# MRSA and Staphylococcal Infections

> Mechanism of methicillin resistance via PBP2a, glycopeptide therapy, decolonisation and infection management for NEET-PG Medicine exam preparation.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/mrsa-and-staphylococcal-infections
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-08
- Last updated: 2026-10-08
- Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Mandell's Principles and Practice of Infectious Diseases; Harrison's Principles of Internal Medicine
- How to cite: "MRSA and Staphylococcal Infections", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/mrsa-and-staphylococcal-infections

## Direct answer

MRSA is Staphylococcus aureus resistant to methicillin and, in practice, to most beta-lactam antibiotics, because it carries the mecA gene that encodes an altered penicillin-binding protein, PBP2a, with low affinity for beta-lactams. Serious infection is treated with a glycopeptide such as vancomycin, with linezolid, daptomycin and teicoplanin as alternatives in selected situations, and abscesses are drained surgically. Decolonisation with nasal mupirocin and chlorhexidine body washes reduces recurrence in people who are repeatedly infected.

## What you must remember

- **mecA gene:** Carried on the SCCmec mobile genetic element; PBP2a continues cell wall synthesis when beta-lactams have bound the normal penicillin-binding proteins.
- **Detection:** A cefoxitin disc screen detects mecA-mediated resistance, while PCR for mecA and mecC and PBP2a latex agglutination confirm it.
- **Glycopeptides:** Vancomycin for serious MRSA infection, dosed with area-under-the-curve-guided monitoring (current guidance) or trough monitoring where AUC is unavailable; reduced susceptibility and heteroresistance can cause treatment failure.
- **Alternatives:** Linezolid is useful for pneumonia because it penetrates the lung, while daptomycin treats bacteraemia and endocarditis but is inactivated by lung surfactant and so is not used for pneumonia.
- **Community versus hospital MRSA:** Community-acquired strains often carry Panton-Valentine leucocidin and SCCmec types IV or V, and cause skin and soft tissue infection; trimethoprim-sulfamethoxazole is an oral option for uncomplicated skin infection.
- **Decolonisation:** Nasal mupirocin for a short course with chlorhexidine body washes, used for recurrent infection and before surgery in carriers.
- **Abscess management:** Incision and drainage is the cornerstone; antibiotics alone rarely cure a large abscess.
- **Vancomycin resistance:** Mediated by the vanA gene in rare strains, and VRSA remains uncommon.

## Worked scenario: MRSA bacteraemia from a central line

A 46-year-old man with a central venous catheter develops fever and rigors, and blood cultures grow Staphylococcus aureus resistant to oxacillin on the cefoxitin screen.

1. **Confirm and treat:** The first step is to confirm methicillin resistance and start a glycopeptide without delay, with vancomycin dosed to trough or area-under-the-curve targets.
2. **Remove the device:** The catheter is removed, since retained lines sustain bacteraemia and antibiotics alone do not clear them, and repeat blood cultures document clearance.
3. **Search for endocarditis:** An echocardiogram looks for endocarditis, and imaging searches for metastatic foci such as spinal or septic emboli.
4. **Adjust the course:** Where endocarditis or a metastatic focus is found, the duration of therapy is extended, and daptomycin may be chosen for right-sided disease.
5. **Reconsider if cultures stay positive:** Repeat blood cultures that stay positive after several days are a signal to reconsider the source, the drug choice or the presence of an abscess.

## Where the examiner tests mechanism and drug

The commonest errors combine mechanism, drug choice and toxicity.

- **Trap:** treating MRSA as a beta-lactamase problem — **Correct:** penicillinase-producing staphylococci are overcome by flucloxacillin or amoxicillin-clavulanate, but MRSA carries an altered target, so the beta-lactamase inhibitor does not help.
- **Trap:** using daptomycin for pneumonia — **Correct:** it is inactivated by surfactant and is not used for lung infection.
- **Trap:** raising the vancomycin dose to fix a nephrotoxicity problem — **Correct:** nephrotoxicity rises with high troughs and concurrent nephrotoxic drugs, so the examiner expects monitoring rather than a higher dose.
- **Trap:** naming a beta-lactamase gene as the cause of methicillin resistance — **Correct:** mecA, encoding PBP2a, confers methicillin resistance, not a beta-lactamase gene.

## Frequently asked questions

### What gene confers methicillin resistance in Staphylococcus aureus?

The mecA gene confers methicillin resistance in Staphylococcus aureus by encoding PBP2a, a penicillin-binding protein with low affinity for beta-lactam antibiotics. It is carried on the SCCmec mobile genetic element. The gene is detected by PCR, and resistance is screened with a cefoxitin disc.

### Why is vancomycin used for serious MRSA bacteraemia?

Vancomycin is used for serious MRSA bacteraemia because it is a glycopeptide that remains active against methicillin-resistant strains and has long-standing evidence in this setting. Dosing is guided by trough or area-under-the-curve monitoring to balance efficacy and nephrotoxicity. Alternatives such as daptomycin are considered when vancomycin fails or is unsuitable.

### Why is daptomycin not used for MRSA pneumonia?

Daptomycin is not used for MRSA pneumonia because pulmonary surfactant inactivates it, so it does not achieve effective concentrations in the lung. Linezolid or vancomycin is preferred for MRSA pneumonia instead. Daptomycin remains useful for bacteraemia and endocarditis.

### How is MRSA carriage eradicated in a patient with recurrent infection?

MRSA carriage is eradicated with a short course of intranasal mupirocin combined with daily chlorhexidine body washes, sometimes with decolonisation of household contacts. Environmental cleaning and attention to skin care support the treatment. Recurrence is common, so decolonisation is repeated or checked when infections return.

### Which laboratory test detects methicillin resistance in Staphylococcus aureus?

A cefoxitin disc diffusion screen detects methicillin resistance in Staphylococcus aureus, and mecA PCR or PBP2a latex agglutination confirms it. The oxacillin minimum inhibitory concentration is also used in many laboratories. Results guide the choice between beta-lactams and glycopeptides.