Gram stain

The Gram stain is a fundamental laboratory technique used to classify bacteria. Scientists use this method to sort bacterial species into two large groups: gram-positive and gram-negative. This classification is based on the physical and chemical properties of the bacterial cell wall. It is often the very first step in identifying a bacterial group. Because it works so quickly, it is a vital tool in medical settings.

Doctors use it to examine body fluids or biopsies when they suspect an infection. For example, it can be used on cerebrospinal fluid to check for meningitis. It can also be used on synovial fluid to check for septic arthritis. This speed helps doctors make important decisions about patient treatment and prognosis.

The process relies on how different cell walls react to specific dyes. Gram-positive bacteria have a very thick, mesh-like cell wall made of peptidoglycan. This layer makes up about 50% to 90% of the cell envelope. In contrast, gram-negative bacteria have a much thinner peptidoglycan layer. This layer only accounts for about 10% of their cell envelope. Gram-negative cells also have an outer membrane containing lipopolysaccharides, or LPS.

This structural difference determines whether a cell retains or loses color during the staining process.

There are four specific steps in the Gram stain mechanism. First, a primary stain called crystal violet is applied to a heat-fixed bacterial smear. Heat fixation helps stick the bacteria to the slide so they do not wash away. Second, Lugol's iodine is added to the sample. The iodine acts as a mordant or trapping agent by binding with the crystal violet. This forms a large, stable complex called CV-I within the cells.

Third, a decolorizer like ethanol or acetone is added. This step is extremely critical and must be timed perfectly. If the decolorizer stays on for even a few seconds too long, it will wash the color out of all cells. Finally, a counterstain like safranin or fuchsine is applied. This gives the decolorized cells a pink or red color.

During decolorization, the two types of bacteria behave very differently. In gram-negative cells, the alcohol interacts with the lipids in the outer membrane. The cell loses its outer lipopolysaccharide membrane, leaving the thin peptidoglycan layer exposed. The CV-I complexes are then washed out of the cell. In gram-positive cells, the ethanol treatment causes the cell to dehydrate. The large CV-I complexes become trapped inside the thick, multilayered peptidoglycan. Consequently, these cells remain purple.

The final counterstain colors the gram-negative cells pink, but the purple of the gram-positive cells is too dark to be seen.

The history of this method begins with Hans Christian Gram. He was a Danish bacteriologist who worked in Berlin. In 1884, he developed this technique while working with Carl Friedländer in a city hospital morgue. Interestingly, Gram did not originally intend to classify different bacterial species. He was actually trying to make bacteria more visible in stained sections of lung tissue. He noticed that certain bacterial cells showed a resistance to decolorization. He published his findings in 1884, noting that the typhus bacillus did not retain the stain.

Many different types of bacteria fall into these two categories. Gram-positive bacteria include groups like Bacillota and Actinomycetota. Many well-known genera, such as Staphylococcus and Streptococcus, are gram-positive. Some of these bacteria use teichoic acids to strengthen their cell walls. Gram-negative bacteria include most phyla, such as Cyanobacteria and Pseudomonadota. These bacteria often rely on their outer membrane and porins to regulate what enters the cell.

Some bacteria, like those in the genus Mycoplasma, lack a cell wall entirely. Because they have no wall, they cannot be classified by this method.

Not every microorganism responds perfectly to the Gram stain. Some bacteria are considered gram-variable, meaning they show a mix of pink and purple cells. This can happen if the peptidoglycan thickness changes as the culture grows older. Other bacteria are called gram-indeterminate because they do not respond predictably. This group includes species like Mycobacterium, which causes tuberculosis and leprosy. These bacteria require different, specialized staining procedures to be identified correctly.