Yes, Streptococcus is gram-positive. Every species in the Streptococcus genus retains the crystal violet dye during Gram staining, appearing purple under the microscope. This is due to a thick peptidoglycan cell wall, 20 to 80 nm thick, compared to only 2 to 3 nm in gram-negative bacteria. That thick wall traps the dye and resists the decolorizing step, which is the whole basis of the Gram stain test.
What Streptococcus Looks Like Under a Microscope
Streptococci are round (coccus-shaped) bacteria that divide in a single plane, which causes them to line up in pairs or chains. The chain formation is one of the quickest visual clues in a lab setting. Individual cells are tiny, ranging from about 0.5 to 1.25 micrometers in diameter depending on the species.
Not all species look identical. S. pyogenes, the species behind strep throat, appears as round-to-ovoid cocci typically 0.6 to 1.0 micrometers across. S. pneumoniae, which causes pneumonia and meningitis, looks slightly different: it forms pairs (diplococci) with a distinctive lancet or flame shape rather than long chains.
How the Gram Stain Works
The Gram stain is a four-step process. First, a sample of bacteria is spread on a glass slide and heat-fixed so the cells stick. Crystal violet, a purple dye, is applied for up to 60 seconds and then rinsed off. Next, an iodine solution is added to lock the dye into the cell wall. Then comes the critical step: an alcohol-acetone solution is washed over the slide to try to strip the dye away. Bacteria with thin cell walls lose the purple color at this point, but gram-positive bacteria like Streptococcus keep it. Finally, a pink-red counterstain is applied, which only shows up in bacteria that lost the purple dye.
Under the microscope at 100x magnification with oil immersion, gram-positive bacteria appear purple-brown. Gram-negative bacteria appear pink-red. When a lab technician sees purple cocci arranged in chains, Streptococcus is the immediate suspect.
Telling Streptococcus Apart From Staphylococcus
Both Streptococcus and Staphylococcus are gram-positive cocci, so the Gram stain alone can’t distinguish them. The first difference is arrangement: Staphylococcus grows in irregular grape-like clusters, while Streptococcus forms chains or pairs. The second key test is the catalase test. A drop of hydrogen peroxide is placed on the bacteria. Staphylococcus produces the enzyme catalase and immediately generates bubbles. Streptococcus is catalase-negative, so nothing happens. This simple reaction is one of the most reliable ways to separate the two genera in the lab.
How Streptococcus Species Are Classified
Once bacteria are identified as Streptococcus, labs use two additional systems to figure out exactly which species is present: hemolysis patterns and Lancefield grouping.
Hemolysis on Blood Agar
When Streptococcus is grown on blood agar plates, different species break down red blood cells in different ways. Beta-hemolytic streptococci completely destroy the red blood cells around each colony, leaving a clear zone. This group includes some of the most aggressive human pathogens, like S. pyogenes. Alpha-hemolytic streptococci only partially break down red blood cells, producing a greenish discoloration around the colony. S. pneumoniae and the viridans group streptococci fall into this category (“viridans” comes from the Latin word for green). Gamma-hemolytic streptococci don’t break down red blood cells at all.
Lancefield Groups
The Lancefield system classifies streptococci by specific carbohydrate molecules on their cell surface. There are 20 groups, but only a handful matter in human medicine. Group A (S. pyogenes) causes strep throat, scarlet fever, skin infections, and in rare cases necrotizing fasciitis. Group B (S. agalactiae) is a leading cause of sepsis and meningitis in newborns. Groups C and G include species that occasionally cause pharyngitis and skin infections in humans, often with an animal origin. The viridans group streptococci don’t fit neatly into any single Lancefield category and are classified separately.
Major Streptococcus Species and Their Diseases
- S. pyogenes (Group A): Strep throat, tonsillitis, scarlet fever, impetigo, wound infections. Can trigger immune complications like rheumatic fever and kidney inflammation weeks after the initial infection.
- S. agalactiae (Group B): The primary concern during pregnancy and childbirth. Causes sepsis, meningitis, and pneumonia in newborns. Also causes urinary tract infections and skin infections in adults.
- S. pneumoniae: The most common bacterial cause of community-acquired pneumonia. Also causes ear infections, sinus infections, and meningitis. Classified by its alpha-hemolysis rather than a Lancefield group.
- Viridans group (S. mutans, S. mitis, S. sanguis, and others): Normal inhabitants of the mouth and gut. S. mutans is a major driver of tooth decay. Several viridans species can cause endocarditis, an infection of the heart valves, particularly in people with pre-existing heart conditions.
Why Gram Status Matters for Treatment
Knowing that Streptococcus is gram-positive has direct treatment implications. The thick peptidoglycan wall that holds onto the Gram stain is also the structure that many common antibiotics target. Gram-positive bacteria are generally more susceptible to antibiotics that interfere with cell wall construction than gram-negative bacteria, which have an additional outer membrane that acts as a barrier. This is why streptococcal infections are often treatable with widely available, well-established antibiotics, while gram-negative infections can be harder to manage.
The Gram stain result is typically available within an hour of a sample reaching the lab, long before full culture results come back. That quick purple-cocci-in-chains identification lets clinicians start targeted treatment without waiting days for a definitive species identification.