The resistance (from the Latin resistentia for "resistance"; English resistance ) is the resistance of a living being against harmful influences of the environment (such as parasites , infections , diseases , climate ), in the case of animal and plant pests also against applied pesticides, as well in bacteria and viruses against drugs. Resistance can be hereditary, but it can also be generated by environmental conditions (e.g. diet). The creation of resistant breeds or varieties through mutation or crossingand continued selection is one of the primary goals of animal and plant breeding .
Resistance to microorganisms
Populations of microorganisms consist of thousands upon thousands of individuals, some of which are less susceptible to e.g. B. are against an antibiotic than others. When an antibiotic is used, the most sensitive individuals die first and the most resistant survive the longest. If antibiotic use is stopped too early, some of the most resistant individuals will survive and multiply. The new population is, on average, more resistant than the old one because all current individuals are descended from mother cells that were selected for low susceptibility to the antibiotic. Diseases that can be traced back to these pathogens, which have already gone through this process several times, can hardly be treated with antibiotics.
Mechanisms of resistance are
- Efflux pumps are located in the outer membrane of the cells and pump antibiotics out of the cells, for example, so that the concentration inside the cell remains below a critical level,
- Gene mutation : Mutations of the gene that is encoded for the protein that represents the binding site / the site of action of the antibiotic in the pathogen and
- Metabolic detoxification : for example, using enzymes that break down the active ingredient.
Occasionally, when a metabolic pathway is blocked by an antibiotic, microorganisms can activate an alternative metabolic pathway and thus survive in an environment containing antibiotics. Resistance can also be acquired by transferring genes for antibiotic resistance , for example encoded on plasmids , from one bacterium to another by conjugation . In hospitals in particular there are pathogens (such as Pseudomonas ) that have acquired several resistance factors in this way, i.e. have become multi-resistant - a consequence of the continuous selection pressure due to the need for antibiotics in infectious or intensive care departments.
Resistance from higher organisms
Today, many weeds are also resistant to herbicides , insects to insecticides or fungi to fungicides . This has consequences for modern agriculture , but also in the fight against diseases spread by insects (e.g. malaria ).
In the case of plants, resistance to diseases and pests is referred to as resistance, while resistance to abiotic damaging factors (e.g. cold, herbicides ) is referred to as tolerance. In cultivated plants , resistance breeding , in which the qualitatively and quantitatively different resistances of the varieties and species against pathogens and pests are newly combined, is of great importance for plant protection .
Resistance reactions of plants to harmful organisms are divided into different types based on the mechanisms of action.
In the case of non-host resistance, all varieties and genotypes of a plant species are resistant to all races of a pathogen .
If a pathogen has the basic ability to cause disease in a plant species (e.g. the pathogen Phytophthora infestans causes late blight on tomatoes and potatoes), individual host genotypes can be resistant to certain strains or races of the pathogen. This resistance is therefore race-specific and characterized by a qualitative interaction (yes-no). Virulent breeds lead to unhindered infestation on susceptible plants; in avirulent breeds, the infection is completely stopped in plants with suitable resistance. Race-specific resistances are usually based on only one or very few genes. Such complete resistances are also referred to as vertical resistances , since their more or less monogenic inheritance is clearly reflected in the offspring. The mechanism of qualitative resistance is often hypersensitive cell death. This means that as soon as a plant cell comes into contact with an avirulent breed, the cell dies and u. U. the neighboring cells before the pathogen can penetrate into the next cell, and thus also kills the pathogen. The qualitative resistance can be overcome quickly by many pathogens.
In contrast to breed-specific resistances, there are also partial resistances that allow infestation, but drastically slow down the infection process. Such gradual resistance reactions ( horizontal resistance ) are based on a large number of polygenically controlled resistance mechanisms that are triggered largely independently of the pathogen races. That is why they are referred to as race-unspecific or quantitative (more / less). In principle, it is much more difficult for pathogens to adapt to these complex quantitative resistances, since several compensatory mutations have to occur at the same time in order to circumvent them, which statistically occurs significantly less often. Crossing combines several partial resistances, which can also lead to complete resistance. Partial resistances are mostly found in plants that are only slightly affected.
Both the qualitative and quantitative resistance reactions are based on biochemical processes in the plant, which, once triggered, are often active for a certain period of time and thus temporarily protect the plants against later infections, analogous to a very short-term vaccination (typical action time three to seven days ). This process is known as induced resistance .
Resistance of humans and animals
In humans and animals, in addition to the individual resistance mechanisms, there is another resistance that an organism acquires in the course of its life against pathogens, the so-called immunity. The process of generating immunity is also called immunization and consists in the immune system adapting to the pathogens to which the organism has been exposed. There is also a temperature resistance to changing temperature influences .
Resistance in crop protection
In the case of fungi, resistance can develop quickly through several generations of fungi per vegetation period. If the spores are spread with the wind, as in the case of powdery mildew (formerly Erysiphe graminis , today Blumeria graminis ) or coffee rust , a new resistance can spread several 100 km within a few weeks. In the case of other pathogens such as apple scab , resistance only spreads locally (e.g. in the Lake Constance area). In order to reduce the risk of pathogens developing resistance in agriculture , it is recommended that the active ingredient group be changed regularly when using fungicides .
In pharmacology and related fields, resistance describes the fact that normally effective influences do not work, for example when a neurotransmitter or a hormone no longer works as usual because the receptors on the target cells are missing or blocked. In order to achieve the same intensity of action, an increase in dose is then necessary, the effective dose increases. A well-known example of this is insulin resistance . In radiochemistry , a possible resistance to low radiation doses due to a hormesis is being discussed, which presumably arise due to the activation of repair mechanisms of the cell from a threshold value of the radiation dose .
- Pesticide resistance
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