This Is AuburnElectronic Theses and Dissertations

Survey of feed mills around the United States for select bacterial pathogens




Escobar Lobo, Cesar

Type of Degree

Master's Thesis


Poultry Science


Feed and feed ingredients are one of the principal sources of microbial contamination in the animal production industry and contamination may occur at any time during growing, harvesting, processing, manufacturing, storage, and distribution. Feed can be a source of Salmonella, Escherichia coli, Clostridium perfringens, and other pathogens of human and animal concern. The objective of this project was to analyze and determine the microbial content of feed ingredients and finished feed (manufactured) from different feed mills around the United States focusing on Salmonella spp., E. coli, and Clostridium spp. Tris phosphate carbonate (TPC), which is a pre-enrichment with an strong buffering capacity, and buffer peptone water (BPW) were evaluated as pre-enrichment mediums for the recovery of Salmonella in feed. A total of 269 samples (feed ingredients and manufactured feed) were collected from 6 feed mills (A, B, C, D, E, and F). Microbial isolation was performed using selective media and colony counts are presented as follows: Clostridium spp. counts (CSC), E. coli counts (ECC), Enterobacteriaceae counts (ENC), aerobic counts (AEC), and anaerobic counts (ANC). For the pre-enrichment evaluation, pH was measured at 0 and 24 h, samples were plated on selective media to confirm the positive or negative presence of Salmonella. Colony forming units, were log10 transformed and analyzed using a GLM model, Proc Glimmix (significant P ≤ 0.05), and means were separated by sample type using Tukey’s HSD. The pH change in time (initial–final) was analyzed using a dependent t-test for each sample type in SAS® 9.4 software. None of the samples were positive for Salmonella without pre-enrichment. Differences (P<0.05) were observed comparing the feed ingredients with meat and bone meal having the higher CSC counts while DDGS, poultry by-product meal, and soybean meal had lower CSC, ECC, and ENC levels. Additionally, DDGS and poultry by-product meal had lower amounts of AEC and ANC. Manufactured feed was compared by feed mill. Post mixing samples of feed mill A and E had higher ECC and ENC compared to post cooling and pellet loadout, therefore the pelleting process was effective at reducing the background bacterial contamination in feed ingredients. However, pellet loadout from feed mill E had higher CSC compared to post mixing and post cooling samples which can be presumptive re-contamination and prevalence of Clostridium spp. due to it being able to form spores. Manufactured feed was also compared by sample type. Post mixing samples were higher in ECC and ENC compared to post cooling and pellet loadout, however, pellet loadout was higher in CSC than post mixing and mash loadout. Pre-enrichment evaluation was performed by comparing pH values by sample type for TPC and BPW. DDGS (TPC 6.98/BPW 5.81) and poultry by-product meal (TPC 7.60/ BPW 6.54) presented lower initial pH values for TPC and BPW compared to the other sample types. For both, TPC and BPW, meat and bone meal (TPC 7.77/ BPW 6.45) presented higher final pH values compared to the other sample types, while soybean meal (TPC 5.59/BPW 4.86) and peanut meal (TPC 5.75/BPW 4.68) had lower final pH values. Furthermore, for BPW, post cooling (BPW 5.11), pellet loadout (BPW 5.05) and wheat middlings (BPW 5.15) reported lower final pH values. Most feed ingredients presented significant differences in pH change after 24 h incubation (P<0.001) except DDGS. From meat and bone meal samples, 4 Salmonella isolates were recovered, 3 using BPW and 1 TPC; for the same samples, initial-final pH for BPW was 6.89-6.40 (S. Oranienburg), 6.89-6.45 (S. Senftenberg), 7.09-6.42 (S. Agona) and for TPC 7.80-7.78 (S. Infantis). Based on these results, the pelleting process is effective in decreasing ECC and ENC, however CSC can persist through this process. Meat and bone meal can be a source of CSC and Salmonella. Also, these results show that TPC provided greater buffer capacity compared to BPW, but BPW was more effective at recovering Salmonella.