Staphylococcus aureus is a common bacterium found on human skin and in nasal passages, with about 30% of healthy individuals carrying it asymptomatically. While often harmless as part of the normal microbiota, it is an opportunistic pathogen. This means it can cause infections ranging from minor skin conditions like pimples and boils to severe, potentially life-threatening issues such as pneumonia, meningitis, and toxic shock syndrome.
Observing Staphylococcus Aureus Under the Microscope
Under a microscope, Staphylococcus aureus reveals its distinctive shape and arrangement. These bacteria are spherical (cocci), typically 0.5 to 1.5 micrometers in diameter. Their most characteristic feature is their tendency to form irregular, grape-like clusters, from which the genus “Staphylococcus” derives its name (Greek “staphule” meaning “bunch of grapes”).
This clustering occurs because the bacteria divide in multiple planes, and the daughter cells do not fully separate after division. Individual cells can also be observed in pairs or short chains, but the predominant grape-like grouping helps distinguish them from other spherical bacteria like streptococci, which typically form chains. When Gram-stained, Staphylococcus aureus retains the primary stain, appearing blue to purple and classifying it as a Gram-positive bacterium. This purple coloration is due to its thick peptidoglycan cell wall.
The Microscopic Identification Process
Initial identification of Staphylococcus aureus in a laboratory often begins with Gram staining, a differential technique developed by Hans Christian Gram in 1884. This process differentiates bacteria based on their cell wall composition, providing a quick, preliminary classification. The first step involves applying a primary stain, crystal violet, to a heat-fixed bacterial smear on a microscope slide.
After a brief incubation, the crystal violet is rinsed off, and Gram’s iodine, a mordant, is added. This iodine solution forms a complex with the crystal violet, helping to fix the dye within the bacterial cell wall. The slide is then decolorized using alcohol or acetone, a step that is crucial for differentiation. Gram-positive bacteria like Staphylococcus aureus, with their thick peptidoglycan layer, retain the crystal violet-iodine complex and remain purple, as the decolorizer dehydrates and shrinks the cell wall, trapping the stain.
Finally, a counterstain, typically safranin, is applied. Because Staphylococcus aureus retains the purple color from the initial stains, it does not pick up the red or pink color of the safranin. This rapid microscopic examination, showing Gram-positive cocci in clusters, provides initial information in clinical and laboratory settings. It guides further specific tests and informs treatment decisions before culture results are available.