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Colony shape as a genetic trait in the pattern-forming Bacillus mycoides
Carmen Di Franco1, Elena Beccari, Tiziana Santini
1Dipartimento di Genetica e Biologia Molecolare, Università La Sapienza, P. le A. Moro 5, 00185 Roma, Italy. carmen.difranco@uniroma1.it
BMC Microbiology
|November 14, 2002
Summary
Bacillus mycoides bacteria form distinct colony patterns, curving either clockwise (DX) or counter-clockwise (SIN), due to early-established asymmetric growth. Mutants reveal that cell division regulation is key to maintaining these unique filamentous structures.
Area of Science:
- Microbiology
- Bacterial Morphology
- Genomics
Background:
- Bacillus mycoides, a soil bacterium from the Bacillus cereus group, exhibits unique growth patterns on agar, forming radial filaments that curve clockwise (DX) or counter-clockwise (SIN).
- The underlying molecular mechanisms driving this asymmetric colony formation remain largely unknown.
- A working hypothesis suggests that the regulation of filamentous growth is a prerequisite for these distinct morphotypes.
Purpose of the Study:
- To investigate the molecular basis of asymmetric colony formation in Bacillus mycoides.
- To understand the role of cell division and growth regulation in establishing SIN and DX morphotypes.
- To characterize morphotype mutants and their relationship to wild-type B. mycoides.
Main Methods:
- Isolation and classification of SIN and DX strains of B. mycoides using biochemical and molecular tests.
- Cultivation of strains on agar with varying hardness and nutrient concentrations.
- Microscopic observation of colony formation and cell division processes.
- Sequencing of the division cell wall (dcw) gene cluster (ftsQ, ftsA, ftsZ, murC) in DX and SIN strains.
Main Results:
- Environmental SIN and DX strains were confirmed as B. mycoides, related to the B. cereus group.
- Colony patterns were consistent across different agar conditions, and no morphotype interconversion was observed.
- Mutants derived from a SIN strain exhibited altered morphology (fluffy or round/compact) and lost aggregation in liquid culture.
- Microscopy revealed differences in cell division and filament elongation between wild-type and mutant strains.
Conclusions:
- The asymmetric DX and SIN morphotypes originate from closely related but not identical genomic contexts.
- Asymmetry in B. mycoides colony formation is established early in growth on agar.
- Wild-type strains form long, uninterrupted filaments by dividing at free ends, enabling extensive "walking" growth, unlike mutants with more frequent cell separation leading to compact colonies.