Staphylococci & Streptococci: Gram-Positive Cocci Guide

Staphylococci are gram-positive cocci that grow in grape-like clusters, while streptococci are gram-positive cocci that grow in chains. Both are among the commonest bacterial pathogens on earth: staphylococci dominate skin and wound infections, and streptococci dominate sore throats, pneumonia and the immune complications of rheumatic fever. Telling them apart, and then identifying the exact species, is the first practical skill of the microbiology laboratory.

This matters because Staphylococcus aureus can turn a pimple into bloodstream sepsis, and Streptococcus pyogenes can turn a sore throat into rheumatic heart disease weeks later. India still sees large numbers of both, from MRSA wound infections to pneumococcal pneumonia in children.

Key Concepts

• Gram stain: both appear as purple (gram-positive) spheres; clusters suggest staphylococci, chains suggest streptococci.
• Catalase test: staphylococci are catalase-positive (bubbles with hydrogen peroxide); streptococci are catalase-negative; this single test splits the two genera.
• Coagulase test: Staphylococcus aureus is coagulase-positive (clots plasma); coagulase-negative staphylococci like S. epidermidis are usually skin flora but infect prosthetic devices.
• MRSA: methicillin-resistant S. aureus carries the mecA gene making PBP2a, resisting all beta-lactams; treated with vancomycin, linezolid or daptomycin.
• Lancefield grouping: streptococci are classified A to V by cell-wall carbohydrate; Group A is S. pyogenes, Group B is S. agalactiae (neonatal meningitis).
• Key streptococci: S. pyogenes (beta-haemolytic, bacitracin-sensitive, causes pharyngitis and rheumatic fever); S. pneumoniae (alpha-haemolytic lancet-shaped diplococci, optochin-sensitive, bile-soluble, causes pneumonia and meningitis); viridans streptococci (dental flora, cause endocarditis after dental procedures).

How It Works

1. A pus or throat swab is Gram stained; purple cocci in clusters or chains set the direction.
2. The catalase test divides the sample: bubbles mean staphylococcus, no bubbles mean streptococcus.
3. For staphylococci, the coagulase test identifies S. aureus; a cefoxitin disc screens for MRSA.
4. For streptococci, blood agar haemolysis is read: complete (beta) clearing suggests S. pyogenes, confirmed by bacitracin sensitivity; greenish (alpha) haemolysis with lancet-shaped diplococci suggests pneumococcus, confirmed by optochin sensitivity and bile solubility.
5. Lancefield grouping and specific antibody tests (ASO titre for recent S. pyogenes infection) complete the identification.

Worked Example

A urine sample from a woman with burning micturition is cultured with a calibrated 0.001 mL loop. Step 1: after incubation, the plate shows 200 colonies of gram-positive cocci in clusters. Step 2: colony count per mL = 200 divided by 0.001 = 200,000 colony-forming units per mL, which is 2 times 10 to the power 5. Step 3: the threshold for significant bacteriuria is 10 to the power 5 (100,000) CFU per mL, so 200,000 exceeds it and confirms a true urinary infection rather than contamination. Step 4: catalase is positive and coagulase is negative, identifying a coagulase-negative staphylococcus; in a young woman this pattern points to Staphylococcus saprophyticus, a classic cause of honeymoon cystitis, treated with nitrofurantoin.

Common Misconceptions

• "All staphylococci in a culture are dangerous." Coagulase-negative staphylococci are normal skin flora and common contaminants
• "Every sore throat needs antibiotics." Most sore throats are viral; antibiotics are for proven Group A streptococcal pharyngitis (Centor criteria plus a rapid test or culture), to prevent rheumatic fever.
• "MRSA exists only in hospitals." Community-associated MRSA causes skin boils and necrotising pneumonia in healthy people with no hospital contact.

Real-World Applications

• Wound and blood cultures: rapid MRSA screening guides vancomycin use.
• Sore-throat clinics: rapid antigen tests for Group A streptococcus prevent rheumatic fever.
• ASO titre above 200 units supports a diagnosis of rheumatic fever or post-streptococcal glomerulonephritis.

Staphylococci cluster and are catalase-positive; streptococci chain and are catalase-negative; coagulase, haemolysis and Lancefield grouping then name the species. Colony counts separate true infection from contamination, and each species carries its own disease and drug pattern.