MRSA Infection: Questions and Answers

Addressing MRSA and how to protect yourself from an infection

MRSA: Bacteria and the human state

The state of being human is anything but static. Humans are home to many bacteria, viruses and fungi. By an order of magnitude, there are more non-human cells than human cells on the surfaces and within the orifices of the body (10 to 14th compared to 10 to 13th). Bacteria colonize any surface that is in contact with the environment, such as the skin, mouth, upper respiratory tract, gastrointestinal system, urinary tract, vagina. Each surface has a predilection for different microbes on the basis of multiple factors, including pH, temperature, protein sources, oxygen content and moisture/salinity. Staphylococcus aureus is one species that colonizes the surface of the skin of the groin, perineum (area between genitals and anus), axillae (armpits) and the nasopharynx (nose and throat) – its primary habitat. As many as 25% or more of the population is colonized with S. aureus in the nose. Much less are colonized with MRSA (2-4%). A discussion of the complex relationship with S. aureus and humans in colonization and infection follows.

Compared to oral and intestinal surfaces, bacterial colonization on the surface of the skin is sparse. Of the 1.2 kg of total body weight amounting from bacteria, the weight of bacteria of skin is approximately 0.2 kilograms; the intestines comprise the bulk of the weight (1 kg). Bacteria reside on the surface of the skin generally as commensalists or “neutralists”, a relationship with the human host that is neither beneficial nor detrimental. Bacteria utilize various elements on the surface such as carbon, oxygen, nitrogen, potassium, magnesium, iron and calcium in order to allow for growth. However, given the right circumstances, such as trauma, skin abrasion or excessive dryness of the skin (xerosis), there is a transition from greater colonization to infection.

The most common bacteria associated with skin infections are Streptococcus and Staphylococcus. Streptococcus commonly cause impetigo (a superficial infection), wound infections frm injury or after surgery and cellulitis, an ascending infection associated with fever, rash and pain and swelling of the site. Staphylococcus aureus are predominantly associated with wound infections, folliculitis, furuncles, carbuncles (skin abscesses or “boils”), but can become deeper, more serious infections, if left unchecked. With both Staph and Strep, these deeper infections may require significant surgical intervention, such as necrotizing fasciitis (deep fascial infection) or a muscle abscess (pyomyositis). An untreated Staphylococcus aureus infection can predispose to abscesses in the epidural space or even heart valve infections (endocarditis). Post-viral respiratory infections can sometimes be complicated by Staphylococcus aureus which can sometimes be necrotizing and life-threatening.

Staphylococcus aureus is easy to see on Gram staining. They stain Gram positive (purple) and appear as grape-like clusters in the microscope. They grow readily on culture media, including blood cultures. Newer methods of testing including a nasal swab for PCR are useful for detecting colonization of Staph aureus in the nares and differentiating between MRSA and MSSA. This has clinical implications for treatment and pre-operative preparation.

Methicillin-resistant Staphylococcus aureus (MRSA). This form of Staphylococcus aureus defined by its resistance to methicillin (including others such as oxacillin; cefazolin), an iteration of penicillin produced in the 1959. It provided a short period of coverage against penicillin-resistant Staph, associated with production of an enzyme beta-lactamase, which was capable of breaking down the penicillin structure (beta-lactam ring), neutralizing its activity. Unfortunately, not long after its use, Staph bacteria capable of growing in its setting appeared, initially in the hospital setting. In the 80’s and 90’s, a growing percentage of community-associated infections from MRSA appeared. Various proteins expressed by MRSA, such as the Panton Valentin Leukocidin (PVL) and others are associated with outbreaks and, to some extent, a greater virulence (more severity) of infection. Co-association with colonization of MRSA and MSSA can be attributable to a risk of infections after surgery.

An approach to treatment of MRSA in a person presenting with an infection is three-fold.

1) Address the infection – for instance, perform an incision and drainage of a boil, if present; or sometimes, allow it to mature and resolve spontaneously. Some data support that antibiotics have a limited role after an incision and drainage of a small <2cm abscess. Though with larger abscesses, there has been found a lower failure rate than with a proper incision and drainage. Antibiotic consideration include trimethroprim-sulfamethoxazole (bactrim), doxycycline and possibly clindamycin (some resistant isolates).

2) Address the colonization state – Again, the usual colonization site is the nose and skin and can be addressed after the infection is treated. For decolonization, I prescribe a surgical cleanser, such as Hibiclens, which can be applied 2 to 3 times weekly – lather on the skin and rinse off. For the nasal carriage, an antibiotic ointment such as bactroban (nasal bactroban) can be used – apply a cotton swab with the ointment in each naris twice daily for 1 week. I usually begin this protocol 2 weeks after the acute infection is treated.

3) Address the risk factor – For a state of colonization to transform to a state of infection, there is often a risk factor that can be identified. Some are more obvious than others. Occasionally, upon examining a patient, I can tell the risk factor. For instance, some men (swimmers, other athletes) are shavers; some women shave closer to the groin or mons pubis. Using a dull razor, not using a cream or moisturizer to shave, or not treating the razor with alcohol prior to use can lead to a break in the skin and allow the bacteria to cause an infection. Sometimes, a new sexual partner of a patient was colonized with Staph and increases the risk of transmission, enough to lead to folliculitis and abscesses. On the examination, I will often observe a patient scratching themselves where they have dry skin or folliculitis and even attempt to express a lesion in front of me. People who are chronic sitters, e.g. truck drivers, writers, bloggers (one of many reasons I have a standing desk…) are also prone to folliculitis on the buttock, which can then cause an abscess. Below. I have listed several ways to reduce the risk of Staph aureus infection.

MRSA: Question and Answer

What is MRSA? Methicillin-resistant Staphylococcus aureus. This is a ubiquitous bacterium resides on the skin and naso-oropharynx of humans and other animals. The ecological niche of Staphylococcus aureus is the nares (nose). Others areas of the skin populated by this bacterium include the axillae (armpits), groin, perineal area (area between genitals and anus). Contamination of fomites (inanimate objects such as keys, computers, phones etc) can represent a source of transmission to other people. Although colonization generally precedes infection, outbreaks have been described where colonization was not evident. I had a Spider bite that got infected with MRSA. Is this possible?

Typically, when someone is in the process of developing an infection with Staphylococcus aureus, the area may feel a sharp pain. Although the sensation is similar to an insect of spider bite, it is almost never the case. Certainly, in various geographic locations, the bite of a brown recluse or hobo spider, to name a few, can result in a erythematous lesion with central necrosis (dark center). This aside, an abscess that cultures Staphylococcus aureus in the wound suggests that this alone was the likely cause of the process.

In one study by Moran er al, a report of a spider bite had an adjusted odds ratio of 3 (that means that those with this history were 3x more likely than those without to have an MRSA infection.

What are some epidemiologic associations of MRSA infection?

  1. Locations: Prisons, athlete groups/gyms, astroturf, hospitalization, military, reservations

  2. Behaviors: Men who have sex with men (MSM), Elicit drug use (meth/heroin), shavers

  3. Conditions: HIV, Hepatitis C, Diabetes mellitus and cirrhosis, Jobs syndrome, Paraplegia

  4. Medications: Recent antibiotic exposure, Steroid use (prednisone) or other immunosuppressants, possible association with nonsteroidal agents (NSAIDS)

  5. Devices: Hemodialysis catheters, Peripherally inserted central catheters (PICC), foleys, PEG tubes, etc

What are risk factors for MRSA infection? This question is one with a difficult answer. Miller and Binh an Diep CID 2008 had a nice way of representing risk factors in three areas: Risk factors of behaviors and exposures: skin-to-fomite contact (shaving, tattoos, rug burns,etc), skin-to-skin contact, antibiotic exposure Host defenses: Skin integrity, Antimicrobial peptides (an area that is in its infancy but essentially looking at proteins that are secreted by our skin which neutralize bacteria,etc, medications that affect the immunity such as biologicals e.g. tnf-a antagonists (etanercept,etc), chemotherapy, steroids, etc. Immunodeficiency states. Pathogen factors: Virulence factors, strain type, etc.

How do I prevent myself from getting MRSA? Approximately 10-25% of the population is colonized with Staphylococcus aureus – sensitive or MSSA, while 3% of the population is colonized with MRSA. Suggestions for avoiding colonization are somewhat similar for those suggestions for avoiding infection: 1. Good hand washing, regular (but not excessive) showering and laundering – increased rates of MRSA in poor hygiene practices. Wash hands after touching or picking nose. 2. Use sharp razors for shaving (to avoid blunt trauma) but avoid cutting trauma. Apply alcohol to the razor before/after use. Do not share razors. Use a moisturizer for skin after shaving. 3. If you have dermatitis such as eczema, psoriasis or others, make sure that it is treated. You have a higher rate of colonization with Staph… 4. Use antibiotics judiciously – you play an important role in limited antibiotic use! 5. If you develop a pimple, DON’T attempt to express it. When the process is deeper, the attempt may fail and lead to more inflammation which means more abscess. The majority of people I see that require an incision and drainage have admitted to having attempted too vigorously to express a furuncle. 6. Use NSAIDS and Steroids judiciously. 7. Keep your skin well lubricated with humectants, as drying can promote breaks in the skin and dermatitis/eczema. Plan on applying skin creams after you have dried yourself from your shower/bath. 8. Move your body around. Sitting on your butt, kneeling, or leaning on your elbows too long promotes skin changes that can lead to breaks in the skin in these areas. It is this reason that I see a lot of MRSA abscesses in paraplegics/quadriplegics or others that have limited motion. 9. Get plenty of quality sleep and exercise (don’t underestimate the role of these factors on the immune system)

After decolonization, there is a fairly high likelihood that a person previously colonized will become recolonized. This does not mean that you will get another abscess. With a greater awareness of cause and prevention measures, it is with hopes that Staph aureus will not cause recurrent problems.

Thanks for reading This! Dr. Cirino

Further reading (below are a few references and links): Boeker S et. al. Methicillin-resistant Staphylococcus aureus: risk factors associated with community-onset infections in Denmark – Clinical Microbiology and Infection. 2008. Clin Micro and Infection. Danish Community-MRSA group.

Moran er al. N Engl J Med 2006; 355:666-674 (Epidemiology)

Please feel free to post any questions or comments.

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