Antibiotics are life savers. In 1928, when Alexander Fleming discovered Penicillin, the first antibiotic, the medical field reached a new level of safety and defense. Common bacterial infections such as ear infections, strep throat, and pneumonia became treatable and life expectancy increased dramatically. However, according to the CDC, every time antibiotics are used, bacteria get increasingly resistant due to the spread of antibiotic resistant genes and mutations from the exposure. Unfortunately antibiotics have been notoriously overprescribed and used to treat illnesses that aren’t caused by bacteria which has driven antibiotic resistance to be an urgent public health threat. With the emergence of antibiotic resistant bacteria, there is a high likelihood of losing the ability to effectively treat and eradicate common infections. The CDC has proposed actionable steps to slow the spread of resistance by preventing spread through washing hands and getting vaccinated. In spite of these mitigation practices, antibiotic resistance will continue to exist, especially in light of the paper by Finkelshtein et al. 2015 which found that motile bacteria are able to form symbioses with resistant bacteria and transport them through surfaces with antibiotics.
The implications of this paper are huge as it is a brand new pathway for spreading antibiotic resistance. It is a very specific mechanism for dispersing resistance since it is only possible due to bacteria being able to move across surfaces. There are a diverse array of movements bacteria can adopt for motility including swimming, swarming, twitching, gliding, and floating. (Videos of each type of motility can be accessed here at the lab website of Dr. Howard Berg at Harvard University.)
Figure 1. A photo of Paenibacillus vortex swarming (Eshel Ben Jacob)
For this blog, the motility that we will focus on is swarming. It is a unique technique since it is actually a social behavior and all the other methods are individual cell movements. Swarming is mediated by flagella which are massive structures that extend from cell membranes to the outside of the organisms. They are made of a protein called flagellin, and when they rotate, the bacteria is propelled forward.
One of the most proficient swarmers is Paenibacillus vortex due to its unique strategy and behavior (Figure 1). One of the reasons it is so advanced at swarming is due to the complementary roles of two subpopulations within the larger colonies: the explorers and builders. Explorers have a low level of resistance, but in the presence of antibiotics it still cannot grow. They are also the drivers of motility due to their increased level of flagella compared to the builders. While the explorers are on the edges of colonies, the builders compose the majority of the inside of the colony, and wait for signals from the explorers on where and when to move. This creates a specific growth oscillation pattern (Figure 2a).
P. vortex is not just special for its swarming method, but also for its ability to transport other microorganisms. It is already known that P. vortex can transport spores from a fungus, Aspergillus fumigatus, to areas not previously colonized and across air gaps in soil. Recently, the same researchers, Colin Ingham, Alin Finkelshtein, Dalit Roth, and Eshel Ben Jacob published a paper that investigated the relationship between the swarming ability of P. vortex in the presence of antibiotic resistant bacteria and ampicillin. Ampicillin is a beta-lactam antibiotic (BLA), meaning that it is in the same family as penicillin and damages the cell wall of bacteria leading to the organism’s death. The resistant bacteria used in the experiment was a known strain of Escherichia coli that has the genes for beta-lactamase (BL). Beta-lactamase is an enzyme that can break down penicillin and similar antibiotics, such as ampicillin by blocking the antibiotic from working. Any bacteria that can make the beta-lactamase enzyme would be resistant to ampicillin.
In this experiment, Ingham et al. inoculated P. vortex on a 14-cm agar plate by itself, and found that it swarmed the entire surface in twelve hours. They did the procedure again, but treated the bacteria with ampicillin and found that P. vortex could not swarm at all. Similarly, when E. coli was placed on the agar with ampicillin, it was unable to spread over the agar plate due to it being non-motile. In the presence of antibiotics, both bacteria could not swarm or colonize the plate. However, when they were inoculated together, they were able to colonize the entire plate in 72 hours (Figure 2a). Ultimately, Ingham et al. found that P. vortex can transport antibiotic resistant bacteria through antibiotics and colonize previously blocked areas of the agar (Figure 2).
One of the most proficient swarmers is Paenibacillus vortex due to its unique strategy and behavior (Figure 1). One of the reasons it is so advanced at swarming is due to the complementary roles of two subpopulations within the larger colonies: the explorers and builders. Explorers have a low level of resistance, but in the presence of antibiotics it still cannot grow. They are also the drivers of motility due to their increased level of flagella compared to the builders. While the explorers are on the edges of colonies, the builders compose the majority of the inside of the colony, and wait for signals from the explorers on where and when to move. This creates a specific growth oscillation pattern (Figure 2a).
P. vortex is not just special for its swarming method, but also for its ability to transport other microorganisms. It is already known that P. vortex can transport spores from a fungus, Aspergillus fumigatus, to areas not previously colonized and across air gaps in soil. Recently, the same researchers, Colin Ingham, Alin Finkelshtein, Dalit Roth, and Eshel Ben Jacob published a paper that investigated the relationship between the swarming ability of P. vortex in the presence of antibiotic resistant bacteria and ampicillin. Ampicillin is a beta-lactam antibiotic (BLA), meaning that it is in the same family as penicillin and damages the cell wall of bacteria leading to the organism’s death. The resistant bacteria used in the experiment was a known strain of Escherichia coli that has the genes for beta-lactamase (BL). Beta-lactamase is an enzyme that can break down penicillin and similar antibiotics, such as ampicillin by blocking the antibiotic from working. Any bacteria that can make the beta-lactamase enzyme would be resistant to ampicillin.
In this experiment, Ingham et al. inoculated P. vortex on a 14-cm agar plate by itself, and found that it swarmed the entire surface in twelve hours. They did the procedure again, but treated the bacteria with ampicillin and found that P. vortex could not swarm at all. Similarly, when E. coli was placed on the agar with ampicillin, it was unable to spread over the agar plate due to it being non-motile. In the presence of antibiotics, both bacteria could not swarm or colonize the plate. However, when they were inoculated together, they were able to colonize the entire plate in 72 hours (Figure 2a). Ultimately, Ingham et al. found that P. vortex can transport antibiotic resistant bacteria through antibiotics and colonize previously blocked areas of the agar (Figure 2).
Figure 2. Fluorescence microscopy of the interaction between P. vortex and E. coli a) the growth oscillation of P. vortex and E. coli b) P. vortex (red) and E. coli (green) together in colonies at the periphery c) Enterobacter aerogenes (green) and P. vortex (red) with cefotaxime d) movement of P. vortex and E. coli in presence of ampicillin (4 circles)
The cooperation between P. vortex and E. coli in the presence of antibiotics is a mutual relationship since both species benefit. P. vortex needs E. coli to break down the ampicillin and E. coli needs P. vortex for general transport. Initially, Ingham et al. believed that E. coli was present at the periphery and throughout the entire colony of P. vortex (Figure 2a and b). It makes sense that E. coli would be in the periphery to break down any ampicillin that would kill P. vortex before there was any contact. However, results from figure 2d actually prove that it is not the case. In fact, Ingham et al. found that initial colonies lacked E. coli and then when P. vortex reached an area with antibiotics, E. coli was transported to the barrier when ampicillin was detected. This led the researchers to conclude that P. vortex is an opportunistic transporter, meaning that it will only transport E. coli once it detects antibiotics and needs E. coli to break it down.
Another complication to the relationship between P. vortex and E. coli is that E. coli needs P. vortex, but P. vortex does not specifically need E. coli. Through a series of studies using different swarming bacteria species, P. vortex was identified to be the most proficient at transporting antibiotic resistance. Therefore, if antibiotic resistant bacteria want to be transported, P. vortex is the most reliable and capable swarmer, ultimately making it a specific relationship for the resistant strains. On the other hand P. vortex does not necessarily need E. coli to swarm in the presence of antibiotics. Figure 2c shows the results of the same type of experiments except with P. vortex, the antibiotic cefotaxime, and the cefotaxime resistant bacteria Enterobacter aerogenes. P. vortex is able to swarm and transport E. aerogenes in the presence of cefotaxime. These experiments highlight that the spreading of antibiotics is not specific to E. coli and ampicillin.
This poses a potential concern in increasing the spread of antibiotic resistance because P. vortex has a strong capacity to transport cargo including highly resistant bacteria. To test the ability of P. vortex to transport infectious bacteria, Ingham et al. inoculated P. vortex with soil isolates of known infectious strains common in hospitals. This particular experiment is very exciting to me as my microbiology class at Mount Holyoke College has been conducting our own soil isolate research throughout the entire semester. Some of my classmates even took their soil microbes from the grave of our dead founder - that’s almost as scary as the spread of antibacterial resistance, but I digress. The results of the experiment are concerning since the swarming ability of P. vortex actually increased. The implications of these results are huge because P. vortex is a soil bacteria, therefore it has a higher potential to come into contact with these microbes naturally and, thus, lead to increased spread and potential human infections.
There are still many unknowns surrounding the ability of P. vortex to spread antibiotic strains. For example, how is the connection between P. vortex and resistant strains mediated? Is there a way we can break that connection to stop the spread? In a previous paper looking at fungal spores, Ingham et al. proved that P. vortex holds and carries the spores through flagella. It is very well possible that the same thing is happening in this study too.
Due to the very possible consequences of this symbiosis, other researchers have followed up on the research from Ingham et al. to investigate the community dynamics within the P. vortex and E. coli community. Gilad Book, Colin Ingham, and Gil Ariel modeled the complex system to demonstrate possible movement strategies and behavior of the multispecies system. They modeled the builder and explorer phases of P. vortex and E. coli in many conditions to predict the changes in behavior. Figure 3 is the result of setting up a simulation with an antibiotic gradient. Although the preliminary data suggests that the multi-species colony would only grow in the area with lower antibiotic concentrations, in theory it would be able to grow on the left side too. A potential way to combat this error that was proposed by Book et al. was to include an equation incorporating P. vortex’s ability to release E. coli when there are no antibiotics present.
Figure 3. Antibiotic gradient simulation a) antibiotics placed only on left side (yellow), b) P. vortex, c) E. coli
Modeling is essential for the advancement of microbiology as it creates an avenue for testing hypotheses in dynamic interactions. For example, the models by Book et al. 2017 can incorporate a P. vortex transport capacity to mimic competition between itself and E. coli. Other results highlighted that while P. vortex and E. coli have to cooperate in toxic environments, they are also competing for the same nutrients which creates a complex relationship. Ultimately, modeling is the next step in understanding the dynamics and potential effects of P. vortex’s transport capability. It is very useful to be able to test a prediction in a model and then try to find it in nature since you can look for it.
Both the lab experiment by Ingham et al. and the model by Book et al. strongly illustrate the ability of P. vortex to spread antibiotic resistance. Although I do not want to give anyone another reason to stress, it is very important to understand the consequences of these results. Antibiotic resistance is serious and while this study focuses on environmental antibiotic resistance, the environment can still easily become medical.
About the Author:
Megan Dear ‘22 is a biology major with an environmental studies minor and 5 College Certificate in Marine and Coastal Sciences. Within marine sciences, her passion is coral reef ecology and conservation. After falling in love with microbiology this semester, her post graduation plans are to pursue research in the coral microbiology field. Outside of academics, you can find her reading literary fiction or talking about her time on SEA Semester (pictured above).
References:
Beta-lactam antibiotics. (2022). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Beta-lactam_antibiotics&oldid=1084528780
Beta-Lactams—Infectious Diseases. (n.d.). Merck Manuals Professional Edition. Retrieved April 25, 2022, from https://www.merckmanuals.com/professional/infectious-diseases/bacteria-and-antibacterial-drugs/beta-lactams
CDC. (2022, March 15). How do germs become resistant? Centers for Disease Control and Prevention. https://www.cdc.gov/drugresistance/about/how-resistance-happens.html
CDC Newsroom. (2016, January 1). CDC. https://www.cdc.gov/media/releases/2016/p0503-unnecessary-prescriptions.html
Flagellum | biology | Britannica. (n.d.). Retrieved April 25, 2022, from https://www.britannica.com/science/flagellum
Flagellin. (2021). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Flagellin&oldid=10509436
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