MRSA in water treatment plants! Panic?!

There is a new study just published in Environmental Health Perspectives that is getting a lot of media attention (here and here and here). The (open-access) study by University of Maryland researchers identified MRSA (and MSSA) in water samples collected during 2009/2010 from two mid-atlantic and two mid-western water treatment plants. They found that 50% of samples contained MRSA and 55% contained MSSA. Among the MRSA strains, 83% were SCCmec type IV and 15% were SCCmec type II, while 68% were pvl gene positive. USA100, USA300 and USA700 PFGE strain types were identified.

Sounds pretty scary, and if you don't read carefully, you might conclude that you shouldn't drink municipal water. However, it appears that most of the MRSA/MSSA was detected in "influent" or pretreatment samples, which were 83% positive, while 17% of post-treatment "effluent" samples were positive and only 8% were MRSA positive. This suggests that water treatment is effective in eliminating S. aureus, so keep drinking the water.

Note:  The positive samples in the effluent were collected during periods when seasonal chlorination was not taking place. Thus, if your municipality uses chlorination, there is likely little S. aureus risk. Also, we should ask if we can generalize beyond this 4-facility sample.

Image Source: EPA via CDC