The news is not good. If you are concerned about the possibility that antibiotic residues in the environment create another setting in which resistance can develop, it is worth reading. It is long (10 pages in pdf) but has a comprehensive bibliography. Also, it's open-access.
Where do these antibiotic residues come from? From us, in some cases: We urinate out up to 90% of some drugs, wash off topical formulations, flush old prescriptions down the toilet. Sometimes from industrial residues, or from leaky hospital sewage, or from sewage treatment plants, or — of course — from industrial-scale agriculture administration and run-off.
And where do they go? According to the paper, over more than 20 years of research, 126 different antibiotics and anti-infectives have been identified in processed waste water, natural surface water and groundwater, and drinking water supplies. Among them are all the antibiotics that we are concerned about here: the drugs that MRSA is already resistant to (beta-lactams, lincosamides, macrolides) and the drugs that still work, for community MRSA at least (sulfonamides, trimethoprim, tetracycline).
Moreover, the trend is expected to get worse, the authors warn: because of increased urbanization; because many urban areas are consciously setting water-saving policies, reducing the volume of wastewater and therefore increasing the concentration of drugs in the water that remains; and because, well, CAFOs aren't exactly going away right now, are they? As they say:
...even if our results show that high concentrations ... of anti-infectives in these waters are more the exception than the rule, the existence of a few locations where these concentrations can be reached are enough to contribute to the global spreading of anti-infective resistance. Given that large populations of bacteria are being exposed to a selective pressure, environmental waters and especially wastewaters become ideal settings for the assembly and exchange of mobile genetic agents encoding for resistance in bacteria. ... Anti-infectives, the miracle drugs of the 20th century, have become environmental contaminants of emerging concern in the 21st.The cite is: Segura PA et al. Review of the Occurrence of Anti-infectives in Contaminated Wastewaters and Natural and Drinking Waters. Environmental Health Perspectives, 117 (5) May 2009.