What's the difference between gram-positive and gram-negative bacteria?

Most species of bacteria can be broadly divided into two groups, known as gram-positive and gram-negative. These categories reflect big differences in the microbes' biology, and they also dictate how doctors treat bacterial infections.

But what are the differences between gram-positive and gram-negative bacteria?

The names themselves date back to 1884, when Danish bacteriologist Hans Christian Gram developed a staining procedure to view bacteria under the microscope. First, he applied a dye, called gentian violet, which penetrates both the protective wall and membrane of bacteria, thus staining the material inside. Then, he added the mineral iodine, which formed a complex with the dye that wouldn't break down in water, thus "fixing" the stain in place.

After being washed with alcohol, some bacteria remained blue or purple, while others did not retain the stain. The first group was dubbed "gram-positive," while the latter was designated "gram-negative."

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Gram's stain experiment pointed toward some sort of difference in the structures of various bacterial cells. However, it was not until the early 1950s that scientists started to understand the differences in the chemical composition of bacterial cell walls that makes a difference in the staining.

Differences between gram-positive and gram-negative bacteria

All bacteria have a cell wall composed of mesh-like strands of a big molecule called peptidoglycan, which surrounds the cell membrane. A cell wall provides the bacterial cell sturdiness and helps maintain its shape and internal pressure. However, there are some key differences between the two classes of bacteria.