Natural and Engineered Halloysite Clay Interact with Bacteria in a Double-Edged Manner

In this paper, we investigated the behavior of a natural, low-cost, and biocompatible clay, focusing on its potential use in biomedical applications, with an eye on its ability as a material that inhibits or promotes bacterial growth. The interaction of raw and acid-treated halloysite (HT) with Gram-positive and Gram-negative bacteria representative of different environments, such as the human body, food, air, soil, water, and marine environments, was explored. Environmental strains of <i>Escherichia coli</i>, <i>Acinetobacter baumannii</i>, <i>Lactococcus lactis</i>, and <i>Staphylococcus aureus</i> were isolated and examined for their responses to HT and its derivatives after acid treatment, including acidic HT (HT (H⁺)), precipitate (P), and supernatant (S). HT before and after acid treatment did not have any effect on the growth of this subset of opportunistic bacteria that mainly inhabit air and water. Bacteria of marine origin (<i>Vibrio</i> spp and <i>Halomonas</i> spp) were isolated from the body lesions of a spotted diseased sea urchin, <i>Paracentrotus lividus</i><i>.</i> These species were highly sensitive to the material tested, showing an opposite survival response under treatment with the raw or the acidic HT forms. Materials were fully characterized by scanning and transmission electron microscopy (SEM and TEM) and X-ray photoelectron spectroscopy (XPS). The responses of marine bacteria exposed to HT and its derivatives were dependent on their structural and physicochemical properties, as elucidated here.