Conjugative plasmids: vessels of the communal gene pool.

Anders Norman, Lars H. Hansen and Soren Sorensen
Phil. Trans. R. Soc. B 2009 364: 2275-2289

Browsing the Internet for a recent review article talking generally about plasmids, I found an article written by the group of Prof. Sorensen from the University of Copenhagen in Denmark. The first surprise was the journal, which full title is Philosophical Transactions of The Royal Society B: Biological Sciences. The “Royal Society” sounds great and the journal is really good with the IF=5.9 (2008), but may be not very popular especially among molecular biologists now days. Philosophical Transactions B is divided into four cluster areas: Cell and Development, Health and Disease, Environment and Evolution, Neuroscience and Cognition. The leading theme of the August 2009 issue, where this review was published, was 'The network of life: genome beginnings and evolution'. Second, the title “…plasmids as vessels of the communal gene pool”: what does this really mean? The explanation can be found in the abstract. The authors point out that evolution of microorganisms is tightly linked to the environment in which they live and the communal (total) pool of genes within that environment. As conjugative plasmids play a major role in horizontal gene transfer (HGT) within and between different bacterial populations, the accessory genes carried by these plasmids belong to the pool of communal genes.
The review consists of seven main chapters. After a short introduction to the current bacterial evolution research based on single genomes or metagenomic DNA sequences, the authors propose some new terms (chapter 2) such as:
“supergenome” – the total pool of genes readily available to a prokaryotic organism within a particular setting;
“private pool” - which consists of the fixed and ‘idiosyncratic’ genes encoded on the chromosome of the prokaryote;
“communal pool” - which consists of genes encoded on mobile genetic elements (MGEs) and that are thus available to all permissive prokaryotes,
and discuss the relations of these new concepts to older terms like: core genome, which define the genes present in all strains of a prokaryotic species; dispensable genome (or flexible genome), which are genes present in some, but not all, strains of the same species; and pan genome—the sum of the former two.
The difference between these new and old terms is that the latter are related to a single species, while the new ones are more related to the population of microorganisms. Actually, I like the idea because it really reflects the natural state. As new DNA sequencing technologies enable us to analyze whole populations generating gigabytes/bases of information, it is better to treat this as a “supergenome-gene pool” than pan genome, especially since assembling single genomes out of this population is not an easy task and can generate many errors. On the other hand, the authors confine the communal pool to genes present on mobile genetics elements, which in my opinion is not really good as we know that the structure of these elements can be very unstable with almost continuous exchange between different genome parts (chapter 3).
I think that in ‘population genetics’/metagenomic kinds of studies based on current technology in DNA sequencing and analysis one really could concentrate on two things:
1) identification of species within the population based on 16SrDNA sequence;
2) supergenome analysis – presentation of all genes available within a population with their relative abundance – which will reflect the physiology of the analyzed population.

But back to the article; in chapter 4 and 5 –‘The tools of genetic mobility’ and ‘Mechanisms of, and barriers to, horizontal gene transfer’, the authors briefly describe different mobile elements and mechanisms that drive HGT. This leads us to the main part describing ‘The world of conjugative plasmids’, where in a few subchapters the authors describe origin of plasmids and its organization (with a description of plasmid modular structure) and discuss the role of conjugative plasmids in the cell. Finally, in the last part they talk about some methods used to study the communal gene pool and how these studies reflect on our understanding of bacterial evolution.
I found this article very well written and really interesting. It is a very current review article talking about horizontal gene transfer and conjugative plasmids with up-to-date references. Simple and relatively broad presentation of HGT and all the processes that lead to genetic exchange within microbial populations as well as a simple description of conjugative plasmids and their role in HGT make this review an ideal article as an introduction for students and researchers new to this field.

Jarek Krol PhD
UofI

Survival of the Fittest

Vriezen JAC, Valliere M, Riley MA. 2009. The evolution of reduced microbial killing. Genome. Biol. Evol. 2009:400-8.

One interesting question in the plasmid world is how to classify plasmids. They are commonly compared to parasites in that they require the use of host machinery for replication and protein production, and can “infect” bacterial hosts even if this reduces host fitness (i.e. the bacteria often have no say in whether a plasmid is admitted into its cell or not). Unlike parasites, however, plasmids can be beneficial to their hosts, depending on what genes they code for and the current environment. For instance, some plasmids code for colicin production, which can kill bacteria that are closely related to the host bacteria(1). This gives plasmid-bearing bacteria an edge on competing strains, but at a slight cost due to plasmid maintenance and colicin production. In this article the authors found that after 253 generations of growth in the absence of competing strains, the killing ability of E. coli was reduced in an attempt to reduce the cost of plasmid maintenance in an environment wherein colicin production is unnecessary.

So far, these results aren’t terribly surprising: if colicin production comes with a cost the bacterial host then any bacterium that can reduce this cost will be at a selective advantage. This will allow bacteria with reduced killing affects to become more dominate in the population over time(c.f. 2,3). What is more interesting is that, even though it is the plasmid that codes for colicin production it is the bacteria’s genes that change to reduce colicin production. After 253 generations the plasmid’s sequence remained completely unaltered, whereas the expression of host genes including those for DNA repair, Mg ion uptake, and late prophage genes displayed changes in their regulation. The authors commented that this was also a wider variety of genes that were involved in this evolutionary response to colicin production pressures than expected.

The interactions between plasmids and their host bacteria appear very complex. The fate of each is closely related to the fate of the other, and there are a variety of ways that the plasmid, the host, or both could change to increase the chances of survival of both together. Changes on one partner, in this case the bacteria, can regulate the expression of the other without altering the other at all. The interactions and coevolution of plasmids and bacteria are dynamic, with countless possibilities that have yet to be explored. What seems ever more clear with each such study is that when faced with a problem, in the words quoted in Jurassic Park, “Nature will find a way.”

Julie Hughes
University of Idaho

References\Further Reading:

1. Cascales E, et al. 2007. Colicin biology. Microbiol Mol Biol Rev. 71:158–229.

2. Lenski RE, Winkworth CL, Riley MA. 2003. Rates of DNA sequence evolution in experimental populations of Escherichia coli during 20,000 generations. J Mol Evol. 56:498–508.

3. Modi RI, Adams J. 1991. Coevolution of bacterial-plasmid populations. Evolution. 45:656–667.

4. Walker D, et al. 2004. Transcriptional profiling of colicin-induced cell death of Escherichia coli MG1655 identifies potential mechanisms by which bacteriocins promote bacterial diversity. J Bacteriol. 186:866–869.

An efficient stress-free strategy to displace stable bacterial plasmids

Lisa Hale, Orestis Lazos, Anthony S. Haines, and Christopher M. Thomas

Plasmid curing is the process of displacing a plasmid from a plasmid-bearing strain. This is useful for studying phenotypic effects of plasmids on their bacterial hosts. Curing is easy to do when plasmids are unstable and easily lost, but more challenging when dealing with stable plasmids, which can be maintained for a long time in the host even in the absence of selection. Traditionally, plasmid curing is achieved by growing the bacterial host under stressors such as high temperature, detergents or mutagens [1]. This process has the disadvantage of inducing mutations in the bacterial host, which is undesirable. The authors propose a method of plasmid curing based on plasmid incompatibility that can avoid chromosomal mutations. Plasmid incompatibility arises when two plasmids having related replication functions find themselves in the same bacterial cell. This results in the displacement of one by the other that has a second, unrelated replicon [2]. However some plasmids have post-seggregational-killing (psk) genes [3], which produce toxins that kill plasmid-free bacteria in the absence of the anti-toxin, which is another hindrance in plasmid curing. To overcome this, the authors included an anti-toxin gene in their displacing plasmid, which can counter the toxin produced in plasmid-free cells and prevent host killing during plasmid curing. They constructed three vectors having similar replication and stability functions to the plasmids that were being displaced. Plasmid pCURE1, which was constructed to cure plasmid pO157 from E. coli O157:H7, had a pO157-related replicon repF1B and an unrelated replicon from pMB1. To prevent psk, anti-toxin genes related to corresponding genes of pO157 were cloned into pCURE1. Plasmid pCURE1 was introduced into the pO157-bearing strain and successfully cured E. coli O157:H7 of plasmid pO157. Thus, while repF1B replicon disrupted replication of pO157, the anti-toxin produced by pCURE1 prevented host lysis. This produced E. coli O157:H7 bearing only pCURE1, which being unstable was lost rapidly in the absence of selection. Similarly two more vectors (pCURE2 and pCURE11) were constructed for displacing an F plasmid and an IncP-1 plasmid. This approach was thus successful in displacing two different kinds of F plasmids as well as those of the IncP-1 family. The authors also ruled out the possibility of chromosomal integration of pO157 through PCR using pO157-specific primers.
In summary, although plasmid curing through incompatibility has been used before [4], this paper presents a new method, which overcomes psk during plasmid curing. The only drawback is that the authors do not mention how frequent psk is and if the inefficiency of the previously used method was shown to be linked to a plasmid-encoded psk system. To show a direct link between plasmid curing and psk, they could have compared the displacing capacity of a vector with a psk system and another without a psk system. Also, while the authors suggest their method to be more efficient than the previously used method, they do not quantitatively compare the two.

References:
1. Stanisich, V.A. 1984. Identification and analysis of plasmids at the genetic level, pp. 5-32. In P.M. Bennett and J. Grinsted (Eds.), Plasmid Technology. Academic Press, London.
2. Novick, R.P. 1987. Plasmid Incompatibility. Microbiol. Rev. 51:381-395.
3. Gerdes, K., S. Ayora, I. Canosa, P. Ceglowski, R. Diaz-Orejas, T. Franch,
A.P. Gultyaev, R. Bugge Jensen, et al. 2000. Plasmid maintenance systems, pp. 49-85. In
C.M. Thomas (Ed.), The Horizontal Gene Pool: Bacterial Plasmids and Gene Spread.
Harwoord Academic Press, Amsterdam.
4. Tatsuno, I., M. Horie, H. Abe, T. Miki, K. Makino, H. Shinagawa, H. Taguchi,
S. Kamiya, et al. 2001. toxB gene on pO157 of enterohemorrhagic Escherichia coli O157: H7 is required for full epithelial cell adherence phenotype. Infect. Immun. 69:6660-6669

Diya Sen
Graduate Student
University of Idaho