This 1965 study examined nutritional control of cellular morphology in an aerobic actinomycete isolated from hamster periodontal disease. The organism exhibited dimorphism, existing as either filamentous branching mycelium or as diphtheroid (coccoid) forms. Researchers tested how different growth conditions affected morphology using six strains. Aerobic incubation favored filamentous mycelial development, while carbon dioxide-enriched conditions promoted diphtheroid formation. Iron depletion induced filament formation in some experiments, though results were inconsistent. Magnesium affected growth rate and amount but not morphology. Sub-inhibitory concentrations of certain antibiotics—particularly streptomycin sulphate and chloramphenicol—induced mycelium formation. The findings demonstrate that environmental and nutritional factors directly regulate morphological switches in this actinomycete, similar to dimorphic control mechanisms observed in other microorganisms. The authors suggest these results have implications for understanding oral actinomycete morphology in vivo versus in vitro, since conditions in the oral cavity might favor the diphtheroid form through elevated CO₂ concentrations.

Key findings

• Aerobic incubation promotes filamentous branching mycelial growth, while CO₂-enriched atmospheres promote diphtheroid colony formation in this actinomycete
• Iron depletion induces filament formation, though results were inconsistent across experiments
• Sub-inhibitory concentrations of streptomycin sulphate and chloramphenicol induce mycelium development instead of diphtheroid forms
• Magnesium concentration affects growth rate and amount but does not affect cellular morphology
• Environmental and nutritional factors regulate morphological dimorphism, with implications for understanding organism morphology in oral cavity environments