Fecal coliform and E. coli in compost or leachate is usually reported in MPN per g compost or MPN per 100 mL water (or leachate). Sometimes we see results in CFU/g, or per 100 mL. Is there a difference?
CFU refers to “colony forming units”, whereas MPN refers to “most probable number”. The difference is that CFU/100ml is the actual count from the surface of a plate, and MPN/100ml is a statistical probability of the number of organisms (American Public Health 2012). The US EPA appears to prefer MPN rather than CFU “because a colony in a CFU test might have originated from a clump of bacteria instead of an individual, the count is not necessarily a count of separate individuals.” (US EPA 2003).
It is important to note that some test methodology for specific organisms report the results in CFU, whereas for fecal coliform and E.coli, MPN is most often used.
Although we would intuitively think that CFU and MPN should be equivalent, and we normally assume that they are, research suggests that this is not always the case. One research report indicated that “especially in fall, E. coli concentrations in MPN are one order of magnitude greater than that in CFU” (Cho et al. 2010).
The Organic Matter Recycling Regulation, the Approved Water Quality Guidelines for British Columbia (BC MOE 2001), the CCME Compost Quality Guidelines (CCME 2005) and the US EPA (US EPA 2003), and the UK Compost Regulation (BSI 2011) report the fecal coliform requirements for compost in MPN per g solids.
BC Ministry of Environment. 2001. Approved Water Quality Guidelines Microbiological Indicators 2001. (http://www2.gov.bc.ca/assets/gov/environment/air-land-water/water/waterquality/wqgs-wqos/approved-wqgs/miceoindicators-or.pdf)
BSI. 2011. PAS 100:2011. Specification for Composted Materials. http://www.wrap.org.uk/sites/files/wrap/PAS%20100_2011.pdf
CCME 2005. Guidelines for Compost Quality. PN 1340 (http://www.ccme.ca/files/Resources/waste/compost_quality/compostgdlns_1340_e.pd)
Cho, K.H., D. Han, Y. Park, S.W. Lee, S.M. Cha, J.H. Kang and J.H. Kim. 2010. Evaluation of the relationship between two different methods for enumeration fecal indicator bacteria: colony-forming unit and most probable number. J. Environ Sci (China) 22: 846-50.
American Public Health Association, American Water Works Association, Water Environment Federation. 2012. Standard Methods for the Examination of Water and Waste Water.
US EPA 2003. Environmental Regulations and Technology. Control of Pathogens and Vector Attraction in Sewage Sludge (Including Domestic Septage) Under 40 CFR Part 503. EPA/625/R-92/013(https://www.epa.gov/sites/production/files/2015-04/documents/control_of_pathogens_and_vector_attraction_in_sewage_sludge_july_2003.pdf)