Staph Wound Infections and Staphylococcus aureus

Staph Wound Infections and Methicillin Resistant Staphylococcus aureus

Staphylococcus aureus, also called S. aureus or “staph,” is a bacterium that frequently colonizes the human skin and is present in the nose of about 25-30% of U.S. 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 or surgery, or if there is a depression in the person’s immune system, then colonizing S. aureus can cause an infection. Staph frequently causes localized skin infections, such as folliculitis, furuncles, and impetigo. It can also cause abscesses and spread into the bones (osteomyelitis), lungs (staphylococcal pneumonia), blood (bacteremia or sepsis), heart (endocarditis – which can damage the heart valves), and other organs. Staph may also infect others as it can be passed from both infected and colonized people to other people through skin contact or through sharing contaminated objects, such as towels or razors.

Hospital-acquired infections
Staph 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. The confined population and the widespread use of antibiotics have led to the development of antibiotic-resistant strains of S. aureus. These strains are called methicillin resistant staphylococcus aureus (MRSA), named after the antibiotic treatment that was developed in 1960 to treat penicillin-resistant strains. Infections caused by MRSA are frequently resistant to a wide variety of antibiotics and are associated with ignificantly 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, exposure to a MRSA-colonized patient or health care worker, being in the hospital more than 48 hours, and having an indwelling catheter or other medical device that goes through the skin.

One strategy that may be used in an effort to control the spread of infection includes active surveillance for the detection of MRSA in 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. Community-acquired infections
MRSA infections have increased in importance in the community in recent years because they have been associated with a growing number of outbreaks and deaths in non-medical settings where individuals are in close contact such as 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 hospitalized for what appears to be a simple but persistent skin infection or for pneumonia that develops after a bout of influenza.

Until recently, part of the problem with community-acquired MRSA (CA-MRSA) has been a lack of awareness in the medical community and the community at large. Historically, physicians have treated staph infections, based on their severity, with either over-the-counter triple-antibiotic ointments or with a standard course of antibiotics. They did not routinely order cultures to identify the organism and its antibiotic susceptibility profile unless the infection appeared extensive or the initial treatment was unsuccessful. With CA-MRSA, however, these conventional therapy options have frequently failed. A significant number of those affected have been hospitalized and a few previously healthy patients have died.

Investigations of these outbreaks have revealed that the CA-MRSA was spread from infected or colonized 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 distinct from those that have been causing hospital-acquired MRSA. The CA-MRSA are resistant to methicillin and related antibiotics (oxacillin, dicloxacillin, nafcillin) and erythromycin but remain susceptible to many other antibiotics.

Laboratory Tests

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 source of the infection. This is especially important in CA-MRSA to 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 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 cultures (collected by inserting a swab inside the nose) used to screen healthy people may also be ordered to determine whether someone has been colonized with MRSA and is a carrier. The cultured samples are incubated and examined for the growth of characteristic S. aureus colonies. If they are present, susceptibility testing is performed to determine whether the strain is MRSA.
* Nasal swabs may be collected to detect MRSA colonization based on rapid molecular tests, which do not grow the bacteria but detect their presence and antibiotic resistance by detecting the genes responsible for the methicillin resistance.

Identifying MRSA can sometimes be challenging. The population of staph that a person has tends to be mixed. This means that even if a patient has CA-MRSA or hospital-acquired MRSA, not all of the staph present will be equally resistant. Since resistant strains may grow more slowly than susceptible strains, there is the potential for missing them.

A variety of methods may be used to track different strains 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.