Should Escherichia coli K-12 substrain MG1655 be classified as NaCl resistant?

Abstract Extreme environments are defined by conditions that challenge cellular machinery, often compromising survival and biological function. Halophiles, a subclass of extremophiles, thrive in high sodium chloride (NaCl) concentrations. However, current knowledge on salt stress resistance is largely derived from studies on extreme halophiles, even though most halophilic microorganisms are classified as slight or moderate halophiles. This fact poses a question regarding the real representation of state-of-the-art information about salt resistance mechanisms and diversity. To bring light to the problem of the lack of information on slight halophiles, we propose the study of Escherichia coli str. K-12 substr. MG1655 to fill the gap. We evaluated the response of E. coli MG1655 to varying NaCl concentrations, including control without NaCl addition. Our results indicate optimal growth at NaCl concentrations up to 0.5 mol·L⁻¹, suggesting that this strain should be classified as a slight halophile, in contrast to its current classification in the literature. Thus, we propose the study of this strain to understand the molecular mechanisms underlying adaptation to low and moderate salt stress.