Resistance to heavy metals and chromium reduction by bacteria isolated from air.

Abstract

Environmental pollution by heavy metals is a major global concern, necessitating the exploration of sustainable bioremediation strategies. Airborne bacteria represent an underexplored resource in this context. This study investigated the potential of bacteria isolated from bioaerosols for the bioremediation of heavy metals. Nine bacterial strains belonging to the genera Exiguobacterium, Kocuria, Rhodococcus, and Staphylococcus were isolated and identified through MaLDI-TOF analysis and 16S rRNA gene sequencing. The minimum inhibitory concentrations (MIC) of chromium, copper, lead, nickel, mercury, and cadmium were determined to evaluate metal resistance. Bioreduction assays were performed to determine the capacity of selected strains to reduce hexavalent chromium [Cr(VI)] in solution. Chromate reductase activity was quantified in Rhodococcus rhodochrous As33. Morphological responses to chromium exposure were examined using scanning and transmission electron microscopy (SEM and TEM). MIC analyses revealed variable but significant resistance to multiple Heavy metals among the isolates. Bioreduction assays demonstrated that five selected strains reduced from 79.9% to 100% of Cr(VI) within 72 h. R. rhodochrous As33 achieved complete Cr(VI) reduction, as confirmed by 1,5-diphenylcarbazide complexation, and inductively coupled plasma mass spectrometry (ICP-MS). Enzymatic analysis indicated a chromate reductase activity of 67.87 U mg⁻¹ of total protein in this strain. SEM and TEM revealed marked cellular adaptations to chromium stress, including pleomorphism, membrane thinning, vesicle formation, and the deposition of extracellular electron-dense precipitates, suggesting active biosorption and bioprecipitation mechanisms. The results highlight the bioremediation potential of airborne bacteria, particularly R. rhodochrous As33, in heavy metal-contaminated environments. Further studies are needed to validate their performance under complex environmental conditions and to support their application in sustainable remediation strategies.