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MRSA-study: bacteria are nesting in households

Portrait of Prof. Michael David, University of Chicago Biological Sciences
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A recent study of MRSA shows an interesting picture: Easily transmissible bacteria remain for years and continue to develop. Carola Timmel spoke with the head of the study, Prof. Michael David from the University of Chicago Biological Sciences.
(short summary in german: MRSA Erreger nisten sich in Haushalten ein: Eine kürzlich veröffentlichte Studie über MRSA zeigt ein interessantes Bild: Leicht übertragbare Bakterien bleiben jahrelang und entwickeln sich weiter. Mit dem Leiter der Studie, Prof. Michael David, von der University of Chicago Biological Sciences sprach Carola Timmel.

Prof. David, how can this findings about MRSA in the households help in relation to the handling of MRSA in the hospitals? How can hospitals benefit from this knowledge?

It is important to note that in North America, infections with MRSA are very common outside of hospitals. The great majority of MRSA infections have their onset outside of hospitals in the U.S.; this is not generally true in Europe.   Many otherwise healthy people who have had no health care contact are infected by MRSA.  The most common strain type of MRSA in the U.S. is USA300, the strain type that we studied in our paper.  This strain type is not common in Europe, although it has been detected in almost every country in Europe although the European community-associated MRSA strain type (ST80) is usually found to be more common and is essentially absent from North America.
Because infections in the U.S. with USA300 are so common, it was of interest to us to determine the reservoir for these strain types, and we have found in our study that the household is a likely reservoir.  We have learned a lot about risk factors for the spread of MRSA from patient to patient in hospitals through many hundreds of studies; we believe that in hospitals, it is transmitted from patient to patient on the hands of care-givers.  It is likely that hand-washing by health care workers is best way to intervene in the spread of MRSA in the hospital.  In contrast, we do not know what the primary means of transmission of MRSA in the household might be, and this makes it difficult to recommend specific preventative strategies.  Transmission could be by direct skin-to-skin contact or it may be by indirect transmission from sharing of towels and other items that have frequent skin contact.  It is not known if the same risk factors for transmission are relevant in the household and in the hospital.
Since about 2005, USA300 MRSA has also become one of the most common MRSA strain types to circulate in hospitals in the U.S., and it is possible that many of the patients with USA300 MRSA infections picked up this strain in their own households.

What is the most important lesson from your study for hospiatls?

I think that the most important lesson from our study for hospitals is that the unit of concern for the introduction of MRSA into the hospital may not be the individual patient, but instead it may be the household unit where the patient lives.   It is not clear if the same risk of household transmission and persistence exists in Europe and other countries where USA300 MRSA strains are not common.   It seems likely to me, however, that it is only a matter of time until USA300 does become common in other parts of the world.  Once this occurs, we would expect household transmission to become increasingly common in other countries as well.

Are you also planning an investigation of other hospital germs, such as clostridium difficile?

I am not planning studies on other pathogens at this time.  Instead, I am planning additional studies on MRSA to assess risk factors for transmission and infection as well as means of preventing transmission and infection in community settings.

Can this extramural investigation be a good approach to learn more about the life and the behavior of bacteria?

Yes, if by extramural you are referring to “outside of healthcare facilities,” then this is certainly a good approach to learning about the life and behavior of bacteria.  By studying transmission dynamics of bacteria when they are silently colonizing the human body and not causing infections, one can come to understand how they spread to put people at risk of infection. This is the first step in determining how to prevent the risk of infection.

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