Siderophore-Mediated Iron Acquisition Plays a Critical Role in Biofilm Formation and Survival of Staphylococcus epidermidis Within the Host

Synthesis and antibacterial activities of baulamycin A inspired derivatives

SbnE is an essential enzyme for staphyloferrin B biosynthesis in Staphylococcus aureus. An earlier study showed that natural product baulamycin A has in vitro inhibitory activity against SbnE and antibacterial potency. A SAR study with analogues of baulamycin A was conducted to identify potent inhibitors of SbnE and/or effective antibiotics against MRSA. The results show that selected analogues, including 11, 18, 21, 24a, 24c, 24m and 24n, exhibit single-digit micromolar inhibitory potencies against SbnE (IC50s = 1.81-8.94 μM) and 11, 24m, 24n possess significant activities against both SbnE (IC50s = 4.12-6.12 μM) and bacteria (MICs = 4-32 μg/mL). Biological investigations revealed that these substances possess potent cell wall disruptive activities and that they inhibit siderophore production in MRSA. Among the selected analogues, 7 has excellent antibiotic activities both gram-positive and -negative bacteria (0.5-4 μg/mL). Moreover, these analogues significantly impede biofilm formation in a concentration-dependent manner. Taken together, the results of the investigation provide valuable insight into the nature of novel baulamycin A analogues that have potential efficacy against MRSA owing to their membrane damaging activity and/or inhibitory efficacy against siderophore production.

Antibacterial MccM as the Major Microcin in Escherichia coli Nissle 1917 against Pathogenic Enterobacteria

Probiotic Escherichia coli Nissle 1917 (EcN) possesses excellent antibacterial effects on pathogenic enterobacteria. The microcins MccM and MccH47 produced in EcN played critical roles, but they are understudied and poorly characterized, and the individual antibacterial mechanisms are still unclear. In this study, three EcN mutants (ΔmcmA, ΔmchB, and ΔmcmAΔmchB) were constructed and compared with wild-type EcN (EcN wt) to test for inhibitory effects on the growth of Escherichia coli O157: H7, Salmonella enterica (SE), and Salmonella typhimurium (ST). The antibacterial effects on O157: H7 were not affected by the knockout of mcmA (MccM) and mchB (MccH47) in EcN. However, the antibacterial effect on Salmonella declined sharply in EcN mutants ΔmcmA. The overexpressed mcmA gene in EcN::mcmA showed more efficient antibacterial activity on Salmonella than that of EcN wt. Furthermore, the EcN::mcmA strain significantly reduced the abilities of adhesion and invasion of Salmonella to intestinal epithelial cells, decreasing the invasion ability of ST by 56.31% (62.57 times more than that of EcN wt) while reducing the adhesion ability of ST by 50.14% (2.41 times more than that of EcN wt). In addition, the supernatant of EcN::mcmA culture significantly decreased the mRNA expression and secretion of IL-1β, TNF-α, and IL-6 on macrophages induced by LPS. The EcN::mcmA strain generated twice as much orange halo as EcN wt by CAS agar diffusion assay by producing more siderophores. MccM was more closely related to the activity of EcN against Salmonella, and MccM-overproducing EcN inhibited Salmonella growth by producing more siderophores-MccM to compete for iron, which was critical to pathogen growth. Based on the above, EcN::mcmA can be developed as engineered probiotics to fight against pathogenic enterobacteria colonization in the gut.

Pseudomonas aeruginosa responds to altered membrane phospholipid composition by adjusting the production of two-component systems, proteases and iron uptake proteins

Membrane protein and phospholipid (PL) composition changes in response to environmental cues and during infections. To achieve these, bacteria use adaptation mechanisms involving covalent modification and remodelling of the acyl chain length of PLs. However, little is known about bacterial pathways regulated by PLs. Here, we investigated proteomic changes in the biofilm of P. aeruginosa phospholipase mutant (∆plaF) with altered membrane PL composition. The results revealed profound alterations in the abundance of many biofilm-related two-component systems (TCSs), including accumulation of PprAB, a key regulator of the transition to biofilm. Furthermore, a unique phosphorylation pattern of transcriptional regulators, transporters and metabolic enzymes, as well as differential production of several proteases, in ∆plaF, indicate that PlaF-mediated virulence adaptation involves complex transcriptional and posttranscriptional response. Moreover, proteomics and biochemical assays revealed the depletion of pyoverdine-mediated iron uptake pathway proteins in ∆plaF, while proteins from alternative iron-uptake systems were accumulated. These suggest that PlaF may function as a switch between different iron-acquisition pathways. The observation that PL-acyl chain modifying and PL synthesis enzymes were overproduced in ∆plaF reveals the interconnection of degradation, synthesis and modification of PLs for proper membrane homeostasis. Although the precise mechanism by which PlaF simultaneously affects multiple pathways remains to be elucidated, we suggest that alteration of PL composition in ∆plaF plays a role for the global adaptive response in P. aeruginosa mediated by TCSs and proteases. Our study revealed the global regulation of virulence and biofilm by PlaF and suggests that targeting this enzyme may have therapeutic potential.

Involvement of the Iron-Regulated Loci hts and fhuC in Biofilm Formation and Survival of Staphylococcus epidermidis within the Host

Staphylococcus epidermidis is a major nosocomial pathogen with a remarkable ability to persist on indwelling medical devices through biofilm formation. Nevertheless, it remains intriguing how this process is efficiently achieved under the host's harsh conditions, where the availability of nutrients, such as essential metals, is scarce. Following our previous identification of two iron-regulated loci putatively involved in iron transport, hts and fhuC, we assessed here their individual contribution to both bacterial physiology and interaction with host immune cells. Single deletions of the hts and fhuC loci led to marked changes in the cell iron content, which were partly detrimental for planktonic growth and strongly affected biofilm formation under iron-restricted conditions. Deletion of each of these two loci did not lead to major changes in S. epidermidis survival within human macrophages or in an ex vivo human blood model of bloodstream infection. However, the lack of either hts or fhuC loci significantly impaired bacterial survival in vivo in a murine model of bacteremia. Collectively, this study establishes, for the first time, the pivotal role of the iron-regulated loci hts and fhuC in S. epidermidis biofilm formation and survival within the host, providing relevant information for the development of new targeted therapeutics against this pathogen. IMPORTANCE Staphylococcus epidermidis is one of the most important nosocomial pathogens and a major cause of central line-associated bloodstream infections. Once in the bloodstream, this bacterium must surpass severe iron restriction in order to survive and establish infection. Surprisingly, very little is known about the iron acquisition mechanisms in this species. This study represents the first report on the involvement of the S. epidermidis iron-regulated loci hts and fhuC in biofilm formation under host relevant conditions and, most importantly, in survival within the host. Ultimately, these findings highlight iron acquisition and these loci in particular, as potential targets for future therapeutic strategies against biofilm-associated S. epidermidis infections.

Fighting Staphylococcus epidermidis Biofilm-Associated Infections: Can Iron Be the Key to Success?

Staphylococcus epidermidis is one of the most important commensal microorganisms of human skin and mucosae. However, this bacterial species is also the cause of severe infections in immunocompromised patients, specially associated with the utilization of indwelling medical devices, that often serve as a scaffold for biofilm formation. S. epidermidis strains are often multidrug resistant and its association with biofilm formation makes these infections hard to treat. Their remarkable ability to form biofilms is widely regarded as its major pathogenic determinant. Although a significant amount of knowledge on its biofilm formation mechanisms has been achieved, we still do not understand how the species survives when exposed to the host harsh environment during invasion. A previous RNA-seq study highlighted that iron-metabolism associated genes were the most up-regulated bacterial genes upon contact with human blood, which suggested that iron acquisition plays an important role in S. epidermidis biofilm development and escape from the host innate immune system. In this perspective article, we review the available literature on the role of iron metabolism on S. epidermidis pathogenesis and propose that exploiting its dependence on iron could be pursued as a viable therapeutic alternative.