Though maybe you have not heard of the bacterium Bordetella pertussis by its Latin name, you are likely familiar with it; this bacterium causes Whooping Cough, a contagious respiratory illness named for its terrible cough. In fact, it is likely that when you were last vaccinated for tetanus, you received the Tdap (or DTap) vaccine which also protects against B. pertussis and diphtheria, another bacterial disease. Additionally, if you have pet cats or dogs, it is likely they have received a Bordetella vaccination - though not B. pertussis, it is a related bacterium B. bronchiseptica that causes kennel cough.
B. pertussis was first isolated by scientists in 1906, so this bacterium has been studied for a long time. Originally, B. pertussis was thought to be non-motile, however, more recently, Hoffman et al. found that B. pertussis does actually use flagella to move around. If you are curious in learning more about that study, check out this prior blog post - new findings: a bacteria that causes whooping cough could have the ability to move. In 2022, scientists in Japan discovered that the flagella actually play a role in how B. pertussis infects people. This blog post will further describe the findings of this study.
The flagella of B. pertussis and B. bronchiseptica can be seen in Figure 1. The long, filamentous flagella are made of the protein “flagellin,” which rotates, allowing the individual bacterial cells to move. But, these flagella are actually important beyond allowing the bacteria to move, as the findings of Hiramatsu et al. indicate. The flagella are one of the factors that contribute to the bacteria knowing it has successfully found a host cell.
B. pertussis was first isolated by scientists in 1906, so this bacterium has been studied for a long time. Originally, B. pertussis was thought to be non-motile, however, more recently, Hoffman et al. found that B. pertussis does actually use flagella to move around. If you are curious in learning more about that study, check out this prior blog post - new findings: a bacteria that causes whooping cough could have the ability to move. In 2022, scientists in Japan discovered that the flagella actually play a role in how B. pertussis infects people. This blog post will further describe the findings of this study.
The flagella of B. pertussis and B. bronchiseptica can be seen in Figure 1. The long, filamentous flagella are made of the protein “flagellin,” which rotates, allowing the individual bacterial cells to move. But, these flagella are actually important beyond allowing the bacteria to move, as the findings of Hiramatsu et al. indicate. The flagella are one of the factors that contribute to the bacteria knowing it has successfully found a host cell.
Figure 1: Images of B. bronchisepta and B. pertussis taken with an electron microscope showing the cell’s flagella, from Hoffman et al. B. pertussis strains are shown in C and E. In C-H, the locations of the flagella are marked by white arrows.
As you might imagine, B. pertussis bacteria swimming around looking for a host need to express different genes than when they are infecting their host. So, what is an easy way for the bacteria to know when it has come into contact with a host cell? The flagella! Once B. pertussis has come into contact with a host cell, the rotation of the long flagella filaments is impeded by the host cells. This triggers a shift of gene expression within the bacteria, and it starts up-regulating genes that contribute to infection of the host cells. This is a convenient mechanism for the bacteria’s strategic infection.
While generally the contact with the host cell inhibits flagellar rotation, you might be wondering how exactly this works within the cell. Bacterial cells have small RNA molecules (sRNAs) that regulate gene expression through binding to messenger RNAs (mRNAs) within the cell and influencing the mRNAs translation, degradation or stabilization. Hiramatsu et al. previously identified one specific sRNA, Bpr4, that is upregulated when the bacteria infected tracheal cells. They then were able to determine the connection to how this aids in the infection of the host cell. Bpr4 up-regulates filamentous hemagglutinin (FHA), by inhibiting the degradation of the mRNA encoding FHA. FHA is mainly how the B. pertussis anchors itself to its host cells. Therefore, when the bacteria makes contact with a potential host cell, the rotation of its flagella is interrupted and the cell begins producing more FHA, so it can attach to the host (Figure 2).
In addition to FHA, gangliosides are important in the connection between the host cell and B. pertussis. Gangliosides are receptors found on the outer surface of eukaryotic cells. The flagella of B. pertussis recognize gangliosides on host cells; this is another factor that provides information to the bacteria to prepare for infection (Figure 2). We can imagine why this would be useful - if only interference of flagellar rotation caused a change in gene expression, what if the bacterial cell had come into contact with something other than a eukaryotic host cell? In this case, it would not be an efficient use of the bacteria’s energy if it started producing proteins and other things needed to infect host cells, if it were not actually in contact with a host cell. As a result, it is helpful to have several factors coming together in order to influence the production of proteins within the cell.
Figure 2: Diagram from Hiramatsu et al. showing interactions between the host cell and bacterial cell, especially its flagella. In A, just B. pertussis is shown, and the rotation of the flagella indicated. In B, the gangliosides are interacting with the flagella and FHA is shown attaching to the host cell; factors also involved in upregulation of Bpr4 are also shown.
Looking at combinations of these specific factors, such as ganglioside presence and flagella presence, Hiramastu et al. were able to determine that both the gangliosides and the immobilization of flagella are required for Brp4 upregulation. Culture plates were treated to create three different overall combinations: just gangliosides, gangliosides and anti-FHA (an antibody that binds to FHA), and gangliosides and anti-flagellin (an antibody that binds to flagellin). Though there were additional combinations within these groups, this main setup provides the big picture of the experiment. When B. pertussis was added to culture plates treated with gangliosides, RNA sequencing revealed that Bpr4 was upregulated in only the bacteria adhered to the plate (Figure 3A). Therefore, the presence of gangliosides alone is not enough for Bpr4 upregulation, but up-regulation requires the flagella to be immobilized. When B. pertussis was added to culture plates treated with anti-FHA, only when gangliosides were also present, resulting in the bacteria being bound to the plate, was there up-regulation of Bpr4 (Figure 3B). In this case, anti-FHA and gangliosides were necessary to immobilize B. pertussis. In addition, the experiment was repeated with plates treated with anti-FHA and anti-flagellin. In this case, upregulation occurred when both anti-FHA and anti-flagellin were present, demonstrating that anti-flagellin can replace the role of gangliosides (Figure 3). This result demonstrates that the gangliosides interact and bind to the flagella of the bacteria (Figure 3). These patterns in upregulation demonstrate that B. pertussis cells need to be immobilized and interact with the host cells in order for the upregulation of Bpr4. These combinations demonstrate that the predicted outcome does occur, as they show flagellar interference leads to the upregulation of Bpr4.
Figure 3: Figure adapted from Hiramatsu et al. showing the upregulation of Bpr4 in relation to flagellar interference. In A, the up-regulation of Bpr4 occurs on plate adhering bacteria when gangliosides are present. In B, when both anti-FHA and gangliosides are present, up-regulation occurs. In C, when both anti-FHA and anti-flagellin are present, up-regulation occurs; in this case, anti-flagellin is assuming the role of gangliosides in B.
Consequently, this up-regulation as a result of flagella interference and immobilization of the bacterial cell plays a role in the regulation of gene expression required for B. pertussis infection. There is still additional information to learn about this signaling pathway, as this research is just the beginning of learning more about the role flagella play in infection. In addition, in this study Hiramatsu et al. focus on one specific sRNA, Bpr4, but B. pertussis has hundreds of sRNAs - some of which might also be connected to flagella interference. Even if these sRNAs are not connected to this pathway connected to the immobilization of the bacterial cell, they likely have important roles in gene regulation and the colonization of B pertussis. Furthermore, identifying sRNAs such as Bpr4 that contribute to pathogenesis create important targets for developing new treatments for B. pertussis infections. Currently, B. pertussis is generally treated using antibiotics, so developing more specific treatments is beneficial due to the rise of antibiotic resistance.
B. pertussis was only recently found to have flagella, so it is exciting that Hiramatsu et al. discovered that flagella play a role in the gene regulation as well. Learning about this mechanism for infection of host cells by B. pertussis is an example of the ways in which bacteria have perfected their ability to efficiently infect host cells without unnecessarily expending their energy. Using a feedback mechanism from the interference of flagellar rotation signifies changes in the bacterial cell that are directly needed when the bacteria can infect its host. This carefully adapted mechanism of B. pertussis shows a straightforward and smooth path for infection. Additionally, though this mechanism seems intuitive, this research was published in 2022; we are still learning things about bacteria, even those we have known about for 100+ years.
About the Author:
Kristen Johnson ‘23 is a Biology and Music double major from Haverford, Pennsylvania. She enjoys photography and spending time outside as well as playing the violin. She hopes to work as a field research technician on various projects before heading to graduate school.
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