StraighterLine Microbiology Practice Test 2025 – Complete Exam Prep Guide

The Firmicutes represent a significant phylum of bacteria that are characterized by their thick cell walls, predominantly composed of peptidoglycan. This feature classifies them as Gram-positive bacteria. When subjected to the Gram staining procedure, Firmicutes retain the crystal violet stain, appearing purple under a microscope, which is indicative of a Gram-positive response. The dense peptidoglycan layer in their cell walls also contributes to their rigidity and structural integrity.

While the term "peptidoglycan-rich" could describe their walls as well, it does not encapsulate the full aspect of their classification as Gram-positive. Firmicutes are distinctively identified based on their ability to retain the Gram stain, defining them more accurately as Gram-positive. Therefore, identifying the cell wall type of Firmicutes as Gram-positive directly reflects both their structural characteristics and their classification in microbiology.