Role of
Tetracycline Speciation in
the Bioavailability to <i>Escherichia
coli</i> for Uptake and Expression of Antibiotic Resistance
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Abstract
Tetracycline contains
ionizable functional groups that manifest
several species with charges at different locales and differing net
charge; the fractional distribution of each species depends on pH-p<i>K</i><sub>a</sub> relationship in the aqueous phase. In nature,
these species interact with naturally abundant cations (e.g., Ca<sup>2+</sup> and Mg<sup>2+</sup>) to form metal-tetracycline complexes
in water. In this study, we used <i>Escherichia coli</i> MC4100/pTGM whole-cell bioreporter to investigate tetracycline uptake
from solution under varying conditions of pH, salt composition and
concentration by quantifying the corresponding expression of antibiotic
resistance gene. The expression of antibiotic resistance gene in the <i>E. coli</i> bioreporter responded linearly to intracellular
tetracycline concentration. Less tetracycline entered <i>E. coli</i> cells at solution pH of 8.0 than at pH 6.0 or 7.0 indicating reduced
bioavailability of the antibiotic at higher pH. Both Mg<sup>2+</sup> and Ca<sup>2+</sup> in solution formed metal-tetracycline complexes
which reduced uptake of tetracycline by <i>E. coli</i> hence
diminishing the bioresponse. Among the various tetracycline species
present in solution, including both metal-complexed and free (noncomplexed)
species, zwitterionic tetracycline was identified as the predominant
species that most readily passed through the cell membrane eliciting
activation of the antibiotic resistance gene in <i>E. coli</i>. The results indicate that the same total concentration of tetracycline
in ambient solution can evoke very different expression of antibiotic
resistance gene in the exposed bacteria due to differential antibiotic
uptake. Accordingly, geochemical factors such as pH and metal cations
can modulate the selective pressure exerted by tetracycline for development
and enrichment of antibiotic resistant bacteria. We suggest that tetracycline
speciation analysis should be incorporated into the risk assessment
framework for evaluating environmental exposure and the corresponding
development of antibiotic resistance