Download Methicillin-resistant Staphylococcus aureus (MRSA)

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MICROBIOLOGY LEGEND
CYCLE 39 ORGANISM 2
Methicillin-resistant Staphylococcus aureus (MRSA)
Methicillin-resistant Staphylococcus aureus (MRSA) is a bacterium responsible for several difficult-to-treat
infections in humans. It is also called oxacillin-resistant Staphylococcus aureus (ORSA). MRSA is any strain of
Staphylococcus aureus that has developed, through the process of natural selection, resistance to beta-lactam
antibiotics, which include the penicillins (methicillin, dicloxacillin, nafcillin, oxacillin, etc.) and the cephalosporins.
Strains unable to resist these antibiotics are classified as methicillin-sensitive Staphylococcus aureus, or MSSA.
The evolution of such resistance does not cause the organism to be more intrinsically virulent than strains of S.
aureus that have no antibiotic resistance, but resistance does make MRSA infection more difficult to treat with
standard types of antibiotics and thus more dangerous.
MRSA is especially troublesome in hospitals, prisons, and nursing homes, where patients with open wounds,
invasive devices, and weakened immune systems are at greater risk of nosocomial infection than the general
public. MRSA began as a hospital-acquired infection, but has developed limited endemic status and is now
sometimes community-acquired. The terms HA-MRSA (healthcare-associated MRSA) and CA-MRSA (communityassociated MRSA) reflect this distinction.
Signs and symptoms
S. aureus most commonly colonizes under the anterior nares. The rest of the respiratory tract, open wounds,
intravenous catheters, and the urinary tract are also potential sites for infection. Healthy individuals may carry
MRSA asymptomatically for periods ranging from a few weeks to many years. Patients with compromised
immune systems are at a significantly greater risk of symptomatic secondary infection.
In most patients, MRSA can be detected by swabbing the nostrils and isolating the bacteria found inside the
nostrils. MRSA may progress substantially within 24–48 hours of initial topical symptoms. After 72 hours, MRSA
can take hold in human tissues and eventually become resistant to treatment. The initial presentation of MRSA is
small red bumps that resemble pimples, spider bites, or boils; they may be accompanied by fever and,
occasionally, rashes. Within a few days, the bumps become larger and more painful; they eventually open into
deep, pus-filled boils. MRSA infection in patients who are in health care facilities tend to be severe. These staph
infections may be in the bloodstream, heart, lungs or other organs, urine, or in the area of a recent surgery.
Risk factors
Some of the populations at risk:
 People who are frequently in crowded places, especially with shared equipment and skin-to-skin contact
 People with weak immune systems (HIV/AIDS, lupus, or cancer sufferers; transplant recipients, severe
asthmatics, etc.)
 Diabetics
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P.O. Box 131375, Bryanston, 2074
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Intravenous drug users
Users of quinolone antibiotics
The elderly
College students living in dormitories
Women with frequent urinary tract or kidney infections due to infections in the bladder
People staying or working in a health care facility for an extended period of time
People who spend time in coastal waters where MRSA is present, such as some beaches in Florida and
the west coast of the United States
People who spend time in confined spaces with other people, including occupants of homeless shelters
and warming centers, prison inmates, military recruits in basic training, and individuals who spend
considerable time in changing rooms or gyms
Veterinarians, livestock handlers, and pet owners
Extended stay in the hospital
Genetics
Antimicrobial resistance is genetically based; resistance is mediated by the acquisition of extrachromosomal
genetic elements containing resistance genes. Examples are plasmids, transposable genetic elements, and
genomic islands, which are transferred between bacteria by horizontal gene transfer. A defining characteristic of
MRSA is its ability to thrive in the presence of penicillin-like antibiotics, which normally prevent bacterial growth
by inhibiting synthesis of cell wall material. This is due to a resistance gene, mecA, which stops β-lactam
antibiotics from inactivating the enzymes (transpeptidases) critical for cell wall synthesis.
Diagnosis
Diagnosis begins with health history assessment and physical examination. Cultures should be obtained from
the site of infection using antiseptic techniques. Types of cultures obtained to diagnose MRSA include the
following methods:
 Cultures of Skin Infections
 Sputum Cultures
 Urine Cultures
 Blood Cultures
Colonies of methicillin-resistant Staphylococcus aureus isolated from a patient's blood.
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Certificate available on request or at www.sanas.co.za
P.O. Box 131375, Bryanston, 2074
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www.thistle.co.za
Tel: +27 (011) 463 3260
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Fax to Email: + 27 (0) 86-557-2232
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Preventing HA-MRSA
In the hospital, people who are infected or colonized with MRSA often are placed in contact precautions as a
measure to prevent the spread of MRSA. Visitors and health care workers caring for people in isolation may be
required to wear protective garments and must follow strict hand hygiene procedures. Contaminated surfaces
and laundry items should be properly disinfected.
Preventing CA-MRSA
 Wash your hands.
 Keep wounds covered.
 Keep personal items personal. Avoid sharing personal items such as towels, sheets, razors, clothing and
athletic equipment. MRSA spreads on contaminated objects as well as through direct contact.
 Shower after athletic games or practices.
 Sanitize linens. If you have a cut or sore, wash towels and bed linens in a washing machine set to the
hottest water setting (with added bleach, if possible) and dry them in a hot dryer. Wash gym and athletic
clothes after each wearing.
Treatment
Both CA-MRSA and HA-MRSA are resistant to traditional anti-staphylococcal beta-lactam antibiotics, such as
cephalexin. CA-MRSA has a greater spectrum of antimicrobial susceptibility, including to sulfa drugs (like cotrimoxazole/trimethoprim-sulfamethoxazole), tetracyclines (like doxycycline and minocycline) and clindamycin
(for osteomyelitis), but the drug of choice for treating CA-MRSA is now believed to be vancomycin, according to a
Henry Ford Hospital Study. HA-MRSA is resistant even to these antibiotics and often is susceptible only to
vancomycin. Newer drugs, such as linezolid (belonging to the newer oxazolidinones class) and daptomycin, are
effective against both CA-MRSA and HA-MRSA.
Several newly discovered strains of MRSA show antibiotic resistance even to vancomycin and teicoplanin. These
new evolutions of the MRSA bacterium have been dubbed vancomycin intermediate-resistant Staphylococcus
aureus (VISA). Linezolid, quinupristin/dalfopristin, daptomycin, ceftaroline, and tigecycline are used to treat more
severe infections that do not respond to glycopeptides such as vancomycin. Current guidelines recommend
daptomycin for VISA bloodstream infections and endocarditis. Studies suggest that allicin, a compound found in
garlic, may prove to be effective in the treatment of MRSA
References
1. http://www.medicalnewstoday.com/articles/10634.php
2. http://en.wikipedia.org/wiki/Methicillin-resistant_Staphylococcus_aureus
3. http://www.mayoclinic.org/diseases-conditions/mrsa/basics/treatment/con-20024479
Questions
1. What is MRSA?
2. Discuss the signs and symptoms of MRSA.
3. Discuss the treatment protocol in MRSA.
Thistle QA is a SANAS accredited organisation, No: PTS0001
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Certificate available on request or at www.sanas.co.za