Preclinical Models and Methodologies for Monitoring Staphylococcus aureus Infections Using Noninvasive Optical Imaging

Complement in breast milk modifies offspring gut microbiota to promote infant health

Breastfeeding offers demonstrable benefits to newborns and infants by providing nourishment and immune protection and by shaping the gut commensal microbiota. Although it has been appreciated for decades that breast milk contains complement components, the physiological relevance of complement in breast milk remains undefined. Here, we demonstrate that weanling mice fostered by complement-deficient dams rapidly succumb when exposed to murine pathogen Citrobacter rodentium (CR), whereas pups fostered on complement-containing milk from wild-type dams can tolerate CR challenge. The complement components in breast milk were shown to directly lyse specific members of gram-positive gut commensal microbiota via a C1-dependent, antibody-independent mechanism, resulting in the deposition of the membrane attack complex and subsequent bacterial lysis. By selectively eliminating members of the commensal gut community, complement components from breast milk shape neonate and infant gut microbial composition to be protective against environmental pathogens such as CR.

Effective decolonization strategy for mupirocin-resistant Staphylococcus aureus by TPGS-modified mupirocin-silver complex

The widespread utilization of mupirocin to treat methicillin-resistant Staphylococcus aureus (MRSA)-caused infectious diseases has led to the emergence of mupirocin-resistant Staphylococcus aureus (MuRSA), posing a serious global medical threat. In order to counteract MuRSA, we develop a d-α-tocopherol polyethylene glycol 1000 succinate (TPGS) modified mupirocin and silver complex (TPGS/Mup-Ag) to combat MuRSA. The surfactivity of TPGS endows Mup-Ag with a homogeneous and small particle size (∼16 ​nm), which significantly enhances bacterial internalization. Silver ions are released from the mupirocin-Ag complex (Mup-Ag) to exert a synergistic antibacterial activity with mupirocin. Results manifest that our strategy reduces the concentration of mupirocin that induces 50% bacterial death from about 1000 ​μmol/mL to about 16 ​μmol/mL. In vitro bacterial infection model suggests that TPGS/Mup-Ag can not only eliminate both intracellular and inhibit bacterial adhesion, but also living cells are not affected. Results of in vivo experiments demonstrate that TPGS/Mup-Ag can effectively inhibit the progression of skin infection and accelerate wound healing, as well as alleviate systemic inflammation in both the subcutaneous infection model and the wound infection model. Furthermore, this study may contribute to the development of therapeutic agents for antibiotic-resistant bacteria and offer ideas for silver-based bactericides.

Humanized Mice Exhibit Exacerbated Abscess Formation and Osteolysis During the Establishment of Implant-Associated Staphylococcus aureus Osteomyelitis

Staphylococcus aureus is the predominant pathogen causing osteomyelitis. Unfortunately, no immunotherapy exists to treat these very challenging and costly infections despite decades of research, and numerous vaccine failures in clinical trials. This lack of success can partially be attributed to an overreliance on murine models where the immune correlates of protection often diverge from that of humans. Moreover, S. aureus secretes numerous immunotoxins with unique tropism to human leukocytes, which compromises the targeting of immune cells in murine models. To study the response of human immune cells during chronic S. aureus bone infections, we engrafted non-obese diabetic (NOD)-scid IL2Rγ^null (NSG) mice with human hematopoietic stem cells (huNSG) and analyzed protection in an established model of implant-associated osteomyelitis. The results showed that huNSG mice have increases in weight loss, osteolysis, bacterial dissemination to internal organs, and numbers of Staphylococcal abscess communities (SACs), during the establishment of implant-associated MRSA osteomyelitis compared to NSG controls (p < 0.05). Flow cytometry and immunohistochemistry demonstrated greater human T cell numbers in infected versus uninfected huNSG mice (p < 0.05), and that T-bet⁺ human T cells clustered around the SACs, suggesting S. aureus-mediated activation and proliferation of human T cells in the infected bone. Collectively, these proof-of-concept studies underscore the utility of huNSG mice for studying an aggressive form of S. aureus osteomyelitis, which is more akin to that seen in humans. We have also established an experimental system to investigate the contribution of specific human T cells in controlling S. aureus infection and dissemination.

Intracellular escape strategies of Staphylococcus aureus in persistent cutaneous infections

Pathogenic invasion of Staphylococcus aureus is a major concern in patients with chronic skin diseases like atopic dermatitis (AD), epidermolysis bullosa (EB), or chronic diabetic foot and venous leg ulcers, and can result in persistent and life-threatening chronic non-healing wounds. Staphylococcus aureus is generally recognized as extracellular pathogens. However, S. aureus can also invade, hide and persist in skin cells to contribute to wound chronicity. The intracellular life cycle of S. aureus is currently incompletely understood, although published studies indicate that its intracellular escape strategies play an important role in persistent cutaneous infections. This review provides current scientific knowledge about the intracellular life cycle of S. aureus in skin cells, which can be classified into professional and non-professional antigen-presenting cells, and its strategies to escape adaptive defense mechanisms. First, we discuss phenotypic switch of S. aureus, which affects intracellular routing and degradation. This review also evaluates potential intracellular escape mechanism of S. aureus to avoid intracellular degradation and antigen presentation, preventing an immune response. Furthermore, we discuss potential drug targets that can interfere with the intracellular life cycle of S. aureus. Taken together, this review aimed to increase scientific understanding about the intracellular life cycle of S. aureus into skin cells and its strategies to evade the host immune response, information that is crucial to reduce pathogenic invasion and life-threatening persistence of S. aureus in chronic cutaneous infections.

IsdB antibody-mediated sepsis following S. aureus surgical site infection

Staphylococcus aureus is prevalent in surgical site infections (SSI) and leads to death in approximately 1% of patients. Phase IIB/III clinical trial results have demonstrated that vaccination against the iron-regulated surface determinant protein B (IsdB) is associated with an increased mortality rate in patients with SSI. Thus, we hypothesized that S. aureus induces nonneutralizing anti-IsdB antibodies, which facilitate bacterial entry into leukocytes to generate "Trojan horse" leukocytes that disseminate the pathogen. Since hemoglobin (Hb) is the primary target of IsdB, and abundant Hb-haptoglobin (Hb-Hp) complexes in bleeding surgical wounds are normally cleared via CD163-mediated endocytosis by macrophages, we investigated this mechanism in vitro and in vivo. Our results demonstrate that active and passive IsdB immunization of mice renders them susceptible to sepsis following SSI. We also found that a multimolecular complex containing S. aureus protein A-anti-IsdB-IsdB-Hb-Hp mediates CD163-dependent bacterial internalization of macrophages in vitro. Moreover, IsdB-immunized CD163-/- mice are resistant to sepsis following S. aureus SSI, as are normal healthy mice given anti-CD163-neutralizing antibodies. These genetic and biologic CD163 deficiencies did not exacerbate local infection. Thus, anti-IsdB antibodies are a risk factor for S. aureus sepsis following SSI, and disruption of the multimolecular complex and/or CD163 blockade may intervene.

Research Techniques Made Simple: Mouse Bacterial Skin Infection Models for Immunity Research

Bacterial skin infections are a major societal health burden and are increasingly difficult to treat owing to the emergence of antibiotic-resistant strains such as community-acquired methicillin-resistant Staphylococcus aureus. Understanding the immunologic mechanisms that provide durable protection against skin infections has the potential to guide the development of immunotherapies and vaccines to engage the host immune response to combat these antibiotic-resistant strains. To this end, mouse skin infection models allow researchers to examine host immunity by investigating the timing, inoculum, route of infection and the causative bacterial species in different wild-type mouse backgrounds as well as in knockout, transgenic, and other types of genetically engineered mouse strains. To recapitulate the various types of human skin infections, many different mouse models have been developed. For example, four models frequently used in dermatological research are based on the route of infection, including (i) subcutaneous infection models, (ii) intradermal infection models, (iii) wound infection models, and (iv) epicutaneous infection models. In this article, we will describe these skin infection models in detail along with their advantages and limitations. In addition, we will discuss how humanized mouse models such as the human skin xenograft on immunocompromised mice might be used in bacterial skin infection research.