Date Published: July 17, 2017
Publisher: Public Library of Science
Author(s): Dali Wang, Zhifen Lin, Ting Wang, Xiruo Ding, Ying Liu, Anderson de Souza Sant’Ana.
Hormesis has aroused much attention during the past two decades and may have great implications on many fields, including toxicology and risk assessment. However, the observation of hormesis remains challenged under laboratory conditions. To determine favorable conditions under which to observe hormesis, we investigated the hormetic responses of Escherichiacoli (E. coli) upon exposure of different concentrations of sulfonamides and erythromycin at different time points and in different culture media: Luria-Bertani (LB) broth and Mueller Hinton (MH) broth. Our results reveal that the antibiotics, both individually and combined, produce hormetic effects on E. coli growth in MH broth at the stationary phase, with the maximum stimulatory response increasing with time. However, in LB broth, the hormetic response was not observed, which can be explained by an analogous “wood barrel theory”. Our study suggests that the culture medium and time should be taken into consideration in hormetic studies, and compound mixtures should also receive more attention for their potential to induce hormesis.
The dose-response relationship is the most fundamental concept in toxicology, and environmental risk assessment is conducted based on this relationship . For a long time, threshold and linear non-threshold (LNT) models have been applied in risk assessment for non-carcinogens and carcinogens, respectively . However, the two traditional models have been challenged with an alternative model that has aroused many concerns over the past two decades . This dose-response model is defined as hormesis, which refers to a biphasic dose-response relationship with low dose stimulation and high dose inhibition . Studies [5,6] have indicated that hormesis can better predict low-dose responses, and the hormetic dose-response model is more common than the threshold model in toxicology. Calabrese  has argued that the hormetic dose response relationship should be the default model in risk assessment for non-carcinogens and especially for carcinogens. Therefore, studies on hormesis may have significant implications for the practice of risk assessment.
In this paper, we investigated SAs, Ery, and their mixtures for their hormetic effects on E. coli under various culture conditions and at different times. It was found that hormesis of the antibiotics, both individually and in combination, only occurred in MH broth at the stationary phase, with the maximum stimulatory response increasing with time. The hormetic effects on E. coli growth were likely due to adaptive effects induced by the antibiotics. The maximum stimulatory response caused by the mixture was comparable to that caused by the individual component. Our study suggests that the culture conditions and time should be taken into consideration in toxicological studies, especially in studies on hormesis, and combination treatments should receive more attention for their potential to induce hormetic effects on organisms.