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Why do bacteria have both of these mechanisms to deal with the same environmental stress: nutrient deprivation? In a culture exposed to this condition, often both competent cells and sporulated cells are found.
Biological Pest Control uses , advantages and disadvantages
Controlling the pests with their natural enemies , including the parasites , the predators , the diseases & competing the organisms , is called the biological control , It is an alternative to using the broad-spectrum pesticides , that kill off the beneficial insects as well as the pest organisms , It is an environmental friendly method & it does not introduce the pollutants into the environment .
Biological Pest Control
The biological control minimizes the environmental , legal & public safety concerns , Integrated pest control uses the bio-agents in combination with the other measures , The biological control agents are called the bio-agents , leave behind no long-lasting residues which remain in the environment , They don’t leach into the groundwater or create the resistant strains of the insects .
It is highly specific to one pest , It can either be less or more expensive than the pesticides , You can incur significant expense studying , choosing , testing & breeding the bio-agent , A long term solution if equilibrium is established , Biological Pest Control is inexpensive over the long term & it can be used in a glasshouse .
The pests do not become resistant , There is no environmental contamination , If the biological control organism is introduced , It does not have to be re-introduced , The chemical pesticides must be used repeatedly , So , more expenses & time consumed , The biological control limits the subsequent use of the pesticides .
It is an alternative to the pesticides & the poisons , When you remove the organism from the food web , It can disrupt all the others around it , so , you have to be very careful , biological control is a better , safer method of control than using pesticides .
Biological pest control is the use of the natural predators as agents to attack the weeds , The biological control agents are the insects , but fungi , bacteria & nematodes are sometimes used , Some fungi attack the insects & kill them , The fungal spore penetrates the insect & grows throughout it , It takes about a week for the insect to die , Fungi are cost-effective unless a high application rate is needed for heavy insect infestations .
Biological Control advantages
Biological Pest control is a very specific strategy , whatever the predator is introduced will only control the population of the pest they are meant to target , making it a green alternative to the chemical or the mechanical control methods , whereas the weed killing chemicals can destroy fruit-bearing plants , The biological control allows the fruit to be left uninterrupted while the weeds are destroyed .
Biological pest control does not have adverse effect on the human health or the environment , It is self-sustaining , It can be cost effective , When the cost of testing & introducing control agents has been met , the on-going costs are small , There is no need to find & identify every individual weed to be treated , An effective agent will search out all suitable plants of the weed .
Development of the host resistance is not a problem , The biological pest control is compatible with most other control techniques ( except sometimes the use of the insecticides & the herbicides ) , The biological control reduces the competitiveness & reproductive capacity of the weed , making it more manageable .
The biological control method of pest management does not use the chemicals , It uses many organisms that are either the predators or the parasites to the pest , The pest is the organism that causes damage to the people and their crops , The biological control should be implemented whenever possible because it does not pollute the environment .
The great benefit of this method is its selectivity , There is a restricted danger of damage to non target plant species , Biological control does not create new problems , like conventional pesticides , Selectivity is the most important factor regarding the balance of the agricultural ecosystems because a great damage to non target species can lead to the restriction of natural enemies’ populations .
The biological control agent (BCA) can be deployed in the agricultural ecosystem , so as not to damage non target pests , depends on appropriate host specificity tests which determine the potential host range .
The ability to self-perpetuate is an interesting advantage of the biological control method , BCAs will increase in number & spread , Because BCAs are self-propagating & dispersing , pest control is self-perpetuating too , This is quite important regarding the economic feasibility of the biological control .
Another benefit of the biological control method is the environmental safety of BCAs , The pest is unable (or very slow) to develop the resistance , The biological control can be cost effective , Its effectiveness is based on the self-perpetuation & self-propagation , So , if we establish a control agent in a specific area , it will reduce the target pest in an acceptable threshold for quite long time .
The financial benefit of the biological control is greatest in the cases when there is no other option , The biological control is very effective in the inaccessible areas , The cost efficiency of this method is that the yield benefit of the biological control is less than yield achieved by the agrochemicals , but the primary cost of BCA is lower than the chemical pesticides .
The biological control is the attractive alternative to the agrochemicals , the use of environmentally friendly alternatives to the chemical pesticides are absolutely required in agriculture , No chemicals are used , so , there’s less pollution , disruption of the food chain & risk to the people eating the food that has been sprayed .
BCAs are more susceptible to the environmental conditions than the chemical control , This consequently causes the fluctuations to the pest populations , It reflects to the product quality , to the crop yield and of course to the price of the product on market , if the annual harvest of the crop is not stable , it will affect the grower’s income stability .
The biological control method is environmentally safe , This method offers less risk of the residues in food chain , biological control is most suited for exotic pest that are not closely related to indigenous beneficial species , Biological control is les s expensive than the chemical control .
Disadvantages of Biological Control
Biological control is a slow process , It takes a lot of time & patience for the biological agents to work their magic on the pest population , whereas the other methods like the pesticides work offer immediate results , The upside to this is the long-term effect biological control provides .
It can be fickle , you can’t control whatever natural enemy , you set loose in an ecosystem , While it is supposed to manage one pest , your predator will switch to a different target , they may decide eating your crops instead of the insects infesting them , but when you introduce a new species to the environment , there is a risk of disrupting the natural food chain .
Although it is cheap in the long run , the process of setting up the biological control system is a costly endeavor , A lot of planning money goes into developing the successful system , The predator you introduce may not eat the pest , It could eat useful species , The predators population may increase & get out of control , The predator may not stay in the area where it is needed .
The biological control has slow action , It lacks the immediacy of chemical control , So , During the period required until the natural enemies control the pest population , the pests may be present in the intolerable populations , The agent may become a pest itself , Frequent input is needed to maintain population balance and it needs to be large scale .
The pesticides can not be used because they will damage the biological control system , The need for a difficult transition period from insecticides to the biological control may make the introduction of the biological control unattractive for a grower , It is expensive to research and It needs a high level of skills & initial set up costs .
The biological control is unpredictable , Its unpredictability lies mostly in the fact that the natural enemies are dependent on the environmental conditions , The deployment of BCAs in a new environment requires a lot of research to succeed the desirable results because of climatic constraints .
It is difficult & sometimes expensive to develop biological control in field because it requires high qualified scientific staff , There is relatively less investment in the biological control research in compare with the chemical pesticides , The variation & changes in behaviour of natural enemies that can be caused by rearing conditions are manifold , This variation leads to inconsistent results in the biological control .
The potential agents are expensive to test for specificity , Host specificity testing may take many years to complete because of the need for thoroughness , The biological control operates over large areas , so , it can not be limited to the individual properties or paddock , It does not eradicate the pest organism completely , because if the control agent reduces the pest population too far , it destroys its own food source .
Cell Biology: Asexual and Sexual Reproduction
During ________, a new plant grows from parts of the parent plant.
During _______, a cell divides to produce a new, genetically identical cell.
_____ is a form of asexual reproduction used by mushrooms and molds.
During ____, the offspring grows from the body of the parent.
____ is a form of asexual reproduction that must be followed by regeneration.
What is the main method of reproduction for these flatworms?
What statement would they most likely make about the new species?
- Asexual reproduction requires two parents and produces non-identical offspring, while sexual reproduction requires only one parent and produces identical offspring.
- Asexual reproduction requires two parents and produces identical offspring, while sexual reproduction requires only one parent and produces non-identical offspring.
- Asexual reproduction requires only one parent and produces identical offspring, while sexual reproduction requires two parents and produces non-identical offspring.
Sporulation of Fungi
Fungi are some of the most studied spore-producing organisms in the world. They produce a wide variety of spores that significantly vary in size, shape and other surface features that suit their environment (for dispersal etc).
Whereas the spores produced sexually (through meiosis) remain dormant for survival (e.g. ascospores), those that are produced asexually (mitospores) are for dispersal.
Produced through mitosis, asexual spores are released in high numbers and are genetically identical. This allows them to play an important role in reproduction when they land on the appropriate substrate in the environment following dispersal.
* Chlamydospore - This is a type of fungal spore that develops from the hyphal structures during unfavorable conditions. Chlamydospores are characterized by a thick, melanized wall that protects the contents of the spore.
Different types of fungi may produce different types of spores.
The following are some of the main groups of fungi and the type of spores they produce:
· Zygomycota - Members of Zygomycota are known as zygomeycetes. They produce both sexual (zygospores) and asexual (sporangiospores) spores.
· Ascomycota - Ascomycetes also produce both sexual (ascospores) and asexual (conidia) spores.
· Basidiomycota - Compared to the other groups of fungi, basidiomyecetes are largely known to produce sexual spores that are known as basidiospores.
Determination of the optimal rate of HGT
For all the examples shown here, we keep N = 500, s = 0.1 and c = 0.01. The rate of origin of new genes is presumed to be slow. In all these examples we fix u = 1/N = 0.002, so that there is only one new gene in the whole population per generation. Lowering this rate further would slow down the time scale of the simulations unnecessarily, but would not qualitatively change the predictions of the model. The key variables to be studied in the simulation are the gene deletion rate, v, and the horizontal transfer rate h. Simulations were begun with a population of identical individuals having one gene each. For each combination of parameters, a simulation was run for many generations until a stationary state was reached. Mean quantities were then determined over 500,000 generations in the stationary state.
Figure 1 shows the mean fitness w ¯ as a function of h for three different values of v. For the largest value, v = 0.01, there is an optimum close to h = 0.6. When v is reduced to 0.001, the optimum reduces to h = 0.035. For the smallest deletion rate, v = 0.0001, the optimum horizontal transfer rate is h = 0 (or is not distinguishable from zero in our simulation). The case v = 0.01 is intended to model the situation in early cells with very inaccurate replication. Note that v is per gene. A genome of 100 genes will lose one gene per generation on average. A high rate of HGT is required to balance this loss, i.e. h is of order 1 per individual. The rate of gain of genes by HGT is much larger than the rate of gain of genes by de novo evolution, which is only u = 0.002 per individual in the simulation (and presumably even smaller in reality).
Mean fitness of the population versus HGT rate, h , for three different rates of gene deletion, v.
Figure 2 helps to explain why there is an optimal h. The mean number of genes per individual, n ¯ , increases with h because gain of genes by HGT balances gene loss. The mean number of types of gene per individual, n t y p e s ¯ , also increases with h, but not as quickly as n ¯ . The difference between these two curves is the number of duplicate genes per individual. This becomes very large when h is high. Figure 2 also shows the size of the pan-genome: n p a n ¯ is the mean number of types of gene in the whole population. Clearly n p a n ¯ is greater than n t y p e s ¯ . A gene acquired by HGT is only useful if it is different from the genes already in the genome. From the simulations, the probability p useful that the acquired gene is useful was found to decrease from about 25% when h = 0 to only a few percent for large h. Thus, if h is small, genomes remain of limited size. Larger, higher-fitness genomes can be maintained if h is larger. However, if h is too large, HGT causes the build-up of large numbers of duplicate genes that reduce the fitness. When genome replication is more accurate (smaller v), the optimum h is reduced, and is found to be zero for very small v. In the latter case, large, high-fitness genomes can be accurately replicated and are maintained in the population by selection, even in absence of HGT. If h = 0, there are no duplicate genes. HGT does more harm than good in this situation because it creates duplicates.
Mean values of properties of the population as a function of h for the case of high gene deletion rate, v = 0.01. n ¯ , number of genes per individual n t y p e s ¯ , number of different types of gene per individual n p a n ¯ , number of different types of gene in the whole population p useful, probability that a horizontally transferred gene is useful to the receiving organism.
Evolution of the rate of HGT
From the above results, we would expect that cells would evolve towards high or low rates of HGT depending on whether the gene loss rate is high or low. In order to show this, we allowed h to be a variable property of individual cells. Simulations were performed in which each new cell inherited the h value of its parent, but with a small probability, the offspring h was mutated to be slightly higher or lower than the parent. When v = 0.01, the mean HGT rate of the population, evolved towards a stable intermediate rate around h ¯ = 0.4 . This is less than the value of h = 0.6 at which the peak in fitness occurs in Figure 1. Similarly, when v = 0.001, the mean HGT rate evolved towards approximately h ¯ = 0.01 , which is less than the value of 0.035 where the peak fitness occurs. For the smallest deletion rate, v = 0.0001, where the optimum in Figure 1 is at h = 0, it was found that h ¯ evolved towards a very low value that depended on the details of the way h was mutated between parents and offspring.
The fact that h ¯ does not evolve straightforwardly to the position of the peak fitness in the first two cases shows that evolution does not automatically optimize the mean fitness of the population. The most likely reason why the dynamics leads to smaller than optimal h is that if a low h value arises on a genome that has higher than average fitness, this low h value is beneficial in the short term because it maintains the integrity of this genome. Therefore the new h will spread, even though the mean fitness of the population will decrease in the long term if all individuals have the new h value. On the other hand, if a higher than optimal h value arises on a fitter than average individual, the descendants of this individual will acquire large numbers of duplicate genes and will not retain a high fitness. Therefore a too-large h value is unlikely to spread.
What is most important for the current argument is that h ¯ does indeed evolve towards smaller values when v is smaller. It is particularly interesting to see what happens when both v and h are allowed to vary and be inherited from parent to offspring. Gene deletion is on average deleterious in this model therefore we expect variants with lower values of v to be selected. We began with individuals with v = 0.01 and h = 0.6, representing early cells with inaccurate replication and frequent HGT. Figure 3 shows mean values v ¯ and h ¯ as a function of time. These are averaged over individuals in the population and over five independent runs of the simulation. Error bars show standard deviations across the simulation runs. The model shows that v ¯ evolves towards very low values because highly accurate replication is advantageous. It is also seen that v ¯ evolves towards very low values because lower h is favoured when v is lower. Limiting values of v ¯ and h ¯ depend on the way v and h are mutated between parents and offspring, and both would tend to zero if only selection were operating.
Variation of mean deletion rate, v ¯ , and mean HGT rate, h ¯ , as a function of time in simulations in which both quantities are heritable. Error bars show standard deviations over five runs.
Emergence of Evolutionary Lineages
As discussed in the background section, evolution is expected to be tree-like in absence of HGT, but will become a tangled web if HGT is frequent. If there is an evolutionary tree, then it should be possible to cluster genomes according to their similarity. The basal split in the tree defines the two largest scale clusters of genomes, indicated schematically as black and white points in Figure 4. Genomes in the same cluster should be closer to one another in genome space than they are to genomes in the other cluster. If the tree is well-defined, there will also be sub-clusters nested hierarchically within the larger clusters (as in the centre of Figure 4). On the other hand, if there is a high HGT rate, then there will not be a clear way to split genomes into clusters. There will be an amorphous cloud of points in genome space, and although some genomes will be slightly closer to one another than others, any attempt that we make to split the population into clusters will be rather ill-defined and unsatisfactory (as on the right of Figure 4).
Clustering individuals according to similarity of genomes.
We will now use the simulations of Figure 3 to show that clusters of genomes are not well defined in our model when h is high, but that they become well defined as h decreases during the progress of the simulation. Our model therefore demonstrates the emergence of separate evolutionary lineages over time, i.e. it goes through Woese's Darwinian threshold .
For each set of genomes generated by the model, we calculated the distance matrix between all pairs of genomes, as described in Methods. This matrix was used as input to the standard UPGMA method of hierarchical clustering. Only the two largest-scale clusters were used, i.e. the two clusters that remain at the penultimate step before the root is reached. Using these two clusters, we measured d 1, the mean distance between pairs of individuals in the same cluster, and d 2, the mean distance between pairs of individuals in different clusters. The clustering ratio, R = d 2/d 1, was used as a measure of the extent of separation of these large-scale clusters. The higher the clustering ratio, the more clearly defined is the split at the base of the evolutionary tree.
Rows 1-8 of Table 1 correspond to values of v ¯ and h ¯ that arose at regularly spaced time intervals in Figure 3. Row 9 corresponds to the smallest v that was used in Figure 1 combined with h = 0. As R fluctuates a lot between populations for any given parameters, it is necessary to generate many populations for each set of parameters. Simulations were performed with v and h fixed at each of the combinations shown in the table. Genome data was printed out at well-spaced intervals, thus generating 100 independent populations for each parameter set. R was calculated for each population. Mean R values are shown in Table 1. For the highest HGT rate (Row 1), R is only 1.54. The clustering algorithm always produces a result even if the input data matrix is very far from tree-like. This means that R is bound to be greater than 1. However, this low value of R indicates that the basal split of the tree is poorly defined (as on the right of Figure 4). Table 1 shows that as h decreases, R becomes much larger. Thus, the basal split is very clearly defined in these latter cases (Rows 8 and 9): separate lineages have emerged.
Principle of Dry heat sterilization using HOT AIR OVEN
Sterilizing by dry heat is accomplished by conduction. The heat is absorbed by the outside surface of the item, then passes towards the centre of the item, layer by layer. The entire item will eventually reach the temperature required for sterilization to take place.
Dry heat does most of the damage by oxidizing molecules. The essential cell constituents are destroyed and the organism dies. The temperature is maintained for almost an hour to kill the most difficult of the resistant spores.
- 170°C (340°F) for 30 minutes,
- 160°C (320°F) for 60 minutes, and
- 150°C (300°F) for 150 minutes or longer depending up the volume.
Bacillus atrophaeus spores should be used to monitor the sterilization process for dry heat because they are more resistant to dry heat than the spores of Geobacillus stearothermophilus. The primary lethal process is considered to be oxidation of cell constituents.
11 Advantages And Disadvantages Of Sexual Reproduction
Sexual reproduction occurs when living organisms combine genetic information from two different types. These types are referred to as “sexes.” For most high-level organism, this occurs between two genders. The male gender produces a mobile gamete which travels to fuse with a stationary gamete that is produced by the female gender.
Gametes are germ cells that are able to unit with germ cells from the opposite gender. Some may refer to them as “sex cells” or “reproductive cells.” For humans, the male gamete would be sperm cells and the female gamete would be egg cells.
The primary advantage of sexual reproduction is that it encourages the survival of a species. Whether discussing people, plants, or animals, mates are attracted to one another based on a hormonal perception of superiority. There is a natural desire to mate with someone from the opposite gender with heterosexual attraction so that the best possible traits can be passed along to the future offspring.
The disadvantage of sexual reproduction is that outside influences can determine the viability of the offspring. In humans, for example, a failure for a mother to consume an adequate amount of folic acid is directly linked to neural tube birth defects. This defect occurs at the earliest stages of development, often when a woman doesn’t know she is pregnant, which means the folic acid must be consumed when attempting to conceive. About 3,000 children in the US are born with neural tube defects in the United States every year.
Here are some additional advantages and disadvantages of sexual reproduction to consider as well.
List of the Advantages of Sexual Reproduction
1. It creates genetic diversity within a species.
In asexual reproduction, a direct copy, a clone, is produced. This allows for reproduction to occur without a mate, but it also increases the chances of a mutation developing within the species. Should the wrong mutation occur, the entire species could eventually be wiped out.
Sexual reproduction prevents this issue from occurring because the genetic materials from two parents, not one, are used to produce an offspring. That prevents genetic bottlenecks from occurring.
2. There is a natural level of disease resistance throughout the species.
A greater level of genetic diversity allows for higher levels of natural disease resistance within a species. That is because the bacteria, parasites, and viruses which may affect the health of a species are unable to adapt to one specific genetic profile. There will always be disease, but genome diversity allows for the immune systems of people, plants, and animals to fight off the intrusions so a healthy life can be maintained.
3. Genetic variation can lead to evolutionary advancements.
Genetic variation incorporates a process that is similar to the “survival of the fittest” principles that Charles Darwin first introduced. Darwin suggested that the animals of a species that are best suited to their environment are the most likely to survive. Through sexual reproduction, those survivors pass on their traits to their offspring, which allows the species to begin to evolve on micro-levels, and potentially on macro-levels as well.
Even if evolution does not occur, sexual reproduction does offer the chance to screen out undesirable traits or genetics from a species. Many tests are available to determine the genetic status of people, plants, and animals. By identifying high-risk individuals and restricting their access to breeding, it is possible to eliminate certain poor genetic profiles.
These restrictions are not generally imposed on humans, but can be seen in other species, such as horses.
4. It is a rewarding experience.
Sexual reproduction makes those experiencing it feel good. It stimulates the pleasure centers of the brain so that more is wanted. This happens because sexual reproduction, especially in humans, releases dopamine. It can even be addictive, much like cocaine, because of how it affects the brain. It can act like a drug in more than one way.
Sexual reproduction can act as an antidepressant. The process of sexual reproduction can help to relieve pain. Vigorous sexual reproduction can even lead to temporary amnesia, but it can also improve personal memory.
In many ways, sexual reproduction is designed to be a rewarding experience. That encourages reproduction to occur so that the cycle of life can continue.
5. It can encourage the growth of brain cells.
In asexual reproduction, a parent essentially clones itself to create an offspring. Both are individuals, but copies of one another. In sexual reproduction, a 2010 finding by Princeton scientists discovered that sexually active creatures experience brain growth compared to creatures that are not sexually active.
Sexual activity increases the production of brain cells within the hippocampus. This is the area of the brain that manages memory. Larger cells and more connections form with a greater frequency of mating.
6. It improves health.
In humans, practicing sexual reproduction is directly linked to better health. Men who have sex 1-2 times per week, for example, have a lower risk of developing heart disease compared to men who have sex 1-2 times per month or less. At the same time, sexual activity can lower blood pressure and reduce the influence of cortisol, a stress hormone, on the body.
Sexual activity can also promote a stronger immune system, stronger muscles, and may even lower certain cancer risks.
One unique benefit of sexual reproduction is that it increases levels of oxytocin, which is often called the “love hormone.” Sexual activity increases the generosity people have when they are emotionally engaged with a mate.
List of the Disadvantages of Sexual Reproduction.
1. It takes time and energy to find a mate and reproduce.
In sexual reproduction, the two genders must find each other to be able to reproduce. It takes time and energy to locate a suitable mate with the preferred traits that are desired so that the offspring produced by the union can thrive. For some species, the process of mating is an all-encompassing task that requires a sole focus on the reproductive cycle until it is completed.
2. Reproduction through sexual means is uncertain.
Sexual reproduction is not a 100% successful method of creating offspring. Some chosen mates may be infertile. Others may not have the gametes come together, despite numerous attempts at creating offspring. Although there are diversity advantages that come through this method of reproduction, it is an uncertain method.
If population numbers are low for a species, it is possible for it to become extinct despite efforts at sexual reproduction because a zygote is unable to form.
3. Favorable genetics might not be passed to the offspring.
The offspring of two parents receives a combination of their genetics. Inheritance patterns are common with sexual reproduction. Patterns include autosomal dominant and recessive, x-linked dominant and recessive, and mitochondrial.
Autosomal dominant patterns occur in every generation and each affected offspring usually has an affected parent. Autosomal recessive patterns require both parents of an offspring to be affected. Huntington’s disease is an autosomal dominant disease and sickle cell anemia is an autosomal recessive disease.
In x-lined dominant diseases, female offspring are more likely to be affected than male offspring. The opposite is true for x-linked recessive diseases.
For mitochondrial inheritance, both males and females are effected, but mothers pass the traits onto their children.
4. Fewer offspring are typically produced.
Sexual reproduction can produce numerous offspring at one time. Humans may typically have one child through reproduction, but twins, triplets, and larger multiples are possible. Horses may typically have one foal, but cats and dogs may have more than a dozen in a litter. Compared to asexual reproduction, however, there are usually fewer offspring produced over time. With asexual reproduction, whenever an offspring is required, it can be produced. The same is not always true by using sexual reproduction.
5. It can be deadly.
Achieving an orgasm is the goal of sexual reproduction, but success sometimes comes with a price. Up to 5% of the human population has a brain aneurysm and the process of sexual reproduction can cause the aneurysm to rupture. There are 8 common triggers that cause a rupture and mating is one of them. That is because the process of sexual reproduction creates an increase in blood pressure.
About half of the people who experience a ruptured aneurysm will die from the bleeding that occurs within their brain. 1 in 4 people who survive will be left with a permanent disability.
Similar traits can be found in various animal species as well.
Sexual reproduction has many advantages and disadvantages to consider from a scientific standpoint. Genetic diversity can be created, but the process is uncertain and somewhat unpredictable. When practiced regularly, it can improve brain power, help to fight off disease, and make those practicing it feel good. There may be health risks involved, but for most species, the reward of sex outweighs its potential disadvantages.
- Pathogenicity is the ability of a microbe to cause disease and inflict damage upon its host virulence is the degree of pathogenicity within a group or species of microbes.
- The pathogenicity of an organism is determined by its virulence factors.
- Virulence factors enable that bacterium to colonize the host, resist body defenses, and harm the body.
- Most of the virulence factors are the products of quorum sensing genes.
- Quorum sensing involves the production, release, and community-wide sensing of molecules called autoinducers that modulate gene expression, and ultimately bacterial behavior, in response to the density of a bacterial population.
- The outcomes of bacteria-host interaction are often related to bacterial population density.
- At a low density of bacteria, the autoinducers diffuse away from the bacteria and there are insufficient quantities of these molecules to activate the quorum sensing genes that enable the bacteria to act as a population. As a result the bacteria behave as individual, single-celled organisms.
- Acting as individual organisms may enable a low density of bacteria to gain a better foothold in their new environment by enabling bacteria to use motility and taxis to contact host cells, use pili to initially adhere to and crawl over host cell surfaces, use adhesins to adhere to host cells and resist flushing, and secrete a glycocalyx to form microcolonies.
- As the bacteria increase in numbers geometrically as a result of binary fission and reach high density, large quantities of autoinducers are produced and are able to bind to the signaling receptors on the bacterial surface in sufficient quantity so as to activate the quorum sensing genes that enable the bacteria to now behave as a multicellular population.
- By behaving as a multicellular population, individual bacteria within a group are able to benefit from the activity of the entire group.
- As the entire population of bacteria simultaneously turn on their virulence genes, the body's immune systems are much less likely to have enough time to counter those virulence factors before harm is done. Virulence factors such as exoenzymes and toxins can damage host cells enabling the bacteria in the biofilm to obtain nutrients.
- As the area becomes over-populated with bacteria, quorum sensing enables some of the bacteria to escape the biofilm and return to individual single-celled organism behavior in order to find a new sight to colonize.
- Quorum sensing enables bacteria to communicate with members of their own species, with other species of bacteria, and with their eukaryotic host cells.
- Most genes coding for virulence factors in bacteria are located in pathogenicity islands or PAIs and are usually acquired by horizontal gene transfer.
- Many bacteria involved in infection have the ability to co-opt the functions of the host cell for the bacterium&rsquos own benefit by producing secretions systems that enable the bacterium to directly inject bacterial effector molecules into the cytoplasm of the host cell in order to alter the host cell&rsquos cellular machinery, cellular function, or cellular communication.
Viruses that infect plant or animal cells may also undergo infections where they are not producing virions for long periods. An example is the animal herpes viruses, including herpes simplex viruses, which cause oral and genital herpes in humans. In a process called latency, these viruses can exist in nervous tissue for long periods of time without producing new virions, only to leave latency periodically and cause lesions in the skin where the virus replicates. Even though there are similarities between lysogeny and latency, the term lysogenic cycle is usually reserved to describe bacteriophages.