Although microbes or animal cells can be visible under the microscope, the individual parts of the tiny cell may be hard to differentiate as the colors under the microscope can appear alike, or the cell appear see-through. Laboratory analysts who specialize in microscopic analysis of cells use staining methods to impart colors to the cell, so they can see the parts clearly. Differential staining refers to the type of staining that allows the analyst to tell different types of the cells apart; it is a general term that encompasses a variety of staining procedures.
When an animal is broken down into its cells, the cells can vary in appearance and in function. Commonly different species have sets of cells that appear individual under the microscope. A common example of the different between species and groups is the way that most bacteria can be split into two groups based on their reaction to a particular sort of stain called the Gram stain. Hans Christian Gram was a Danish microbiologist who first invented the stain in 1844, which is still in common usage as a useful first step of bacterial identification in a laboratory setting.
In differential staining, a sample of a bacterial population is treated with sets of dyes in a process that involves steps like heating and washing the samples, so that the dye gets into all of the cells. The various steps use dyes like crystal violet and fuchsin, along with other substances like alcohol and iodine to help fix the color. Cells that appear pink are identified as Gram negative, whereas blue-colored cells at the end of the process are Gram positive. This color difference helps the microbiologist pinpoint which type of cell wall the species has, which helps to narrow down the possible list of species an unknown sample belongs to. As well as indicating Gram type, the differential staining process makes the shapes and arrangements of the cells more obvious, which also helps in identification.
Animal cells can also be sorted under the microscope with differential staining. For example, cells that circulate in the blood react differently to certain stains. An example is Wright's stain, which incorporates dyes like eosin and methylene blue, and which can tell a lab analyst which types of blood cells are present in a sample and at what concentration. For example, an eosinophil cell tends to take up a lot of eosin coloration compared to other blood cells. Analysts generally use stain coloration identified through differential staining along with other cell characteristics such as size, shape and internal structures to figure out which cells are in a sample.