MRSA Methicillin-Resistant Staphylococcus aureus
© Kenneth Todar, PhD
Staphylococcus aureus is the quintessential superbug.
Hospital strains of Staphylococcus aureus are usually resistant to a
variety of different antibiotics. A few strains are resistant to all
clinically useful antibiotics except vancomycin, and
vancomycin-resistant strains are increasingly-reported. The term MRSA refers to Methicillin resistant Staphylococcus aureus.
Methicillin resistance is widespread and most methicillin-resistant
strains are also multiply drug-resistant. In addition, S. aureus exhibits resistance to
antiseptics and disinfectants, such as quaternary ammonium compounds,
which may aid its survival in the hospital environment.
Staphylococcal disease has been a perennial problem in the hospital
environment since the beginning of the antibiotic era. During the
1950's and early 1960's, staphylococcal infection was synonymous with
nosocomial infection. Gram-negative bacilli (e.g. E. coli and Pseudomonas aeruginosa) have
replaced staph as the most frequent causes of nosocomial infections,
although the staphylococci have remained a problem, especially in
surgical wounds. S aureus
responded to the introduction of antibiotics by the usual bacterial
means to develop drug resistance: (1) mutation in chromosomal genes
followed by selection of resistant strains and (2) acquisition of
resistance genes as extrachromosomal plasmids, transducing particles,
transposons, or other types of DNA inserts. S. aureus expresses its resistance
to drugs and antibiotics through a variety of mechanisms.
MRSA are strains of the Staphylococcus
aureus that are resistant to the action of methicillin and
related beta-lactam antibiotics (e.g. penicillin, oxacillin,
amoxacillin). MRSA have evolved resistance not only to beta-lactam
antibiotics, but to several classes of antibiotics. Some MRSA are
resistant to all but one or two antibiotics, including vancomycin.
Reports of VRSA (Vancomycin-Resistant Staph aureus) or VRSA are
troublesome in the ongoing battle against staph infections.
MRSA are often sub-categorized as Hospital-Associated
MRSA (HA-MRSA) or Community-Associated
MRSA (CA-MRSA), depending upon the circumstances of acquiring
disease. Based on current data, these are distinct strains of the
HA-MRSA occurs most frequently among patients who undergo invasive
medical procedures or who have weakened immune systems and are being
treated in hospitals and healthcare facilities such as nursing homes
and dialysis centers. MRSA in healthcare settings commonly causes
serious and potentially life threatening infections, such as
bloodstream infections, surgical site infections or pneumonia.
In the case of HA-MRSA, patients who already have an MRSA infection or
who carry the bacteria on their bodies but do not have symptoms (are
colonized) are the most common sources of transmission. The main mode
of transmission to other patients is through human hands, especially
healthcare workers' hands. Hands may become contaminated with MRSA
bacteria by contact with infected or colonized patients. If appropriate
hand hygiene such as washing with soap and water or using an
alcohol-based hand sanitizer is not performed, the bacteria can be
spread when the healthcare worker touches other patients.
MRSA infections that occur in otherwise healthy people who have not
been recently (within the past year) hospitalized or had a medical
procedure (such as dialysis, surgery, catheters) are categorized as
community-associated (CA-MRSA) infections. These infections are usually
skin infections, such as abscesses, boils, and other pus-filled
About 75 percent of CA-MRSA infections are localized to skin and soft
tissue and usually can be treated effectively. However, CA-MRSA strains
display enhanced virulence, spread more rapidly and cause more severe
illness than traditional HA-MRSA infections, and can affect vital
organs leading to widespread infection (sepsis), toxic shock syndrome
and pneumonia. It is not known why some healthy people develop CA-MRSA
skin infections that are treatable whereas others infected with the
same strain develop severe, fatal infections.
Studies have shown that rates of CA-MRSA infection are growing fast. In
1999, four children in Minnesota and North Dakota were reported to have
died from fulminant CA-MRSA infections One study of children in south
Texas found that cases of CA-MRSA increased 14-fold between 1999 and
2001. By 2007, CA-MRSA was the most frequent cause of skin and
soft-tissue infections seen in emergency departments in the United
Although most MRSA cases are skin and soft-tissue infections, some are
more serious with septicemia and pneumonia. It was reported in 2005
that previously healthy adolescents without any predisposing risk
factors presented more frequently with severe Staph infections (mostly
the USA 300 strain) since 2002.
CA-MRSA skin infections have been identified among certain populations
that share close quarters or experience more skin-to-skin contact.
Examples are team athletes, military recruits, and prisoners. However,
more and more, CA-MRSA infections are being seen in the general
community as well, especially in certain geographic regions.
Also, CA-MRSA are infecting much younger people. In a study of
Minnesotans published in The Journal of the American Medical
Association, the average age of people with MRSA in a hospital or
healthcare facility was 68. But the average age of a person with
CA-MRSA was only 23.
In the United States it is estimated that 31.8 out of 100,000 people are being infected by MRSA each year, more infections than meningitis, bacterial pneumonia and flesh-eating strep put together. More people in the U.S. now die from MRSA infection than from AIDS. Methicillin-resistant Staphylococcus aureus was responsible for an estimated 94,000 life-threatening infections and 18,650 deaths in 2005, as reported by CDC in the Oct. 17, 2007 issue of The Journal of the American Medical Association. The national estimate is more than double the invasive MRSA prevalence reported five years earlier. That same year, roughly 16,000 people in the U.S. died from AIDS, according to CDC.
While most invasive MRSA infections could be traced to a hospital stay or some other health care exposure, about 15% of invasive infections occurred in people with no known health care risk. Two-thirds of the 85% of MRSA infections that could be traced to hospital stays or other health care exposures occurred among people who were no longer hospitalized. People over age 65 were four times more likely than the general population to get an MRSA infection. Incidence rates among blacks were twice that of the general population, and rates were lowest among children over the age of 4 and teens.
In 2010, encouraging results from a CDC study published in the Journal of the American Medical Association showed that invasive (life-threatening) MRSA infections in healthcare settings are declining. Invasive MRSA infections that began in hospitals declined 28% from 2005 through 2008. Decreases in infection rates were even bigger for patients with bloodstream infections. In addition, the study showed a 17% drop in invasive MRSA infections that were diagnosed before hospital admissions (community onset) in people with recent exposures to healthcare settings.
A parallel study from the National Healthcare Safety Network (NHSN) found that rates of MRSA bloodstream infections occurring in hospitalized patients fell nearly 50% from 1997 to 2007. Furthermore, a March 2011, CDC Vital Signs article reported that bloodstream infections from staph in ICU patients with central lines were reduced by 73%, more than from any other cause.
Taken altogether these reports provide evidence that rates of invasive MRSA infections in the United States are falling. While MRSA remains an important public health problem and more remains to be done to further decrease risks of developing these infections, this decrease in healthcare-associated MRSA infections is encouraging.
On the other hand, rates of CA-MRSA infections have increased rapidly during the past decade and there is little evidence that the risk of developing infection is following the same downward trend as HA-MRSA.
Beginning with the use of the penicillin in the 1940's, drug resistance
has developed in the staphylococci within a very short time after
introduction of an antibiotic into clinical use. Some strains are now
resistant to most conventional antibiotics, and there is concern that
new antibiotics have not been forthcoming. New strategies in the
pharmaceutical industry to find antimicrobial drugs involve identifying
potential molecular targets in cells (such as the active sites of
enzymes involved in cell division), then developing inhibitors of the
specific target molecule. Hopefully, this approach will turn up new
antimicrobial agents for the battle against staph infections. Indeed,
since 2003, alternatives to vancomycin have been approved for treatment
Textbook of Bacteriology Index