Literature Commentary by Dr. John G. Bartlett: Methicillin-resistant Staphylococcus aureus infections, January 2008

John G. Bartlett, MD


January 11, 2008

Klevens RM, Morrison MA, Nadle J, et al, for the Active Bacterial Core surveillance (ABCs) MRSA Investigators. Invasive methicillin-resistant Staphylococcus aureus infections in the United States. JAMA. 2007;298:1763-1771.

Purpose: The goal of the study was to determine the incidence and distribution of invasive methicillin-resistant Staphylococcus aureus (MRSA) disease in 9 US communities and to use these data to estimate the burden of MRSA infections in the United States.

Methods: The study investigators, who were from the US Centers for Disease Control and Prevention (CDC), used data from the Active Bacterial Core surveillance program from July, 2004 through December, 2005. This database is populated with data from MRSA strains recovered from sites that are normally sterile, primarily blood cultures. There were 9 communities represented, with a total population of 16.5 million people, representing about 5.6% of the total US population. The effort is supported by 123 laboratories that sent the strains to the CDC for selective testing. The cases were classified as hospital onset; community-associated; or the newer category of "healthcare-associated, community-onset," which refers to MRSA that is acquired in long-term care facilities, during recent hospitalization, or patients receiving dialysis or other continuing contact with the healthcare system.

Results: There were 8987 observed cases of invasive MRSA during the review period of approximately 18 months. Of these, 58% were healthcare-associated, community-onset infections; 27% were hospital-onset, and 14% were community-associated. Demographics indicated a preponderance of men, elderly people, and African-American race. There were 1598 deaths in the hospital, giving a standardized mortality rate of 3.6/100,000 people. The distribution of cases in the 3 categories and the data provided based on host demographics are summarized in Table 1 and Table 2 .

In terms of pulsed-field gel electrophoresis (PFGE) strain typing, the dominant strain for community-acquired infection was the USA300 strain while the USA100 strain dominated in the hospital-associated and hospital onset cases. These data are provided in Table 3 .

Conclusions: The authors conclude that MRSA infection is a "major public health problem primarily related to healthcare, but no longer confined to intensive care units, acute care hospitals or any healthcare institution."

Comment: Of particular interest in this report is the observation that the dominant strain type for invasive MRSA was the USA100 family, which is nosocomial, although most of the cases were acquired outside the hospital in patients with contact with the healthcare system. Nevertheless, there is substantial concern about the USA300 family of MRSA that represents most of the "community-acquired infections." These different strains of MRSA are distinct, although there does appear to be a merging of the 2 within the hospital, at least according to some reports. The difference in the 2 types of MRSA are summarized in Table 4 .


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