MSSA vs MRSA: what's the difference?
MSSA and MRSA are the same bacterium — Staphylococcus aureus. The difference is one letter and one gene: MRSA carries the mecA gene and is resistant to methicillin and most related antibiotics. MSSA is not. Symptoms look identical; treatment does not.
The short answer
MSSA is methicillin-sensitive Staphylococcus aureus. MRSA is methicillin-resistant Staphylococcus aureus. Both cause the same spectrum of illness — from a boil on the leg to bloodstream infection, endocarditis, pneumonia, and sepsis. What separates them is which antibiotics still work. That single fact drives every practical decision, from which drug the pharmacy sends to the ward to whether a patient goes into contact isolation.
At a glance
| Attribute | MSSA | MRSA |
|---|---|---|
| Full name | Methicillin-sensitive Staphylococcus aureus | Methicillin-resistant Staphylococcus aureus |
| Antibiotic resistance | Sensitive to standard beta-lactams | Resistant to methicillin and most beta-lactams |
| First-line treatment | Nafcillin, oxacillin, cefazolin, dicloxacillin | Vancomycin, daptomycin, linezolid; oral: doxycycline, clindamycin, Bactrim |
| Typical settings | Community and hospital | Community (CA-MRSA) and healthcare (HA-MRSA) |
| Contagiousness | Spreads by skin contact and shared items | Same routes, similar transmissibility |
| Nasal carriage | ~30% of healthy people | ~2% of healthy people |
| Bacteraemia mortality | ~15–20% with appropriate therapy | ~20–25% with appropriate therapy |
| Diagnosis | Culture + susceptibility, or PCR | Culture + susceptibility, or PCR (mecA/mecC) |
Are the symptoms different?
No. Clinically, MSSA and MRSA look the same. A skin abscess caused by MSSA and one caused by MRSA present with the same redness, swelling, warmth, and pus. Invasive syndromes — bacteraemia, endocarditis, pneumonia, osteomyelitis — produce the same fevers, rigors, and organ failure regardless of resistance status. You cannot look at an infection and tell which one it is.
Diagnosis therefore rests on the laboratory. A swab, blood culture, or tissue sample is grown on agar; the isolate is then tested against oxacillin (a methicillin surrogate) or checked by PCR for the mecA or mecC gene. The report comes back as MSSA or MRSA and treatment is adjusted accordingly.
Treatment differences
MSSA is easier to treat. For serious infection, IV nafcillin, oxacillin, or cefazolin are first-line — they penetrate tissues well, kill quickly, and have decades of outcome data behind them. Skin MSSA is treated with oral dicloxacillin or cephalexin.
MRSA does not respond to any of those drugs. Serious MRSA is treated with IV vancomycin, daptomycin, linezolid, or ceftaroline; skin MRSA is typically treated with oral doxycycline, clindamycin, or trimethoprim-sulfamethoxazole (Bactrim). Empiric therapy in a seriously ill patient usually covers MRSA from the start — the antibiotic is narrowed down to a beta-lactam only after cultures confirm MSSA. More information on our Treatment page →
Why the distinction matters
The choice of drug changes outcomes. In S. aureus bacteraemia, MSSA patients treated with a beta-lactam do better than MSSA patients treated with vancomycin — vancomycin is a good MRSA drug but a mediocre MSSA drug, and time on the wrong agent costs survival. Rapid PCR testing of blood cultures has become standard in many hospitals partly to shorten the window before therapy is right-sized.
Resistance also has downstream consequences: MRSA carriage triggers contact precautions in most healthcare settings, extends hospital stays, and complicates surgical planning. MSSA carriage does not.
Where each is found
MSSA is ubiquitous — about 30% of healthy people carry S. aureus in the anterior nares, and the great majority of those isolates are methicillin-sensitive. MRSA carriage is much rarer in the general population (around 2%) but is concentrated in specific groups: people recently hospitalised, dialysis patients, IV drug users, nursing-home residents, athletes in close-contact sports, and military recruits. Community-associated MRSA (CA-MRSA) has changed the picture over the last twenty years: many MRSA skin infections now arise in people with none of the traditional healthcare risk factors. Read more about the bacterium and how it spreads.
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MRSA boil
Deep, painful, pus-filled lumps and how they are drained.
MRSA cellulitis
Spreading deeper-skin infection and urgent warning signs.
MRSA folliculitis
Small pus-filled bumps around hair follicles and prevention.
MRSA pneumonia
Hospital, ventilator-associated and necrotising lung infection.
MRSA bacteremia
MRSA in the bloodstream — how it gets there and how it's treated.
MRSA sepsis
Can MRSA cause sepsis? Warning signs and survival rates.
Invasive MRSA
How MRSA reaches the bloodstream, lungs and bone.
MRSA test
PCR nasal swab, wound culture and susceptibility testing.
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How to tell a normal pimple from a staph or MRSA skin lesion.