Here's why:
* Gram-positive bacteria have a thick layer of peptidoglycan in their cell wall, which is what makes them stain purple in the Gram stain test.
* Firmicutes is a large phylum that includes many well-known bacteria, such as *Bacillus*, *Clostridium*, *Staphylococcus*, and *Streptococcus*.
* While there are some exceptions, the majority of bacteria within Firmicutes have gram-positive cell walls.
It's important to note that other phyla also contain gram-positive bacteria, but Firmicutes is the defining phylum for this characteristic.
