Time-dependent hormetic dose responses of skin care product mixtures to Vibrio qinghaiensis sp.-Q67: Appearance and quantification

Hormesis is a widely recognized and extensively studied phenomenon. However, few studies have described the quantitative characteristics of hormesis required for appropriate risk assessment. Although skin care product (SCP) mixtures and their active ingredients can induce the hormesis of Vibrio qinghaiensis sp.-Q67 (Q67), the quantitative characteristics of time-dependent hormetic dose responses in SCPs have not yet been investigated. In this study, 28 SCP mixtures were tested for luminescence toxicity against Q67 after five exposure durations (0.25, 3, 6, 9, and 12 h). With increasing exposure duration, the concentration response curves (CRCs) were classified as constant monotonic nonlinear (S-shaped) for four SCPs, S- to hormetic (J-shaped) for 13 SCPs, and constant J-shaped for 11 SCPs. Of 140 CRCs, 98 were J-shaped. An increased frequency of SCPs inducing hormesis was observed. The toxicity (pEC50) of the SCPs was independent of the exposure duration and product type. The maximum stimulatory effect (Emin) of the 12 SCPs increased with exposure duration. We proposed a modified parameter, the width of inhibition dose zone (WIDZ; EC50/EC10), to depict the width of inhibition dose zone. The WIDZ of S-shaped CRCs were significantly larger than that of J-shaped CRCs. In addition, the characteristic parameters reported in the general literature were analyzed. The good linear relationship between EC50 and the maximum stimulatory effective concentration (ECmin) indicated that toxicity may be transformed into stimulatory effects over exposure durations. The width of stimulation dose zone (WSDZ) and Emin of the seven SCPs had the same increasing trends with increasing exposure duration. The combination of WIDZ with other characteristic parameters (e.g., zero effective concentration point, ECmin, etc.) could better depict hormesis with low-dose stimulation and high-dose inhibition. The quantitative characteristics of the dose-responses of hormesis-inducing SCPs could provide reference basis for the risk assessment of SCP mixtures.

Pollen biology and hormesis: Pollen germination and pollen tube elongation

This paper evaluated the occurrence of hormetic dose responses in pollen reported over the past eight decades. Hormetic doses responses were induced by a wide range of chemical and physical agents in 34 plant species for pollen germination and pollen tube growth/elongation. Agents inducing such hormetic dose/concentration responses in pollen included nutrients, growth-promoting agents, plant and animal hormones, toxic substances, including heavy metals such as cadmium, gaseous pollutants such as ozone, as well as ionizing and non-ionizing radiation. This paper provides further evidence for the broad generality of the hormesis dose response, supporting substantial prior findings that the hormetic response is independent of biological model, inducing agent, and endpoints measured. Given the widespread potential of inducing hormetic dose responses in pollen, these findings indicate the need to explore their emerging biological, ecological, agricultural, economic and public health implications.

Bt-induced hormesis in Bt-resistant insects: Theoretical possibility or factual concern?

The biphasic dose-response of a stressor where low amounts of a toxicant may stimulate some biological processes is a recent focus of attention in insecticide ecotoxicology. Nonetheless, the importance and management consequences of this phenomenon of pesticide-induced hormesis remain largely unrecognized. Curiously, the potential induction of hormesis by insecticidal proteins such as Bacillus thuringiensis toxins (i.e., Bt toxins), a major agriculture pest management tool of widespread use, has been wholly neglected. Thus, we aimed to circumvent this shortcoming while assessing the potential occurrence of hormesis induced by the Bt toxin Cry1Fa in its main target pest species - the fall armyworm Spodoptera frugiperda. Concentration-response bioassays were carried out in a Bt-susceptible and a Bt-resistant population providing the purified Cry1Fa toxin in artificial diet and recording the insect demographic parameters. As significant hormetic effect was detected in both populations with a significant increase in the net reproductive rate and the intrinsic rate of population growth, the potential occurrence of Bt-induced hormesis was subsequently tested providing the insects with leaves from transgenic Bt maize expressing the toxic protein. The performance of the Bt-resistant insects was not different in both maize genotypes, indicating that the leaf expression of the Bt protein did not promote hormesis in the resistant insects. Thus, despite the Bt-induced hormesis detected in the purified protein bioassays, the phenomenon was not detected with current levels of Bt expression in maize minimizing the risk of this additional efficacy constraint besides that of field occurrence of Bt resistance.

Stress response and population dynamics: Is Allee effect hormesis?

Hormesis is a fundamental notion in ecotoxicology while competition between organisms is an essential notion in population ecology and species adaptation and evolution. Both sub-disciplines of ecology deal with the response of organisms to abiotic and biotic stresses. In ecotoxicology, the Linear-non-Threshold (LNT), Threshold and Hormetic models are used to describe the dominant responses of a plethora of endpoints to abiotic stress. In population ecology, the logistic, theta-logistic and the Allee effect models are used to describe the growth of populations under different responses to (biotic) stress induced by population density. The per capita rate of population increase (r) measures species fitness. When it is used as endpoint, the responses to population density seem to perfectly correspond to LNT, Threshold and Hormetic responses to abiotic stress, respectively. Our analysis suggests the Allee effect is a hormetic-like response of r to population density, an ultimate biotic stress. This biphasic dose-response model appears across different systems and situations (from molecules to tumor growth to population dynamics), is highly supported by ecological and evolutionary theory, and has important implications in most sub-disciplines of biology as well as in environmental and earth sciences. Joined multi-disciplinary efforts would facilitate the development and application of advanced research approaches for better understanding potential planetary-scale implications.

Low Doses of Tetracycline Trigger the E. coli Growth: A Case of Hormetic Response

Hormesis is a biphasic dose-response relationship, occurring when low concentrations of toxic agents elicit apparent improvements. In this work, the ability of sub-inhibitory concentrations of Tetracycline to induce hormetic response in a model organism was investigated. To this aim a reference strain of Escherichia coli, MG1655, was exposed to six decreasing doses of Tetracycline (between 0.12 and 0.00375 μg/ml), much lower than the Minimal Inhibitory Concentration (4 μg/ml). An hormetic increase was observed at the intermediate concentrations (0.015-0.03 μg/ml) of the tested range. The Colony Forming Unit number, indeed, rose up to 141% and 121% as compared to the control. At the highest (0.12 μg/ml) and lowest (0.00375 μg/ml) concentrations a slight decrease in CFU number was found. Results demonstrated that, in Escherichia coli, low concentrations of Tetracycline bias the bacterial numerical increase through a hormetic response; the dose-response curve describing this numerical increase is an U-inverted curve. Furthermore, these data confirm that hormesis is common to many - if not all - living systems, including bacteria; they underline the relevance of a deepened knowledge of both the effects and the possible consequences of exposure to low doses of contaminants.