A microfluidic
gradient chamber (MGC) and a homogeneous batch culturing
system were used to evaluate whether spatial concentration gradients
of the antibiotic ciprofloxacin allow development of greater antibiotic
resistance in Escherichia coli strain
307 (E. coli 307) compared to exclusively
temporal concentration gradients, as indicated in an earlier study.
A linear spatial gradient of ciprofloxacin and Luria–Bertani
broth (LB) medium was established and maintained by diffusion over
5 days across a well array in the MGC, with relative concentrations
along the gradient of 1.7–7.7× the original minimum inhibitory
concentration (MICoriginal). The E. coli biomass increased in wells with lower ciprofloxacin concentrations,
and only a low level of resistance to ciprofloxacin was detected in
the recovered cells (∼2× MICoriginal). Homogeneous
batch culture experiments were performed with the same temporal exposure
history to ciprofloxacin concentration, the same and higher initial
cell densities, and the same and higher nutrient (i.e., LB) concentrations
as in the MGC. In all batch experiments, E. coli 307 developed higher ciprofloxacin resistance after exposure, ranging
from 4 to 24× MICoriginal in all replicates. Hence,
these results suggest that the presence of spatial gradients appears
to reduce the driving force for E. coli 307 adaptation to ciprofloxacin, which suggests that results from
batch experiments may over predict the development of antibiotic resistance
in natural environments.
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