Explain how a bacterial pathogen might evolve to become able to exploit a new host.

In order for a pathogenic bacterium to exploit a new host it needs to adapt to the new environment. It needs to acquire the ability to:
·         Invade the host
·         Attatch to and colonise the host tissue
·         Survive within the host avoiding both the hosts innate and adaptive immune system
·         Acquire nutritients within the host
·         Disseminate from the host and allow transmission.
Often it is these attributes of the bacteria that cause its virulence (although evolution would favour the bacteria that can complete the above tasks without causing virulence as this would allow for greater transmission)
The central dogma of biology states that the characteristics of an organism are determined by the DNA in the following way:
DNA->RNA-> Protein
And subsequently a change in the DNA is required for the acquisition of the above characteristics. This essay will discuss the mechanisms by which this change in DNA can occur as this underlies the ability for bacterial pathogens to exploit new hosts.
Mechanisms of by which the DNA can change
The mechanisms by which the DNA can change are by:
·         Spontaneous mutation (due to incorrect replication during vertical transmission or due to mutagens)
·         Mutations induced by insertion sequences
·         Horizontal gene transfer:
o   Transformation
o   Transduction (generalised or specialised)
o   Conjugation
An idea that is central to the theory of evolution by natural selection is that these above events can occur randomly and also result in the acquisition of unhelpful mutations, however these will be selected against very quickly. Furthermore ‘neutral mutations’ will not be selected for and hence will be only present in a small number of the population. A helpful mutation will be selected for and be increased by the population.
Spontaneous mutation
Any bacterial mutation will be expressed (as bacteria are haploid). The rapid nature of replication means that as well as this being common, any bacteria with an unhelpful mutation will be selected against while those with helpful mutations may be selected for.
Insertion sequences
Both insertion sequences and transposons are transposable elements that can move around the genome. Insertion sequences however only consist of the DNA required for this movement while transposons contain other genes and allow the movement of genes. Hence one of the roles of the IS is to disrupt DNA (in phase variation) and also cause gene loss. Also there are examples of pathogens which use IS to encourage gene loss. An example is salmonella, Y. Pestis and Bordetella. The advantage of this means that any DNA that is not useful is removed as it consumes resources!
Transformation
This is gene transfer resulting from the uptake of naked fragment of DNA. It is seen in H. Influenzae, Strep pneumonia, N. Pharyngitidis. Normally this only occurs withing a species. In order for transformation to occur:
·         Bacteria must be ‘competent’
·         Species specific uptake
This allows bacteria to take up DNA from those around them however it only allows them to acquire the characteristics of bacteria that have died. And hence acquire characteristics enabling survival in a new host. These techniques were important in avery’s experiments.
Transduction
This is where DNA is carried by bacteriophages into bacterial cells. Bacteriophages are viruses that can infect the bacteria. They have 2 cycles:
·         Lytic cycle – multiply within the host cell and cause lysis
These can cause generalised transduction where there is infection. Lysis of host DNA and packaging of DNA, mostly the viral DNA but some host dna is taken as well.
·         Lysogenic cycle – enter a quiescent state where genome is integrated into the host. This is done by using site specific recombination. However, the host will repress the genome.
·         In adverse conditions this repression is released (e.g in diphtheria at low iron conc)
The DNA is then excised and sometimes neighbouring DNA can be taken as well. This is specialised transduction.
Conjugation
Gene transfer from a donor to a recipient by direct physical contact between cells. How does this work
Donor bacteria contains a plasmid. This is a piece of circular DNA that contains a variety of genes and also genes involved in their transfer. The plasmid encodes an F-pilus and a conjugate bridge is formed.
This can occur between species and shigella species in japan become resistant to antibiotic due to conjugation from E.coli
Movement within the organism
So far we have only considered the movement between organisms however the bacteria only expresses the genes that are in its chromosome. To allow movement, transposable elements mentioned earlier are required. They mediate site specific recombination to allow the movement of genes from:
·         1 plasmid to another
·         Plasmid to chromosome

Integrons – in addition there are gene capture mechanisms by which genes can be taken up into the genome. This allows the formation of pathogenicity islands.
Originally written by willamena Withering

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