Breaking down the microbiology world one bite at a time
Health in space and why bacteria play a role in it
It takes hundreds of people and thousands of hours of work to prepare a single space mission. Pages of mathematical calculations, careful calibration of equipment, and, of course, prediction of potential threats to mission objectives. Success of a space mission also depends on astronauts, who act as highly-trained data scientists, engineers, doctors, and even athletes. An astronaut’s health is on top of the priorities list, and much effort is put into maintaining his or her wellbeing. And this is where microorganisms play a crucial role…
Microbes are not only ubiquitous on Earth. They are also found on structures inhabited by humans in a strictly sanitized and organized spacecraft environment. Why? Because the crew’s normal microbiome “contaminates” the area. Under any unfavorable conditions and in the absence of gravity, microbial composition can suddenly change, which can lead to the development of infections. One such organism of concern is Streptococcus mutans. This bacterium is a causing agent of dental caries, and it has been demonstrated that long-term space missions and increased exposure to microgravity and radiation [are correlated with] an increased incidence of oral disease in astronauts. In addition, it is predicted that dental emergencies will be one of the top conditions impacting future space missions.
Fernander with her team (2022) conducted an interesting study, in which they analyzed the adaptive response of S. mutans to the conditions of simulated microgravity. Although this organism is well-studied on Earth, scientists are wondering how this bacterium responds to the space environment. Do S. mutans have the potential to pose a risk to human health once in space?
But what is “microgravity” and how can it be simulated? As NASA explains it, “The condition of microgravity comes about whenever an object is in free fall” (think of the amusement park rides that move vertically). In the study, authors created something similar to a free fall state. They tested two populations of S. mutans: one that grew under normal gravity conditions (control group) and the other one under the conditions of simulated microgravity.
The formation of dental caries is reliant on two factors: 1) an ecological shift that favors the growth of acid producing bacteria and 2) the presence of sucrose that bacteria will feed on. Because sugars tend to create a more acidic environment, the group also focused on the analysis of changes that happened in acid tolerance levels, adhesion levels, and antibiotic susceptibility under the conditions of simulated microgravity.
To develop dental caries, bacterium S. mutans first has to adhere to a tooth surface and form a biofilm known as a plaque (Figure 2). If a plaque is continuously maintained below a pH value of 5.4 (which is acidic), it favors demineralization of a tooth enamel.
The researchers assessed changes in acid tolerance by exposing bacterial populations to acidic environments for 0, 20, 30 and 45 minutes. Then, they counted the number of colonies to evaluate the survival and ability to tolerate low pHs (Figure 3).
They found out that adaptation to normal gravity showed a clear reduction in acid tolerance, while in simulated microgravity the results were quite variable. It is worth mentioning that the researchers grew cells planktonically, meaning that bacteria were free-living (compared to biofilms, which have higher resistance to fluctuations that happen in the environment). Dental plaque is an example of a biofilm, and thus, the results of the same experiment might have differed, if S. mutans were grown in biofilms instead.
Now, let’s talk about adhesion. It happens in the presence of sucrose via sucrose-dependent (SDA) and sucrose-independent (SIA) mechanisms. While the first one is required for initial attachment, the second is responsible for virulence and pathogenicity. The researchers grew bacterial colonies under the conditions of normal gravity (NG) and simulated microgravity (MG) for 100 days and then analyzed the results. As seen on Figure 4, sugar-dependent and sugar-independent mechanisms behaved in different ways. The SDA group was barely affected by microgravity conditions. On the contrary, the SIA group showed some phenotypic variations, whereas certain bacterial colonies (in both, normal gravity and microgravity) did not successfully adhere to the surface. However, this does not mean that these results would be universal for all the similar experiments conducted in the future. Authors of the article discuss how it can be problematic to predict “the influence of the selection environment (i.e. space environment) on this phenotype [adhesion of S. mutans to surfaces]”.
S. mutans are greatly studied on Earth, and are treated using antibiotics. However,since space is an extraterrestrial environment, the research group posed the question: How will this bacteria react to treatments while in space? They tested six different antibiotics: amoxicillin, penicillin, clindamycin, erythromycin, methicillin, and vancomycin. It turned out that the conditions of simulated microgravity did not provoke a significant change in susceptibility! Scientists only registered a slight increase in erythromycin susceptibility and an increase in resistance towards clindamycin.
Based on gathered results, the research team concluded that the microgravity environment displayed greater selection force than a normal gravity. This means that the conditions in the absence of gravity are harder to change, significantly slowing the rate of a mutation and, on the contrary, if the environment is susceptible to change,a greater number of mutations could take place.
The next step is to study genes (and resultant proteins) that allow S. mutans to adapt to such unusual conditions. Enhanced fitness of these bacteria in space can pose some risks for astronauts, such as the development of unwanted dental problems even in space. And while we wait for the solution, let’s hope that astronauts don’t indulge in too much sugar on the board of their spacecraft!
Link to the original post: Fernander, M.C., Parsons, P.K., Khaled, B. et al. Adaptation to simulated microgravity in Streptococcus mutans. npj Microgravity 8, 17 (2022).