Bacteriological survey of sixty health foods

Persistence of Escherichia coli in the microbiomes of red Romaine lettuce (Lactuca sativa cv. 'Outredgeous') and mizuna mustard (Brassica rapa var. japonica) - does seed sanitization matter?

BACKGROUND

Seed sanitization via chemical processes removes/reduces microbes from the external surfaces of the seed and thereby could have an impact on the plants' health or productivity. To determine the impact of seed sanitization on the plants' microbiome and pathogen persistence, sanitized and unsanitized seeds from two leafy green crops, red Romaine lettuce (Lactuca sativa cv. 'Outredgeous') and mizuna mustard (Brassica rapa var. japonica) were exposed to Escherichia coli and grown in controlled environment growth chambers simulating environmental conditions aboard the International Space Station. Plants were harvested at four intervals from 7 days post-germination to maturity. The bacterial communities of leaf and root were investigated using the 16S rRNA sequencing while quantitative polymerase chain reaction (qPCR) and heterotrophic plate counts were used to reveal the persistence of E. coli.

RESULT

E. coli was detectable for longer periods of time in plants from sanitized versus unsanitized seeds and was identified in root tissue more frequently than in leaf tissue. 16S rRNA sequencing showed dynamic changes in the abundance of members of the phylum Proteobacteria, Firmicutes, and Bacteroidetes in leaf and root samples of both leafy crops. We observed minimal changes in the microbial diversity of lettuce or mizuna leaf tissue with time or between sanitized and unsanitized seeds. Beta-diversity showed that time had more of an influence on all samples versus the E. coli treatment.

CONCLUSION

Our results indicated that the seed surface sanitization, a current requirement for sending seeds to space, could influence the microbiome. Insight into the changes in the crop microbiomes could lead to healthier plants and safer food supplementation.

Diversity of the spinach (Spinacia oleracea) spermosphere and phyllosphere bacterial communities

The bacterial diversity of seeds, transmission of bacteria from seed to phyllosphere, and fate of seed-transmitted bacteria on mature plants are poorly characterized. Understanding the dynamics of microbial communities is important for finding bio-control or mitigation strategies for human and plant pathogens. Bacterial populations colonizing spermosphere and phyllosphere of spinach (Spinacia oleracea) seedlings and plants were characterized using pyrosequencing of 16S rRNA gene amplicons. Spinach seed microbiota was composed of three bacterial phyla: Proteobacteria, Firmicutes and Actinobacteria, belonging to > 250 different operational taxonomic units (OTUs). Seed and cotyledon bacterial communities were similar in richness and diversity. Richness of 3-4 leaf-stage of development plants increased markedly to > 850 OTUs classified within 11 phyla. Although some bacterial OTUs were detected on seeds, cotyledons and plants, the breadth of new sequences indicates the importance of multiple sources outside the seed in shaping phyllosphere community. Most classified sequences were from previously undescribed taxa, highlighting the benefits of pyrosequencing in describing seed diversity and phyllosphere bacterial communities. Bacterial community richness increased from 250 different OTUs for spinach seeds and cotyledons, to 800 OTUs for seedlings. To our knowledge this is the first comprehensive characterization of the spinach microbiome, complementing previous culture-based and clone library studies.

Evaluation of aerated steam treatment of alfalfa and mung bean seeds to eliminate high levels of Escherichia coli O157:H7 and O178:H12, Salmonella enterica, and Listeria monocytogenes

Sprouts contaminated with human pathogens are able to cause food-borne diseases due to the favorable growth conditions for bacteria during germination and because of minimal processing steps prior to consumption. We have investigated the potential of hot humid air, i.e., aerated steam, to treat alfalfa and mung bean seeds which have been artificially contaminated with Escherichia coli O157:H7, Salmonella enterica subsp. enterica serovar Weltevreden, and Listeria monocytogenes Scott A. In addition, a recently collected E. coli O178:H12 isolate, characterized by a reduced heat sensitivity, was exposed to the treatment described. Populations of E. coli O157:H7 and S. enterica on alfalfa and mung bean seeds could be completely eliminated by a 300-s treatment with steam at 70 ± 1°C as revealed by enrichment studies. L. monocytogenes and E. coli O178:H12 could not be completely eliminated from artificially inoculated seeds. However, bacterial populations were reduced by more than 5 log CFU/g on alfalfa and by more than 4 log CFU/g on mung bean seeds. The germination rate of mung beans was not affected by the 300-s treatment compared to the germination rate of untreated seeds whereas that of alfalfa seeds was significantly lower by 11.9%. This chemical-free method is an effective alternative to the 20,000-ppm hypochlorite treatment presently recommended by the U.S. Food and Drug Administration (FDA).

Growth of Salmonella during sprouting of alfalfa seeds associated with salmonellosis outbreaks

Growth of Salmonella was assessed during sprouting of naturally contaminated alfalfa seeds associated with two outbreaks of salmonellosis. Salmonella was determined daily in sprouts and sprout rinse water samples by a three-tube most probable number (MPN) procedure and a commercial enzyme immunoassay (EIA). Growth of Salmonella in the sprouts was reflected in the rinse water, and the MPNs of the two samples were generally in agreement within approximately 1 log. The results from EIA testing of sprouts and water samples were also in agreement. The pathogen was present in the seed at less than 1 MPN/g, and it increased in number to maximum population levels of 102 to 10(3) MPN/g in one seed lot and 10(2) to 10(4) MPN/ g in the other seed lot. Maximum populations of the pathogen were apparent by day 2 of sprouting. These results show the ability of the pathogen to grow to detectable levels during the sprouting process, and they provide support for the recommendation to test the sprout water for the presence of pathogens 48 h after starting seed sprouting. The effectiveness of a 10-min, 20,0000-microg/ml (ppm) calcium hypochlorite treatment of the outbreak-associated seeds was studied. For both seed lots, the hypochlorite treatment caused a reduction, but not elimination, of Salmonella contamination in the finished sprouts. These results confirm the need to test each production batch for the presence of pathogens, even after 20,000 microg/ml (ppm) hypochlorite treatment of seeds, so that contaminated product is not distributed.

Response to trisodium phosphate treatment ofSalmonellaChester attached to fresh-cut green pepper slices

A laboratory model using green pepper disks was developed to investigate the attachment of Salmonella Chester on plant tissue and to evaluate the effectiveness of sanitizer agents in inactivating attached bacteria on fruits. Pepper disks (14 mm in diam, and 3-4 mm in thickness) were immersed in a bacterial suspension containing 1.5 × 107cfu·mL-1of S. Chester for 30 s and subsequently air-dried at room temperature for 10 min. Approximately 30% of the bacteria retained on the disk after immersion were firmly attached and could not be removed by two washes and agitation. A positive correlation was observed between the number of bacteria attached and the concentration of bacteria in the suspension. Population studies and scanning electron microscopic examinations revealed that attachment of S. Chester on pepper disks occurred mainly on the surfaces of injured (cut) tissue but rarely on the unbroken skin. When inoculated disks were treated with 3% to 12% (w/v) of trisodium phosphate (TSP) at pH 12.3 for 5 min, the population of bacteria on the disk was reduced by 10- to 100-fold. A small portion (0.7% to 7.1%) of bacteria attached to the disk were either resistant to or protected from the TSP treatment. When the pH of TSP solution was reduced from 12.3 to 4.5, the effectiveness of TSP in inactivating S. Chester on pepper disks was reduced by 26%. This study shows that surfaces of injured fruit tissue are the principal sites for bacterial attachment, and a small portion of the bacteria attached to the tissue are resistant to the sanitizer treatment. Avoiding mechanical injuries to fresh fruits during and after harvest would reduce the chance of pathogen attachment and contamination on green pepper and fruits of similar nature.Key words: Salmonella, attachment, detachment, plant tissue, sanitizer treatment.

Microbiological safety evaluations and recommendations on sprouted seeds. National Advisory Committee on Microbiological Criteria for Foods

In 1997, the National Advisory Committee on Microbiological Criteria for Foods (NACMCF/the Committee) was asked to review the current literature on sprout-associated outbreaks: identify the organisms and production practices of greatest public health concern: prioritize research needs: and provide recommendations on intervention and prevention strategies. In response to this charge, the Fresh Produce Work Group (FPWG) documented the relevant epidemiology and microbial ecology of sprout-associated outbreaks and reviewed current industry practices and initiatives related to the growing of seed and the production of sprouts. Sprouts have been identified as a special problem because of the potential for pathogen growth during the sprouting process. If pathogens are present on or in the seed, sprouting conditions may favor their proliferation. There is no inherent step in the production of raw sprouts to reduce or eliminate pathogens. Contaminated seed is the likely source for most reported sprout-associated outbreaks. Research has been initiated on methods to reduce or eliminate pathogenic bacteria on seeds and sprouts and some treatments show promise. However, to date, no single treatment has been shown to completely eliminate pathogens under experimental conditions used. Finally, the Committee found that, at the time of the charge, there was a lack of fundamental food safety knowledge along the continuum from seed production through sprout consumption. More recently, many have become aware of the potential for this food to be a vehicle for foodborne illness and the need for appropriate controls: however, such awareness is not universal. Although seed appears to be the most likely source of contamination in sprout associated outbreaks, practices and conditions at the sprouting facility may also impact on the safety of the finished product. In recent sprout-associated outbreak investigations, facilities associated with outbreaks did not consistently apply seed disinfection treatments prior to sprouting. Conversely, facilities that used seed from the same lot as an implicated facility, but had not been associated with reported illnesses, appear to have been consistently using seed disinfection treatments, such as 20,000 ppm calcium hypochlorite, to disinfect seed prior to sprouting. The Committee has developed a number of specific recommendations, including: 1. The knowledge of all interested parties pertaining to the microbiological safety of sprouted seeds must be enhanced; government and industry should develop education programs for seed and sprout producers on basic principles for microbiological food safety, good agricultural practices, good manufacturing practices, and hazard analysis and critical control point (HACCP) systems. 2. Good agricultural practices should be systematically implemented to reduce the potential for microbial contamination of seeds for sprout production. 3. Seed cleaning, storage, and handling practices that minimize the potential for microbial contamination should be developed and implemented. 4. Seeds should be treated with one or more treatments that have been shown to reduce pathogenic bacteria that may be present. Intervention strategies that deliver less than a given reduction (at this time, 5-log) in levels of Salmonella spp. and enterohemorrhagic Escherichia coli O157 should be coupled with a microbiological testing program. 5. Establish good manufacturing practices and food safety systems, including regulatory oversight, microbial testing, adoption of HACCP, and improved traceback, that systematically look for means to prevent seeds from serving as the vehicle for foodborne disease, and 6. Conduct research related to the microbiological safety of sprouted seeds, particularly in the areas of pathogen reduction or elimination, sources of contamination and its prevention, and preventing or retarding pathogen growth during sprouting.

Current microbiological status of 'health foods' sold in Canada

A follow-up survey was conducted across Canada to evaluate the current status of 'health foods' sold in Canada. A total of 1239 sample units of 'health foods' were analysed for a variety of bacteria, including aerobic colony counts (ACC), coliforms, aerobic and anaerobic sporeformers, Escherichia coli and Bacillus cereus. Results presented indicate that 16.8-18.4% of the 'health foods' exceed ACC guidelines, 16.0-17.8% exceeded coliform guidelines, 35.7% exceeded aerobic sporeformer guidelines, 81.4% exceeded anaerobic sporeformer guidelines, and 9% exceeded B. cereus guidelines. Some ACC were further identified and found to be opportunistic pathogens, including the genera Bacillus, Enterococcus, and Staphylococcus. It was concluded that more extensive surveillance of this industry by health officials is needed.

Efficacy of chlorine and heat treatment in killing Salmonella stanley inoculated onto alfalfa seeds and growth and survival of the pathogen during sprouting and storage

The efficacy of chlorine and hot water treatments in killing Salmonella stanley inoculated onto alfalfa seeds was determined. Treatment of seeds containing 10(2) to 10(3) CFU/g in 100-micrograms/ml active chlorine solution for 5 or 10 min caused a significant (P < or = 0.05) reduction in population, and treatment in 290-micrograms/ml chlorine solution resulted in a significant reduction compared with treatment in 100 micrograms of chlorine per ml. However, concentrations of chlorine of up to 1,010 micrograms/ml failed to result in further significant reductions. Treatment of seeds containing 10(1) to 10(2) CFU of S. stanley per g for 5 min in a solution containing 2,040 micrograms of chlorine per ml reduced the population to undetectable levels (< 1 CFU/g). Treatment of seeds in water for 5 or 10 min at 54 degrees C caused a significant reduction in the S. stanley population, and treatment at > or = 57 degrees C reduced populations to < or = 1 CFU/g. However, treatment at > or = 54 degrees C for 10 min caused a substantial reduction in viability of the seeds. Treatment at 57 or 60 degrees C for 5 min appears to be effective in killing S. stanley without substantially decreasing germinability of seeds. Storage of seeds for 8 to 9 weeks at 8 and 21 degrees C resulted in reductions in populations of S. stanley of about 1 log10 and 2 log10 CFU/g, respectively. The behavior of S. stanley on seeds during soaking germination, sprouting, and refrigerated storage of sprouts was determined. An initial population of 3.29 log10 CFU/g increased slightly during 6 h of soaking, by about 10(3) CFU/g during a 24-h germination period, and by an additional 10 CFU/g during a 72-h sprouting stage. A population of 10(7) CFU/g of mature alfalfa sprouts was detected throughout a subsequent 10-day storage period at 5 degrees C. These studies indicate that while populations of S. stanley can be greatly reduced, elimination of this organism from alfalfa seeds may not be reliably achieved with traditional disinfection procedures. If S. stanley is present on seeds at the initiation of the sprout production process, populations exceeding 10(7) CFU/g can develop and survive on mature sprouts exposed to handling practices used in commercial production and marketing.