MRSA

Background

Staph aureus is a commensal gram +ve coccus. After the advent of penicillin it rapidly acquired resistance to penicillins via penicillinase and alteration of its penicillin binding proteins. B-lactamase resistant penicillins (methicillin 1959, flucloxacillin, and oxacillin) were developed to treat these strains. In 1961 the first strain resistant to these was discovered in the UK. Although methicillin is no longer manufactured, its name continues to be used to describe resistant Staph aureus strains.

Most common HCAI. HCAIs cost 1 billion a year and cause 5000 deaths.
UK has one of the highest prevalences in Europe with a dramatic rise over the last 15 years (not just attributable to higher reporting). Thought to have caused 1652 deaths in 2006.
Up to 1/3 critically ill patients are are colonised or infected on ICU admission.
MRSA causes increased LOS, cost, risk of death. Mortality rates are higher in those with MRSA but it is difficult to distinguish whether MRSA causes death or people die with, rather than from, MRSA. Evidence is conflicting.
Trusts in England are now set targets for bacteraemia rates and reporting is mandatory.
Rates are now thankfully falling because of the preventative measures below. Screening is probably the single main reason although this is very difficult to prove.

Resistance

MRSA
Resistant to B-lactam antibiotics (penicillins, cephalosporins and carbapenems) and also macrolides and quinolones (mostly gram -ve).
It is sensitive to tetracyclines, aminoglycosides, glycopeptides and oxazolidinones (linezolid)
MSSA
Sensitive to flucloxacillin, cephalosporins, carbapenems
Variable resistance to other penicillins.

Classification

Classified into CA-MRSA (community acquired) and HA -MRSA (hospital acquired).
CA-MRSA strains are more virulent due to toxins such as PVL (Panton-Valentine leukocidin) and PSM (phenol-soluble modulin).
The 2 main strains in the UK are EMRSA15 and 16.

Screening

MRSA is detected by screening. Nasal swabs are subjected to latex agglutination or more recently PCR which speeds up reporting from 2 days to a few hours.
From March 2009, Dept. of Health requires all elective patients to be screened. This allows control measures such as topical decontamination. Patients should be screened weekly thereafter.

Risk factors

Patient factors
Recent exposure to health care environments, long term care facilities, previous MRSA colonisation, foreign bodies, wounds, males, old age, obesity
Environmental factors
Large hospital size, patient transfers, LOS, physiotherapy, RRT, ETTs, nebulisers, humidifiers and vasopressors.

Prevention

Code of practice is now a legal requirement. Care bundles help measure compliance. Examples in ICU include use of 2% chlorhexidine for skin prep, transparent dressings, no routine catheter changes, replacement of peripheral cannulae after 72-96h, documentation of date of insertion.
58% of health care workers are colonised with MRSA (BMJ 2007)

Hand hygiene
Evidence to show limits spread and incidence of bacteraemia.
Gloves and aprons.
Gloves have been shown to significantly reduce hand contamination.
MRSA can survive and be transported on clothing hence apron use.

Local environment
Areas closest to patients have a high incidence of MRSA contamination as well as patients and health care workers.

Isolation
DOH guidelines advocate patient isolation but there is debate about this with conflicting evidence. Some studies show isolation reduces infection and transmission but this difficult to separate from other interventions used. Isolated patients have reduced interactions with staff so is not without risk and facilities for isolation are often lacking.

Antibiotic prescribing
Quinolones and cephalosporins are both implicated as risk factors for antibiotic resistance including the acquisition and virulence of MRSA. Mechanisms are unclear but quinolones may upregulate fibronectin adhesions which promotes bacterial attachment to indwelling devices.

Treatment

Glycopeptides (vancomycin and teicoplanin) but have poor tissue penetration. Tetracyclines (doxycycline).
Linezolid
Tigecycline
Rifampicin (must be used in combination or resistance develops very quickly).

Some vancomycin strains are now becoming evident.

Decolonisation

Chlorhexidine soap head to toe.
Mupirocin 2% nasal ointment.

Evidence
“Narrow-spectrum” antibiotic policy
Decrease in C.diff
Stable MRSA
Fowler et al (UK)

Limit use of fluoroquinolones
Lower MRSA
Madras-Kelly et al (USA)

Education, antibiotic-control
Significant decrease MRSA
Apisarnthanarak (Thailand)
Fluoroquinolones restriction
Significant decrease MRSA
Charbonneau et al (France)

Education, restriction fluoroquinolones, IV cephalosporins
Significant decrease MRSA
Liebowitz, Blunt (UK)

References
Fowler S, Webber A, Cooper BS et al. J Antimicrob Chamother. 2007; 59:990-5
Madras-Kelly KJ, Remington RE, Lewis PG, Stevens DL. Infect Control Hosp Epidemiol. 2006; 27(2):155-69
Apisarnthanarak A, Danchaivijitr S, Khawcharoenporn T et al. Clin Infect Dis 2006; 42:768-75
Charbonneau P et al. Clin Infect Dis 2006; 42: 778-84
Liebowitz LD, Blunt MC. J.Hosp Infect2008; 69: 328-336