Overview
Staphylococcus aureus, also called S. aureus or “staph,” is a bacterium that frequently colonises the human skin and is present in the nose of about 25-30% of U.K. adults. S. aureus can exist in this form without harming its host or causing symptoms. However, if there is a break in the patient’s skin from a wound surgery or indwelling device (including intravenous catheters), or if there is a depression in the person’s immune system, then colonising S. aureus can cause an infection. S.aureus frequently causes localised skin infections, such as folliculitis, furuncles, and impetigo. It can also cause abscesses and spread into the bones (osteomyelitis), lungs (staphylococcal pneumonia), blood (bacteraemia or sepsis), heart (endocarditis – which can damage the heart valves), and other organs. S.aureus may also be transmitted to close contact from both infected and colonised people to other people through inadequate hand hygiene or throughsharing contaminated objects, such as towels or razors.
Hospital-acquired infections (Healthcare-associated infections, HAI)
S.aureus infections acquired while a patient is in a hospital, long-term care facility, or other health care setting have been a challenge for many years. Hospital conditions, including the widespread use of antibiotics, have selected for antibiotic-resistant strains of S. aureus. These strains are called Meticillin Resistant Staphylococcus Aureus (MRSA), named after the antibiotic treatment that was developed in 1960 to treat penicillin-resistant strains. MRSA is inherently resistant to flucloxacillin and most other antibiotics belonging to a class of antibiotics known as the 'beta-lactams', which includes penicillin. MRSA is also frequently resistant to a wide variety of other antibiotics. Infections are associated with significantly higher rates of morbidity and mortality, higher health care costs, and longer hospital stays than infections caused by methicillin susceptible S. aureus.
Risk factors for MRSA infection in the hospital include surgery, prior antibiotic therapy, admission to intensive care, close contact with MRSA-colonised patients and health care workers, being in the hospital more than 48 hours, and having an indwelling catheter or other medical device that goes through the skin. The risk of spread of MRSA is reduced through adequate hand hygiene amongst all close contacts, particularly healthcare professionals.
One strategy that may be used in an effort to control the spread of MRSA includes active surveillance for the detection of MRSA infection or colonisation amongst patients admitted to intensive care units (ICUs) and other high risk areas. Another approach is to screen all patients admitted to a health care facility. Public Health England (PHE; formerly the Health Protection Agency) actively manages mandatory surveillance of MRSA on behalf of the Department of Health. In 2014, the latest figures show that the total number of MRSA blood stream infections reports have decreased compared to the same period last year.
Community-acquired infections
MRSA infections have been associated with outbreaks and deaths in non-medical settings where individuals are in close contact, including prisons, day care facilities, military units, and contact sports. These infections are occurring in people who do not have classic MRSA risk factors as described above. A significant number of those affected have had to be hospitalised for what appears to be a simple but persistent skin infection or for pneumonia that develops after a bout of influenza.
There is increased awareness of community-acquired MRSA (CA-MRSA) amongst the medical community and the community at large. With CA-MRSA, conventional therapy options have frequently failed. Investigations of outbreaks have revealed that the CA-MRSA was spread from infected or colonised patients to those around them through skin contact (such as sports-related cuts and abrasions), through droplets from the respiratory tract, or through exposure to contaminated objects, such as shared sports equipment, towels, toys, or playground equipment. Investigations have also revealed that the S. aureus strains involved in CA-MRSA are genetically different from those that have been causing hospital-acquired MRSA. Effective treatment is usually readily available.
The purpose of culturing wound infections is to identify the bacteria causing the infection and to determine the susceptibility of the microorganism to available antibiotics. If an infection is due to MRSA, it should be investigated to determine the primary source of the infection. This is especially important in CA-MRSA to break the chain of infection and prevent further cases from occurring.
The primary laboratory tests performed are:
- Cultures of the affected areas. Fluid or pus from a wound, sputum, blood, joint fluid, or even breast milk (in the case of an infected breast) is collected and spread onto a thin layer of nutrient gel (agar) and/or grown in a nutrient broth. Sometimes, multiple samples are collected to evaluate different body sites or to attempt to detect bacteria that may be present in small numbers.
- Nasal (collected by inserting a swab inside the nose), throat, axilla and groin cultures may also be requested to determine whether someone is colonised with MRSA. The cultured samples are incubated and examined for the growth of characteristic S. aureus colonies, often on agar plates that change colour in the presence of MRSA. If suspicious-looking organisms are isolated on culture, susceptibility testing is usually performed to confirm that the strain is MRSA and determine appropriate treatment options, should these be required.
- Nasal swabs may be collected to detect MRSA colonisation by means of rapid molecular tests, which do not grow the bacteria but detect the genes responsible for the meticillin resistance.
Identifying MRSA can sometimes be challenging. The population of S.aureus that a person is colonised with tends to be mixed. This means that even if a patient has CA-MRSA or hospital-acquired MRSA, not all of the S.aureus present will be equally resistant. Since resistant strains may grow more slowly than susceptible strains, laboratories may employ additional methods for detection.
A variety of methods may be used to monitor the spread of MRSA. These are used in the investigation of the spread of MRSA within a community or region but are not used in the treatment of an individual patient.
Prevention of infection is the most important target for the UK healthcare system. National efforts are underway to raise awareness of MRSA and to encourage the adoption and maintenance of preventative measures, particularly adequate hand hygiene. For more on the proper way to wash your hands, visit the Health Protection (Scotland) and Public Health England (PHE) websites.
Many institutions have put procedures in place to more rapidly recognise and address MRSA infections. Health care providers are being urged to request cultures and susceptibility testing with outpatient skin and wound infections, to monitor them carefully for effectiveness of treatment, and to be alert for the possibility of CA-MRSA. Outbreaks of CA-MRSA are being investigated and traced back to their source in order to identify the cause, to determine whether other patients may have unrecognised MRSA infections or colonisation, and to reduce the potential for additional cases.
There are multiple treatment options for patients with MRSA infections. Some antibiotics may be given orally, whilst others must be administered intravenously (IV), often for several weeks’ duration. Treatment options for significant MRSA infections include, but are not limited to, vancomycin, teicoplanin, linezolid, daptomycin, ceftaroline, ceftobiprole and chloramphenicol. .