MIC of the resistant strain (a 17-fold concentration range
for Ser83Leu). The antibiotic concentration at which this
mutation is selected is similar to those found in many in
outside environments such as rivers, lakes and soils where
concentrations are typically measured in the mg/L to ng/L
range [16,51]. An elegant chromogenic assay system suitable
for qualitative discrimination between strains competing
in liquid medium has also been developed [52].
Although the system is not designed to quantify small
differences in selection coefficient, it also showed that
mutant strains were selected at antibiotic concentrations
significantly below the MIC of a susceptible strain, and
it may prove very useful in assaying environmental
contamination by antibiotics.