Plasmid Capture by the Bacillus thuringiensis Conjugative Plasmid pXO16

Plasmid Capture by the Bacillus thuringiensis Conjugative Plasmid pXO16
Sophie Timmery, Pauline Modrie, Olivier Minet, and Jacques Mahillon
Journal of Bacteriology, April 2009, p. 2197-2205, Vol. 191, No. 7


In this article authors describe the ability of Bacillus thuringiensis plasmid pXO16 to transfer as well as mobilize three other plasmids. It is very important issue to study plasmid transfer in Gram positive bacteria as this process is not as well known as plasmid transfer in Gram negative bacteria. Gram positive bacteria are very important as they can be as deadly as Bacillus anthracis – the causative agent of anthrax, but also can be opportunistic pathogens associated with food, like Bacillus cereus. The widely used B. thuringiensis plays very important role as the source of insecticidal toxins. Other Gram positive bacteria like Streptococcus sp. and Staphylococcus sp. are also pathogenic for human. The presence and horizontal transfer of mobile genetic elements that can play a role in spreading an antibiotic resistance as well as pathogenic determinants is very important issue.
So this paper gives us some information about pXO16 plasmid transfer and its ability to mobilize other plasmids. I would like to point out some parts of this paper. First, I was intrigued by the title of this paper. The title could suggest that pXO16 can capture other plasmids DNA molecules and incorporate into its own DNA. From the second sentence we can figured out what authors have on mind talking about capture of other plasmids by pXO16, and that it is the mobilization and retromobilization of other plasmids to the host cells by this Bacillus thuringiensis plasmid.
In the introduction section authors presented very briefly the “state of art” in the plasmid transfer and mobilization field. The special interest is put on retromobilization which is very interesting event. Retromobilization occurs when recipient DNA, either plasmid or chromosomal markers are transferred to the donor during conjugation. This reciprocal DNA transfer is very interesting and it was discussed as it is against the unidirectional DNA transfer rule. The schematic representation of retromobilization models on Fig. 1. is very easy and easy understandable. In the results section we have a very nice story about pXO16 plasmid transfer, and mobilization of “mob” plasmids pUB110, and pE194, as well as no mobilizable plasmid pC194. One thing that bothers me is the way to present conjugation efficiency as the transconjugant to recipient ratio. I know that it is the way, but it does not show overall number of donor, recipient and transconjugant cells in the conjugation mixture and I personally do not like it. In the triparental matings with the plasmid free cells as recipients only two plasmids were used, pUB110 and pC194. Generally lower frequencies of plasmid transfer were observed in triparental than in biparental matings. Authors described also the ability of pXO16 to mobilize pUB110 in three different “media” cow, soy and rice milk and showed the influence of media on mobilizable plasmid transfer. It was also shown that plasmid pXO16 can be used to capture the pUB110 plasmid from other bacteria in all 3 kinds of milk. This is quite interesting that food products can be used as media in this kind of experiments and that biological events can occur in such an environment. We should consider this and look on the food quality especially on expiration date in “milk related” products.
Very interesting results are presented in the section describing plasmid transfer kinetics where it is shown that the plasmid transfer occurs only in a short period of time reaching plateau after certain period of time. It is consistent with other results and proves that plasmid transfer is controlled by specific mechanisms in the cell and these functions are not the only plasmid related.
In discussion authors focused on the mode of retrotransfer and showed that retrotransfer in the case of B. thuringiensis pXO16, and plasmids used in the paper support the two step model. It is interesting as we always thought that we need two kinds of cells, donor and recipient for conjugation to occur. And now we have two kinds of cells but both contain the same conjugative plasmid. Mobilizable plasmid can be transfer only if two cells form mating pair. Mobilizable plasmid encode no functions related to mating pair formation so it means that two “donor” cells can form mating pair and maybe reciprocally transfer DNA molecules. That is really exciting…
To summarize I found this paper quite interesting but some weak points, like poorly written materials and methods, as well as graphical representation of results depreciate the quality of this work and should be corrected before publication.

Jarek Krol, PhD
UofI