Staphylococcus is gram-positive. Under a microscope after gram staining, staph bacteria appear blue to purple, confirming their gram-positive classification. This applies to all species in the Staphylococcus genus, including S. aureus, S. epidermidis, and every other member of the family.
What Makes Staph Gram-Positive
The gram stain is a lab technique that sorts bacteria into two camps based on the structure of their cell walls. Gram-positive bacteria like staph have an unusually thick outer layer made of a mesh-like material called peptidoglycan, roughly 30 to 100 nanometers thick and stacked in many layers. Gram-negative bacteria, by comparison, have a peptidoglycan layer only a few nanometers thick.
During staining, the bacteria are first flooded with a purple dye, then treated with iodine, which causes the dye molecules to form large complexes inside the cell. Next comes the critical step: a decolorizing solution (usually ethanol) is applied. In gram-negative bacteria, this washes away the thin cell wall and lets the purple dye leak out. In gram-positive bacteria like staph, the thick peptidoglycan layer dehydrates and tightens, trapping the purple dye complexes inside. That’s why staph stays purple and gram-negative bacteria lose the color (they pick up a pink counterstain instead).
How Staph Looks Under the Microscope
Beyond the purple color, staph has a distinctive shape and arrangement. The cells are round (cocci) and tend to cluster in irregular, grape-like clumps rather than forming neat chains. This happens because staph divides in three alternating perpendicular planes, and the daughter cells stay loosely attached to each other after splitting apart. Because the attachment points shift slightly and the cells can change position relative to one another while still connected, the result is the disorganized clusters that give the genus its name (“staphyle” is Greek for “bunch of grapes”).
This clustering pattern is one way lab technicians distinguish staph from streptococcus under the microscope. Strep bacteria also appear purple on a gram stain, but they line up in chains or pairs instead of clumps.
How Labs Tell Staph Apart From Other Gram-Positive Bacteria
Since several common bacteria are gram-positive, the stain alone doesn’t identify staph. Labs use a simple follow-up called the catalase test. A small amount of hydrogen peroxide is dropped onto a sample of the bacteria. Staph produces an enzyme that breaks down hydrogen peroxide, so the sample bubbles vigorously. Strep lacks this enzyme, so nothing happens. A purple, clustered, catalase-positive result points clearly to staph.
From there, labs need to figure out which species of staph is involved. The most important distinction is between S. aureus and the dozens of other species collectively called coagulase-negative staphylococci. S. aureus produces an enzyme that causes blood plasma to clot. In a study of bloodstream staph infections, about 34% were caused by S. aureus (coagulase-positive), while the remaining 66% were caused by coagulase-negative species like S. epidermidis. S. aureus is generally the more dangerous of the two groups.
Why Gram Status Matters for Treatment
Knowing that staph is gram-positive directly shapes how doctors choose antibiotics. Several major antibiotic classes work by attacking the thick peptidoglycan cell wall that defines gram-positive bacteria. These drugs disrupt the construction of that wall, causing the bacterial cell to burst. This is why antibiotics in the beta-lactam family (which includes penicillin-type drugs) are a frontline choice for staph infections. For bloodstream infections caused by staph strains that respond to standard antibiotics, beta-lactams are the preferred treatment, typically given for a minimum of 14 days.
The challenge is that some staph strains have evolved defenses. MRSA, or methicillin-resistant Staphylococcus aureus, has acquired the ability to resist beta-lactam antibiotics. MRSA rates vary enormously by country. In 2023, WHO data showed that Scandinavian countries like Finland and Denmark reported MRSA in fewer than 2% of staph bloodstream infections. The United States and Canada fell in the 15 to 18% range. In parts of South Asia, the Middle East, and sub-Saharan Africa, MRSA rates exceeded 50 to 70% of tested samples. These resistant strains are still gram-positive, but they require different, often more limited antibiotic options.
Common Staph Infections
Most staph infections are minor skin problems: pimples, boils, and small abscesses that resolve with drainage or topical treatment. Staph bacteria live on the skin and in the nose of roughly 30% of healthy people without causing any trouble. Infections typically start when the bacteria enter through a cut, scrape, or hair follicle.
In hospital settings, staph can cause far more serious problems. Bloodstream infections can progress to sepsis. Staph pneumonia primarily affects people with underlying lung disease or those on breathing machines. The bacteria can also settle on heart valves, causing endocarditis, or travel through the bloodstream to bone, causing osteomyelitis. These invasive infections are why rapid identification of the bacteria, starting with that simple gram stain, matters so much for guiding the right treatment quickly.