In a recently published article in PNASThe researchers examined the evolutionary trajectory of a community of bacteria emerging in the laboratory. Escherichia coliwith two interacting strains.
The excessive use of antibiotics over the years has resulted in disease-causing bacteria developing the ability to grow in higher concentrations of antibiotics. Although less studied, bacteria have also evolved a tolerance, that is, the ability to continue to be exposed to antibiotics for a long time. The researchers showed that under clinical and laboratory conditions, bacteria can develop resistance and tolerance. However, laboratory studies have primarily examined single bacterial strains.
However, to understand how community interactions shape the bacterial response to antibiotics, researchers needed laboratory-engineered bacterial strains with a variable composition. In addition, it has evolved to give rise to strains that can distinguish it.
In this study, the researchers examined laboratory development coli The strains were exposed to ampicillin using a simplified mathematical model. While one community was exposed to ampicillin, the other was resistant to the antibiotic. Ampicillin resistant bacteria were added to the culture media to protect the susceptible strains.
The team performed an experimental development under high concentrations of ampicillin, which far exceed the minimum inhibitory concentration (MIC) for allergens. coli torsion. However, the MIC was significantly lower than that of the resistant strain. In this way, they characterized the evolutionary response of the susceptible coli Stress upon exposure to ampicillin within in vitro cultured communities.
The researchers serially propagated these communities for 10 growth and thinning cycles (about 100 generations) to periodically measure the abundance of the two strains. Next, they isolated representatives of the highly susceptible strains. Furthermore, the researchers measured the growth rates of the evolved isolates during evolution.
Resistance coli The strain, due to its atrophy towards lysine, cannot grow well without its supplementation to the growth medium. The team controlled combinations of resistant and susceptible strains by adding different amounts of lysine to the medium, distinguishable by selective coating and color differences.
Expose susceptible strains in monocultures to ampicillin concentrations 50-fold the minimum inhibitory concentration (MIC). These bacteria were rapidly depleted and extinct within three dilution cycles each day. Conversely, susceptible bacterial strains survived similar ampicillin exposure, but coexisted stably alongside resistant cells under similar daily growth dilution cycles.
sensitive coli Strains have evolved tolerance in the presence of antibiotics, which are characterized by an increased survival rate but a decreased growth rate, highlighting the trade-off between the two processes.
The mathematical model reasoned about why tolerance would be beneficial in this societal context and predicted that tolerance would become harmful if we increased the protection provided by the resistant strain. However, the study model did not find any tolerance arising under manipulated experimental conditions.
The study model demonstrated that the observed reduction in mortality, although combined with a decrease in growth rate, was beneficial in a population with poor protective interactions. The model also predicted that this trade-off would be harmful in the presence of strong protective interactions. However, tolerance will not appear, which the researchers also achieved experimentally. Interestingly, whole-genome sequencing of evolving tolerant isolates revealed two significant genomic points at which mutations accumulated in parallel, suggesting their association with tolerance.
Compared to the wild-type isolates, the evolved bacterial isolates developed lower death rates and only a moderate increase in MIC, from 1 to 2 μg/mL. Two-modulus regression showed that a decrease in growth rates better rationalized the amplified survival of the evolved bacterial isolates, which is calculated by their survival frequency after five hours of antibiotic exposure. It should be noted here that increases in MIC, which are indicative of resistance, were far removed from the exposed antibiotic concentrations (100 μg/mL). The increased survival is likely due to tolerance. Susceptible strains slowed their growth to develop tolerance, i.e. lower mortality with lower growth rate in response to antibiotics.
Theoretically, the lower mortality rate favors the survival of susceptible cells when antibiotic exposure exceeds that in microscopy. However, in this study, tolerance to bacterial strains had a benefit and a cost, that is, lower mortality and growth rate, respectively. Together, the study models showed that when the resistant cells provided weak protection from the antibiotic manifesting as slow lysis of susceptible cells in the population, tolerance appeared in the evolved isolates.
Under increasing concentrations of lysine, susceptible and resistant strains coexist. Above 0.0001% Lysine, eight of nine coli The strains did not evolve, and susceptible strains isolated after 10 development cycles showed no decrease in mortality or increase in MIC. The model predicted that a high antibiotic degradation rate ensured no death phase for susceptible cells, which in turn offered no advantage to increasing the MIC, although there was no cost. Moreover, the concentration of antibiotics also controls how and to what extent tolerance develops.
The study highlighted the importance of examining the relationship between environment and evolution in relation to antibiotic treatments and their failure. The researchers demonstrated an interaction between resistance and tolerance, with tolerance serving as the basis for resistance. Moreover, an antibiotic-resistant bacterial strain protects the susceptible strain. Notably, this protective interaction is under the development of tolerance. More importantly, studying the interactions between bacterial strains and their dynamics over longer periods of time is likely to be fruitful for future research.