Williams and Rittenberg document a biphasic life cycle in Spirillum lunatum, a marine bacterium, characterized by alternation between motile spiral vegetative cells and dormant microcysts. Using phase-contrast microscopy, they observed three distinct pathways for microcyst formation: fusion of two entwined organisms producing one or more microcysts at fusion points; formation of a protuberance that gradually absorbs the remainder of the organism; and gradual shortening and rounding of the entire cell into oval or spherical bodies. Microcyst formation begins approximately 24 hours after inoculation into fresh medium and predominates by day four. When mature microcysts from old cultures are transferred to fresh media, they germinate via either unipolar or bipolar emergence of a germ tube, regenerating normal spiral organisms within 6–16 hours depending on culture conditions. The authors establish that microcyst formation is not synchronous, with freely motile organisms observable for up to three weeks. This represents a complete life cycle common to several Spirillum species and provides evidence that eubacteria employ diverse developmental strategies beyond simple binary fission.

Key findings

• Spirillum lunatum exhibits a distinct life cycle alternating between vegetative spiral cells and dormant microcysts, with three mechanisms of microcyst formation: organism fusion, protuberance formation, and gradual shortening
• Microcyst formation initiates around 24 hours post-inoculation and becomes predominant by day four in broth culture
• Microcysts germinate via unipolar or bipolar germ tube emergence to regenerate spiral vegetative organisms, with germination occurring 6–16 hours after transfer to fresh medium
• This life cycle pattern occurs in multiple Spirillum species and demonstrates that bacterial development involves strategies beyond transverse fission