Bacteria in Organic vs. Conventional Dairy: A Review from U.S. and Europe

Background and methodology 

The objective of this review was to identify, evaluate, and summarize findings of all primary research published in English or French that investigated the prevalence of zoonotic or potentially zoonotic bacteria, bacterial resistance to antimicrobials, and somatic cell count (SCC) in organic dairy production, or comparing organic and conventional dairy production. Potentially zoonotic bacteria were defined for purposes of this review as those bacteria capable of causing illness in humans following ingestion or contact. 

Among 47 studies included in the review, 32 comparison studies were suitable for a quality assessment. Fifteen studies were not assessed because of their quality, descriptive nature or low sample size (two or fewer farms). Overall, bacterial outcomes were reported in 17 studies, and the prevalence of antimicrobial resistance (AMR) and multidrug resistance (MDR) of zoonotic or potentially zoonotic bacteria in 12 and 7 studies, respectively. Campylobacter spp., Escherichia coli includingShiga toxin–producing strains, Salmonella spp.Staphylococcus aureus, and SCC were investigated in 2, 7, 4, 6, and 15 studies, respectively. 


Contradictory findings were reported for differences in bacterial outcomes and the SCC between dairy production types. Lower prevalence of AMR on organic dairy farms was reported more consistently in studies conducted in the United States, as opposed to those conducted in Europe. These conflicting findings may result from geographic differences in organic production regulations governing antimicrobial usage, use of antimicrobials in conventional dairy production, and baseline prevalence, as well as laboratory methods, study designs, or methods of analysis employed. The majority (four of seven) of MDR investigations reported no significant differences in prevalence. Overall, only 9 of 32 studies met all five methodological soundness criteria. This study recommended that more well designed, executed, and reported primary research is needed at the farm and post-farm levels to reach to a conclusion.


Wilhelm, B., Rajić,  A., Waddell, L., Parker, S., Harris, J., Roberts, K. C., Kydd, R., Greig, J., & Baynton, A., (2009). Prevalence of zoonotic or potentially zoonotic bacteria, antimicrobial resistance, and somatic cell counts in organic dairy production: Current knowledge and research gaps. Foodborne Pathogens and Disease, 6(5), 525-539. Available on-line at: