Physiology and ultrastructure of Leptothrix discophora SS-1

Abstract

Leptothrix discophora strain SS-1 (ATCC 43182) is a Gram-negative, Mn²⁺-oxidizing, aerobic heterotroph which lost its sheath-forming ability after 18 months of cultivation on laboratory media. SS-1 possesses high 6-phosphogluconate dehydratase and KDPG aldolase activities, and a very low level of phosphofructokinase, indicating carbohydrate catabolism by the Entner-Doudoroff pathway. The strain is polarly flagellated, accumulates PHB up to 67% of its dry weight when grown in pyruvate-containing medium, and has a G+C content of 69.8 mol%. These properties indicate that L. discophora is essentially a pseudomonad which can form a sheath and oxidize Mn²⁺. Ultrastructural observations made before SS-1 lost its sheath-forming ability indicated two cell types. Short, flagellated, non-sheathed cells seen under the electron microscope probably corresponded to swarmer cells observed under phase-contrast microscopy. These cells contained plate organelles and PHB granules, and produced extracellular blebs approx. 25–50 nm in diameter. Larger sheathed cells also contained plate organelles, PHB granules, and blebs that were often sandwiched between the outer membrane and the sheath. Cells grown in the presence of added Mn²⁺ were surrounded by an extensive fibrillar matrix, rendered electron dense by precipitation of manganic oxide. The matrix was connected to various points of the cell by outer membrane evaginations or electron dense threads. We propose that the outer membrane blebs represent vehicles for excretion of unorganized sheath material and/or Mn²⁺-oxidizing protein produced by L. discophora.