Does low abundance of both Firmicutes and Bacteroidetes indicate reduced microbial biomass?

The clinical focus on the Firmicutes-to-Bacteroidetes (F/B) ratio often overlooks a critical ecological dimension: the absolute quantity of the microbiota. Maintaining a robust, resilient gut microbiome is vital for preserving systemic health and defending against pathogens.

Absolute biomass vs. relative ratios

• Core phyla dominance: Firmicutes and Bacteroidetes constitute the overwhelming majority of the healthy human gut microbiota.
• Global depletion: A concurrent decline in the absolute abundance of both phyla directly reflects a systemic depletion of global microbial biomass rather than a simple taxonomic imbalance.
• Limitations of relative metrics: Relative abundance metrics (such as the F/B ratio) can mask this depletion; a mathematically "normal" ratio can persist even in severely low-biomass states, such as those following intensive antibiotic therapy or bowel preparation.

Mechanistic pathways of ecosystem resilience

• Loss of niche occupancy: Reduced overall microbial biomass directly impairs colonization resistance by leaving crucial nutritional and spatial niches vacant.
• Impaired pathogen defense: In a high-biomass ecosystem, dense bacterial populations physically occupy the mucosal niche and aggressively compete for resources, preventing pathogen invasion.
• Vulnerability to dysbiosis: A depleted microbial load lacks this functional redundancy and competitive exclusion capability, leaving the gut ecosystem highly vulnerable to opportunistic pathogens and destabilizing dysbiotic shifts.

Bottom line

• Simultaneous low abundance of both Firmicutes and Bacteroidetes indicates a profound depletion of overall gut microbial biomass, which compromises ecosystem resilience and colonization resistance. Evaluating absolute microbial load provides a much more accurate assessment of gut stability than relying on relative taxonomic ratios.