Chronic Exposure to an Environmentally Relevant Triclosan Concentration Induces Persistent Triclosan Resistance but Reversible Antibiotic Tolerance in Escherichia coli

The
major concern regarding the biocide triclosan (TCS) stems from
its potential coselection for antibiotic resistance. However, environmental
impacts are often investigated using high concentrations and acute
exposure, while predicted releases are typified by chronic low concentrations.
Moreover, little information is available regarding the reversibility
of TCS and derived antibiotic resistance with diminishing TCS usage.
Here, the model Gram-negative bacterium Escherichia
coli was exposed to 0.01 mg/L TCS continuously for
more than 100 generations. The adapted cells gained considerable resistance
to TCS as indicated by a significant increase in the minimal inhibitory
concentration (MIC50) from 0.034 to 0.581 mg/L. This adaptive
evolution was attributed to overexpression and mutation of target
genes (i.e., fabI) as evidenced by transcriptomic
and genomic analyses. However, only mild tolerance to various antibiotics
was observed, possibly due to reduced membrane permeability and biofilm
formation. After TCS exposure ceased, the adapted cells showed persistent
resistance to TCS due to inheritable genetic mutations, whereas their
antibiotic tolerance declined over time. Our results suggest that
extensive use of TCS may promote the evolution and persistence of
TCS-resistant bacterial pathogens. A quantitative definition of the
conditions under which TCS selects for multidrug resistance in the
environment is crucially needed.