Strain Diversity in the Human Microbiome: Personal Variation, Pathobionts, Therapeutics, and Methodological Challenges.

Advances in sequencing technologies have transformed human microbiome research, yet most analyses still rely on species-level profiles. However, strains rather than species represent the true ecological and functional units of the microbiome. Individual strains can vary substantially in gene content, metabolic capacity, virulence factors, antimicrobial resistance, and host-interaction properties. These differences critically influence immune responses, epithelial barrier integrity, disease susceptibility, and therapeutic outcomes. Here, we synthesize recent human microbiome studies that provide robust strain-resolved evidence, focusing on three major themes: (i) the emergence and long-term persistence of personalized strain repertoires, (ii) strain-specific pathobiont traits that drive host pathology, and (iii) the implications of strain-level ecology for the development of next-generation microbiome therapeutics. We also highlight key methodological innovations including high-resolution amplicon profiling, advanced metagenomic and single-cell genomics, and culture-based functional approaches that collectively enable strain-level resolution and are reshaping the field.