This study examined how Pelobacter acidigallici, a strictly anaerobic bacterium that ferments gallic acid, colonizes surfaces and forms biofilms using a custom continuous-flow microchamber. The bacterium displayed a complex life cycle involving reversible and irreversible attachment to agar-coated glass surfaces. At low gallic acid concentrations (<0.1 mM), nonmotile cells attached irreversibly and formed small microcolonies (≤128 cells) that maintained their size by releasing motile swarmer cells. At higher substrate concentrations (>5 mM), continuous biofilms developed. Growth rates on surfaces were comparable to suspended cells across most substrate concentrations. Under starvation, motile cells elongated, ceased growth, and were washed away, while irreversibly attached cells continued dividing without cell growth. The study revealed that P. acidigallici undergoes a sophisticated developmental cycle including attachment, detachment, and swarmer cell formation—processes previously unstudied in anaerobic bacteria. The heterogeneous biofilm structure observed at environmentally relevant low substrate concentrations challenges existing biofilm formation models based on homogeneous film structures.

Key findings

• Pelobacter acidigallici exhibits a complex life cycle with reversibly attached motile cells and irreversibly attached nonmotile cells that respond differently to surface colonization
• Microcolony size is maintained at low substrate concentrations by peripheral release of motile swarmer cells, while continuous biofilms form only at gallic acid concentrations >5 mM
• Growth rates on surfaces are comparable to suspended cultures across substrate concentrations of 0.1-10 mM gallic acid, but decline significantly below 0.1 mM
• Biofilms developing at low substrate concentrations maintain heterogeneous structures with distinct microcolonies, challenging existing homogeneous biofilm models
• Under starvation, motile cells undergo morphological changes and washout while attached cells continue dividing without growth, indicating distinct survival strategies