Japonica
03-07-10, 06:07 PM
I didn't know whether to post this here or not, but since the bacteria are changing as a result of vaccination programs, I suppose this is as good a spot as any.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2834637/?tool=pubmed
Changes in the genomic content of circulating Bordetella pertussis strains isolated from the Netherlands, Sweden, Japan and Australia: adaptive evolution or drift?
Audrey J King,1 Tamara van Gorkom,1 Han GJ van der Heide,1 Abdolreza Advani,2 and Saskia van der Lee1
MC Genomics. 2010; 11: 64.
Published online 2010 January 26. doi: 10.1186/1471-2164-11-64.
PMCID: PMC2834637
Copyright ©2010 King et al; licensee BioMed Central Ltd.
Three main forces have been found to shape genome evolution; gene gain, gene loss and gene change [52]. Gene loss seems to be an important event in the genomic evolution of the B. pertussis species. B. pertussis evolved from a B. bronchiseptica-likeancestor probably by large scale gene loss [47]. Much of this loss is most likely due to ISE-mediated deletion events and/or ISE mediated rearrangements, which reshaped the genome presumably to benefit from increased virulence expression...It seems that gene loss began during the evolution from B. bronchiseptica, and is continuing during the evolution of B. pertussis. This conclusion is supported by the results presented in this study that illustrate a progressive decrease in the genome size over time in strains isolated in different countries. Moreover, the genes that were lost exhibited similar functions as the genes lost in the evolution from B. bronchiseptica to B. pertussis (see above). Gene loss in B. pertussis has been previously reported for Finnish [44] and French B. pertussis strains [48]. However, to our knowledge, such a rapid decline in gene loss during a period of 60 years has not yet been described. The loss of genetic material is a dynamic, ongoing process that is not specific for one country, but observed in several areas of the world. In other examples of bacteria gene loss, the process has been described as a progressive purging of unnecessary genes from the genome [53].
Bacteria appear to prefer gene deletions, which could account for a general drive to lose DNA. It is generally assumed, that the bacterial deletion which offers the least negative fitness effect on the host will be selected. However, gene loss can lead to benefits for the pathogen as well, because some gene products are detrimental to pathogenic lifestyle. For example, loss of the cadA gene from the Shigella bacterium has been correlated with an increase of Shigella pathogenicity [54]. Additionally, the preferential loss of bacterial cell surface determinants has been shown to result in an increase in virulence by reducing the number of targets that could be recognized by the human immune system [55].
During the last 15 years, the expansion of B. pertussis strains in multiple countries that carry the ptxP3 allele [18,45], may also have been influenced by intensive pertussis vaccination for half a century. Since pertussis has shifted to older age groups [11,56] in immunized populations during the last decade, it has been suggested that B. pertussis has adapted to the host population with waning immunity, in order to maintain the bacterial reservoir among older hosts [18,57]. Mooi et al [18] proposed that strains surviving better in these hosts are being selected for by vaccination of young infants. Recently, it was shown that the rise of strains carrying the ptxP3 allele, which are also characterized by a particular gene content [45] (GC-types 2, and its derivatives) and seem to have a benefit in older hosts, has contributed to the resurgence of pertussis in the Netherlands [18]. Thus, the size and gene content of the B. pertussis genome appear to be influenced by vaccination by herd immunity. Immune pressure may select for certain strains with a particular advantage, and which may be linked to specific gene content. The fact that we see the same or similar strains in different countries during certain time periods suggests that an important advantage for these strains may be their capability to spread throughout the immunized population.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2834637/?tool=pubmed
Changes in the genomic content of circulating Bordetella pertussis strains isolated from the Netherlands, Sweden, Japan and Australia: adaptive evolution or drift?
Audrey J King,1 Tamara van Gorkom,1 Han GJ van der Heide,1 Abdolreza Advani,2 and Saskia van der Lee1
MC Genomics. 2010; 11: 64.
Published online 2010 January 26. doi: 10.1186/1471-2164-11-64.
PMCID: PMC2834637
Copyright ©2010 King et al; licensee BioMed Central Ltd.
Three main forces have been found to shape genome evolution; gene gain, gene loss and gene change [52]. Gene loss seems to be an important event in the genomic evolution of the B. pertussis species. B. pertussis evolved from a B. bronchiseptica-likeancestor probably by large scale gene loss [47]. Much of this loss is most likely due to ISE-mediated deletion events and/or ISE mediated rearrangements, which reshaped the genome presumably to benefit from increased virulence expression...It seems that gene loss began during the evolution from B. bronchiseptica, and is continuing during the evolution of B. pertussis. This conclusion is supported by the results presented in this study that illustrate a progressive decrease in the genome size over time in strains isolated in different countries. Moreover, the genes that were lost exhibited similar functions as the genes lost in the evolution from B. bronchiseptica to B. pertussis (see above). Gene loss in B. pertussis has been previously reported for Finnish [44] and French B. pertussis strains [48]. However, to our knowledge, such a rapid decline in gene loss during a period of 60 years has not yet been described. The loss of genetic material is a dynamic, ongoing process that is not specific for one country, but observed in several areas of the world. In other examples of bacteria gene loss, the process has been described as a progressive purging of unnecessary genes from the genome [53].
Bacteria appear to prefer gene deletions, which could account for a general drive to lose DNA. It is generally assumed, that the bacterial deletion which offers the least negative fitness effect on the host will be selected. However, gene loss can lead to benefits for the pathogen as well, because some gene products are detrimental to pathogenic lifestyle. For example, loss of the cadA gene from the Shigella bacterium has been correlated with an increase of Shigella pathogenicity [54]. Additionally, the preferential loss of bacterial cell surface determinants has been shown to result in an increase in virulence by reducing the number of targets that could be recognized by the human immune system [55].
During the last 15 years, the expansion of B. pertussis strains in multiple countries that carry the ptxP3 allele [18,45], may also have been influenced by intensive pertussis vaccination for half a century. Since pertussis has shifted to older age groups [11,56] in immunized populations during the last decade, it has been suggested that B. pertussis has adapted to the host population with waning immunity, in order to maintain the bacterial reservoir among older hosts [18,57]. Mooi et al [18] proposed that strains surviving better in these hosts are being selected for by vaccination of young infants. Recently, it was shown that the rise of strains carrying the ptxP3 allele, which are also characterized by a particular gene content [45] (GC-types 2, and its derivatives) and seem to have a benefit in older hosts, has contributed to the resurgence of pertussis in the Netherlands [18]. Thus, the size and gene content of the B. pertussis genome appear to be influenced by vaccination by herd immunity. Immune pressure may select for certain strains with a particular advantage, and which may be linked to specific gene content. The fact that we see the same or similar strains in different countries during certain time periods suggests that an important advantage for these strains may be their capability to spread throughout the immunized population.