms, their toxic substances and the effects of these.
|Aflatoxins||Aspergillus flavus, Aspergillus parasiticus||strong carcinogenic effect, often the cause of food poisoning (traces on nuts, grains, spices)|
|Amanitine||Death cap mushroom||Inhibition of RNA polymerase II, even a small dose is lethal|
|Muscarines||Fly agaric||Effect as a parasympathomimetic on the nervous system|
|Ergotamines||Ergot (Claviceps purpurea)||Effect on veg. Nervous system, hallucinogenic effect, contracts uterine muscles|
|Cyclosporine A||Cylindrocarpon, Tolypocladium||immunosuppressive effect (clinical use after organ transplants)|
Image: “Knollenblattpilz Heidelberg Germany” by Grossbildjaeger. License: CC BY-SA 3.0
The pathogenic effects of mushrooms
The infectious diseases caused by fungi are called mycoses. In most cases they are not a problem in healthy people. However, when the immune system is depressed, they break out as opportunistic infections.
- Dermatophytes: Keratinophilic fungi attack human skin, hair and nails.
- Yeasts and molds Affects the mucous membranes of the intestinal and respiratory tract (high risk after antibiotic treatment!).
- Systemic mycoses arise from the absorption of fungal spores from the air we breathe, which can manifest themselves in various internal organs: The result is severe infections that can be fatal. (Risk patients are e.g. HIV patients)
Synthesis of antibiotics
In 1928, Flemming found out that some mushrooms are able to produce antibiotic substances: penicillin from Penicillium notatum, Cephalosporin from the genus Acremonium and Griseofulvin from Penicillium griseofulvum. 50 of these approx. 2000 characterized antibiotic substances are classified as Chemotherapy drugs used.
Viruses are between 20 nm and 300 nm in size infectious particles. Viruses cannot grow or multiply on their own. Instead, they use host cells. They penetrate into it and use the foreign metabolism for their own reproduction. Bacteriophage are special viruses: They use bacteria as hosts by not penetrating them, but instead injecting their genome into the bacterium.
Structure and classification of the virus
The genetic information of the virus is in the form of a nucleoid made of DNA or RNA. The nucleoid is surrounded by a capsid, a protein shell, and partly by an additional shell made of lipids and glycoproteins. Some viruses have enzymes, e.g. reverse transcriptase.
Viruses are classified according to the following characteristics:
- RNA or DNA?
- Single-stranded or double-stranded genome?
- Naked or wrapped (additional cover)?
- Cubic, helical or complex capsid symmetry?
- Animals, plants or humans as hosts?
- Immunological properties?
- Sensitive to chemical or physical properties?
The multiplication of viruses
In terms of the type of reproduction, a distinction must be made between bacteriophages and viruses of eukaryotic cells.
Multiplication cycle of bacteriophage
Phages are made up of a head with a single or double strand and a tail, which is used to attach to the host bacterium. After penetrating the bacterial cell wall, this hollow tail injects its genome into the bacterium. Then two different cycles can be run through: The lytic reproduction cycle and the temperate cycle.
During the lytic replication cycle, the DNA is transcribed immediately. The protein structures and the envelopes of the phage are reproduced by the host, and later also the DNA. The bacterium dies in the process.
In the temperate cycle, lysogeny occurs. The term lysogeny describes the process of integrating phage DNA into the bacterial chromosome. Prophages emerge, which are initially replicated and inherited together with the bacterial DNA.
Replication cycle of the viruses of eukaryotic cells
In this case, the viruses completely penetrate the host cell. The genome is released inside the infected cell.
The 6 stages in the virus replication cycle:
- adsorption: Attachment to the host cell
- penetration: Infiltration into the cell
- Uncoating: Capsid / shell are degraded.
- Replication: Viral nucleic acid and viral proteins are synthesized.
- Maturation: Synthesis components form new viruses.
- Liberation: New viruses leave the cell by lysing the membrane or by pinching it off from the cell membrane.
Retroviruses, a special case
With retroviruses, the genetic information is in the form of RNA instead of DNA. Retroviruses have a special enzyme called reverse transcriptase. The RT transcribes the RNA into DNA before it is introduced into host cells. The best known example of a retrovirus is this HI virus, Causer of AIDS.
The term Prions is from English Proteinaceous Infectious Particle derived. The name is also a definition: prions are the smallest pathogenic infectious proteins. Prions are associated with some degenerative diseases, e.g. with the Bovine spongiform encephalopathy(BSE), the "mad cow disease".
Other diseases are Scrapie in sheep that Creutzfeld-Jakob disease and the Kuru Kuru Disease (Occurred only in a tribe that practices cannibalism). The transmission mechanism of prions has not yet been conclusively clarified.
Popular Biology Exam Questions for Medical Professionals: Microbiological Fundamentals
The solutions can be found below the references.
1. Endotoxin shock can occur in the course of infectious diseases. It is created by the release of lipopolysaccharides. With which infection is this most likely to be feared?
- Gram-negative chopsticks
- Gram-positive chopsticks
- Gram-positive diplococci
- facultative anaerobes
2. Fungi can release toxins. Which assignment is correct?
- Aflatoxin - Aspergillus flavus
- Amanitin - fly agaric
- Ergotamine - death cap mushroom
- Cyclosporin A - Ergot
- Muscarin - Tolypocladium
3. The Retroviridae represent a specific group of viruses. Which stage in the replication cycle of viruses was decisive for the name of this particular virus?
Course tip on microorganisms in biology for medical professionals
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Wenisch (2013): mediscript short textbook biology - Elsevier Verlag.
Poeggel (2009): Short textbook Biology -Thieme Verlag.
Solutions to the questions: 1B, 2A, 3B
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