MRSA & Staphylococcus aureus

MSSA and MRSA — one organism, two problems

Staphylococcus aureus is a Gram-positive coloniser of skin and the anterior nares in roughly a third of healthy adults. Methicillin resistance is conferred by the mecA gene, which encodes an altered penicillin-binding protein (PBP2a) with low affinity for β-lactam antibiotics. The organism is otherwise the same; what changes is which antibiotics work.¹

The distinction that matters clinically is therefore narrower than it sounds. Methicillin-sensitive S. aureus (MSSA) and MRSA cause the same spectrum of airway disease. MRSA adds a treatment constraint and an infection-control obligation.

Where it matters

Cystic fibrosis

S. aureus is typically the first organism cultured from the CF airway, often in infancy, and remains among the most prevalent throughout childhood. MRSA prevalence in CF rose through the 2000s, peaked, and has since declined.²

Bronchiectasis

Less common than Haemophilus influenzae or Pseudomonas aeruginosa, but not rare, and it has been comparatively neglected — a 2026 review set out the epidemiology, clinical impact and management uncertainty explicitly.³ Isolation of S. aureus in an adult with bronchiectasis should also prompt a thought about undiagnosed CF or ABPA, both of which it accompanies.

Pneumonia

An uncommon but aggressive cause of community-acquired pneumonia, classically following influenza, capable of cavitating and necrotising disease and rapid deterioration.

Why it matters — and the honest limits of what is known

Registry studies in cystic fibrosis have associated persistent MRSA with lower lung function and worse survival. Whether MRSA causes that decline, or marks a population already declining for other reasons, has never been settled — the association is consistent, the causal inference is not.

In bronchiectasis the picture is less developed still: S. aureus is associated with more severe disease, but its independent contribution is unclear.³

This uncertainty is not a footnote. It is the reason the eradication evidence below reads the way it does.

Should a new MRSA isolate be treated?

There is no established eradication guideline for MRSA in cystic fibrosis or in bronchiectasis — a striking contrast with P. aeruginosa, where eradication is a graded recommendation.⁵

The evidence that does exist:

• STAR-too, a randomised controlled trial of early MRSA eradication in CF, showed microbiological efficacy at short-term follow-up — the protocol did clear the organism in a significant proportion — without establishing durable clinical benefit.⁶
• A Cochrane review of interventions for MRSA eradication in people with CF concluded that early eradication can achieve culture negativity in the short term, while finding insufficient evidence that this translates into improved lung function, fewer exacerbations or better survival.⁷

The practical position, and it should be stated as a position rather than a recommendation: many centres attempt eradication at first isolation on the reasoning that clearance is achievable and the downside is a defined course of antibiotics, while accepting that the clinical benefit is unproven. Others do not. Both are defensible on the current evidence.

Anti-staphylococcal prophylaxis in CF

Continuous anti-staphylococcal prophylaxis in infancy reduces MSSA isolation but has been associated with earlier acquisition of P. aeruginosa — trading a manageable organism for a more consequential one. It is not standard practice in Australia or the UK, and this is the classic example of an intervention that succeeds on its stated endpoint and harms on the one that matters.

Chronic infection and long-term therapy

The 2025 ERS bronchiectasis guideline makes a conditional recommendation in favour of long-term inhaled antibiotics for patients at high risk of exacerbations with chronic infection by pathogens other than P. aeruginosa, which is where chronic S. aureus sits.⁵ Long-term macrolides carry a strong recommendation for high exacerbation risk irrespective of organism, and long-term oral non-macrolide antibiotics are suggested against for routine use.⁵

For an exacerbation, send sputum first where possible and let previous cultures guide the empirical choice; the 2026 CHEST guideline suggests duration be guided by clinical response rather than a fixed course length.⁸

Infection control

MRSA is managed with contact precautions — gown and gloves, single room where available, dedicated or decontaminated equipment — in both CF and general respiratory settings.⁹ In CF services this sits within the wider rule that people with CF are not cohorted with one another at all, whatever they are growing.

Nasal and skin carriage matters: decolonisation regimens using nasal mupirocin and chlorhexidine body wash are used around surgery and in some eradication protocols, but carriage recurs frequently and decolonisation alone does not clear airway infection.

What has changed since the older guidance

The UK CF Trust's 2008 MRSA document was written when MRSA prevalence in CF was rising and the question was whether it mattered at all.¹⁰ Since then:

• The infection-control standard was consolidated into the 2013 CF Foundation guideline.⁹
• Eradication was actually tested, in STAR-too and then synthesised in Cochrane — and returned a more equivocal answer than the field expected.^6,7
• MRSA prevalence in CF peaked and fell, alongside better infection control and, latterly, modulator therapy.¹¹
• S. aureus in non-CF bronchiectasis became a subject in its own right, including the high-inoculum resistance finding that explains a class of treatment failure.^3,4

Role of the physiotherapist

• Contact precautions are the practical difference — gown, gloves, single room where possible, and scheduling that avoids contact with other respiratory patients.
• Airway clearance is unchanged by the organism. Do not reduce or defer treatment because a patient is growing MRSA; the clearance need is identical.
• Decontaminate everything the patient breathes through or touches. PEP and oscillating PEP devices, spirometers, nebulisers, plinths and gym equipment.
• Flag apparent treatment failure to the medical team rather than assuming poor adherence — the inoculum effect is a real and under-recognised explanation.⁴
• Manage the label. Patients frequently interpret an MRSA result as meaning they are dangerous to their family. Household transmission risk to healthy contacts is low, and the precautions are about other people with lung disease. Saying so directly relieves a disproportionate amount of distress.

References & evidence base

1. Lowy FD. Antimicrobial resistance: the example of Staphylococcus aureus. J Clin Invest 2003;111(9):1265–1273.
2. Dasenbrook EC, Checkley W, Merlo CA, et al. Association between respiratory tract methicillin-resistant Staphylococcus aureus and survival in cystic fibrosis. JAMA 2010;303(23):2386–2392.
3. Staphylococcus aureus in bronchiectasis: epidemiology, clinical impact and challenges in management. ERJ Open Res 2026;12(2):00740-2025.
4. Mossman AK, Svishchuk J, Waddell BJM, et al. Staphylococcus aureus in non-cystic fibrosis bronchiectasis: prevalence and genomic basis of high inoculum β-lactam resistance. Ann Am Thorac Soc 2022;19(8):1284–1293.
5. Chalmers JD, Haworth CS, Flume P, et al. European Respiratory Society clinical practice guideline for the management of adult bronchiectasis. Eur Respir J 2025;66:2501126.
6. Muhlebach MS, Beckett V, Popowitch E, et al. Microbiological efficacy of early MRSA treatment in cystic fibrosis in a randomised controlled trial (STAR-too). Thorax 2017;72(4):318–326.
7. Lo DK, Muhlebach MS, Smyth AR. Interventions for the eradication of meticillin-resistant Staphylococcus aureus (MRSA) in people with cystic fibrosis. Cochrane Database Syst Rev 2022;12(12):CD009650.
8. Management of adults with non-cystic fibrosis bronchiectasis: CHEST clinical practice guideline. Chest 2026.
9. Saiman L, Siegel JD, LiPuma JJ, et al. Infection prevention and control guideline for cystic fibrosis: 2013 update. Infect Control Hosp Epidemiol 2014;35(S1):S1–S67.
10. UK Cystic Fibrosis Trust Infection Control Working Group. Methicillin-resistant Staphylococcus aureus (MRSA). Bromley: Cystic Fibrosis Trust; April 2008.
11. Middleton PG, Mall MA, Dřevínek P, et al. Elexacaftor–tezacaftor–ivacaftor for cystic fibrosis with a single Phe508del allele. N Engl J Med 2019;381(19):1809–1819.

References are numbered in citation order (Vancouver/BMJ style) and were current at the time of writing. Guidelines are living documents — verify against the latest version before clinical use.