You wake up very excited this morning as you are going to start your solo road trip across the country from California to Rhode Island. At 4:30 am, you get in your car and start driving through your sparsely populated city. All of a sudden your car makes a loud noise and stops moving. You start to panic since you are 90 minutes from home, and you are starting to get hungry and thirsty, and there is no food nearby. Later, you finally see some car lights. You stick your thumb out hoping the driver will see you. They slow down, and you run up to the car explaining your situation. They explain to you that unfortunately your car will not be able to make the journey, but they agree to drive you to your destination. Happily, you enter the car and restart the journey with your new companion.
In situations where one does not have the resources they need, hitchhiking may be a necessary form of transportation. Humans are not the only living beings that use hitchhiking, microbes do as well! Two examples of microbes that use hitchhiking are Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa).
S. aureus is a bacteria that can cause human infections especially in those who are immunocompromised. This bacteria lives in the nose of 30% of the human population. Furthermore, this bacteria causes the disease Methicillin-resistant Staphylococcus aureus (MRSA), which is resistant to antibiotics. This bacteria is gram positive, which means that it has a thick cell wall with peptidoglycan on the outside, and a cell membrane on the inside.
Despite this bacteria’s strengths, one weakness that it has is that it lacks pili and flagella. Therefore, S. aureus is nonmotile, which is harmful if the bacteria needs to leave the environment due to a lack of resources. However, S. aureus can move on surfaces by releasing biosurfactants which reduces surface tension, allowing it to spread.
Unlike S. aureus, P. aeruginosa is a bacteria that is mobile due to its flagella. Therefore, P. aeruginosa moves by swimming. This bacteria also tends to share the same environment as S. aureus. Furthermore, this bacteria can cause infections in the blood or lungs as pneumonia. P. aeruginosa is gram negative, which means that it has an outer and inner plasma membrane with a thin peptidoglycan cell wall in the middle.
S. aureus and P. aeruginosa often compete with one another for resources by inhibiting each other’s growth. However, there are other times in which they work together. P. aeruginosa is able to help S. aureus become mobile when it swims. How does this occur? What structures are used for this swimming together to happen? In Liu and Lin’s study, they show that S. aureus hitchhikes onto P. aeruginosa because of S. aureus’ wall teichoic acids (WTA) and P. aeruginosa’s lipopolysaccharides (LPS). The WTA are found on the surface of gram positive bacteria, and they help make the peptidoglycan cell wall more stiff and repel harmful substances. The LPS are on the surface of gram negative bacteria and also repels harmful substances. By conducting this study, Liu and Lin help us to understand better how these certain strains of this bacteria are resistant and how they are able to survive in unfavorable environments.
Figure 1
Liu and Lin (2023) set out to investigate the underlying mechanisms of the interactions between S. aureus and P. aeruginosa. In order to do this, they had to acquire and transform different bacterial strains. They used Staphylococcus aureus SA113 (wildtype strain), a ΔtagO mutant of S. aureus which had a deletion in tagO and doesn't make WTA. In order to complement the mutant strain, they transformed the tagO-expressing plasmid (which does make WTA). As for P. aeruginosa, they used a wildtype strain of P. aeruginosa PAO1 and a mutant, ΔfliA which does not have flagellum.
Figure 1 shows the hitchhiking motility which measures the ability of P. aeruginosa to carry S. aureus during swimming. Different mixtures of S. aureus and P. aeruginosa were added into the wells of a Calgary Biofilm Device (CBD). The wells had S. aureus wild type only, a mix of wild type S. aureus and wildtype P. aeruginosa, and a mix of wildtype S. aureus and P. aeruginosa without flagella. These bacterial mixtures were separated into two layers: nonmotile bacteria at the bottom and motile bacteria at the top. After being incubated for an hour, the pegs on the lid were immersed into the wells for 30 seconds. They collected the bacteria from the pegs for counting. The count was 17 times larger when wild type S. aureus was mixed with wildtype P. aeruginosa. P. aeruginosa without flagella carried 100 times less than wildtype S. aureus on the pegs of the lid. Therefore, hitchhiking only happens if P. aeruginosa has its flagella.
Figure 2
Figure 2 highlights the role of WTA in the hitchhiking motility of Staphylococcus aureus. Referring back to the methods used in figure 1, the same three mixtures were used, excluding the variant of P. aeruginosa without flagella. First, investigated hitchhiking swimming motility with P. aeruginosa. The same methods from figure 1 were used for counting the amount of motility that occurred. The results showed that the number of S. aureus on the peg had decreased in the mixture containing the S. aureus without WTA. However, the mixture with S. aureus with WTA had an increased cell count showing the vital role of WTA in hitchhiking motility. Next, the team examined if P. aeruginosa carried S. aureus during swarming motility. Both S. aureus and P. aeruginosa were added to the middle of the agar plates. Samples were collected from the center and the edge of the plate, then cells were counted to determine motility. S. aureus hitchhiked with P. aeruginosa away from the middle of the plate. The plate with S. aureus with WTA added back in had the highest cell count while the plate with the S. aureus without the WTA had the lowest.
From the second experiment, the scientists found that the WTA of the S. aureus is important for mobility. In the third experiment, the scientists analyze how the WTA of S. aureus interacts with the LPS of P. aeruginosa to be mobile. The scientists placed LPS that was not attached to the P. aeruginosa into a mixture with S. aureus and observed how many S. aureus would adhere to some pegs that contained P. aeruginosa. They found as they increased the amount of LPS, less S. aureus stuck to the P. aeruginosa that was on the pegs. These results show that the LPS competes for the binding of S. aureus. In this analysis, the LPS and S. aureus were mixed, and the fluorescence intensity was detected by staining the cells. With a higher intensity and more staining, there is more binding which occurred when S. aureus was mixed with labeled LPS. Lastly, when P. aeruginosa, S. aureus, and LPS were all mixed together, there was more P. aeruginosa associated with S. aureus. This number decreased if the WTA of the S. aureus was mutated, indicating that the WTA and LPS are both needed for motility.
Lastly, the scientists wanted to see if there would be mobility of P. aeruginosa and S. aureus not only in lab settings in CBD and pegs but also in animals. Caenorhabditis elegans, a worm, was fed either S. aureus, P. aeruginosa, a mix of S. aureus and P. aeruginosa, a mix of S. aureus without WTA and P. aeruginosa, the same mix without P. aeruginosa, a mix of S. aureus with WTA and P. aeruginosa, and the same mix without P. aeruginosa. Then, the displacement of the S. aureus in the worm was visualized with green autofluorescence. There was more displacement shown when S. aureus was combined with P. aeruginosa, and when the WTA was not mutated or restored.
They also saw similar results in mice. Mice were injected with S. aureus alone, a mix of S. aureus and P. aeruginosa, and the mix with the two bacteria in which the S. aureus did not have WTA. Skin tissue was taken 0 cm and 2 cm from the injection sites, and the scientists found that movement was higher when S. aureus was combined with P. aeruginosa overall, but the movement decreased if the WTA was mutated, emphasizing the importance of the WTA. These results show how these bacteria could be involved in skin infections and can cause infections in animals.
Transportation is especially important when you need resources for survival. The bacteria S. aureus and P. aeruginosa use hitchhiking as a way of transportation. The non motile S. aureus has WTA that interacts with the LPS of P. aeruginosa to move from one place to another. With this transportation, the bacteria are able to move to different environments to find more resources or to leave an unfavorable environment.
The results of this study are not consistent because previous studies have shown that these two bacteria are competitive when they interact with each other. Therefore, more research is needed to figure out when and how these bacteria move together and compete against each other. However, this study is still important because it shows one bacteria can help the other move, so co-infection is common. An example of this is the mucus that’s found in cystic fibrosis patients has both S. aureus and P. aeruginosa. By continuing research on the motility of these two bacteria, scientists can develop better strategies for treatment of chronic polymicrobial infections.
About the authors:
Noelani Noël
- Amherst College Class of 2023
- Major in Psychology and French
- Plans after graduating: Going to nursing school at Johns Hopkins
Valeria Serna-Solis
- MHC Class of 2023
- Biology major and Latine Studies minor
- Plans after graduating: Research Assistant at Johns Hopkins University
Sarah Luiz
- MHC Class of 2023
- Neuroscience and behavior major
- Plans after graduation: attending Columbia School of Nursing
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