A New Genus of the Actinomycetales—Intrasporangium gen.nov.

Kalakoutskii, Kirillova, and Krassilnikov describe a novel genus and species of actinomycete, Intrasporangium calvum, isolated from air in a school dining room. The organism produces fine branching mycelia 0.4–1.2 μm in diameter that fragment easily. Its defining characteristic is the formation of sporangia intercalary (embedded within) mycelial hyphae, rather than on sporangiophore tips as in other sporangium-forming actinomycetes. Mature sporangia are round, oval, or lemon-shaped (5–15 μm diameter) and contain 1–20 round or oval spores (1–2 μm diameter) that are non-motile and capable of germination. The organism grows slowly on meat-extract peptone agar and broth, growing poorly on most standard actinomycete media. It is Gram-positive, not acid-fast, and shows no antibiotic activity. The authors propose classifying this organism within the family Actinoplanaceae based on its sporangium-forming capability and non-motile spores, establishing a new subdivision of that family defined by intercalary sporangium formation.

Key findings

• Intrasporangium calvum is a new genus and species of actinomycete characterized by sporangia formed intercalary within mycelial hyphae, distinguishing it from all other known Actinoplanaceae
• The organism produces non-motile spores within mature sporangia that can germinate and serve as dispersal units
• I. calvum grows slowly and is nutritionally fastidious, requiring meat-extract peptone medium for optimal growth
• The intercalary mode of sporangium formation is consistent, regular, and previously unreported in actinomycetes, indicating it is a stable taxonomic character rather than an involution form