Pleomorphism is defined in bacteria as the property of having a number of morphological forms. The authors explain how pleomorphism exists in the three homogenous groups by stating that there is very little genetic differences between the clades. Therefore there are similar genes in the three groups but the genes still allow for different ways of life. This means that the genes of B. cereus can be regulated by their environment, and that it is mostly their environment that affects their differences. The authors give the example of the PlcR gene which is a transcriptional regulator and allows the organism to sense and respond to the environment.
2. State each of Darwin’s four postulates. Fully explain how the various populations (phage-infected vs. uninfected cells) of Bacillus anthracis in the “competitive microbial environment in the worm gut” (page 17) meet each of the four postulates for evolution by natural selection.
Darwin's four postulates are: 1. There is variation among individuals of the same species, 2. At least some of these variations are hereditary, 3. In every generation there are more offspring produced than can survive, and 4. Natural selection operates on populations (survival and reproduction are not random).
With regard to Darwin's first postulate, among the Bacillus anthracis there are specimens who, after being exposed to a group of bacteriophages, have emerged as lysogens. A lysogen is "a bacterial cell strain that has been infected with a temperate virus, one that does not cause destruction of the cell" (Dictionary.com). In this study specifically the exposure to a bacteriophage has given the anthrax unique "survival capabilities" including the ability to "block or promote sporulation"; "induce exopolysaccharide expression and biofilm formation"; and finally, "enable the long-term colonization of both an artificial soil environment and the intestinal tract of the invertebrate redworm, Eisenia fetida." Not all of the anthrax cells obtain every one of these characteristics when they are infected but even a single one could improve their survival.
Darwin's second postulate is that some of this variation must be able to be passed on to the next generation. Once certain specimens have been infected by the phage, the bacterium can then takes over the "machinery" of the cell and can incorporate some of it's own DNA into the cell's genome. In the case of the Bacillus athracis, the bacterium "encoded bacterial sigma factors and the expression of at least one host-encoded protein predicted to be involved in the colonization of invertebrate intestines" into the anthrax cells allowing them to pass this on to the next generation. Also, the "shedding of phages by B. anthracis" scientists in this study theorized "could reflect a mechanism for DNA transfer (and thus niche expansion) among these organisms."
When evolution is occurring, there will always be more offspring produced in each generation than can survive based on the resources or environment. This is Darwin's third postulate. In this study one of the most prominent abilities that the bacterium enabled the anthrax to do was to thrive in certain soil environments. The scientists conducted a test in which they placed Bacillus anthracis specimens into a soil environment "without a phage source as a control, [and there was] little or no survival was apparent at 24 weeks." When they placed anthrax cells that had been "co-cultured' with the bacterium, however, "there was a pronounced improvement in survival." As a result, they concluded that "B. anthracis phages that are either shed or added as free particles can lysogenize susceptible strains and confer the long-term survival phenotype in the soil milieu." The scientists also repeated this test in other environments and found that "regardless of the environment, all of the lysogenic derivatives tested survived up to six months while the parental ΔSterne strain declined steadily from the outset."
And lastly, Darwin's fourth postulate is a little more difficult to apply to this scenario because this paper discusses a very controlled, detailed experiment. So natural selection is operating on the population of B. anthracis but survival is certainly not random as the scientists are choosing which specimens are infected with phages and which are not. However, the overall conclusion that they came to is that "B. anthracis phages that are either shed or added as free particles can lysogenize susceptible strains and confer the long-term survival phenotype in the soil milieu." So while survival was not random in this experiment, if the process were to occur naturally in a real-world environment it would agree with Darwin's last postulate.
3. The bacteriophages described in this paper are able to confer “fitness genes” (page 2) to their Bacillus hosts. What do these genes allow B. anthracis to do ecologically? Compare Figures 1 and 10 to help address this question.
Figure 1:
Figure 10:
Fitness genes, or lysogen conversion factors, are genes that are not essential for the phage lifecycle and are usually discussed with respect to virulence proteins and the evolution of pathogenic phenotypes. However, they have recently been considered more with respect to "promoting environmental functions for bacteria and including metabolic enzymes, and transcriptional repressors for metabolic downshifts in nutrient-poor environments." So basically, B. anthracis potentially goes through lysogeny as seen in Figure 10, which is the biological process where a bacterium is infected by a bacteriophage and the bacteriophage integrates its own DNA into the host sto that the host and the bacteriophage are not destroyed (dictionary.com). The effects of this lysogeny with respect to ecological terms is that B. anthracis can be resistant to the soil antibiotic fosfomycin. So, through this lysogeny, the B. anthracis become resistant to the fosfomycin antiobiotic and this would likely transmit to an animal that the anthrax was transferred to if it infects a host.
4. Think about the hosts for Bacillus species. Their anatomies serve as selection pressures on Bacillus populations. Apply Darwin’s four postulates to the coevolution of Bacillus anthracis and its hosts.
Darwin's four postulates are: 1. There is variation among individuals of the same species, 2. At least some of these variations are hereditary, 3. In every generation there are more offspring produced than can survive, and 4. Natural selection operates on populations (survival and reproduction are not random).
Among the hosts that are affected by Bacilus anthracis there are
varying traits that affect the hosts probability of infection. One example of a
trait that could vary and affect the Bacillus anthracis bacteria’s
ability to survive is the intestine (specifically from the paper the red worm
intestine). Bacillus antracis for part of its lifecycle resides in the
intestine of its host. The
intestine could vary in surface area (number of folds) and this could affect
how Bacillus anthracis is able to survive in the host. Bacillus
anthracis for part of its lifecycle resides in the intestine of its host.
There are no doubt variations in the intestines of species that are caused by
genetic differences. These differences in the anatomy of the intestine will
likely affect the bacteria’s ability to survive. These traits are encoded for
by genes that are passed down from generation to generation. The hosts
reproduce and produce more species than are capable of surviving. The hosts
with the variations in their intestine that make it less susceptible to a Bacillus
anthracis infection are more likely to survive and reproduce. The hosts
that are more fit and do survive will mate with other fit hosts and will pass
on their genes to their offspring. Meanwhile at the same time natural
selection is operating on the Bacillus
anthracis and the Bacillus anthracis that
are able to reside in the intestine will survive and pass on their genes that
allowed them to better adapt to the intestine.
Bonus: Biofilms are kind of a hot topic in multiple biological disciplines right now. Define and briefly discuss biofilms.
Biofilms are an
assortment of microorganisms that grow on a surface. Biofilms have complex
interactions within themselves between the varied organisms that they are made
of. Biofilms begin with microorganisms adhering to a substrate. More and
varying microorganisms that come along the substrate are then able to attach
themselves to the substrate or, through some physical or chemical interaction,
attach to the other microorganisms. The adhesion of one microorganism to
another allows for a diverse amount of microorganisms to inhabit the substrate
instead of only having microorganisms that are able to bind to the substrate
directly. Secretions from these microorganisms can facilitate the transfer of
nutrients to the microorganisms, protect the organisms or provide another
avenue of attachment for microorganisms to the biofilm. The secretions, along
with the layers of microorganisms help protect the biofilm as a whole. The
secretions can protect the microorganisms from antibiotics and the outer layer
of microorganisms help protect the inner layers from microorganisms as well.
These forms of protection make biofilms resistant to conventional methods of
disinfection. A common biofilm is the microorganisms in your mouth and teeth.
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