We know from the horticultural literature that high populations of harmless microorganisms in peat-based plant soils have significant effects on the quality and yield of vegetable crops. When light peat that is rich in microorganisms is replaced with dark peat that contains comparatively lower microbial levels, the developing plants tend to become more susceptible to diseases. Peat products are widely used in the mushroom industry to prepare casing soils and in recent years, partial replacement of light peat with dark types has become more common. In earlier studies conducted in our laboratory, we observed that when Listeria and Salmonella were inoculated into casing soil, populations of these pathogenic bacteria decreased by over 4 log units (a 10,000-fold reduction) within 10 days. In contrast, if the casing soil had been steam sterilized, pathogen levels remained relatively constant for up to 58 days.
Growers are increasingly supplementing traditional light peat in mushroom casing formulations with deep-dug dark peat. The impact this may have on the microbial ecology of casing soil has not been studied. We therefore expanded upon our earlier work to determine if dark peat supplementation might affect the survival of human pathogens in casing soils. We also determined the potential for pathogens to transfer from casing soil to mushrooms as well as the effects of irrigation treatments on pathogen survival. In contrast to our earlier work, we utilized a laboratory scale growing system that allowed us to follow pathogen levels at the same time that Agaricus bisporus was actively growing and producing mushrooms. Results of our study are summarized.
A study to compare background microbial levels in uninoculated casing soils was initially conducted using one casing soil prepared with light peat and two soils prepared with dark peat. Light peat, obtained from Acadian Peat (New Brunswick, Canada), was neutralized with calcium carbonate (Pulverized High Calcium Limestone, Con-Lime Inc. Bellefonte, PA) at the Mushroom Test Demonstration Facility (MTDF). The two dark peat casing soils were formulated by the manufacturers. Harte dark peat (Monaghan, Ireland) was amended with limestone flour and spent sugar beet lime to raise the pH to approximately 7.0. Sylvan dark peat (Ontario, Canada) was similarly neutralized with Canadian lime. Each of the casing soils were mixed with Casing Inoculum (CI) (Phoenixx, Amycel, Madisonville, TX) in the laboratory.
Populations of harmless aerobic bacteria, actinomycetes, and yeasts + molds in the light peat casing soils were on average 8.4, 7.8, 6.8 log CFU/g, respectively. Microbial levels in light peat soil were significantly higher compared to either of the dark peat casing soils which averaged 6.6, 6.7, and 5.2 log CFU/g, respectively. At no time during the course of this study were human pathogens found in any samples of un-inoculated peat casing soils. These results confirm earlier studies demonstrating that deep-dug dark peats typically contain lower levels of microorganisms compared to light peats which are dug from deposits closer to the surface that are in a aerobic environment.
Because of cross contamination concerns, casing soil inoculation studies were not possible on a commercial farm or at the Penn State Mushroom Test Demonstration Facility (MTDF). Therefore a small-scale laboratory system for growing Agaricus bisporus in 32-ounce containers was developed so that the fate of human pathogens could safely be studied over the course of a complete spawn to harvest production cycle. Phase II substrate and materials for the preparation of casing soils were obtained from the MTDF and transported to the Penn State Food Safety Pilot Plant. Casing soils with light:dark peat ratios of 100:0, 80:20, and 60:40, were prepared, spawned, and then inoculated with L. monocytogenes and Salmonella spp. each to approximately 108 CFU/gram. Pathogen levels were sampled periodically over the course of a complete mushroom growing cycle. Mushrooms emerging from inoculated soil were also tested for the presence of pathogens.
Pathogen levels in soil samples taken between casing and harvesting were significantly higher in dark peat soil mixtures compared to 100% light peat soil. However, in all three light:dark peat formulations, pathogen levels decreased by 3-4 logs between during this period. Transfer of pathogens from inoculated casing soil to emerging mushrooms was observed for all peat types. Average pathogen transfer frequencies were between 45 and 66 percent for Salmonella spp. and between 53 and 56 percent for L. monocytogenes.
In order to create a scenario where mushrooms might become contaminated with Listeria or Salmonella spp. after they emerge from the casing soil, mushrooms were grown in uninoculated soil and individual pins were spot-inoculated with pathogens. The effects of irrigation treatments were also studied. Between pinning and harvesting, three successive watering treatments resulted in average Listeria monocytogenes and Salmonella spp. log reductions of 1.2 and 1.6, respectively. Presumably this was due to exposure of the pathogens to an unfavorable environment on the surface of the mushrooms and possibly cells washing off the mushrooms during each watering. When chlorine dioxide, sodium hypochlorite, hydrogen peroxide, and peroxyacetic acid sanitizers were applied to mushrooms at allowable levels in the third and final irrigation application, no significant additional reductions were observed.
The results of this study support the hypothesis that naturally present populations of harmless microorganisms in peat casing soils have an inhibitory effect on the survival of human pathogens during a complete mushroom growing cycle. Although pathogen reductions generally occurred more rapidly in the light peat casing soil, the small difference between light and dark peat types does not warrant serious concern. Further reductions in pathogen levels did occur during water irrigation treatments. However, addition of sanitizers unexpectedly did not significantly add to this effect.
It is good news for the industry that peat casing soils, formulated from both light and dark peat, do not provide a favorable environment for Listeria monocytogenes or Salmonella spp. However complete destruction over the course of a mushroom growing cycle was not observed. Since adhering particles of peat are often present on the surface of unwashed mushrooms, growers should assume that consumers may actually ingest casing soil materials. When viewed through this perspective, it is essential that growers take all possible actions to prevent casing ingredients and soil formulations from becoming a source of contamination. A Certificate of Analysis should be obtained for each shipment that demonstrates absence of pathogens in peat and lime materials. Thereafter, Mushroom Good Agricultural Practices (MGAP) should be implemented to protect these materials from potential sources of contamination. It is especially important to keep ingredients and finished casing soil as far away as possible from Phase I areas since harmful bacteria can become windborne and spread through the air or wash into storage areas from water run-off and flooding. Overhead contamination from birds can be prevented by storing casing ingredients and soils in an enclosed area or at least under a protective tarp.
When it comes to food safety, there is one piece of advice that has repeatedly proven to be useful. Never wait for a problem to occur. Be proactive – not reactive. Look for all potential sources of contamination in your operation and take action now. Aggressive efforts to keep Listeria out of growing and packing areas are in everyone’s best interest.
Contact Dr. Luke LaBorde at 814-863-2298 or firstname.lastname@example.org for more information on this and other mushroom food safety research. For additional food safety resources, visit the Penn State Farm Food Safety website.
This study was made possible through support from the Giorgi Mushroom Research Fund and the American Mushroom Institute through funding from the USDA/PDA Specialty Crop Block Grant.
Originally printed in Mushroom News Magazine, January 2012.