Cd2+ tolerance and removal mechanisms of Serratia marcescens KMR-3.

Heavy metal contamination is a global issue, with cadmium (Cd2+) and its treatment becoming major environmental challenge that could be solved by microbial restoration, an eco-friendly technique. Serratia marcescens KMR-3 exhibits high tolerance and removal rate of Cd2+ (≤500 mg/L). Here, we aimed to explore mechanisms underlying tolerance to and removal of Cd2+ by KMR-3. Scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectrometry were conducted to analyze characteristics of the KMR-3 biofilm and Cd2+ combined forms. The results revealed varying degrees of cell adhesion, membrane thickening, and shrinkage on the surface of the bacteria. The binding elements, electronic binding energy, and functional groups on the surface of the bacteria exhibited changes. Furthermore, the biofilm amount following treatment with Cd2+ was 1.5-3 times higher than that in the controls, treatment with Cd2+ substantially enhanced biofilm generation and increased Cd2+ adsorption. Cd2+ adsorption by its own secondary metabolite prodigiosin produced by KMR-3 was enhanced by 19.5 % compared with that observed without prodigiosin. Through transcriptome sequencing and RT-qPCR, we observed that Znu protein-chelating system regulated gene expression (znuA, znuB, and znuC), and the efflux mechanism of the P-type ATPase regulated the expression of genes (zntA, zntB, and zntR), which were significantly enhanced. Through the combined action of various strategies, KMR-3 demonstrated a high tolerance and removal ability of Cd2+, providing a theoretical basis to treat Cd2+ pollution.

Mechanisms underlying the inhibitory effects of Cd2+ on prodigiosin synthesis in Serratia marcescens KMR-3.

Prodigiosin (2-methyl-3-pentyl-6-methoxyprodiginine), a red-colored microbial pigment, is produced in large quantities by Serratia marcescens KMR-3. This bacterium can grow in a medium with a Cd2+ concentration of 500 mg/L, but it does not produce prodigiosin when the Cd2+ concentration in the medium is higher than 140 mg/L. Therefore, we investigated the mechanisms by which Cd2+ inhibits prodigiosin synthesis. Upon addition of Cd2+ to the medium, the expression of the prodigiosin (pig) gene cluster was significantly downregulated. Simultaneously, genes encoding proteins related to the synthesis of arginine and proline(prodigiosin precursors) were significantly downregulated, while the degradation-related genes were upregulated. Furthermore, PigF, which encodes a key enzyme involved in the synthesis of 4-methoxy-2,2'-bipyrrole-5-carboxaldehyde and PigC, which encodes a key enzyme involved in the last step of prodigiosin synthesis, were downregulated by 80% and 55%, respectively, following Cd2+ treatment. As PigC and PigF are located on the cell membrane and are involved in the final steps of prodigiosin synthesis, the cell membrane might be presumed to be the site of prodigiosin synthesis. The bacterial membrane exhibited different degrees of elongation, folding, fragmentation, and sagging after the addition of Cd2+, while likely destroying the site of prodigiosin synthesis.

Autoinducer-2-mediated quorum-sensing system resists T4 phage infection in Escherichia coli.

In response to the restriction of nutrients and predation by natural enemies, bacteria have evolved complex coping strategies to ensure the reproduction and survival of individual species. Quorum sensing (QS) is involved in the bacterial response to phage predation and regulation of cellular metabolism. However, to date, no clear evidence exists regarding the involvement of autoinducer-2 (AI-2)-mediated QS systems in Escherichia coli in response to the challenges of nutrient restriction and phage infection. In this study, the role of the AI-2-mediated QS system in resisting T4 phage infection and regulating cell mechanisms in E. coli was revealed for the first time. This effect of the AI-2-mediated QS was achieved by simultaneously downregulating the T4 absorption site and carbon and glucose metabolism. Additionally, we found that lsrB, a metabolic brake, participates in AI-2-mediated regulation and maintenance of the normal metabolic balance of cells. The novel phage defense strategy and regulation and maintenance of cellular metabolism effectively limited the expansion of the phage population.

Serratia marcescens-derived fluorescent carbon dots as a platform toward multi-mode bioimaging and detection of p-nitrophenol.

Carbon dots (CDs) have excellent application prospects in various fields such as fluorescent dyes, but expanding their application, especially in bioimaging and the detection of organic pollutants, is still a major research objective. In this study, fluorescent CDs were successfully synthesized via the hydrothermal method using Serratia marcescens KMR-3. The platform based on CDs-KMR3 exhibited excellent stability, good biocompatibility, and low biotoxicity, and can be effectively applied to the imaging of bacteria, fungi, plant cells, protozoa and mammalian cells, and can specifically stain the membranes of all tested cells. In this study, for the first time, bacteria-derived CDs were used to image the representative species of organisms ranging from lower-order to higher-order organisms, thereby proving the feasibility of the application of CDs in the fluorescence imaging of Paramecium caudatum. Additionally, CDs-KMR3 can rapidly diffuse into all the parts of the leaf through diffusion into the veins and intercellular interstitium in response to the induction of transpiration. Moreover, the data illustrate that CDs-KMR3 are likely to enter the digestive tracts of microworms by ingestion through the oral cavity and pharynx, and spread to the pseudocoelom and somatic cells, and finally to be excreted from microworms through the anus. Furthermore, this platform can be utilized as fluorescent probes for the rapid and highly selective detection of p-nitrophenol (p-NP). Moreover, this study contributed to the increased application of bacteria-derived CDs in bioimaging and detection of p-NP.