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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Davis Electronic Theses and Dissertations (2023)

Antimicrobial resistance (AMR) in bacteria is a natural phenomenon that has been developing for thousands of years. However, intensive use of antibiotics in humans and animals starting in the middle of the last century has resulted in increased AMR, which has become a global threat to public health. Excessive use and misuse of antibiotics in food animals are blamed for the emergence of AMR. In recent years, multidrug resistance (MDR) in bacteria has become a serious issue by limiting the treatment options, making hospital stays longer and healthcare costs more expensive. In some cases, infectious diseases caused by AMR are impossible to treat. Antimicrobial-resistant bacteria (ARB) and their genetic determinants can be transferred from animals to the environment, workers, and animal products such as meat. Governments and various global organizations have taken the AMR issue seriously and have started to implement measures to limit the spread of AMR. In the U.S., the National Antimicrobial Resistance Monitoring System (NARMS) was founded in 1996 to track Antimicrobial-resistant bacteria (ARB )in humans, animals, and retail food and identify resistance trends, patterns, and mechanisms to make efficient interventions and drug development. However, the exposure of farm workers has been largely neglected by government agencies, industries, and researchers. Among the ARB, antimicrobial-resistant Salmonella poses a serious threat to public health. California is the most populated state in the U.S. However, antimicrobial resistance patterns of Salmonella in various retail meats have not been examined adequately. Therefore, two studies were conducted: 1) characterize AMR patterns in a farm environment and identify possible transmission routes of ARB from the farm environment to the farm workers; 2) characterize AMR patterns of antimicrobial-resistant Salmonella in retail meat from California. In the first study, environmental and worker samples were collected from the Hopkins Avian facility of the University of California, Davis.Overall, 5 types of environmental (fecal samples, cage and eggs swab from layer house (LH) and fecal samples and door handle swabs from floor house (FH)) and 2 types of worker's samples (outwear and boots swabs). Samples were processed to isolate Salmonella and Generic Escherichia coli (E. coli) to assess their prevalence and test for antimicrobial resistance using the microbroth dilution method. Additionally, E. coli and aerobic bacteria were counted to evaluate the overall bacterial load in the facilities and on worker's personal protective equipment (PPE). Salmonella was not detected in any of the collected samples. Generic E. coli was present in all the samples except LH cage and egg swab samples. Counts of E. coli and aerobic bacteria were higher in fecal samples from both houses compared to other samples. Thirty-five isolates out of one hundred tested isolates were resistant to one drug, 9 isolates were resistant to two drugs and 6 isolates were multidrug-resistant (MDR) (resistant to at least three or more drugs). Higher resistance in E. coli isolates was observed to ampicillin (15 %) and nitrofurantoin (13 %), among other tested drugs. Antimicrobial-resistant E. coli isolates from LH fecal samples, FH fecal and door handle swab samples shared similar antimicrobial resistance patterns with worker's outwear and boots swab samples. The study results showed that door handles of FH pose a high risk of exposure as the prevalence of ARB was high in isolates of E. coli from the door handle swabs. Moreover, our results demonstrated that workers' PPE can serve as a protective measure against the transmission of ARB to workers. In the second study, a total of 849 meat samples (chicken, pork, ground turkey and beef) were collected from Northern and Southern California. One hundred thirty-two Salmonella isolates were recovered from the meat sample. Antimicrobial susceptibility test (AST) and whole genome sequencing were conducted to identify antimicrobial resistance patterns of Salmonella isolates. The recovery rate of Salmonella was high in chicken samples (24.01%) compared to ground turkey (5.42%) and pork samples (3.08%) (P < 0.001). Ground beef samples were not contaminated with Salmonella. Prevalence of Salmonella was higher in meat samples with reduced antibiotic claim (20.35%) compared to conventional (11.96%) ((P < 0.001). Out of 132 isolates, 32 isolates (24.24%) were susceptible to all the tested drugs, while 17 isolates (12.88%) were resistant to one drug, 69 isolates (52.27%) to two drugs and 14 isolates (10.61%) to three or more drugs. Salmonella isolates were more resistant to tetracycline and streptomycin compared to other drugs. Whole Genome Sequencing (WGS) identified a total of 24 antimicrobial resistance genes, including the gyrA mutation as a notable resistance mechanism, along with the identification of 23 plasmid replicons in Salmonella isolates. Among the plasmid replicons, IncFIB (pN55391) was detected in 7 MDR S. Infantis isolates. IncFIB (pN55391) has been linked to the worldwide dissemination of pESI-like mega plasmid carriage in an emerging S. Infantis clone in previous studies. WGS results showed that the correlation between phenotypic and genotypic resistance was very high (96.85%). Overall, this study characterized AMR patterns and trends in Salmonella from retail meat in California, which might be helpful for public health protection, infection control, and clinical decision-making.

    Cover page: Characterizing Antimicrobial-Resistant Enterobacteriaceae Reservoirs from Farm to Plate in California
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2021)
    This study was conducted to evaluate the effect of red macroalgae Asparagopsis taxiformis supplementation for cattle on the shelf life of fresh beef steaks (longissimus dorsi). Three treatment groups (seven steers per treatment) included: 1) Control diet, 2) Control diet + 0.25% of macroalgae inclusion (low dose, LD), and 3) Control + 0.5% of macroalgae inclusion (high dose, HD). After the animals were harvested, the strip loins from all animals were collected and aged for 14 days at the meat lab of the University of California, Davis. Then the strip loins were cut into steaks, packaged, and placed on a retail display case for 6 days. During a retail display, instrumental color (L*, a*, and b*) of lean muscle and external fat surfaces were measured every 12 h. Bacterial counts for total aerobic mesophilic bacteria (AMB), aerobic psychrotrophic bacteria (APB), and lactic acid bacteria (LAB) were assessed on days 0, 3, and 6 of retail display. The thiobarbituric acid reactive substances (TBARS) analysis was conducted to measure the lipid oxidation and the pH was also assessed on days 0, 3, and 6. No interactive effect between treatments and time on the shelf life of steaks was observed. The HD red macroalgae supplement decreased (P < 0.05) the lightness (L*) of the surface muscle of the steaks, while the lightness of the external fat was not affected (P < 0.05) by treatments throughout the retail display. The external fat yellowness of the steaks was lower (P < 0.05) in LD and HD treatment groups compared with the control group. An increase (P < 0.05) in counts of AMB, APB, and LAB was observed on the steaks from the steers in the HD treatment group while steaks in Control and LD group had similar bacterial numbers throughout the retail display. The results indicated that the shelf life of steaks from cattle in LD group remained the same as that of the Control group, but the HD of A. taxiformis caused a darker color of steaks with higher microbial counts, which may lead to a shortened shelf life due to undesirable appearance and faster microbial spoilage.
      Cover page: Effects of red macroalgae Asparagopsis taxiformis supplementation on the shelf life of fresh whole muscle beef
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2022)
      Consumer interest in grass-fed beef has been steadily rising due to consumer perception of its potential benefits. This interest has led to a growing demand for niche market beef, particularly in the western United States. Therefore, the objective of this study was to assess the impact of feeding systems on the change in microbial counts, color, and lipid oxidation of steaks during retail display, and on their sensory attributes. The systems included: conventional grain-fed (CON), 20 months-grass-fed (20GF), 25-months-grass-fed (25GF) and 20-months-grass-fed + 45-day-grain-fed (45GR). The results indicate that steaks in the 20GF group displayed a darker lean and fat color, and a lower oxidation state than those in the 25GF group. However, the feeding system did not have an impact on pH or objective tenderness of beef steaks. In addition, consumers and trained panelist did not detect a difference in taste or flavor between the 20GF or 25GF steaks but expressed a preference for the CON and 45GR steaks, indicating that an increased grazing period may improve the color and oxidative stability of beef, while a short supplementation with grain may improve eating quality.
        Cover page: Evaluating the Shelf Life and Sensory Properties of Beef Steaks from Cattle Raised on Different Grass Feeding Systems in the Western United States
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