Have you ever wondered why you were told not to eat raw cookie dough? It’s probably because there might be Salmonella Typhimurium in the eggs used in your recipe. Not all eggs are contaminated with S. Typhimurium, but it is harmful to ingest eggs that have been contaminated. The cooking process, if done properly, removes the S. Typhimurium from the egg, making your freshly baked cookies safe to eat!
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| Salmonella Egg Contamination |
Salmonella Typhimurium is an important foodborne pathogen that can cause diarrheal infections all around the world. When someone is exposed to S. Typhimurium, most times this causes benign symptoms, but some patients develop symptoms. Symptoms that develop are characterized as “Salmonella diarrhea” which is characterized by fever, abdominal cramps, and excessive watery stool. S. Typhimurium can typically be found in the intestinal lumen, or the inner space of the intestines.
You might then ask why or how does S. Typhimurium contaminate the egg? The egg has two natural barriers: a physical barrier created by the egg shell and a chemical barrier created by the shell membrane as well as the albumen. The albumen is the protein that creates the egg white.
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| Labeled Raw Egg |
Even with the two natural barriers, S. Typhimurium still finds a way to get in and therefore contaminate the egg. The mechanism of how Salmonella contaminates eggs is currently unknown, however Okuno, Xu, Isogai, and Nakamura postulate that the motility and migration of the bacteria towards the yolk plays a significant role in bacterial growth in eggs. Salmonella are covered in approximately 5-10 flagella, an organelle that is anchored to the bacterial cell, per cell, which allows them to be motile. Flagella can rotate counterclockwise (CCW) or clockwise (CW). When flagellum rotate in a CCW manner, they bundle together and allow the bacterium to travel straight. When the flagellum switches from a CCW to CW rotation, the bundle unravels causing the bacterium to tumble. After the tumbling subsides, the cell will travel in random directions. The direction of flagellar rotation is controlled by an intracellular pathway stimulated by changes in chemical concentrations and light within the environment. Through studying the motility, speed and direction of Salmonella Typhimurium, the bacteria is found to be attracted to the yolk of eggs and will travel through the albumen to infect the yolk.
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| Scanning Electron Microscope of S. Typhimurium |
Using a capillary assay to measure chemotaxis, this paper demonstrates that S. Typhimurium cells are strongly attracted to the yolk as demonstrated by the lines with circles and triangles (Figure 1). This suggests that chemicals such as sugars and amino acids from the yolk attract S. Typhimurium cells. Thick and thin albumen, in contrast, do not attract S. Typhimurium cells (Figure 1). Having albumen in the bacterial suspension did not affect chemotaxis of the S. Typhimurium cells to the yolk, even though the albumen does not have an attractive component. Interestingly, the albumen has been characterized to contain antimicrobial materials that form a chemical barrier.
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| Figure 1B. Chemotaxis of S. Typhimurium cells towards egg extracts using the yolk, thick albumen (Thick), thin albumen (Thin), and motility buffer contained in the capillary (C) and bacterial suspension (BS). |
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| Figure 2. The effect of thick and thin albumen on swimming motility and swimming speed in comparison to motility buffer. |
Researchers were then interested in understanding how the albumen might affect the motility of S. Typhimurium. This was tested by comparing the swimming trajectories and speeds of individual bacterium in three different environments: the motility buffer (a common medium used for observing bacterial motility), the thin albumen and the thick albumen. Researchers were unable to measure the motility of S. Typhimurium in the presence of yolk, because the density prevented any observation of the cells. S. Typhimurium in the motility buffer moved without noticeable changes in their usual swimming directioning and patterning (Figure 2A). When measured in the thin albumen, they found that in comparison to the buffer, the cells appeared to travel shortened distances (Figure 2B). The most apparent difference was the hindered cell motility in the thick albumen (Figure 2C). Upon enlarging of the swim trajectories, there was evidence of more tumbling due to the repellant nature of the thick albumen (Figure 2C). This demonstrates that there may be chemical components within the albumen that conferred resistance within the bacterial cells.
The swimming speeds of S. Typhimurium were also observed within the three environments based on straight swimming from CCW rotation. Cells in the motility buffer were measured at the highest speeds of 29.7 ± 4.3μm/s (n = 20 cells) (Figure 2D). Similar to the directionality in the thin and thick albumen, speeds were slightly slower in the thin albumen than the buffer at 27.2 ± 3.9μm/s (n = 26 cells) and in the thick albumen, the swimming speed was calculated at 15.3 ± 2.5μm/s (n = 12 cells) (Figure 2D). Researchers reasoned that the viscosity of the three mediums affect the speed of the cells, and was the reason little motility was observed for cells in the thick albumen. However, researchers were unable to determine which parts of the albumen were affecting flagellar torque generation.
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| Figure 3. S. Typhimurium flagella alternate between CCW rotation and CW rotation in thick albumen. CCW rotation is presented on the graph as positive revolutions and CW rotation is represented as negative revolutions. |
Now that S. Typhimurium swimming behaviors were analyzed, the researchers investigated how thick or thin albumen affects S. Typhimurium flagella rotation. When placed in motility buffer alone, the flagella only rotated in the CCW direction (Figure 3A,B). Given that the motility buffer does not have a repellant, flagella should easily rotate through the buffer, and it is therefore expected to see only one direction of rotation. In thin albumen, some tumbling, or alternating between CCW and CW rotations, was observed (Figure 3A,B). Interestingly, in thick albumen, the researchers observed almost zero net rotation, because the flagella frequently switched between CCW and CW directions (Figure 3A). Figure 3B illustrates that the flagella spent almost as much time rotating CW as they did rotating CCW, suggesting frequent tumbling. Given that flagella tumble when repelled, they concluded that albumen might be a repellant.
When the researchers looked at rotation speed, they found that S. Typhimurium flagella in thick albumen rotated slower than flagella in the motility buffer or thin albumen (Figure 3C). This signifies that the bacteria’s movement was in part hindered by the thick albumen. It was shown that flagella rotated CW most often in thick albumen compared to the motility buffer or albumen, supporting the previous evidence that more tumbling occurred in the presence of albumen (Figure 3D).
Given that S. Typhimurium causes disease when humans consume eggs contaminated with the bacteria, it is therefore important to better understand S. Typhimurium colonization and motility in eggs. The paper concluded that the bacteria used chemotaxis to move towards the egg yolk and away from the albumen. The research also showed that albumen caused increased tumbling, therefore hindering S. Typhimurium movement in the egg. This article highlighted how the bacteria uses chemotaxis to navigate through the egg, and proposed a model where S. Typhimurium is attracted to the egg yolk but repelled by the albumen. While this study found that S. Typhimurium is attracted the yolk, it did not proposed which chemicals or proteins are secreted by the yolk that attract S. Typhimurium. It would also be interesting to know what repellent is secreted by the albumen to repel the bacteria. In the future, I would be interested to find out more about how S. Typhimurium gets into the egg, through the two natural barriers, and how this might affect virulence and its ability to cause infection. Next time you eat raw cookie dough, let’s hope that the albumen repelled S. Typhimurium from reaching the yolk!
About the Authors:
Courtney Hegner ‘19 is a Biochemistry major at Mount Holyoke College. She spends most of her time working in the Berry Lab, where she has conducted research in for the past 2 and a half years. Next year she will be attending graduate school as part of the Scripps Research Institute’s Chemical and Biological Sciences PhD program.
Catherine Peabody ‘20 is a Biology and East Asian Studies double major at Mount Holyoke College. She is currently on the tennis team and during her free time, she is involved in the the college’s SPLASH program where she teaches swim lessons to kids with disabilities. After graduation, she plans on attending medical school.
Clara Wang ‘19 is a Biology major and a Chinese minor at Mount Holyoke College. She is on the tennis team and plays oboe in the Mount Holyoke Symphony Orchestra. After graduation, she is going to be working in a lab at the National Institute of Health (NIH) conducting biomedical research.
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