Beyond the Usual Suspects: Actinotignum schaalii and Aerococcus urinae as Emerging Uropathogens - Epidemiology, Diagnostics, Antimicrobial Resistance, and Pathogenic Mechanisms

The field of urinary tract infections (UTIs) is undergoing a significant paradigm shift as next-generation diagnostics illuminate organisms previously dismissed as contaminants or commensals. Actinotignum schaalii and Aerococcus urinae exemplify this transition, emerging as clinically important uropathogens that disproportionately affect at-risk populations. This review examines taxonomic reclassification, epidemiological trends, clinical presentations, diagnostic challenges, antimicrobial resistance profiles, and pathogenic mechanisms of both species. Evidence from the past decade, alongside foundational studies, demonstrates that their clinical neglect has been driven primarily by methodological limitations rather than any inherent lack of virulence. MALDI-TOF mass spectrometry and enhanced culture protocols have substantially broadened clinicians' and microbiologists' understanding of UTI aetiology. This review underscores the urgent need for standardised diagnostic approaches, robust epidemiological research, and mechanistic studies of virulence factors to prevent the misdiagnosis and suboptimal treatment of infections caused by these pathogens.