Gram positive refers to a category of bacteria that turn dark purple or violet under a microscope after being treated with a specific staining technique called the Gram stain. The color difference comes down to the physical structure of the bacterial cell wall: gram-positive bacteria have an unusually thick outer wall that traps the purple dye, while gram-negative bacteria have a thinner wall and lose it. This distinction is one of the first and most useful pieces of information doctors get when identifying a bacterial infection, because it immediately narrows down what type of organism is involved and which antibiotics are likely to work.
How the Gram Stain Works
The Gram stain is a four-step process developed in the 1880s by Hans Christian Gram, and it remains a cornerstone of microbiology labs today. First, a sample of bacteria is spread onto a glass slide and flooded with a purple dye called crystal violet. At this stage, all bacteria, positive and negative alike, absorb the dye and turn purple.
Next, an iodine solution is applied. The iodine reacts with the crystal violet to form a larger, bulkier chemical complex inside the cell. Think of it like the dye locking into place. In the third step, a decolorizing solvent (typically a mix of ethanol and acetone) is washed over the slide. This is the critical moment. In gram-negative bacteria, the solvent dissolves the thin, fatty outer layer of the cell wall, and the dye-iodine complex washes right out. In gram-positive bacteria, the solvent actually dehydrates the thick cell wall, shrinking its pores shut and trapping the purple complex inside.
Finally, a pink counterstain is applied. Gram-negative bacteria, now colorless after losing their purple dye, pick up the pink color instead. Gram-positive bacteria, still holding onto the purple, are unaffected. The end result: purple cells are gram positive, pink cells are gram negative.
What Makes the Cell Wall Different
The reason gram-positive bacteria hold onto the dye is their peptidoglycan layer, a rigid mesh of sugars and amino acids that forms the structural skeleton of the cell wall. In gram-positive species, this layer is roughly 20 to 36 nanometers thick, made up of 10 or more stacked layers. Gram-negative bacteria, by comparison, have just one or two layers of peptidoglycan.
Gram-negative bacteria compensate with a different feature: a second outer membrane that sits on top of their thin peptidoglycan. This outer membrane contains a molecule called lipopolysaccharide, which plays its own role in infection and inflammation. Gram-positive bacteria lack this outer membrane entirely. Their thick peptidoglycan wall is essentially exposed to the surrounding environment, which is precisely why they respond so differently to the staining process and to certain antibiotics.
Gram-positive cell walls also contain molecules called teichoic acids, though research using atomic force microscopy has shown that these don’t significantly change the wall’s overall architecture. The peptidoglycan itself is the dominant structural feature.
Why It Matters for Treating Infections
Knowing whether bacteria are gram positive or gram negative gives doctors a head start on treatment, often before the specific species has been identified. The Gram stain produces results almost immediately, while full culture and identification can take a day or more.
That thick peptidoglycan wall is both a strength and a vulnerability. Antibiotics in the beta-lactam family, which includes penicillins and cephalosporins, work by blocking the enzymes bacteria use to build and cross-link their peptidoglycan. Since gram-positive bacteria depend so heavily on this thick wall for survival, disrupting it causes the cell to weaken and burst. Gram-negative bacteria are often naturally more resistant to these antibiotics, partly because their outer membrane acts as an additional barrier that drugs must cross before even reaching the peptidoglycan.
This doesn’t mean all gram-positive infections are easy to treat. Some gram-positive bacteria, most notably certain strains of Staphylococcus aureus (MRSA), have developed resistance to beta-lactam antibiotics and require alternative treatments.
Common Gram-Positive Bacteria and Their Infections
Gram-positive bacteria fall into two broad shape categories: cocci (round) and rods (elongated). Many of the most familiar human pathogens are gram positive.
- Staphylococcus aureus causes skin infections, pneumonia, bloodstream infections, bone infections, and abscesses. Certain strains produce toxins responsible for food poisoning, toxic shock syndrome, and scalded skin syndrome.
- Streptococcus pneumoniae is the most common bacterial cause of ear infections, sinus infections, pneumonia, and meningitis.
- Streptococcus pyogenes (group A strep) causes strep throat, cellulitis, impetigo, scarlet fever, and in rare cases necrotizing fasciitis. It can also trigger immune-related complications like rheumatic fever.
- Enterococci live naturally in the gut and typically cause urinary tract infections or biliary tract infections, especially in hospital settings.
- Clostridioides difficile (C. diff) causes severe diarrhea and colon inflammation, often after antibiotic use disrupts the normal gut bacteria.
- Listeria monocytogenes spreads through contaminated deli meats and unpasteurized dairy, causing serious illness particularly in pregnant women, newborns, and people with weakened immune systems.
Some gram-positive bacteria are also part of the body’s normal flora and only cause problems in specific circumstances. Staphylococcus epidermidis, for instance, lives harmlessly on the skin but can infect implanted medical devices by forming sticky biofilms on their surfaces. Staphylococcus saprophyticus is a normal resident of the genital tract but is the second most common cause of uncomplicated urinary tract infections.
Gram Positive vs. Gram Negative at a Glance
- Cell wall thickness: Gram-positive bacteria have a peptidoglycan layer roughly 20 to 36 nm thick. Gram-negative bacteria have a single thin layer.
- Outer membrane: Only gram-negative bacteria have one.
- Stain color: Gram positive appears purple or violet. Gram negative appears pink.
- Antibiotic susceptibility: Gram-positive bacteria are generally more susceptible to penicillins and related drugs, though resistance varies by species and strain.
When Results Aren’t Clear-Cut
The Gram stain is fast and practical, but it’s not perfect. Some bacteria can appear “gram variable,” meaning they stain inconsistently, sometimes showing a mix of purple and pink cells on the same slide. This often happens when cultures are old, because aging bacteria may have damaged or thinning cell walls that no longer trap the dye reliably. Over-decolorizing the slide (leaving the solvent on too long) can also wash the purple dye out of genuinely gram-positive bacteria, producing a false gram-negative result.
A small number of medically relevant bacteria don’t stain well with the Gram method at all. Mycobacteria, the group that includes the tuberculosis bacterium, have waxy cell walls that resist standard staining and require a different technique entirely. For these reasons, the Gram stain is typically a starting point rather than the final answer, with culture results and other tests following to confirm the exact species and its antibiotic sensitivities.