Infectious Diseases & Family Medicine & Infection Control
MRSA — Methicillin-resistant Staphylococcus aureus
MRSA—acronym for Methicillin-Resistant Staphylococcus aureus Staphylococcus aureus) — is a strain of Staphylococcus aureus having acquired the gene mecA or mecC) carried by the mobile genomic element SCCmecStaphylococcal Cassette Chromosome mec), which codes a modified penicillin-binding protein (PBP2a) with low affinity for all beta-lactams—penicillins, cephalosporins, and carbapenems—rendering these bacteria resistant to the entire class of beta-lactams, which form the backbone of staphylococcal antibiotic therapy. Staphylococcus aureus is already a particularly virulent bacterium thanks to its arsenal of pathogenicity factors (toxins + Panton-Valentine leukocidin + TSST-1 + adhesion proteins + polysaccharide capsule + protein A) - and the acquisition of multi-antibiotic resistance by MRSA drastically reduces the available therapeutic arsenal, increasing the mortality and morbidity of infections compared to susceptible strains (MSSA). Two distinct epidemiologies coexist in Quebec and Canada: hospital-acquired MRSA (HA-MRSA) - acquired in healthcare facilities + affecting patients with risk factors (recent hospitalizations + dialysis + surgery + catheter + immunodepression) + often resistant to multiple antibiotic classes + SCCmec types I-III - and community-acquired MRSA (CA-MRSA) - acquired outside healthcare + affecting young, healthy individuals + often carrying Panton-Valentine leukocidin (PVL), which causes necrotizing skin infections + furuncles + recurrent abscesses + necrotizing pneumonia ++ + mainly SCCmec type IV. The prevalence of MRSA nasal carriage in the Canadian general population is estimated at 1-3 %, and MRSA infections account for 15-30 % of all infections with S. aureus in Quebec hospitals according to data from recent years.
Hospital-acquired MRSA vs. Community-associated MRSA
| Characteristic | Hospital-acquired Methicillin-resistant Staphylococcus aureus | Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) |
|---|---|---|
| Affected population | Hospitalized patients + elderly + immunocompromised + dialysis patients + catheter or prosthesis wearers + recent surgery | Young adults and children without medical risk factors + athletes + military + inmates + daycare centers + closed indigenous communities |
| Typical infections | Bacteremia + ventilator-associated pneumonia + catheter-associated infection + surgical site infection + endocarditis + prosthetic joint infection | Skin and soft tissue infections (boils + carbuncles + subcutaneous abscesses + necrotizing cellulitis) + post-influenza necrotizing pneumonia + necrotizing fasciitis |
| Panton-Valentine leukocidin (PVL) | Generally absent | Present in 70–80% of CA-MRSA strains + responsible for tissue necrosis + recurrent infections + severe necrotizing pneumonia |
| Resistance profile | Multidrug-resistant (beta-lactams + aminoglycosides + macrolides + fluoroquinolones + tetracyclines) + treatment = vancomycin or daptomycin | Beta-lactam resistant + often susceptible to TMP-SMX + doxycycline + clindamycin → outpatient oral treatment often possible |
| SCCmec type | Types I + II + III (long genomic elements carrying additional resistances) | Types IV + V (smaller → easier to transfer + less associated resistance) |
Clinical manifestations
- Skin and soft tissue infections (Community-acquired MRSA++) recurrent boils + carbuncle (coalescent boil) + subcutaneous abscesses with thick yellow pus (spontaneously collected or after minor wound) + cellulitis + bullous impetigo + folliculitis + PVL causes particularly necrotic and recurrent skin infections
- Bacteremia and endocarditis (hospital-acquired MRSA++) MRSA bacteremia = medical emergency + mortality 20–30% % despite treatment + infectious endocarditis (native valve + prosthetic valve + IVDU) + septic emboli + metastatic abscesses
- Necrotizing pneumonia (CA-MRSA + PVL++) often post-influenza + hemoptysis + lung necrosis + rapid cavitation + septic shock + high mortality (>50 %) without aggressive treatment + clindamycin or linezolid for their anti-toxin effect (inhibition of PVL synthesis)
- Necrotizing fasciitis Rapid destruction of fascia + severe septic state + surgical emergency + debridement + IV vancomycin
- Bone and joint infections Osteomyelitis + septic arthritis + prosthetic joint infections + spondylodiscitis + blood cultures + MRI + bone biopsy for confirmation + prolonged treatment (minimum 6 weeks)
Treatment as presented
| Clinical situation | Recommended antibiotic therapy | Duration and remarks |
|---|---|---|
| Cutaneous abscess without cellulitis — Community-acquired MRSA | Incision and Drainage (I&D) = primary treatment + TMP-SMX DS 1–2 tabs PO BID (if confirmed or probable community MRSA) + OR doxycycline 100 mg BID + OR clindamycin 300–450 mg TID (test for inducible resistance D-zone phenotype) | 5 to 7 days plus incision and drainage alone is sufficient for simple abscesses (< 2 cm) without cellulitis. Adjuvant antibiotic therapy reduces recurrences in recent studies (NEJM 2017). |
| Cellulitis + non-purulent soft tissue infection | If community-acquired MRSA is suspected: TMP-SMX DS or doxycycline + if severe: IV vancomycin + If MSSA probable (simple cellulitis without abscess): IV cefazolin or PO cephalexin — DO NOT automatically treat for MRSA without culture | 5 to 7 days for mild cellulitis + 10 to 14 days if severe + culture of an abscess or wound guides adjustment |
| MRSA bacteremia + endocarditis | Vancomycin IV (target AUC/MIC 400–600 + or trough 15–20 mg/L) + or daptomycin 6–10 mg/kg/day IV (superior to vancomycin for right-sided endocarditis — IDSA) + duration according to source and septic metastases | Uncomplicated bacteremia: 14 days IV minimum + Valvular endocarditis: 6 weeks + Prosthetic infection: 6 weeks + surgery if possible |
| Necrotizing pneumonia with PVL | Vancomycin IV + clindamycin or linezolid (anti-toxin effect on PVL - protein synthesis inhibition) + intensive care | Intensive ICU treatment + duration based on response + clindamycin or linezolid are added TO REDUCE PVL PRODUCTION + not just for their antibiotic activity |
| SARM in the nostrils (carriage) - decolonization | Mupirocin nasal ointment 2 % × 2/day × 5 days in each nostril + chlorhexidine 4 % soap for shower × 5 days + close contacts also if recurrent family MRSA | Decolonization recommended if recurrent MRSA in the same family + or before elective surgery in a known carrier + 1-month culture control |
ℙ️
The D-zone test (disk diffusion induction test) is essential before prescribing clindamycin to treat MRSA. Some MRSA strains exhibit inducible resistance to clindamycin mediated by the erm gene—they appear susceptible to clindamycin on standard antibiograms but become resistant in vivo after exposure. The D-zone test reveals this inducible resistance by flattening the inhibition zone into a "D" shape around the clindamycin disk. A positive D-zone = inducible clindamycin resistance → DO NOT use clindamycin despite apparent susceptibility on the antibiogram.
Infection prevention and control
- In a hospital setting: Hand hygiene with alcohol-based hand rub before and after all patient contact + contact precautions (gown + gloves) for known or suspected MRSA patients + systematic screening upon admission to high-risk units (intensive care + dialysis units + surgery) + cohorting of MRSA patients in single rooms or with other MRSA patients
- In the community (Community SARMs): Do not share personal items (towels + razors + sports clothes) + cover wounds with clean bandages + regular hand washing + shower after contact sports + disinfect shared surfaces and equipment in locker rooms
- Preoperative decolonization of known carriers: Nasal mupirocin + chlorhexidine bath × 5 days before any elective surgery → reduces the risk of MRSA surgical site infection
- Antibio-governance Do not use anti-MRSA antibiotics (vancomycin + linezolid + TMP-SMX) without culture confirming resistance or strong epidemiological suspicion → avoid selection of additional resistance + preserve the activity of last-resort antibiotics
Medical emergency — MRSA sepsis
Call 911 or go to the ER immediately if a known or suspected MRSA skin infection is accompanied by high fever (> 39°C) + chills + tachycardia + hypotension + or rapid deterioration of general condition — these signs suggest MRSA bacteremia or severe sepsis requiring urgent IV vancomycin + blood cultures + fluid resuscitation. Also consult the ER if severe pneumonia occurs within days after influenza in a young person — PVL-MRSA necrotizing pneumonia is a resuscitation emergency. For moderate MRSA skin infections, Clinique Omicron offers medical consultations at its service points in Quebec. To make an appointment, visit cliniqueomicron.ca.
Consult at Clinique Omicron
Clinique Omicron's physicians and nurse practitioners (NPs) diagnose and treat MRSA skin infections (boils + abscesses + cellulitis), order incision and drainage when indicated, and prescribe oral TMP-SMX or doxycycline for mild to moderate cases. They also order cultures to identify MRSA and guide antibiotic therapy, initiate nasal decolonization (mupirocin) for documented recurrent carriage, and refer patients with bacteremia and severe cases to the emergency department or infectious disease specialists. Consultations are available at several service locations across Quebec and via telemedicine. To book an appointment, visit cliniqueomicron.ca.
The content of this page is for informational purposes only and does not substitute for advice from a physician or infectious disease specialist. Treatment of MRSA should always be guided by a full antibiogram plus a D-zone test for clindamycin and the epidemiological context (hospital-acquired vs. community-acquired). Never use penicillins or cephalosporins to treat a confirmed MRSA infection.
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