Staphylococcus aureus infection: pathogenesis, virulence, immune evasion, persistence, and therapeutic strategies

Staphylococcus aureus can cause persistent or recurrent infection despite in vitro antimicrobial susceptibility in both methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA), indicating that acquired antimicrobial resistance is only one determinant of eradication. Whether infection is cleared also depends on how bacterial virulence and immune evasion interact with lesion architecture, bacterial physiological state, local antimicrobial activity, host immunity, and source control. Fibrin-encased abscess communities, surface-associated biofilms, intravascular vegetations, and intracellular populations impose distinct barriers to immune and antimicrobial clearance, whereas antibiotic tolerance, persister-cell survival, and small-colony-variant (SCV) phenotypes describe bacterial responses or states rather than anatomical niches. The mechanisms that predominate vary with bacterial lineage, local environmental regulation, anatomical site, and host-specific receptor and coagulation biology. In livestock and companion animals, colonization, host adaptation, strain sharing, and epidemiologically supported transmission therefore represent distinct biological and epidemiological phenomena and should not be inferred from one another. These differences also shape the interpretation of emerging interventions. Toxin-neutralizing antibodies, anti-biofilm approaches, bacteriophages, phage-derived lysins, vaccines, and host-directed therapies may address defined barriers to clearance, but their preventive or therapeutic value depends on target activity in the relevant lesion and host context. Active antimicrobial therapy and appropriate source control remain the basis of treatment, and clinical benefit from adjunctive approaches requires confirmation in defined populations. Integrating pathogenesis across lesions, bacterial states, and hosts provides a basis for interpreting persistent and recurrent infection, cross-species differences in disease mechanisms, and the settings in which emerging therapies may plausibly improve bacterial clearance.