Bacillus subtilis is indeed a Gram-positive bacterium, a classification based on a widely used staining technique in microbiology. This common microorganism is typically found in soil and various environments, including the gastrointestinal tracts of humans and animals. Its widespread presence makes it a well-studied model organism in scientific research.
Understanding the Gram Stain
The Gram stain, developed by Hans Christian Gram, is a fundamental laboratory procedure used to categorize bacteria into two main groups: Gram-positive and Gram-negative. The process begins by applying crystal violet, a purple dye, to a bacterial sample, staining all cells purple. Next, an iodine solution, acting as a mordant, is added to form a crystal violet-iodine complex within the bacterial cells.
Following the mordant, a decolorizing agent, such as alcohol or acetone, is applied to the slide. This step differentiates bacteria by washing the dye complex out of some cells but not others. Finally, a counterstain like safranin (pink or red) is applied to stain cells that lost the initial purple dye. Gram-positive bacteria retain the purple color, while Gram-negative bacteria appear pink or red after the counterstain.
The Cell Wall of Bacillus Subtilis
Its Gram-positive staining is due to the specific structure of its cell wall. Unlike Gram-negative bacteria, Bacillus subtilis possesses a thick layer of peptidoglycan, a robust polymer of sugars and amino acids. This peptidoglycan layer, 20 to 80 nanometers thick, forms a protective barrier around the cell.
It also lacks an outer membrane, characteristic of Gram-negative organisms. During Gram staining, its thick peptidoglycan layer traps the crystal violet-iodine complex. The decolorizing agent, typically alcohol, causes dehydration and shrinkage of this peptidoglycan, sealing the dye complex within the cell and preventing its wash-out.
Why Its Gram Status Matters
Its Gram-positive status, particularly its thick peptidoglycan cell wall, contributes to its resilience. This robust cell wall provides structural integrity and protection against environmental stresses, including osmotic pressure and some antimicrobial agents. This strong outer layer helps the bacterium survive in diverse and often harsh conditions, such as soil or food processing environments.
Furthermore, Bacillus subtilis is known for its ability to form dormant endospores, a characteristic common among Gram-positive bacteria within the Bacillus genus. These endospores are highly resistant structures that allow the bacterium to endure extreme temperatures, desiccation, radiation, and disinfectants. This resilience, partly due to its cell wall and spore-forming capability, makes Bacillus subtilis valuable in industrial applications, such as enzyme production (e.g., amylases and proteases) and as a probiotic in human and animal health.