Woods End Laboratories

Healthy Soil

Healthy Science

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Hygiene of Products


Research designed by Woods End examines the fate of potato pathogens during windrow composting of culled potatoes in Maine and P.E.I.

Testing Composts for Bacterial Content and Hygiene Quality

There is increasing consumer awareness about bacteria in the food chain due to constantly recurring instances of avoidable contamination. The challenge is understanding the appropriate role of human intervention in relation to safety intrinsic to natural biological systems functioning properly.  Often the knee jerk reaction to public concerns is hastily constructed standards.  There is a need to properly assess the science of standards outside the push-pull of politics and panic.  For industry and farming, the best defense in a quality control program is to be well informed by appropriate tests.

“High-end-use” applications of recycled organic matter such as consumer growing media, gardens, playgrounds and parks and sports turf, comprise uses where certainty of absence of bacterial pathogens is extremely important. Such products must be adequately tested following processing and before sales. While composting has the great potential to reduce pathogenic content, it is now generally recognized by microbiologists to be an unreliable means to do so, due to natural variability in biological systems.

Woods End Laboratories has had three decades of intensive laboratory and field experience for examining the presence and fate of fungal and bacterial organisms in recycled wastes. The firm has also recently published extensive bacterial hygiene test data on recycled organic matter composts in the Journal of Food Protection (Vol 2:2009 – see reprint section).

Decision Tree for Testing Organisms

IN EVENT OF NO TESTS: Many products are not tested or no indication of cleanliness is provided. If hygiene information does not exists it is prudent to assume that the product is not hygienic and to act accordingly – anything less that this has increased risk potential.

IF CHOICE FOR TESTING EXISTS: It is best to plan to test by selecting an organism that gives maximum information under the circumstances and from this, to draw conclusions as to whether or not pathogens could be there that are significant for the end-use. With compost and soil amendments fecal coliforms continue to be a very appropriate indicator for pathogen cleanliness. A more conservative tests would use  Clostridium perfringens since it is pathogenic, it  survives extreme temperatures and will grow under varied conditions and is virtually always present in feces. If it is absent, then most likely other pathogens will also be absent.

INDICATOR TESTS: It is common to choose a microbe that gives information from which extrapolation of pathogen status is performed. This is called INDICATOR testing. For example, fecal coliform and E.coli are always found in bird and mammal feces but only some E. coli are pathogenic. The presence of E. coli therefore indicates generally the presence of feces and the potential for toxigenic factors. If E .coli is not present, then the assumption is that pathogenic strains of E. coli such as E. coli-O157:H7 are also not present. Woods End data shows that many composted achieve true pathogenic-free status; the data also reveal that green-waste may contain as much pathogen potential as manure-based materials.

Is the Presence of One Type of Non-pathogenic Bacterium a Valid Indication of Likely Pathogen Presence?

  1. Presently, the field of pathogen testing is changing rapidly. All things are and should always be open for challenge and discovery of error, of course.
  2. A National Research Council report (2002) recommends using C. perfringens as indicator bacterium for compost pathogen evaluation.
  3. Agencies in Europe presently require that compost be Salmonella-free (non-detect on 50g) and have established limits for E.coli, E.coli 0157:H7, Listeria and Campylobacter.


  • TM. Scott, J B. Rose, T M. Jenkins, S R. Farrah, and J Lukasik. “Microbial Source Tracking: Current Methodology and Future Directions”. Applied and Env Microbiol. Dec 2002. p.5796-5803.
  • EPA Health Effects Criteria for Fresh Recreational Waters. EPA-600/1-84-004. Section 2.
  • V J. Harwood, A D. Levine, T M. Scott, V Chivukula, J Lukasik, Sl Farrah, J B. Rose (2005) “Validity of the Indicator Organism Paradigm fort Pathogen Reduction in Reclaimed Water and Public Health Protection.” Appl Env Microbiology, p. 3163-3170.
  • Brinton et al. (2009) Occurrence and Levels of Fecal Indicators and Pathogenic Bacteria in Market-Ready Recycled Organic Matter Composts. Journal of Food Protection, Vol. 72, No. 2, 2009, Pages 332–339

(c) 2005-2010 Woods End Lab. Permission to reprint granted with attribution.

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