Although strictly defined as any inflammatory process in bone, osteomyelitis is most commonly caused by infection. Osteomyelitis is paradigmatic of treatment-refractory invasive infectious disease, in that cure is notoriously difficult and requires prolonged antibiotic therapy in conjunction with surgical débridement. Even with such extreme treatment measures, many patients experiencing osteomyelitis eventually experience chronic infection or sustain highly morbid complications. Treatment failure or recurrence of infection
occurs in an estimated 13% to 30% of cases.³,⁴ Thus, the development of new strategies to manage osteomyelitis and mitigate associated disease morbidity is a high priority.

Osteomyelitis occurs following one of three general pathologic mechanisms: (1) secondary to a contiguous focus of infection; (2) secondary to vascular insufficiency or neuropathy; or (3) secondary to hematogenous seeding of bone.⁵ Osteomyelitis that occurs secondary to a contiguous focus of infection is the most common mechanism in adults and encompasses an array of pathology such as surgical site infection, orthopaedic implant-associated osteomyelitis, and infection following penetrating trauma (eg, contaminated open fractures). Hematogenous osteomyelitis is predominantly a disease of the pediatric population, with two important exceptions. First, hematogenous seeding is the presumed inciting event for native vertebral osteomyelitis, which is the most common form of hematogenous osteomyelitis in patients older than 50 years.⁶ Second, persons who inject drugs are at risk for hematogenous seeding of bones and joints and resultant osteomyelitis and septic arthritis. Osteomyelitis that occurs secondary to vascular insufficiency or neuropathy characteristically follows skin and soft-tissue ulceration that results from repetitive trauma, leading to bone exposure and subsequent microbial contamination. This mechanism of osteomyelitis is most common in patients with diabetes; foot ulcers that involve bone develop in a large percentage of these patients.

In addition to classifying osteomyelitis based on the inciting pathologic mechanism, additional schemes have been proposed that codify osteomyelitis based on factors such as location of disease, patient comorbidities, radiographic features, and histologic findings.⁷,⁸ The Cierny-Mader classification scheme characterizes osteomyelitis based on four anatomic types and three host physiologic classes to comprise 12 clinical stages.⁹ Anatomic types are (I) medullary, in which the disease is endosteal; (II) superficial, in which a contiguous focus of infection causes disease on the superficial surface of the bone; (III) localized, in which disease is full thickness with cortical sequestration and/or cavitation, combining elements of both medullary and superficial disease; and (IV) diffuse, in which disease is permeative, circumferential, and associated with skeletal instability.⁹ Patient classification in the Cierny-Mader system includes normal hosts (A), compromised hosts (B), and hosts in which the treatment is more injurious than the bone disease itself (C). Class B hosts are further classified based on the presence of local (B^L, eg, lymphedema, venous stasis, scarring), systemic (B^S, eg, malnutrition, immunodeficiency, diabetes), or local and systemic (B^LS) comorbidities.⁹ The 12 resulting clinical classifications are then used to infer prognosis and drive treatment algorithms, including surgical débridement, dead space management, and antibiotic therapy.⁹

A precise classification of osteomyelitis as acute versus chronic has been more elusive and controversial.⁷ Suggested definitions of chronic osteomyelitis based solely on disease duration have been variable, ranging from 2 weeks to 6 months.⁷,⁸ Acute versus chronic osteomyelitis is also segregated based on radiographic findings and histologic evaluation. Classic radiographic features of chronic osteomyelitis include the presence of necrotic fragments of bone known as sequestra, areas of surrounding reactive bone formation known as involucrum, and sinus tract formation.⁵ The classic histopathologic finding of acute osteomyelitis is a neutrophilic infiltrate, whereas features associated with chronic osteomyelitis include lymphocytic inflammation and marrow fibrosis. However, histologic features of both acute and chronic osteomyelitis can be observed in the same lesion and may not perfectly correlate with the disease duration.⁷ Accordingly, many experts recommend avoiding classifications of disease that are solely based on time since symptom onset.²

The inciting pathologic mechanisms leading to osteomyelitis (eg, hematogenous versus secondary to a contiguous infection) strongly influence the resulting microbial pathogens present in infected bone. Across all pathologic mechanisms, the gram-positive bacterium Staphylococcus aureus is the most common etiologic agent of bone infection. Accordingly, much of the current knowledge regarding bacterial virulence mechanisms that contribute to the pathogenesis of osteomyelitis come from studies of S. aureus. Table 1 lists the most commonly isolated microorganisms from musculoskeletal infections, as classified based on inciting disease mechanism. Importantly, current knowledge of infectious etiologies of osteomyelitis is largely based on traditional microbiologic culture methods, whereas newer analyses leveraging molecular diagnostics, 16s ribosomal RNA (rRNA) sequencing, or metagenomics suggest that such traditional methods may underestimate the microbial diversity encountered in osteomyelitis. One study compared conventional culture methods with 16s rRNA sequencing for the diagnosis of diabetic foot osteomyelitis and found that culture failed to identify a pathogen in 23.5% of cases, yet Staphylococcus species were detected by 16s rRNA sequencing in 75% of the culture-negative bone samples.¹³ The most
commonly detected microbial genera in this study were Staphylococcus, Corynebacterium, Streptococcus, and Cutibacterium (formerly Propionibacterium). In addition, significantly more anaerobic bacteria were detected by 16s rRNA sequencing (86.9% of samples) than by traditional culture methods (23.1% of samples).¹³ One study also used 16s rRNA sequencing to characterize the microbiota of open fractures.¹⁷ A diverse microbiota was observed in the wound center and adjacent skin, including six genera (Staphylococcus, Corynebacterium, Streptococcus, Acinetobacter, Anaerococcus, and Pseudomonas) present at greater than 1% of the median relative abundance.¹⁷ Notably, bacterial community structure differed significantly in complicated versus uncomplicated cases, suggesting that 16s rRNA-based molecular diagnostics might have prognostic value.¹⁷ These studies highlight that osteomyelitis stemming from a contiguous source or vascular disease is frequently polymicrobial, and although conventional culture techniques can identify dominant pathogens, such methods typically underestimate the diversity of infecting microbes. In contrast, hematogenous and vertebral osteomyelitis are typically monomicrobial diseases.

Atypical etiologies of osteomyelitis, including rare bacterial pathogens as well as mycobacterial and fungal infection, are often suggested by a careful exposure history. Approximately 1% to 5% of Mycobacterium tuberculosis infections involve the skeleton.¹⁰ M tuberculosis osteomyelitis presents subacutely and frequently involves vertebrae or periarticular surfaces. Patients may have a prior history of positive skin testing, travel to endemic areas, known contact exposure, or prior latent tuberculosis. Disease can occur as a function of primary disseminated infection or through reactivation
of an osteoarticular nidus that was hematogenously seeded during a primary infection.¹⁸ More than 50% of tuberculous osteomyelitis cases involve the spine, although in contrast with other infectious etiologies of vertebral osteomyelitis, systemic symptoms are typically absent.¹⁰,¹⁸ Importantly, the absence of abnormal findings on chest radiograph does not exclude the diagnosis of tuberculous osteomyelitis.¹⁹ Nontuberculous mycobacteria are also important causes of musculoskeletal infection. Like tuberculous osteoarticular infection, nontuberculous mycobacteria musculoskeletal infection can occur via hematogenous seeding in compromised patients, but infection also occurs in healthy patients following traumatic inoculation of contaminated water or soil, where nontuberculous mycobacteria are ubiquitously found.¹⁹ Dimorphic fungi are important causes of osteoarticular infection in endemic regions of the United States. Coccidioides species are dimorphic fungi endemic to the Southwestern United States that typically cause respiratory infection, although musculoskeletal tissues are a frequent site of extrapulmonary disease.¹⁰ Bone involvement most frequently follows a primary septic arthritis, but hematogenous seeding of long bones and vertebrae are also reported.¹⁰ Histoplasma capsulatum and Blastomyces dermatitidis are dimorphic fungi that are most highly endemic to the Ohio and Mississippi River valleys. Like Coccidioides species, both fungi typically cause pulmonary infection but can cause disseminated disease, with bone and joint tissues frequently involved.¹⁰,²⁰ B dermatitidis in particular has a high tropism for skin and bone during disseminated disease, and concomitant cutaneous and skeletal disease is common.¹⁰ Risk factors for both histoplasmosis and blastomycosis include exposure to recent soil excavation or construction. Histoplasmosis is also strongly linked to exposure to bird and bat guano, and therefore entry into chicken coops and caves, respectively.¹⁰ Other important causes of fungal osteomyelitis include Candida and Aspergillus species. Candidal osteomyelitis is typically the result of prior hematogenous seeding, and bone disease can manifest years after the initial infection.²⁰ Aspergillus musculoskeletal infection is most common in immunocompromised adults and can occur via spread from a contiguous focus or as the result of hematogenous seeding.²⁰ Atypical bacterial pathogens should also be considered in the context of particular exposures and medical comorbidities. Osteoarticular involvement is observed in approximately one-half of patients with Brucellosis, which is caused by the gram-negative pathogens Brucella melitensis, Brucella suis, and Brucella abortus.¹⁰ Infection occurs following ingestion of contaminated goat or sheep milk, or via direct inoculation of skin, lungs, or conjunctivae following contact with infected animals. Musculoskeletal involvement comprises a spectrum of clinical findings including vertebral osteomyelitis, sacroiliitis, diskitis, and septic arthritis.¹⁰ Nontyphoidal serovars of Salmonella enterica are important causes of long bone osteomyelitis in patients with sickle cell anemia, although S aureus remains a frequent pathogen in this population.¹⁰

Given that S aureus is the predominant pathogen causing osteomyelitis in both children and adults, it is not surprising that most of the current knowledge of the virulence mechanisms, host immune responses, and pathophysiology of osteomyelitis comes from basic and translational studies that focus on this pathogen. Animal studies of osteomyelitis have been particularly useful in dissecting the microbial factors that contribute to the establishment of osteomyelitis, bacterial survival in bone, bone loss, and treatment recalcitrance.²¹ S aureus has a broad armamentarium of virulence factors to facilitate invasive infection, and molecular dissection of these factors defines strategies that can be generalized to other successful pathogens causing musculoskeletal infection. Staphylococcal virulence factors promoting osteomyelitis can be broadly divided into those that (1) facilitate adherence to skeletal tissues or orthopaedic implants, (2) promote biofilm formation, (3) evade host immune responses, and (4) resist antimicrobial therapy. To facilitate binding to host tissues, S aureus possesses a group of adhesins known as microbial surface components recognizing adhesive matrix molecules. Microbial surface components recognizing adhesive matrix molecules enable adhesion to components of the extracellular matrix in skeletal tissues, including fibrinogen, fibronectin, collagen, elastin, and bone sialoprotein.²¹,²² These proteins also facilitate the formation of biofilms, which are defined as structured microbial communities that are encased within an extracellular matrix.²³ Biofilm formation is widely accepted as a critical mechanism underlying the treatment recalcitrance of musculoskeletal infection. Biofilm growth is a highly regulated process in S aureus, with bacteria cycling through successive phases of adhesion to host and abiotic surfaces, proliferation and extracellular matrix production, and biofilm structuring and detachment.²³ The composition of the staphylococcal biofilm matrix, or extracellular polymeric substance, is strain dependent and consists mostly of polysaccharide, protein, and extracellular DNA.²³ The most well-characterized polysaccharide component of the staphylococcal biofilm
matrix is poly-β(1-6)-N-acetylglucosamine, which is produced by the intracellular adhesin (ica) operon.²³ However, many clinical isolates of S aureus produce a largely proteinaceous matrix, and the role of poly-β(1-6)-N-acetylglucosamine is less pronounced.²³,²⁴ The final stage of the biofilm cycle, detachment, is facilitated by the regulated expression of a variety of bacterial proteases as well as two nucleases that degrade proteins and extracellular DNA in the matrix, respectively.²³