Two Processing Methods for the Isolation of Salmonella from Naturally Contaminated Alfalfa Seeds

Abstract

Two processing methods were examined for the recovery of Salmonella from naturally contaminated alfalfa seed. Seed samples, from each of three investigations, were processed by sprouting and shredding before preenrichment and culture. In lot A, Salmonella serotype Newport was isolated from 3 of 30 sample units with the sprouting method and 2 of 30 with the shredding method. In lot B, three serotypes in various combinations were isolated from 10 of 30 sample units with the sprouting method and 9 of 30 with the shredding method. In lot C, Salmonella group C1 was isolated from 27 of 30 sample units with the sprouting method and 24 of 30 with the shredding method. Additionally, serotype Newport was found in one lot C sample unit. Using shredded seed data, a most probable number (MPN) for Salmonella contamination per lot was calculated. Serotype Newport was estimated at 0.07 MPN/100 g in lot A; the concentration for three serotypes was estimated to be 0.36 MPN/100 g in lot B; Salmonella group C1 was estimated at 1.8 MPN/100 g in lot C. Our success in isolating Salmonella from alfalfa seeds was likely attributed to the volume of material tested and the quick acquisition of the seeds after the outbreak was identified. Shredding the seeds was easier and yielded definitive results more quickly than sprouting.

Monte Carlo Simulation of Pathogen Behavior during the Sprout Production Process

“…The sample size here (100 g) is larger than for the calculations shown in Table 2 (25 g), but the pathogen concentrations and prevalences were similar. Rerunning the simulation using the data from the study of Inami et al (20) produces nearly identical results when no disinfection is used (data not shown). When disinfection is assumed, the total fraction of positive batches simulated from seeds contaminated with the level found in the work of Inami et al (20) is lower than in Table 2 but is not zero, and the simulated results for the detection of pathogens in irrigation water finds a similar fraction of positive batches (data not shown).…”

“…Testing of three different lots of seeds implicated in three different outbreaks showed a wide range in pathogen prevalence (20): in seeds from the first outbreak, 10% of 30 samples of 100 g each were positive for Salmonella, in seeds from the second outbreak, 33% of 30 samples of 100 g each were positive for Salmonella, while in seeds from the last outbreak, 90% of 30 samples of 100 g each were positive for Salmonella. The most probable numbers of Salmonella organisms present were estimated to be 0.07, 0.36, and 1.8 CFU/100 g, respectively.…”

“…The method of seed processing (stomaching or blending) was deemed insignificant based on published comparisons (59). The probability that a pathogen, if present, would be detected from the seed was calculated using presence and absence data where a large number of samples of both seeds and sprouts from the same lot were tested (20). The number of positive seed samples (40) was divided by the number of positive samples after sprouting (50, assumed to be the true number of positive samples) giving a probability of 0.8.…”

“…Relevant literature was grouped into two main subject categories: growth of pathogens during sprouting (1,8,16,19,33,41,42,47,49) and pathogen detection on seeds or sprouts (8,15,20,21,23,36,45,48,55,59). Epidemiological literature was reviewed to assemble a comprehensive understanding of primary risk factors in the sprouting process (3,4,9,25,29,30,35,37,52,53,58).…”

“…Inami et al used three different lots of naturally contaminated alfalfa seed to compare isolations of Salmonella by seed shredding and sprouting. Shredding samples yielded 35 positive out of 90 samples, whereas sprouting yielded 40 positive out of 90 samples (20).…”