Pathogenicity Islands, the Tip of the Iceberg
By : Raghad Alach
“Pathogenicity Islands, the Tip of the Iceberg”, is an article which just as the name implies discusses genomic islands and their implications on the evolution of the microorganisms as well as what information they could provide.
Pathogenicity islands belong to a larger group known as genomic islands, which are common mobile genetic elements sharing a set of unifying features that are foreign to the microorganism which they are found in.
The advent of gene sequencing has made such discoveries possible.
By comparing sequences of different microorganism it has been found that unrelated organisms may often contain similar genetic structures, and these structures are found in both pathogenic and nonpathogenic organisms.
These genomic islands depending on function could be pathogenic, symbiotic, metabolic, toxic, help entry into the cell, or resistant.
Some encode for several functions depending on the environment.
Regardless of the function that a genomic island lends, they all have similarities.
Genomic islands are large and are present on many different kinds of bacteria but absent from closely related strains.
They also deviate in GC content from the rest of the chromosome.
Another feature of these islands is that they are usually flanked by specific sequences as well as being located near tRNA coding sequences. Finally the islands contain genes coding for mobility and are sometimes unstable.
Genomic islands are transmitted through horizontal gene transfer, but this event is rarely observed in the lab and so the process is not well understood.
It is important to understand this process because sometimes the islands become permanently integrated into the chromosome and transfer through generations while some other times they get deleted and are highly unstable.
Discovering reasons for this could help fight diseases.
Once acquired, the new DNA needs to be integrated and expressed.
One would think that acquiring new DNA would occur and be advantageous only if it imparts extra functions previously unavailable to the microorganism; however this is not always the case.
Many times the islands contain repeated sequences and even non-coding sequences.
The final step for an island is to be regulated with the rest of the chromosome, in most cases pathogenicity islands seem to be well integrated with the rest of the genome of the microorganism.
According to Darwin, evolution is the result of the need to reach a more fit state, and this evolution is the result of changes in both the genotype and the phenotype of the organism. These changes are dependant on the environment the organism is subjected to.
According to this, horizontal gene transfer occurs when the organism needs to increase fitness to survive and if one is discussing pathogenicity then disease is the result of the microorganism needing more fitness in that particular environment rather than it being virulence.
Furthermore, by horizontal gene transfer in many cases the genome is permanently altered indicating that evolution occurs in large steps and contribute to the emergence of new species and not in point mutations.
An important note, is that genomes retain their overall size which means that an equal amount of gaining and losing DNA is occurring.
This exchange probably enables the microorganism to maintain its flexibility and allows it to adapt to changing environments.
With the progress in sequencing and the new advances being made, much new information will be gained about DNA transfer between microorganisms.