Changing seasons changing communities: A preliminary Study

A review of: Pereira, C., Salvador, S., Arrojado, C., Silva, Y., Santos, A., Cunha, A., Gomes, N., Almeida, A.. (2011). Evaluating seasonal dynamics of bacterial communities in marine fish aquaculture: a preliminary study before applying phage therapy. Journal of Environmental Monitoring. 13 (4), 1053-1058.

The use of phage therapy presents a very promising alternative to the use of antibiotics and harmful chemicals to control pathogenic bacteria in aquaculture. Before phage therapy can be effectively used a comprehensive understanding of bacterial community structure and bacterial population density is necessary. This study aims to develop the understanding of how seasonal dynamics affects overall bacterial community structure, microbiological water quality and population dynamics of disease-causing bacteria. This information will allow them to identify the optimum time period that phage therapy should be applied. The study was carried out in Marine aquaculture system off of the Ria de Aveiro in Portugal. The aquaculture is located near the densely populated Aveiro and as a result the water is subjected to contamination by introduced human wastes. Currently as a countermeasure the water is selectively treated with chemicals, depending on the bacterial community structure, to maintain good water quality.

Samples were collected at different seasons in the year over 2 years. They measured temperature, salinity, dissolved oxygen on site and then within 1-2 hours they processed the samples in the lab. This provided detailed information on the seasonal changes of the water properties. They quantified the relative abundance of specific groups of bacteria using FISH analysis with 16S rRNA target probes. They evaluated the seasonal dynamics of bacterial community structure by comparing DGGE profiles of 16S rDNA fragments. They analysed bacterial indicators such as faecal coliforms and faecal enterococci by filtering and culturing samples. The results were expressed as colony forming units.

DGGE analysis showed that there was highest diversity of bacterial ribotypes occurred during the spring season. The quantification of bacterial indicators showed that in the coldest months (October, November and December) there was a decrease in the number of faecal coliforms, However the highest value of enterococci was obtained in October. The most dominant pathogenic bacteria found in the culture varied between seasons and years. Overall total bacterial numbers were fairly constant over the year but the relative abundance of specific bacterial groups varied significantly during the sampling period. There was a significant amount of variation community structure in the Vibrio genus. As Vibrio bacteria are known to be play an integral role in fish disease outbreaks it will be vital to consider these changes when phage therapy is applied. The effects of chemical disinfection also need to be taken into account as it has a serious effect of community structure. From this preliminary study they conclude that the most effective time to apply phage therapy in this system would be in the spring season.

It is interesting that the quantity of the faecal indictor enterococci was found to be highest in a time period that coliforms were lowest. I assume this is the result enterococci outcompeting coliforms because they are more adapted to thrive in the October conditions. The study has been effectively executed providing reliable information by taking triplicate samples and comparisons against controls. The high number of variables that would have had an impact on bacterial communities has been closely monitored and controlled in this study. This is a preliminary study which will hopefully lead onto the further research. I have had a look for this next paper but was unsuccessful finding it. I will keep looking and hopefully I will be able to review it and make it my next blog post.