Staphylococcus versus Streptococcus on Gram stain: clusters versus chains
On a Gram stain, which shows bacterial colour and shape, staphylococci are purple spherical bacteria usually arranged in grape-like clusters, whereas streptococci are purple spherical bacteria commonly arranged in chains.
Remember
Staphylococci and streptococci are both purple spherical bacteria, so arrangement separates them: grape-like clusters suggest Staphylococcus and chains suggest Streptococcus.
Memory aid
STREPS are STRIPS
Streptococci are commonly arranged in strips of cocci, which are chains, whereas staphylococci are usually arranged in bunches similar to grapes.
A Gram stain gives two pieces of information: colour and shape. Bacteria that keep the crystal violet dye look purple and are called Gram-positive, whereas Gram-negative bacteria look pink. The dye is held by the thick peptidoglycan layer, a mesh of sugars and short protein chains in the bacterial wall.
A coccus (plural cocci) is round, a bacillus (plural bacilli) is a rod, and a spirillum is spiral, which is uncommon. Staphylococci and streptococci are both purple cocci, so it is the arrangement of the cocci that separates them.
Staphylococci are usually arranged in bunches, similar to grapes. Staphylococcus aureus, a common cause of skin abscesses and cellulitis (spreading infection of the skin), is the best-known example. Streptococci are commonly arranged in chains, like strips of cocci, which is why STREPS are STRIPS.
Streptococcus pneumoniae is the commonest exception to learn, because it usually appears as pairs of cocci (diplococci). So purple clusters from a skin swab suggest Staphylococcus aureus, while purple pairs or chains from a chest sample would point towards streptococci.
The stain is an early clue, not a final answer. It narrows the differential diagnosis before culture results are back, but culture and further tests are needed to confirm the species.
Key facts
- Staphylococci
- Gram-positive cocci that are usually arranged in bunches, similar to grapes.
- Streptococci
- Gram-positive cocci that are commonly arranged in chains.
- Colour
- purple means Gram-positive and pink means Gram-negative.
- Pairs
- Streptococcus pneumoniae usually appears as pairs of cocci (diplococci).
- Skin clue
- purple cocci in clusters from a skin abscess are most often Staphylococcus aureus.
- Limit
- the stain narrows the differential diagnosis, but culture and further tests confirm the species.
Exam traps
- Gram-positive cocci in long chains do not describe Staphylococcus. Both Staphylococcus and Streptococcus are Gram-positive cocci, so the arrangement is what separates them, and cocci in chains describe Streptococcus.
- Purple cocci in clusters from a skin abscess do not point to Streptococcus pneumoniae. It is usually seen in pairs and causes pneumonia and meningitis rather than skin abscesses.
- Rod-shaped bacteria in chains are not Streptococcus. Streptococci are commonly arranged in chains, but their cells are spherical, not rods.
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