How can you determine if colonies on a spread plate are indeed Bacillus subtilis?

To determine if colonies on a spread plate are indeed Bacillus subtilis, comparing the color and morphology of the colonies is a reliable method. Bacillus subtilis has distinctive colony characteristics that can be visually assessed. Typically, colonies of B. subtilis appear as opaque, irregular, and often have a rough surface texture. Their color can range from white to yellowish, and they can show a particular pattern of growth on the agar.

This method of visual observation is practical because it allows for rapid identification based on known colony characteristics of the species. While other factors like size and growth rates can provide some clues, they are not definitive on their own due to variability in colony growth influenced by their environment or media. The shape of the petri dish has no relevance to the identification of bacterial species, so it is not a useful criterion in this context.