Friday, 21 February 2014

A Review: Bacteriophages infecting Bacteroides as a marker for microbial source tracking.

Identifying the sources of faecal pollution is necessary in order to control the spread of human pathogens in the environment, and especially ground and surface water resources are affected by it. Human faecal pollution is a major factor contributing to disease outbreaks and tracking the sources of pollution can also help to estimate potential health risks. Microbial source tracking (MST) is a developing tool for this and this paper focused on the tracking bacteriophages infecting strains of Bacteroides spp. which are very common in human faeces and therefore waters contaminated with human waste are expected to have high abundance of bacteriophages.

As discussed in the last lecture, faecal pollution indicators have to fulfil certain criteria so that they can be used as most accurate markers. For instance, they have to be countable and associated with the source of contamination (e.g. human gut). However, none of the currently used indicators are perfect and hence several markers are used to track the sources of pollution and give good estimates of the abundance of contaminants. Phages that infect gut bacteria in warm blooded animal are not expected to be able to replicate outside the gut environments and therefore the abundance detected can be considered as somewhat accurate indicators. In addition, studies have shown that Bacteroides spp. infecting phages were steady in abundance throughout the year.

Most phages that infect Bacteroides species have a narrow range of strains they infect which can allow tracking specific sources of contamination when infecting strains are host-specific as well (e.g. strains that are only present in humans). For example, phages B40-8 and ΦB124.14 infecting strains B. fragilis HSP40 and Bacteroides sp. GB-124 (respectively) have been found to be human gut specific phages which was revealed by metagenomic analyses. Until now, it is not fully understood why some phages infecting Bacteroides are only found in certain host animals. It was hypothesised that host animal associated bacteria and their phages may have co-evolved more separately than other bacterial groups.

Studies have investigated bacteriophages infecting B. fragilis and were able to detect and enumerate them with commonly utilised plaque assays and enrichment methods after the bacteria were cultivated under anaerobic conditions. Identifying specific bacteriophages associated to hosts and therefore determining their source can also be done with molecular techniques sequencing their DNA which is more useful to examine non-culturable bacteria and their phages. 

In summary, bacteriophages infecting Bacteroides spp. have shown to make reasonably good indicators of faecal pollution in the environment, however there need to be more research in this field to give more representative data. Host-specificity of these phages was found in the example mentioned above and in other studies described in the paper, which is very useful for determining the sources of contamination. This paper summarises very nicely what kind of studies have been conducted with these bacteriophages and why and how they are used. Unfortunately, it is also a fairly long review paper so I couldn’t highlight all aspects of it, but I would highly recommend reading it to get a better understanding of this current field of research. 

Jofre, J. et al. (2014) Bacteriophages infecting Bacteroides as a marker for microbial source tracking. Water Research, doi: 10.1016/j.watres.2014.02.006.

The article came out recently and hasn't been published in a journal yet but is accessible here:

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