Alexander R Horswill

University of Colorado | UCD · Department of Immunology and Microbiology

The composition of the respiratory track microbiome is a notable predictor of infection-related morbidities and mortalities among both adults and children. Species of Corynebacterium, which are largely present as commensals in the upper airway and other body sites, are associated with lower colonization rates of opportunistic bacterial pathogens su...

Biofilms are a cause of chronic, non-healing infections. Staphylococcus aureus is a proficient biofilm forming pathogen commonly isolated from prosthetic joint infections that develop following primary arthroplasty. Extracellular adhesion protein (Eap), previously characterized in planktonic or non-biofilm populations as being an adhesin and immune...

Chronic, non-healing wounds are a leading cause of prolonged patient morbidity and mortality, often due to biofilm- associated, polymicrobial infections. Staphylococcus aureus and Pseudomonas aeruginosa are the most frequently co-isolated pathogens from chronic wound infections. Competitive interactions between these pathogens contribute to enhance...

Staphylococcus aureus causes approximately 80% of skin and soft tissue infections (SSTIs). Collagen is the most abundant human extracellular matrix protein with critical roles in wound healing, and S. aureus encodes a collagen binding adhesin (Cna). The role of this protein during skin infections is unknown. Here we report that inability to bind co...

Neutrophils, polymorphonuclear leukocytes (PMN), express numerous pattern recognition receptors, including TLRs, capable of recognizing a wide variety of pathogens. Receptor engagement initiates a cascade of PMN responses with some occurring in seconds, and some requiring de novo protein synthesis over the course of many hours. Although numerous sp...

Diabetic wound infections including diabetic foot ulcers (DFUs) are a major global health concern and a leading cause of non-traumatic amputations. Numerous bacterial species establish infection in DFUs, and treatment with antibiotics often fails due to widespread antibiotic resistance and biofilm formation. Determination of bacterial species that...

Biofilm formation is an important virulence factor for methicillin-resistant Staphylococcus aureus (MRSA). The extracellular matrix of MRSA biofilms contains significant amounts of double-stranded DNA that hold the biofilm together. MRSA cells secrete micrococcal nuclease (Nuc1), which degrades double-stranded DNA. In this study, we used standard m...

Staphylococcus aureus is a Gram-positive pathogen responsible for the majority of skin and soft tissue infections (SSTIs). S. aureus colonizes the anterior nares of approximately 20%–30% of the population and transiently colonizes the skin, thereby increasing the risk of developing SSTIs and more serious infections. Current laboratory models that m...

Staphylococcus aureus is one of the leading causes of hospital-acquired infections, many of which begin following attachment and accumulation on indwelling medical devices or diseased tissue. These infections are often linked to the establishment of biofilms, but another often overlooked key characteristic allowing S. aureus to establish persistent...

Staphylococcus aureus, the most frequent cause of skin infections, is more common in men than women and selectively colonizes the skin during inflammation. Yet, the specific cues that drive infection in these settings remain unclear. Here we show that the host androgens testosterone and dihydrotestosterone promote S. aureus pathogenesis and skin in...

20–41% of the world's population is either transiently or permanently colonized by the Gram-positive bacterium, Staphylococcus aureus. In 2017, the CDC designated methicillin resistant S. aureus (MRSA) as a serious threat, reporting ∼300 000 cases of MRSA-associated hospitalizations annually, resulting in over 19 000 deaths, surpassing that of HIV...

Lipoproteins of the opportunistic pathogen Staphylococcus aureus play a crucial role in various cellular processes and host interactions. Consisting of a protein and a lipid moiety, they support nutrient acquisition and anchor the protein to the bacterial membrane. Recently, we identified several processed and secreted small linear peptides that de...

Bacterial pneumonia is a common clinical syndrome leading to significant morbidity and mortality worldwide. In the current study, we investigate a novel, multidirectional relationship between the pulmonary epithelial glycocalyx and antimicrobial peptides in the setting of methicillin-resistant Staphylococcus aureus (MRSA) pneumonia. Using an in viv...

Staphylococcus aureus causes the majority of skin and soft tissue infections, but this pathogen only transiently colonizes healthy skin. However, this transient skin exposure enables S. aureus to transition to infection. Initial adhesion of S. aureus to skin corneocytes is mediated by surface protein G (SasG). Here, phylogenetic analyses reveal the...

Dihydrofolate reductase (DHFR) catalyzes the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate. Bacterial DHFRs are targets of several important antibiotics as well as model enzymes for the role of protein conformational dynamics in enzyme catalysis. We collected 0.93 Å resolution X-ray diffraction data from both Bacillus subtilis (Bs)...

Biofilm formation is an important virulence factor for methicillin-resistant Staphylococcus aureus (MRSA). The extracellular matrix of MRSA biofilms contains significant amounts of double-stranded DNA. MRSA cells also secrete micrococcal nuclease (Nuc1) which degrades double-stranded DNA. In this study we used a nuc1 mutant strain to investigate th...

Streptococcus pyogenes is an obligate human pathobiont associated with many disease states. Here, we present a model of S. pyogenes infection using intact murine epithelium. We were able to perform RNA sequencing to evaluate genetic changes undertaken by both the bacterium and host at 5 and 24 h post-infection. Analysis of these genomic data demons...

https://authors.elsevier.com/c/1i821L7PXmeD1

Staphylococcus aureus is a Gram-positive pathogen responsible for the majority of skin and soft tissue infections (SSTIs). S. aureus colonizes the anterior nares of approximately 20-30% of the population and transiently colonizes the skin, thereby increasing the risk of developing SSTIs and more serious infections. Current laboratory models that mi...

Modulation of the immune response to initiate and halt the inflammatory process occurs both at the site of injury as well as systemically. Due to the evolving role of cellular metabolism in regulating cell fate and function, tendon injuries that undergo normal and aberrant repair were evaluated by metabolic profiling to determine its impact on heal...

Staphylococcus epidermidis is a common microbe on human skin and has beneficial functions in the skin microbiome. However, under conditions of allergic inflammation, the abundance of S. epidermidis increases, establishing potential danger to the epidermis. To understand how this commensal may injure the host, we investigate phenol-soluble modulin (...

Streptococcus pyogenes is an obligate human pathobiont associated with many disease states. Here, we present a novel model of S. pyogenes infection using intact murine epithelium. From this model, we were able to perform RNA sequencing to evaluate the genetic changes undertaken by both the bacterium and host at 5- and 24-hours post infection. Analy...

Group B Streptococcus (GBS) is a Gram-positive pathobiont that can cause adverse health outcomes in neonates and vulnerable adult populations. GBS is one of the most frequently isolated bacteria from diabetic (Db) wound infections but is rarely found in the non-diabetic (nDb) wound environment. Previously, RNA sequencing of wound tissue from Db wou...

Staphylococcus epidermidis is a ubiquitous skin commensal that has the potential to become pathogenic and cause disease. Here, we report the complete genome sequence of a S. epidermidis strain isolated from adult healthy skin that shows high expression of the virulence factor extracellular cysteine protease A (EcpA).

Pseudomonas aeruginosa and Staphylococcus aureus are among the most frequently isolated bacterial species from polymicrobial infections of patients with cystic fibrosis and chronic wounds. We apply mass spectrometry guided interaction studies to determine how chemical interaction shapes the fitness and community structure during co-infection of the...

DNASE1 (D1) and DNASE1L3 (D1L3) synergistically reduce the severity of systemic infections caused by Staphylococcus aureus. In this issue of JEM, Lacey et al. (2023. J. Exp. Med.https://doi.org/10.1084/jem.20221086) develop D1-/-, D1L3-/-, and D1-/-D1L3-/- mice to show that exogenous addition of the DNase formulation Dornase alfa can facilitate rem...

Prior research has focused on host factors as mediators of exaggerated sepsis-associated morbidity and mortality in older adults. This focus on the host, however, has failed to identify therapies that improve sepsis outcomes in the elderly. We hypothesized that the increased susceptibility of the aging population to sepsis is not only a function of...

Considerable progress has been made toward elucidating the mechanism of Staphylococcus aureus aggregation in synovial fluid. In this study, aggregate morphology was assessed following incubation under several simulated postsurgical joint conditions. Using fluorescently labeled synovial fluid polymers, we show that aggregation occurs through two dis...

Staphylococci, whether beneficial commensals or pathogens, often colonize human skin, potentially leading to competition for the same niche. In this multidisciplinary study we investigate the structure, binding specificity, and mechanism of adhesion of the Aap lectin domain required for Staphylococcus epidermidis skin colonization and compare its c...

Techniques for studying the clearance of bacterial infections are critical for advances in understanding disease states, immune cell effector functions, and novel antimicrobial therapeutics. Intracellular killing of Staphylococcus aureus by neutrophils can be monitored using a S. aureus strain stably expressing GFP, a fluorophore that is quenched w...

Prior research has focused on host factors as mediators of exaggerated sepsis-associated morbidity and mortality in older adults. This focus on the host, however, has failed to identify therapies that improve sepsis outcomes in the elderly. We hypothesized that the increased susceptibility of the aging population to sepsis is not only a function of...

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are difficult to treat due to their resistance to many -lactam antibiotics, and their highly coordinated excretion of virulence factors. One way in...

Staphylococcus aureus is one of the leading causes of hospital acquired infections, many of which begin following attachment and accumulation on indwelling medical devices or diseased tissue. These infections are often linked to establishment of biofilms, but another often overlooked key characteristic allowing S. aureus to establish persistent inf...

Methicillin-resistant Staphylococcus aureus (MRSA) is an opportunistic pathogen and frequent colonizer of human skin and mucosal membranes, including the vagina, with vaginal colonization reaching nearly 25% in some pregnant populations. MRSA vaginal colonization can lead to aerobic vaginitis (AV), and during pregnancy, bacterial ascension into the...

The accumulation phase of staphylococcal biofilms relies both on the production of an extracellular polysaccharide matrix and on the expression of bacterial surface proteins. A prototypical example of such adhesive proteins is the long multidomain protein Aap from Staphylococcus epidermidis which mediates zinc-dependent homophilic interactions betw...

The accumulation phase of staphylococcal biofilms relies both on the production of an extracellular polysaccharide matrix and on the expression of bacterial surface proteins. A prototypical example of such adhesive proteins is the long multidomain protein Aap from Staphylococcus epidermidis which mediates zinc-dependent homophilic interactions betw...

Diabetic wounds have poor healing outcomes due to the presence of numerous pathogens and a dysregulated immune response. Group B Streptococcus (GBS) is commonly isolated from diabetic wound infections, but the mechanisms of GBS virulence during these infections have not been investigated. Here, we develop a murine model of GBS diabetic wound infect...

Staphylococcus aureus is an opportunistic pathogen that causes the majority of wound and soft tissue infections. The accumulation-associated protein (Aap) from S. epidermidis and surface protein G (SasG) from S. aureus are cell wall-anchored (CWA) proteins known to be important in adhesion to healthy corneocytes from human skin. We investigated the...

Within a bacterial population, a stochastically generated heterogeneity of phenotypes allows continual survival against current and future stressors. The generation of a sub-population of quasi-dormant Small Colony Variants (SCV) in Staphylococcus aureus is such a mechanism, allowing for persistent or relapse of infection despite initial interventi...

The coagulase-negative bacterium Staphylococcus epidermidis is a member of the human skin microbiota. S. epidermidis is not merely a passive resident on skin but actively primes the cutaneous immune response, maintains skin homeostasis and prevents opportunistic pathogens from causing disease via colonization resistance. However, it is now apprecia...

Diabetic wounds have poor healing outcomes due to the presence of numerous pathogens and a dysregulated immune response. Group B Streptococcus (GBS) is commonly isolated from diabetic wound infections, but the mechanisms of GBS virulence during these infections have not been investigated. Here, we develop a murine model of GBS diabetic wound infect...

Staphylococcus aureus is a common cause of severe infections, and its widespread antibiotic resistance necessitates search for alternative therapies, such as inhibition of virulence. As S. aureus produces multiple individual virulence factors, inhibition of an entire regulatory system might provide better effects than targeting each virulence facto...

Cephalexin and cefadroxil are oral first-generation cephalosporins used to treat methicillin-susceptible Staphylococcus aureus (MSSA) infections. Despite its shorter half-life, cephalexin is more frequently prescribed, although cefadroxil is an appealing alternative, given its slower clearance and possibility for less frequent dosing. We report com...

Skin health is influenced by the composition and integrity of the skin barrier. The healthy skin surface is an acidic, hypertonic, proteinaceous, and lipid-rich environment that microorganisms must adapt to for survival, and disruption of this environment can result in dysbiosis and increase risk for infectious diseases. This work provides a brief...

Human skin is home to a variety of commensal bacteria, including many species of coagulase-negative staphylococci (CoNS). While it is well established that the microbiota as a whole maintains skin homeostasis and excludes pathogens (i.e., colonization resistance), relatively little is known about the unique contributions of individual CoNS species...

The USA300 and USA600 clonal lineages are the cause of many serious Staphylococcus aureus infections. Here, we report the complete genomes of two methicillin-sensitive S. aureus strains isolated from the healthy skin of adults in Colorado, which are most phylogenetically similar to the USA300 and USA600 lineages.

Early bacterial survival in the post-surgical joint is still a mystery. Recently, synovial fluid-induced aggregation was proposed as a potential mechanism of bacterial protection upon entry into the joint. As synovial fluid is secreted back into the joint cavity following surgery, rapid fluctuations in synovial fluid concentration, composition, and...

Early bacterial survival in the post-surgical joint is still a mystery. Recently, synovial fluid-induced aggregation was proposed as a potential mechanism of bacterial protection upon entry into the joint. As synovial fluid is secreted back into the joint cavity following surgery, rapid fluctuations in synovial fluid concentration, composition, and...

Implant-associated Staphylococcus aureus infections are difficult to treat because of biofilm formation. Bacteria in a biofilm are often insensitive to antibiotics and host immunity. Monoclonal antibodies (mAbs) could provide an alternative approach to improve the diagnosis and potential treatment of biofilm-related infections. Here, we show that m...

Pseudomonas aeruginosa and Staphylococcus aureus are among the most frequently isolated bacterial species from polymicrobial infections of patients with cystic fibrosis and chronic wounds. We apply mass spectrometry guided interaction studies to determine how chemical interaction shapes the fitness and community structure during co-infection of the...

Superficial cutaneous Staphylococcus aureus (S. aureus) infection in humans can lead to soft tissue infection, an important cause of morbidity and mortality. IL-17A production by skin TCRγδ+ cells in response to IL-1 and IL-23 produced by epithelial and immune cells is important for restraining S. aureus skin infection. How S. aureus evades this cu...

Neutrophils, polymorphonuclear leukocytes (PMN), play a critical role in the innate immune response to Staphylococcus aureus, a pathogen that continues to be associated with significant morbidity and mortality. Neutrophil extracellular trap (NET) formation is involved in ensnaring and killing of S. aureus, but this host-pathogen interaction also le...

Staphylococcus aureus interacts with fibrinogen in plasma to form macroscopic clumps of cells. A simple and rapid slide agglutination test using rabbit plasma has been employed in clinical labs to distinguish S. aureus from most coagulase-negative Staphylococci. The method described here is a quantitative clumping assay in which S. aureus cells are...

Staphylococcus aureus is the most significant cause of skin and soft tissue infections yet it rarely colonizes the skin of healthy individuals. This is believed to be due, in part, to inhibition of colonization via toxic substances produced by normal skin flora, including by S. epidermidis .

Skin is one of the most common sites of host immune response against Staphylococcus aureus infection. Here, through a combination of in vitro assays, mouse models, and intravital imaging, we find that S. aureus immune evasion in skin is controlled by a cascade composed of the ArlRS two-component regulatory system and its downstream effector, MgrA....

Methicillin-resistant Staphylococcus aureus (MRSA) represents one of the most serious infectious disease concerns worldwide, with the CDC labeling it a “serious threat” in 2019. The current arsenal of antibiotics works by targeting bacterial growth and survival, which exerts great selective pressure for the development of resistance. The developmen...

Neutrophils (PMN) regulate inflammation in many ways, including communication with other immune cells via extracellular vesicles (EVs). EVs released by human neutrophils activated with N‐formylmethionyl‐leucyl‐phenylalanine (fMLF) (PMN‐fMLF EVs) had an outside‐out orientation and contained functionally important neutrophil plasma membrane proteins,...

Implant-associated Staphylococcus aureus infections are difficult to treat because of biofilm formation. Bacteria in a biofilm are often insensitive to antibiotics and host immunity. Monoclonal antibodies (mAbs) could provide an alternative approach to improve the diagnosis and/or treatment of biofilm-related infections. Here we show that mAbs targ...

Biofilm formation by bacterial pathogens is associated with numerous human diseases and can confer resistance to both antibiotics and host defenses. Many strains of Staphylococcus epidermidis are capable of forming biofilms and are important human pathogens. Since S. epidermidis coexists with abundant Cutibacteria acnes on healthy human skin and do...

Author summary Our skin is our first line of defense against environmental and pathogenic challenges. It is densely populated by a flora of bacteria, fungi, and viruses that normally interact with each other and with our immune system to promote skin health and homeostasis. Staphylococcus epidermidis is one of the most abundant bacterial colonizers...

Infections caused by multi-drug resistant Staphylococcus aureus, especially MRSA, are responsible for high mortality and morbidity worldwide. Resistant lineages were previously confined to hospitals, but are now also causing infections among healthy individuals in the community. It is therefore imperative to explore therapeutic avenues that are les...

Polymorphonuclear leukocytes (PMN) phagocytose and kill individual bacteria but are far less efficient when challenged with bacterial aggregates. Consequently, growth within a biofilm affords Staphylococcus aureus some protection, but PMN penetrate S. aureus biofilms and phagocytose bacteria, suggesting that enzymes released through neutrophil degr...

Periprosthetic joint infection (PJI) occurring after artificial joint replacement is a major clinical issue requiring multiple surgeries and antibiotic interventions. Staphylococcus aureus is the bacterium most commonly responsible for PJI. Recent in vitro research has shown that staphylococcal strains rapidly form aggregates in the presence of syn...