This study characterizes strain Dimbovitza, a leptospira-like spirochete isolated from the Dimbovitza River in Romania in 1953. Using serological, biological, and morphological analyses, the researchers compared strain Dimbovitza with illini strain 3055, another atypical leptospira previously proposed as representative of a new genus, Leptonema. Strain Dimbovitza showed no serological relationship to saprophytic Leptospira strains or pathogenic Leptospira serogroups, but shared similar biological characteristics with illini strain 3055, including growth in Trypticase soy broth, growth at 13°C, resistance to 8-azaguanine, hemolysis of rat erythrocytes, and lecithin degradation. Electron microscopy revealed morphological features including regular wavy outline (12-20 μm long, 0.1 μm wide), two terminal flagella, and cytoplasmic tubules characteristic of treponemes and illini strains. Notably, the flagellar basal complex resembled that of typical leptospires rather than the treponeme-like structure observed in illini strains. The authors conclude that strain Dimbovitza represents a second serotype within the proposed genus Leptonema and represents the first known isolate of this genus outside the United States.

Key findings

• Strain Dimbovitza is serologically distinct from illini strain 3055 and other saprophytic Leptospira strains, indicating the genus Leptonema comprises at least two serotypes.
• Biological characteristics of strain Dimbovitza closely resemble illini strain 3055, including growth in Trypticase soy broth and 8-azaguanine resistance, distinguishing both from typical saprophytic leptospires.
• Electron microscopy revealed flagellar basal complexes in strain Dimbovitza similar to typical leptospires and gram-negative bacteria, unlike the treponeme-like structures in illini strains.
• Strain Dimbovitza contains cytoplasmic tubules similar to those found in treponemes and illini strains, supporting its assignment to genus Leptonema.
• This is the first documented isolate of genus Leptonema from Europe, expanding knowledge of this atypical spirochete's geographic distribution.