Abstract:
Experimental evolution is an important tool used to study the long-term effects of
multidrug resistance in bacterial populations. Two major factors that can potentially
affect the evolution of resistance are fluctuations in exposures to antibiotics and
differences in population sizes. To investigate this, replicate populations of Escherichia
coli were subjected to either a single antibiotic or fluctuations between a pair of
antibiotics. After these selections, the increase in antibiotic resistance was measured as
an increase in MIC, growth rate and carrying capacity compared to the ancestral
populations. Contrary to prior expectations, we found that large populations did not
show an increase in antibiotic resistance compared to small populations when selected
under increasing concentrations of a single antibiotic. Also, large populations showed a
lack of costs when exposed to novel antibiotics. When exposed to fluctuations between
a pair of antibiotics, large populations were able to evolve a higher resistance than small
populations, but again showed a lack of costs in novel antibiotics. These results indicate
that there might be a few factors with different levels of influence on large and small
populations, such as the presence of resistant subpopulations, upregulation of efflux
genes and differences in the distribution of fitness effects. Further experiments are
required to clarify the effect of these individual factors.
