Plasmid Conjugation in an Activated Sludge Microbial Community

By Ruoting Pei and Claudia K. Gunsch

ENVIRONMENTAL ENGINEERING SCIENCE,

Vol 26, No 4, 2009, p. 825

This is jet another paper describing plasmid transfer and their role in so called bioaugmentation in natural kind of environments.

Bioaugmentation involves the addition of exogenous micro-organisms that have the ability of degrading the compound of interest. Using this process, new biodegradation pathways can be added, which inherently improve the metabolic conversion of the contaminants. The main problem of this method is that the laboratory strains used to introduce new genetic traits usually cannot grow in natural environmental conditions. This means that bioaugmentation can fail due to the poor establishment and=or survival of new strains under field environmental conditions. The second process used for enhancing natural biodegradation capabilities is biostimulation which consists of adding nutrients (e.g., carbon, nitrogen, electron acceptor, etc.) to promote the growth of indigenous microorganisms. This method is preferred over bioaugmentation because no additional bacterial strains are introduced but it requires the presence of indigenous microorganisms, which are capable of breaking down the contaminant of interest. To overcome the limitations associated with biostimulation and bioaugmentation, it might be possible to combine these two strategies into a technique called genetic bioaugmentation. This technique would consist of adding bacteria with the needed genes and inducing their horizontal gene transfer (HGT) to indigenous bacterial species utilizing natural prokaryotic adaptation mechanisms.

In this paper authors described pWWO plasmid transfer from Pseudomonas putida strain to bacterial population present in activated sludge. The pWWO plasmid is well known for its toluene degradation capability. The plasmid DNA was tagged with transposone containing gfp gene driven by lac promoter. In the donor strain which overproduce LacI repressor gfp is not expressed. In other bacterial hosts gfp is expressed and gives bright green fluorescence which can be measured. Using flow cytometry authors showed that plasmid was actually transferred into bacterial community of activated sludge with number of bacteria expressing GFP protein up to 6%. They showed that the highest number of transconjugants is reached at the third day of experiment and it depends on the donor to recipient ratio with the best results at the 1:20 ratio. Authors tried to connect the plasmid transfer with the actual capability of activated sludge to toluene degradation. This experiment did not really work as the level of toluene degradation remained unchanged during the time of the experiment.

The paper stays in a main stream of microbial ecology and engineering facing the problem of global pollutions and waists utilization. It is very important to use new methods for detecting plasmid transfer in natural environment. Authors were facing the same problem as in previously published papers that there are some limitations in using fluorescent proteins as a plasmid infection marker. The true plasmid bearing cells are not known until gfp gene is expressed and protein is properly matured to give strong fluorescence, which can be detected by flow cytometry. Authors cannot also link the presence of TOL phenotype (toluene degradation) with the number of plasmid bearing cells as the sludge microbial community shows already high toluene degradation activity.

Perhaps, it is not a very “big” paper, but it is a small step toward understanding how the plasmid transfer occurs in natural environment and how can we use this to save our polluted world.

Jaroslaw Krol, PhD

UofI

The SOS Response Controls Integron Recombination

Émilie Guerin, Guillaume Cambray, Neus Sanchez-Alberola, Susana Campoy, Ivan Erill, Sandra Da Re, Bruno Gonzalez-Zorn, Jordi Barbé, Marie-Cécile Ploy, and Didier Mazel

Science, May 2009 p 1034, Vol. 324, No. 5930.

An ever-growing concern in the medical world is the rise of antibiotic-resistant pathogens. One source of comfort that we have had in this fight against the spread of resistance is that the expression of resistance genes confers a cost to bacteria. In the absence selection for antibiotics it is thought that this cost will give a selective advantage to non-resistant bacteria, and so bacterial populations will loose resistance over time in the absence of antibiotics. The authors of this paper suggest that this is unfortunately not always the case.

In the words of Jurassic Park, “Nature will find a way.” For bacteria, one of the weapons in their arsenal against antibiotics is the SOS response. Under normal conditions the protein LexA binds to the operator region controlling SOS genes to repress their expression. In the event of DNA damage LexA is cleaved from this region and a series of DNA repair mechanisms are activated, some of which are low- fidelity and therefore lead to an increase of mutations, some of which may be beneficial in resisting further DNA damage.

In terms of antibiotic resistance, this paper explains that LexA repression and SOS expression also influences recombination of genes cassettes (which often code for antibiotic resistance. During SOS expression cassette recombination is induced. This can result in either silencing or reactivation of cassettes. In other words, a cassette carrying resistance genes can be reactivated through recombination under times of stress and then revert to a “dormant” form that provides no cost to the bacterium once the SOS response is again repressed.


Additional Reading:
Erill, S. Campoy, J. Barbe, FEMS Microbiol. Rev. 31, 637 (2007).
C. M. Collis, R. M. Hall, Antimicrob. Agents Chemother. 39, 155 (1995).
A. Aertsen, C. W. Michiels, Trends Microbiol. 14, 421 (2006).
D. I. Andersson, Curr. Opin. Microbiol. 9, 461 (2006).


Julie Hughes,
Graduate Student, University of Idaho