Monday, 8 April 2013

Nematodes to test water quality


It is important for human health to analyse waters for faecal contamination, however the methods used for this such as arrays or PCR can be expensive and many water quality or public health laboratories may not have such facilities. This study used a microarray to analyse the virulence genes of environmental isolates (n=148) collected from the South National River in Ontario Canada, of E.coli as well as a positive control of clinical isolates (n=8). They then investigated the technique of an infection assay using the nematode Caenorhabditis elegans to determine potential pathogenesis of these isolates on the basis of these virulence genes. This specific species was used as it was cheap, quick and there is a broad knowledge concerning Caenorhabditis elegans bacterial pathogen interactions.

Overall 29% of the environmental isolates were pathogenic to the C. elegans. A Cox propositional hazards model was used to determine the specific genes and phenotypes related to pathogenicity. They found no link between antibiotic resistance and pathogenicity. They were unable to conduct this test to analyse specific antimicrobial genes due to the lack of isolates that contained them, however those that did were non-pathogenic. This confirmed the importance of specific virulence genes in determining pathogenicity. Some specific genes were observed at higher frequencies in the pathogenic isolates. These included genes encoding for adhesion, siderophores receptors and autotransporter proteins. All of these genes were linked with strains of E. coli that caused extraintestinal infections and some with urinary tract infections. However, these single genes may not necessarily have directly been related to pathogenicity, but could be due to combinations of genes that correlate with genes on pathogenicity islands however were not specifically screened for in the microarray.  

The microarray analysis of the environmental isolates demonstrated 15 extaintestinal and one enteric virotype. The infection assay found 11 of these to be pathogenic to C. elegans. Nonetheless, some of the clinical isolates did not kill the nematode, suggesting this model has some limits. Also, some other studies found the level of killing to be insignificant in comparison to the negative control. However the ability to cause disease also depends on the conditions of infection, therefore this may be responsible for some the variations in results observed between studies. Although 26% of the strains determined non-pathogenic were observed to enhance nematode survival, worryingly 24% of the isolates that were categorised as non-pathogenic actually killed the nematodes. This represents the complexity of the relationships between the virulence gene complements and death of the nematode and suggests virotyping should not be used alone, since the current lack of knowledge of all virulence genes may lead to some pathogenic strains being missed.

Overall, the high percentage of pathogenesis of the isolated E. coli strains on the nematode suggests this water demonstrates a potential risk to human health. Although the identification of virotypes using microarrays was useful in predicting the risk of pathogenesis, using this technique to search for single virulence genes was less successful. However as increasing amount of virotypes are characterised, subsequent identification of gene combinations should reduce the false negatives produced by microarrays, therefore enhancing this technique. The infection assay using C. elegans helped to distinguish isolates that may have a pathogenic effect on humans. Although this technique may be cheaper and easier than techniques such as PCR, the results may not necessarily be as accurate and further work would be needed to investigate the limitations observed in this study before it could be confirmed as a viable method for testing water quality. 

Markx-Jacques, A., Coors, A., Brousseau, R., Masson, L., Mazza, A., Tien, Y. And Topp, E. (2013). Evaluating the Pathogenic Potential of Environmental Escherichia coli by Using the Caenorhabditis elegans Infection Model. Applied and Environmental Microbiology. 79, 2435-2445.


2 comments:

  1. Hi Aimee,

    Does the paper mention how fast this method is? I know that scientists are looking for quicker to methods to test if water at a beach is contaminated as it could be a risk to human health.

    Thanks,

    Sophie

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  2. Hey Sophie,

    The nematodes were added to the bacterial culture and allowed to feed overnight, then observed every other day and recorded as live or dead. So this technique doesn't contribute to solving the issue of time efficiency of testing water quality. I think the main aim of the study was to investigate the use of an infection assay to reduce costs of previous methods such as PCR in addition to how efficient microarray analysis is when testing for specific virulence genes.

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