The Role of Iron in Staphylococcus aureus Infection and Human Disease: A Metal Tug of War at the Host—Microbe Interface

Staphylococcus aureus: A Review of the Pathogenesis and Virulence Mechanisms

Staphylococcus aureus is a formidable human pathogen responsible for infections ranging from superficial skin lesions to life-threatening systemic diseases. This review synthesizes current knowledge on its pathogenesis, emphasizing colonization dynamics, virulence mechanisms, biofilm formation, and antibiotic resistance. By analyzing studies from PubMed, Scopus, and Web of Science, we highlight the pathogen's adaptability, driven by surface adhesins (e.g., ClfB, SasG), secreted toxins (e.g., PVL, TSST-1), and metabolic flexibility in iron acquisition and amino acid utilization. Nasal, skin, and oropharyngeal colonization are reservoirs for invasive infections, with biofilm persistence and horizontal gene transfer exacerbating antimicrobial resistance, particularly in methicillin-resistant S. aureus (MRSA). The review underscores the clinical challenges of multidrug-resistant strains, including vancomycin resistance and decolonization strategies' failure to target single anatomical sites. Key discussions address host-microbiome interactions, immune evasion tactics, and the limitations of current therapies. Future directions advocate for novel anti-virulence therapies, multi-epitope vaccines, and AI-driven diagnostics to combat evolving resistance. Strengthening global surveillance and interdisciplinary collaboration is critical to mitigating the public health burden of S. aureus.

Interpreting roles of mutations associated with the emergence of S. aureus USA300 strains using transcriptional regulatory network reconstruction

The Staphylococcus aureus clonal complex 8 (CC8) is made up of several subtypes with varying levels of clinical burden; from community-associated methicillin-resistant S. aureus USA300 strains to hospital-associated (HA-MRSA) USA500 strains and ancestral methicillin-susceptible (MSSA) strains. This phenotypic distribution within a single clonal complex makes CC8 an ideal clade to study the emergence of mutations important for antibiotic resistance and community spread. Gene-level analysis comparing USA300 against MSSA and HA-MRSA strains have revealed key horizontally acquired genes important for its rapid spread in the community. However, efforts to define the contributions of point mutations and indels have been confounded by strong linkage disequilibrium resulting from clonal propagation. To break down this confounding effect, we combined genetic association testing with a model of the transcriptional regulatory network (TRN) to find candidate mutations that may have led to changes in gene regulation. First, we used a De Bruijn graph genome-wide association study to enrich mutations unique to the USA300 lineages within CC8. Next, we reconstructed the TRN by using independent component analysis on 670 RNA-sequencing samples from USA300 and non-USA300 CC8 strains which predicted several genes with strain-specific altered expression patterns. Examination of the regulatory region of one of the genes enriched by both approaches, isdH, revealed a 38-bp deletion containing a Fur-binding site and a conserved single-nucleotide polymorphism which likely led to the altered expression levels in USA300 strains. Taken together, our results demonstrate the utility of reconstructed TRNs to address the limits of genetic approaches when studying emerging pathogenic strains.

Decoding growth inhibitory associated pathways of xenometal-siderophore antibiotic conjugates in S. aureus

Pathogenic Staphylococcus aureus causes most infectious disease related deaths in the developed world. Continuously evolving resistance to clinically approved antibiotics and combination therapies limits treatment efficacy; new strategies that evade and slow resistance or produce resistant mutants with reduced fitness are needed. We employ antibiotics conjugated to bacterially recognized siderophores to potentiate their efficacy. Acting as a Trojan horse, the siderophore antibiotic conjugates efficiently deliver the antibiotic inside the bacterial cytoplasm by hijacking the iron transport system pathways which are crucial for bacterial survival. Here, we investigated the mechanism of action of gallium xenometallomycins (siderophore antibiotic conjugates incorporating non-endogenous metal ions), Ga-DFO-Cip and Ga-LDFC-Cip, which have demonstrated high potency compared to the parent antibiotic's efficacy in vitro in S. aureus infection. Employing physicochemical, synthetic and transcriptomic analysis studies, this work reveals that kinetically inert, gallium-containing xenometallomycins targeting cytoplasmic bacterial targets impart differential resistance and gene expression profiles when compared to their parent antibiotic in S. aureus bacterial strains. Both Ga-DFO-Cip and Ga-LDFC-Cip effectively disrupt iron-siderophore biosynthesis and uptake machinery. We affirm our results with the radioactive surrogate 67/68Ga-DFO-Cip and demonstrate that the bacterial uptake in Ga-DFO-Cip-resistant S. aureus strains is impaired, leading to diminished compound accumulation in vitro and in vivo.

Enterobactin and salmochelin S4 inhibit the growth of Staphylococcus aureus

There is increasing demand for novel antimicrobial agents to tackle the antimicrobial resistance crisis. Here we report that two Enterobacteriaceae-produced siderophores, enterobactin and salmochelin S4, inhibit the growth of Staphylococcus aureus isolates, including methicillin-resistance S. aureus (MRSA) clinical isolates. The IC50 for different S. aureus isolates were 2-5 µM for salmochelin S4 and 5-10 µM for enterobactin. This inhibitory activity was partially repressed by adding Fe+3. These siderophores also inhibited the growth of Enterococcus strains, including vancomycin-resistant enterococci (VRE) clinical isolates, though less effectively than for S. aureus. The growth of various Gram-negative bacteria was barely affected by these siderophores. These results shed new light on the role of enterobactin and salmochelin in bacterial physiology and ecology and have potential for the development of novel strategies to combat the rapid rise of multidrug-resistant bacteria.

The Staphylococcus aureus non-coding RNA IsrR regulates TCA cycle activity and virulence

Staphylococcus aureus has evolved mechanisms to cope with low iron (Fe) availability in host tissues. Staphylococcus aureus uses the ferric uptake transcriptional regulator (Fur) to sense titers of cytosolic Fe. Upon Fe depletion, apo-Fur relieves transcriptional repression of genes utilized for Fe uptake. We demonstrate that an S. aureus Δfur mutant has decreased expression of acnA, which codes for the Fe-dependent enzyme aconitase. This prevents the Δfur mutant from growing with amino acids as sole carbon and energy sources. We used a suppressor screen to exploit this phenotype and determined that a mutation that decreases the transcription of isrR, which produces a regulatory RNA, increased acnA expression, thereby enabling growth. Directed mutation of bases predicted to facilitate the interaction between the acnA transcript and IsrR, decreased the ability of IsrR to control acnA expression in vivo and IsrR bound to the acnA transcript in vitro. IsrR also bound transcripts coding the alternate tricarboxylic acid cycle proteins sdhC, mqo, citZ and citM. Whole-cell metal analyses suggest that IsrR promotes Fe uptake and increases intracellular Fe not ligated by macromolecules. Lastly, we determined that Fur and IsrR promote infection using murine skin and acute pneumonia models.

Risk of Postoperative Infection in Total Knee Arthroplasty Patients with Preoperative Methicillin-Resistant Staphylococcus aureus (MRSA) Colonization

Background/Objectives: While methicillin-resistant Staphylococcus aureus (MRSA) colonization is a known risk factor for surgical site infections, no definitive recommendations exist regarding preoperative S. aureus screening and decolonization protocols due to inconclusive evidence in Orthopedic Surgery. This study aimed to examine the correlation between preoperative MRSA colonization and postoperative infections in Total Knee Arthroplasty (TKA) patients. Methods: Data from 2005 to 2023 were collected from TriNetX, a global health research network, reviewing 237,360 unique patients. TKA patients were assigned under Current Procedural Terminology, while International Statistical Classification of Diseases Codes were used to identify preoperative comorbidities and postoperative complications. Demographic and analytical statistics were compared between MRSA-positive and control groups before and after propensity matching. Results: The MRSA-positive group had a significantly (p < 0.001) higher proportion of patients over 65 years (47.17% vs. 38.46%), obesity (41.76% vs. 34.67%), smoking disorders (33.36% vs. 19.73%), and diabetes (25.06% vs. 19.85%) compared to the control group. Postoperative complications were significantly (p < 0.001) more frequent in the MRSA-positive group, specifically periprosthetic joint infection (PJI) (4.11% vs. 0.79%, OR = 5.40), deep (0.12% vs. 0.01%, OR = 11.15) and superficial (0.37% vs. 0.09%, OR = 4.17) surgical site infections, and wound dehiscence (1.11% vs. 0.52%, OR = 2.13). The matched analysis confirmed significantly (p < 0.001) higher rates of PJI (4.39% vs. 1.18%, OR = 3.59). Conclusions: Our results illustrated preoperative colonization of MRSA as associated with an increased risk of wound-related complications. Surgeons and patients must consider preoperative MRSA colonization status when deciding if TKA is an optimal treatment option.

Artificial Intelligence Methods in Infection Biology Research

Despite unprecedented achievements, the domain-specific application of artificial intelligence (AI) in the realm of infection biology was still in its infancy just a couple of years ago. This is largely attributable to the proneness of the infection biology community to shirk quantitative techniques. The so-called "sorting machine" paradigm was prevailing at that time, meaning that AI applications were primarily confined to the automation of tedious laboratory tasks. However, fueled by the severe acute respiratory syndrome coronavirus 2 pandemic, AI-driven applications in infection biology made giant leaps beyond mere automation. Instead, increasingly sophisticated tasks were successfully tackled, thereby ushering in the transition to the "Swiss army knife" paradigm. Incentivized by the urgent need to subdue a raging pandemic, AI achieved maturity in infection biology and became a versatile tool. In this chapter, the maturation of AI in the field of infection biology from the "sorting machine" paradigm to the "Swiss army knife" paradigm is outlined. Successful applications are illustrated for the three data modalities in the domain, that is, images, molecular data, and language data, with a particular emphasis on disentangling host-pathogen interactions. Along the way, fundamental terminology mentioned in the same breath as AI is elaborated on, and relationships between the subfields these terms represent are established. Notably, in order to dispel the fears of infection biologists toward quantitative methodologies and lower the initial hurdle, this chapter features a hands-on guide on software installation, virtual environment setup, data preparation, and utilization of pretrained models at its very end.

Genomic exploration of the fermented meat isolate Staphylococcus shinii IMDO-S216 with a focus on competitiveness-enhancing secondary metabolites

BACKGROUND

Staphylococcus shinii appears as an umbrella species encompassing several strains of Staphylococcus pseudoxylosus and Staphylococcus xylosus. Given its phylogenetic closeness to S. xylosus, S. shinii can be found in similar ecological niches, including the microbiota of fermented meats where the species may contribute to colour and flavour development. In addition to these conventional functionalities, a biopreservation potential based on the production of antagonistic compounds may be available. Such potential, however, remains largely unexplored in contrast to the large body of research that is available on the biopreservative properties of lactic acid bacteria. The present study outlines the exploration of the genetic basis of competitiveness and antimicrobial activity of a fermented meat isolate, S. shinii IMDO-S216. To this end, its genome was sequenced, de novo assembled, and annotated.

RESULTS

The genome contained a single circular chromosome and eight plasmid replicons. Focus of the genomic exploration was on secondary metabolite biosynthetic gene clusters coding for ribosomally synthesized and posttranslationally modified peptides. One complete cluster was coding for a bacteriocin, namely lactococcin 972; the genes coding for the pre-bacteriocin, the ATP-binding cassette transporter, and the immunity protein were also identified. Five other complete clusters were identified, possibly functioning as competitiveness factors. These clusters were found to be involved in various responses such as membrane fluidity, iron intake from the medium, a quorum sensing system, and decreased sensitivity to antimicrobial peptides and competing microorganisms. The presence of these clusters was equally studied among a selection of multiple Staphylococcus species to assess their prevalence in closely-related organisms.

CONCLUSIONS

Such factors possibly translate in an improved adaptation and competitiveness of S. shinii IMDO-S216 which are, in turn, likely to improve its fitness in a fermented meat matrix.

The application of synthetic antibacterial minerals to combat topical infections: exploring a mouse model of MRSA infection

The development of new antibiotics has stalled, and novel strategies are needed as we enter the age of antibiotic resistance. Certain naturally occurring clays have been shown to be effective in killing antibiotic resistant bacteria. However, these natural clays are too variable to be used in clinical settings. Our study shows that synthetic antibacterial minerals exhibit potent antibacterial activity against topical MRSA infections and increase the rate of wound closure relative to controls. The antibacterial minerals maintain a redox cycle between Fe2+/Fe3+ and the surfaces of pyrite minerals, which act as a semiconductor and produce reactive oxygen species (ROS), while smectite minerals act as a cation exchange reservoir. Acidic conditions are maintained throughout the application of the hydrated minerals and can mitigate the alkaline pH conditions observed in chronic non-healing wounds. These results provide evidence for the strategy of 'iron overload' to combat antibiotic resistant infections through the maintained release of Fe2+ and generation of ROS via distinct geochemical reactions that can break the chronic wound damage cycle.

Host-microbe interactions in chronic rhinosinusitis biofilms and models for investigation

Chronic rhinosinusitis (CRS) is a debilitating condition characterized by long-lasting inflammation of the paranasal sinuses. It affects a significant portion of the population, causing a considerable burden on individuals and healthcare systems. The pathogenesis of CRS is multifactorial, with bacterial infections playing a crucial role in CRS development and persistence. In recent years, the presence of biofilms has emerged as a key contributor to the chronicity of sinusitis, further complicating treatment and exacerbating symptoms. This review aims to explore the role of biofilms in CRS, focusing on the involvement of the bacterial species Staphylococcus aureus and Pseudomonas aeruginosa, their interactions in chronic infections, and model systems for studying biofilms in CRS. These species serve as an example of how microbial interplay can influence disease progression and exemplify the need for continued investigation and innovation in CRS research.

Iron Metabolism of the Skin: Recycling versus Release

The skin protects the body against exogenous stressors. Its function is partially achieved by the permanent regeneration of the epidermis, which requires high metabolic activity and the shedding of superficial cells, leading to the loss of metabolites. Iron is involved in a plethora of important epidermal processes, including cellular respiration and detoxification of xenobiotics. Likewise, microorganisms on the surface of the skin depend on iron, which is supplied by the turnover of epithelial cells. Here, we review the metabolism of iron in the skin with a particular focus on the fate of iron in epidermal keratinocytes. The iron metabolism of the epidermis is controlled by genes that are differentially expressed in the inner and outer layers of the epidermis, establishing a system that supports the recycling of iron and counteracts the release of iron from the skin surface. Heme oxygenase-1 (HMOX1), ferroportin (SLC40A1) and hephaestin-like 1 (HEPHL1) are constitutively expressed in terminally differentiated keratinocytes and allow the recycling of iron from heme prior to the cornification of keratinocytes. We discuss the evidence for changes in the epidermal iron metabolism in diseases and explore promising topics of future studies of iron-dependent processes in the skin.