Colonic ecosystem by pH

Reading the chart: Panels A and B share the same x-axis. At any pH, read vertically to see which populations are active (Panel A) and what they are producing (Panel B). At pH 6.0: Firmicutes dominate, butyrate peaks, ammonia is at baseline, SCFA protonation is strong. At pH 6.5: Bacteroidetes have overtaken Firmicutes, butyrate is declining, the TGN sigmoid is beginning to lift. At pH 7.0+: the positive feedback loop is active — ammonia rising, butyrate collapsed, toxic BCFA and biogenic amines appearing. Panel C shows the survival consequence: peak survival near pH 6.5 in the Osuka ICU cohort, dropping sharply on both sides for different reasons.

The SCFA antimicrobial mechanism: The protonated (undissociated) form of SCFAs penetrates gram-negative membranes and acidifies the bacterial cytoplasm. This form only exists in sufficient quantity at acidic pH. As pH rises, SCFAs deprotonate, lose membrane-penetrating ability, and Enterobacteriaceae escape suppression. pH does not suppress these organisms directly — the protonated SCFAs do. The pH tells the practitioner whether the weapon is loaded.

The positive feedback loop: Above ~pH 6.5: urease output rises (enzyme optimum ~7.4) → ammonia produced → pH rises → SCFA deprotonates → antimicrobial pressure drops → more Enterobacteriaceae grow → more urease → more ammonia. Self-accelerating. Breaking it requires intervention at multiple points simultaneously.

The lactate-to-butyrate relay and the three lactate peaks: The lactate curve in Panel B shows three distinct peaks. The small middle peak (~pH 5.8) is healthy: LAB produce lactate and the cross-feeders consume it as fast as it is made — steady-state flux through the relay, never accumulating. The acidic peak (~pH 5.0) is cross-feeder failure: LAB are still producing lactate but the Firmicutes cross-feeders (E. hallii, Anaerostipes) have been depleted, typically by antibiotics. Lactate accumulates because the receiving end of the relay is gone. Lactic acid (pKa 3.86, stronger than SCFAs at pKa ~4.8) drives pH lower than healthy SCFA acidification — same low number, different pathology. The alkaline peak (~pH 7.5) is D-lactate from ecosystem collapse: pathological fermentation by different organisms producing D-lactic acid, succinic acid, and formic acid. This is the critically ill patient's acidic stool — not the relay at all. Bacteroidetes are not part of the relay at any pH.