Understanding How Silent Phages Can Prevent Detection of Potentially Deadly Food Contaminants

Many bacteria are detected by culturing, or growing them
out on plates of artificial media.

Contamination of food with bacteria is a huge issue that can sometimes cause life-threatening illness. The bacterial culprits can include E. coli, as well as Listeria monocytogenes. L. monocytogenes is a potent bacteria that can very effectively infect its human host. This bacterium is especially problematic for pregnant women whose newborn children can develop meningitis that can lead to complications as severe as death. Because this is a serious infectious agent, there have been a lot of quality control efforts towards detecting this bacterium in food before it is sold. In this week's post, we are going to discuss a relatively recent study that highlights the role of phages in these efforts. The study does this by showing that nutrients used in the tests can activate silent phage infections and prevent bacterial detection.

L. monocytogenes, as well as other bacteria, are often detected using culturing techniques. This means that the bacteria are actually grown on special media in a petri dish. Simply put, if we streak our sample across the plate and see the dangerous bacteria growing, we conclude that the bacteria is present and can potentially cause an infection. Like most tests, these culture techniques are not 100% accurate and can have erroneous results. Of these incorrect results, false negatives (failure to detect bacteria that are present) are of particular concern because they allow the contaminated food to be sold.

In a recent report, a group led by Letaitre et al investigated a potential cause of false negative results by evaluating the roles of phages in the culturing process. We know that bacteria are capable of being silently infected by phages that can come out into an active infection and kill the host bacterium when it is stressed. These stresses can include nutrient conditions. In their recent paper, Letaitre et al found that these mechanisms may be responsible for false negative results in L. monocytogenes tests.

Phages and tails that were detected in the study.

The study is overall fairly straightforward and a good read. The group tested a variety of components from standard test media that are widely used in the detection of L monocytogenes contaminants. They found that many components are in fact capable of inducing bacteriophages, which means the compounds in the media are capable of killing the bacteria by activating silent phage infections. By killing the bacteria through phage induction, the contaminating bacteria will not be detectable by culture test, and the final result will incorrectly indicate a lack of L monocytogenes. In the end, this highlights the importance of understanding how phages impact the quality control tests that are being used, and also suggests a different media should be considered in quality control detection of L monocytogenes.

As I mentioned above, this is a fairly straightforward read and I would suggest checking it out, especially if you are interested in the details. Any questions, comments, or concerns? Let me know in the comments below, or feel free to shoot me an email anytime. And remember to always consider the phage component.

Works Cited

Lemaître JP, Duroux A, Pimpie R, Duez JM, & Milat ML (2015). Listeria phage and phage tail induction triggered by components of bacterial growth media (phosphate, LiCl, nalidixic acid, and acriflavine). Applied and environmental microbiology, 81 (6), 2117-24 PMID: 25595760