Induction of inflammatory response in human cell lines by arsenic-contaminated soil-isolated bacterium Micrococcus sp. KUMAs15
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An arsenic-resistant microbial strain, Micrococcus sp. KUMAs15 isolated from West Bengal, India, has demonstrated high resistance to arsenic due to its arsenic accumulation and adsorption ability, establishing the strain as a potential arsenic bioremediation candidate for arsenic-contaminated niche. The successful field application of the microbe necessitates evaluation of probable immunotoxicological reactions on human cells. The present study determines expression profiles of pro-inflammatory and anti-inflammatory cytokines in cells exposed to KUMAs15.
The present study explored the alterations in expressions of the pro-inflammatory and anti-inflammatory cytokines in two human cell lines exposed to KUMAs15. The expression profile of the cytokine genes demonstrated that Micrococcus sp. KUMAs15 does not significantly induce inflammatory effects in these human cell lines. The upregulated expression of IL-8 and downregulated expression of IL-6 were observed in HaCaT. The HepG2 have shown downregulated IL-12 gene expression. These observations indicate the non-pathogenicity of KUMAs15 on the human cell lines.
The observations from the study extend the applicability of the arsenic-resistant Micrococcus sp. KUMAs15 for environmental arsenic decontamination. The isolate KUMAs15 was observed to be non-pathogenic to the human cell lines, as the strain does not initiate inflammatory reactions in these cell lines.
KeywordsMicrococcus Cytokines Pathogenicity
Analysis of variance
Basic Local Alignment Search Tool
Dulbecco’s modified Eagle’s medium
Granulocyte-macrophage colony-stimulating factor
Minimum essential medium
Phosphate buffer saline
Polymerase chain reaction
Reverse transcriptase PCR
Tumor necrosis factor
Arsenic (As) is a naturally occurring metalloid often considered as one of the most hazardous chemical pollutants in the contemporary scientific era. The As contamination is now being appropriately called the largest mass poisoning in human history . Arsenic, the most abundant hazardous toxic metalloid , is widely distributed in the lithosphere, hydrosphere, and biosphere since the ancient geological period. The anthropogenic activities, together with the spontaneous geological activities like volcanic eruptions and weathering, are responsible for the dissipation of this metalloid throughout the globe . The As removal technologies, combining chemical and biological approaches, are changing the scientific landscape for environmental As decontamination. The microbe-assisted remediation for As-contaminated systems are making significant progress towards the development of sustainable technologies that are applicable to different agro-ecological regions [4, 5].
Microbes possess resistance adaptations to resist high concentrations of heavy metals present in its ecological niche [6, 7, 8]. The metal tolerance adaptations of bacteria have potentialities for heavy metal bioremediation [9, 10] in different environments. Despite tremendous potentialities for microbial bioremediation, earlier studies reported that heavy metal-tolerant bacteria could be pathogenic to humans . One of the major concerns in the field application of the As-resistant microbial strains for As decontamination of the environment is the probable pathogenic response with other organisms, especially to humans. Thus, for sustainable microbe-assisted bioremediation management, knowledge on probable toxicological or immunogenic reactions with the microbial strain and human cells should be elucidated to establish the isolated microbial strains as non-pathogenic before their application in As-contaminated fields where they might interact with humans.
A highly As-resistant microbial isolate has been previously reported from our laboratory. The microbial isolate is designated as KUMAs15 identified as a strain of Micrococcus  following 16SrDNA sequencing and analysis of homology by the NCBI BLAST function  and Ribosomal Database Project . The isolated microbial strain has shown arsenite [As(III)] oxidation capability, by accelerating the spontaneous conversion of As (III) to a less toxic form arsenate [As(V)]  which could be removed from water with ease due to its lower solubility than As (III) [16, 17]. The microbe Micrococcus sp. KUMAs15 could also remove As from the culture medium in a laboratory condition by surface adsorption as well as arsenic accumulation at the environmentally relevant concentration of As; the results have been earlier reported . The present study aimed to validate the possible pathogenic effect of the isolated arsenic-decontaminating bacterium on human cell lines.
Cell culture and co-culture with KUMAs15
The present study included two adherent human cell lines, keratinocytes (HaCaT) and liver epithelial cells (HepG2), obtained from the National Centre for Cell Science, Pune, India. The HaCaT cell line was cultured in Dulbecco’s modified Eagle’s medium (DMEM; HiMedia) supplemented with high glucose, sodium pyruvate, and 10% fetal bovine serum (FBS; Gibco, USA), while the HepG2 cell lines were cultured in Eagle’s minimum essential medium (MEM; HiMedia) supplemented with 10% FBS. The cells were inoculated without antibiotics to prevent interference in microbial culture in co-culture experiments in subsequent assays. The cells were maintained at 37 °C with 5% CO2 in a standard CO2 incubator (Thermo Scientific, USA). The present study has been waived from ethical permissions according to the guidelines of the Indian Council of Medical Research (ICMR) for the biomedical research studies conducted in India .
Cellular toxicity assay
The cytotoxicity of Micrococcus sp. KUMAs15 on the HaCaT and HepG2 cells was determined by the MTT assay for the analysis of cellular viability. In the experiment, HaCaT and HepG2 cell lines treated only with PBS served as the negative control, whereas cells treated with Triton-X served as the positive control.
The inflammatory reactions of the HaCaT and HepG2 after the exposure to KUMAs15 were determined by cellular cytokine assay. ELISA analysis of the pro-inflammatory cytokines TNF-α, IL-8, IL-2, IL-6, IL-10, and IL-12p70 in the human cell lines HaCaT and HepG2 exposed to isolated Micrococcus sp. KUMAs15 was performed in a 4-h and 24-h time span. The PBS-treated human cell line not exposed to KUMAs15 served as the negative control group whereas cells co-cultured with E. coli served as the positive control due to its lipopolysaccharides, which elicit a cytokine response in these cell lines .
Transcriptional analysis of cytokine genes
The transcriptional expression of cytokine genes have been determined by semi-quantitative reverse transcriptase PCR. The HaCaT and HepG2 cells have been cultured and subjected to treatment with either PBS, served as the negative control, or the Micrococcus sp. KUMAs15 and have been incubated overnight. Total RNA isolation was carried out using Trizol reagent (Invitrogen, USA) following the standard protocol  and was eluted with 20 μL of RNase-free water (Thermo Scientific, USA) and quantified at 260 nm (Evolution 201 UV-Visible Spectrophotometer, Thermo Scientific, USA). The cDNA has been synthesized by reverse transcription, using the total RNA as the template with the RevertAidTM First Strand cDNA Synthesis Kit (Thermo Scientific, USA) following the manufacturer’s protocol, followed by the PCR amplification of the genes of interest using the cDNA as the template and appropriate primers  and followed by the agarose gel electrophoresis. The constitutive expression of the housekeeping gene, β-actin, was used as the loading control in this experiment.
Translational analysis of cytokine genes
The expressional analysis at the transcriptional level of cytokine genes was further confirmed at the protein level by western blot analysis. The HaCaT and HepG2 were cultured and exposed to the isolated Micrococcus sp. KUMAs15 for 24 h followed by protein isolation and western blotting.
The statistical analysis was performed using GraphPad Prism 6 software. The GraphPad algorithm has been used to perform one-way ANOVA followed by Tukey’s multiple comparison tests. All the experiments were carried out in triplicate, and p < 0.05 was considered as statistically significant.
Cellular toxicity assay
Cytokine profile of the human cell lines
Transcriptional analysis of cytokine genes
Translational analysis of cytokine genes
The eco-friendly microbe-mediated arsenic bioremediation demands suitable microbial candidates for successful field administration with optimum safety to the ecologically interacting organisms, most importantly humans. The essential criteria for any microbial candidate for sustainable heavy metal decontamination are the cytotoxic or immunogenic non-reactivity of the isolated microbial candidate with the human cells . The present study dealt with the effect of isolated microbial strain Micrococcus sp. KUMAs15 on human keratinocytes, HaCaT, and liver epithelial cells, HepG2. The characterization of the isolated microbial strain, KUMAs15, has been earlier reported , which also established Micrococcus sp. KUMAs15 as a potential candidate for environmental decontamination of arsenic.
An initial step in understanding the effect of the microbial strain KUMAs15 on the human cells is to examine its cytotoxic or inflammatory effect on the cell lines, representative of human tissues. The cell lines were initially incubated with KUMAs15 followed by analysis of the change in cell viability and inflammatory responses. Following the cytotoxicity assay to ascertain cellular viability, cytokine assay was performed on two human cell lines namely human keratinocyte (HaCaT) and human liver epithelial cell line (HepG2) to ascertain the role of the isolated strain in these cells for finding the suitability of the isolated strain for sustainable microbial bioremediation.
Application of the bacterial isolate to decontaminate ambient arsenic contamination could be limited due to any possible health concern that may emerge during field application if the bacteria are pathogenic to humans . To gain knowledge on toxicological or immunogenic reactions of KUMAs15 exposure, immune-toxicological parameters were studied after the exposure of the isolated microbial strain Micrococcus sp. KUMAs15 on human cell lines. The present study demonstrated that Micrococcus sp. KUMAs15 did not have any significant toxic effect on the viability of the human cells, when compared to the viability of the PBS-treated control cells assessed by the MTT assay (Fig. 1). The human cell lines were directly exposed to the isolates bacteria Micrococcus sp. KUMAs15 in culture condition and then removed by washing as well as antibiotic treatment. The bacterium is completely safe as confirmed by the assay as no increase in cellular apoptosis or necrosis was observed after exposure to the isolated strain. To maintain consistency, the cytokine profile of the cell line after exposure to the isolate was also measured by ELISA; the observation (Fig. 2) was further confirmed by gene expression profiling at the RNA and protein level by RT-PCR (Figs. 3 and 4) and western blotting (Figs. 5 and 6) respectively. The cytokine gene expression shows no alteration in TNF, IL-10, and IL-8 levels. This is due to the suppressive effect of IL-10 on pro-inflammatory cytokine responses [23, 24] in the HepG2 cell line, while the HaCaT cells showed no alteration in TNF expression earlier reported to be induced by GMCSF . It is well evident that exposure to Micrococcus sp. KUMAs15 did not provoke a sustained elevation of any of the inflammatory cytokines in the human cell lines studied. The present observations bring us to the inference that arsenic-resistant Micrococcus sp. KUMAs15 seems to be inherently non-pathogenic to mammalian cells as it does not induce a robust immunogenic response in the human cell lines. The immunotoxicological feature of the isolated strain does not show any alteration in the cytokine profile in human cells in vitro. These results indicate that Micrococcus sp. KUMAs15 is a non-pathogenic strain and along with its potential could be used as a potential arsenic-decontaminating agent.
The cytotoxicity assay, cytokine assay, and gene expression studies of cytokine genes at the transcriptional and translational levels indicate the non-pathogenic nature of the Micrococcus sp. KUMAs15 on the human cell lines at culture conditions. Thus, the present work could be considered as an extension of the previous findings of our laboratory, indicating the extent of safety in applying the Micrococcus sp. KUMAs15 in fields, by establishing it as a non-pathogenic microbial candidate for sustainable As bioremediation.
TP carried out the entire study. TP and SKM participated in the design of the study and performed the statistical analysis. TP and SKM conceived the study, participated in its design and coordination, and helped to draft the manuscript. All authors read and approved the final manuscript.
The study was supported by the financial grant received from the extramural project of the Department of Biotechnology (DBT) under the Government of India (No. BT/PR4693/BCE/8/894/2012). The abovementioned project supports all the experimentations and the stipend.
Ethics approval and consent to participate
The present study has been waived from ethical permissions according to the guidelines of the Indian Council of Medical Research (ICMR) for the biomedical research studies conducted in India.
Consent for publication
The authors declare that they have no competing interests.
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