Testing Composts for Bacterial Content and Pathogens

Woods End Microbiology Laboratory

There is increasing awareness of bacterial content and potential hazards in the food chain from incomplelety processed manures and recycled organic matter (compost). The anticipated end-use of a manure or natural soil amendment has a significant bearing in determining the need for and significance of a bacterial hygiene test.

High-end use applications, including green-leafy vegetable growing, potting media, gardens, playgrounds and parks and sports turf, comprise uses where certainty of absence of bacterial pathogens is extremely important.

A large proportion of the environmentally aware public has very positive feelings about recycled organic matter composts, but this can obscure the fact that compost is not, of course, clean enough to eat, and increasingly, unacceptable for intensive high-impact vegetable production. If recycled organic mamter compost is bagged and sold to the general public it either should be as pathogen-free as possible or there should be a disclaimer warning on the label.

Woods End has had more than a decade of intensive laboratory and field experience for examining the presence and fate of bacterial organisms in recycled wastes.

Decision Tree for Testing Pathogens

1. If testing is not contemplated, not planned for, or not desired, then it is prudent to assume the product is non-hygienic. Customers will assume so and the seller must warn so.
- OR

2. Plan to test by selecting an organism that gives maximum information; draw conclusions from that as to whether or not pathogens could be there. For example, Clostridium perfringens is pathogenic, survives extreme temperatures and conditions and is always present in feces. If it is absent from freshly finished compost, then probably all pathogens will be too.
- OR

3. Select a microbe that gives information from which extrapolation of pathogen status can be performed [eg. E.coli is always found in bird & mammal feces; most E. coli are not pathogenic but many are. The presence of E. coli indicates the presence of feces and the potential for toxigenic accompaniment. Further, if E.coli is not present, then the assumption is that pathogenic strains of E. coli such as E. coli-0157:H7 aren’t either. Other microbes used for this include enterococci, fecal streptococci, fecal coliforms, and C. perfringens.

Is the Presence of One Type of Nonpathogenic
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. Previous research findings indicate that fecal coliforms are actually present in very low densities both in biosolids and even sewage sludge. Woods End Lab has found a considerable amount of fecal coliform in finished composts.
3. Agencies in Europe require that compost be Salmonella-free (non-detect on 50g) and have established limits for E.coli, E.coli 0157:H7, Listeria and Campylobacter.
4. Presence of E. coli in meat carcasses is assumed to indicate possibility of toxigenic E.coli-0157 by USDA, but in sprouts, actual E.coli 0157 is sought in testing.

U.S. Agencies are presently asking this question regarding water pollution evaluation:

• Enterococci are capable of independent growth in the environment after introduction, and reservoirs can build.
• E.coli is able to replicate in contaminated soils.
  “A direct linear relationship was observed between highly credible gastrointestinal illness and bacterial densities of two indicators of fecal contamination, enterococci and E. coli. Fecal coliforms showed no relationship to the rate of swimming-associated gastrointestinal illness.“
• Following a study of wastewater reclamation facilities in USA examining correlations between certain indicator species and certain pathogens, no correlation between any single pair was observed. However, correlation was strong between the group of indicators and pathogens.

Citations

1. TM. Scott, J B. Rose, T M. Jenkins, S R. Farrah, and J Lukasik. “Microbial Source Tracking: Current Methodology and Future Directions”. Applied and Environmental Microbiology, Dec 2002. p.5796-5803.
2. Ibid.
3. “Health Effects Criteria for Fresh Recreational Waters. EPA-600/1-84-004. Section 2.
4. V J. Harwood, A D. Levine, T M. Scott, V Chivukula, J Lukasik, Sl Farrah, J B. Rose. “Validity of the Indicator Organism Paradigm fort Pathogen Reduction in Reclaimed Water and Public Health Protection.” Applied and Environmental Microbiology, June 2005, p. 3163-3170.
   

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