Drug Resistant Bacteria
Relationship Between Community and Hospital Acquired Infections
MRSA acquired by persons who have not been recently hospitalized or had a medical procedure are known as community-acquired (CA-MRSA).
The first reports of methicillin resistance in S. aureus (MRSA) began in the mid-1960s among hospitalized patients. S. aureus is a frequent cause of infection in hospitals and other healthcare facilities such as nursing homes and dialysis centers, because the people in these facilities generally have weakened immune systems. It is really no surprise that the first accounts of MRSA occurred in the healthcare environment where antibiotic pressure helped to select for a larger population of methicillin resistant S. aureus. Serious or invasive S. aureus infections were more often treated in-hospital, and most often with methicillin. Over time strains of MRSA were increasingly found as a cause of staphylococcal infection acquired in the healthcare environment.
In some geographic locations, hospital-acquired MRSA (HAMRSA) still remains low (e.g. Nova Scotia); whereas, in larger cities, the percentage of hospital-acquired staphylococcal infections caused by MRSA is much higher. In many large cities in the U.S. the proportion of S. aureus infections caused by MRSA is 60% or higher. What does this mean? If you developed infection in a surgical wound while in hospital, chances are the organism responsible would be S. aureus, because S. aureus is a major cause of nosocomial infections. AND, because the percentage of S. aureus hanging around the hospital environment are resistant to methicillin, the likelihood that the staph causing – something wrong with this sentence - your wound infection is greater than the flip of a coin if you are hospitalized in a large city in the U.S. In Mexico, some areas of South America, the Mid-East and Asia, the risk of having a wound infection due to MRSA is even greater. And hospital strains of MRSA tend to have resistance not only to methicillin, but also to multiple other antibiotics that might otherwise be useful in treating S. aureus infections.
How are infections spread among patients in a healthcare environment? Close contact – skin to skin, hand to wound, hand to nose – is required to pass S. aureus from one individual to another. There is generally not a lot of patient to patient contact in healthcare facilities, so an in-between carrier is required – the healthcare worker. S. aureus and MRSA are carried on the hands of healthcare workers who don’t glove when performing procedures on patients, and/or don’t wash their hands between patients. Once introduced, even if not directly to a wound where it may set up infection, MRSA has a greater chance of colonizing our nasal passages, skin and digestive tract, so that if and when we’re predisposed to infection, there is a better chance of MRSA being the causative agent.
Within 20 years of the first reports of MRSA within healthcare facilities, infections due to MRSA were also being reported in the community. These first infections were likely due to spread of hospital strains. The risk factors for these earlier community-acquired (CAMRSA) infections included injection drug use, previous antibiotic therapy and recent hospitalization. In the 1990s, MRSA was seen more often in community-acquired infection without the traditional risk factors. Why? The greater the population of MRSA among the total population of S. aureus, the greater the colonization rate with resistant staph rather than methicillin susceptible staph, and the greater the chance of having a staph infection caused by a resistant strain of staph.
We now know more about the mechanism by which bacteria develop antibiotic resistance. Strains of methicillin-resistant S. aureus have developed through acquisition of mobile genetic elements which carry the mecA gene, which encodes a protein (penicillin-binding protein 2a) that penicillin does not bind or attach to. The effectiveness of penicillin antibiotics relies on the ability to bind to the surface of the bacteria. The mecA gene is carried on a mobile genetic element called a cassette (SCCmec cassette). A genetic cassette may carry a number of genetic elements, much like a compact disc holds more than one song – in the case of MRSA, the cassette carries the mecA gene, and depending on the strain of S. aureus may also carry other resistance factors that provide protection against other antibiotics, as well as a variety of virulence factors.
Currently, there are 4 recognized clones or types of SCCmec - I through IV, with SCCmec IV being the most common. The structure of the mecA genes are similar, but the gene structure is significantly different than the hospital-acquired strains. Because of the differences in mecA gene structure and cassette components, hospital-acquired strains tend to be resistant to multiple antibiotics, and are associated more often with surgical wound infections, urinary tract infections, bloodstream infections, and pneumonia. Community-acquired strains do not share the same antibiotic resistance patterns as the strains circulating in the hospital, and tend to cause infections associated with methicillin susceptible strains of S. aureus in the community – skin infections (pimples, boils, impetigo, infected cuts, etc.), but can also cause more serious infections like pneumonia.
As time goes on, the lines between hospital-acquired strains and community-acquired strains will become gradually more blurred, until at some point there is little difference. Hospital infection control practitioners are already beginning to struggle with determining whether infections are hospital-acquired, healthcare-associated, community-acquired or community-associated. At some point, it will no longer matter and we will have to accept that methicillin, and no doubt a number of other antibiotics, are no longer useful in treating staph infections. However, until such time, prudent infection control measures like good hand hygiene and contact measures with patients who are colonized with MRSA will help reduce spread among patients so that methicillin remains an effective treatment for staph infections for as long as possible. Not only will this save tens of thousands of dollars each year, it will also reduce the length of stay in hospital, reduce adverse drug effects related to stronger antibiotics, and delay the speed at which organisms commonly found in the healthcare environment develop resistance.
Take home message – wash your hands!
