Hass avocados may become contaminated with Salmonella and Listeria monocytogenes at the farm and the packing facility or later during transportation and at retail. In Mexico, avocados are frequently sold in bulk at retail markets, where they are stored at room temperature for several hours or days and exposed to potential sources of microorganisms. These conditions may favor the entry, adhesion, survival, and biofilm formation of Salmonella and L. monocytogenes. The aim of this study was to determine the occurrence of Salmonella, L. monocytogenes, and other Listeria species and the levels of indicator microorganisms on the surface of avocados sold at retail markets. A total of 450 samples (Persea americana var. Hass) were acquired from retail markets located in Guadalajara, Mexico. One group of 225 samples was evaluated for the presence of Salmonella and for enumeration of aerobic plate counts, yeasts and molds, Enterobacteriaceae, coliforms, and Escherichia coli. The other 225 samples were processed for isolation of L. monocytogenes and other Listeria species. Microbial counts (log CFU per avocado) were 4.3 to 9.0 for aerobic plate counts, 3.3 to 7.1 for yeasts and molds, 3.3 to 8.2 for Enterobacteriaceae, 3.3 to 8.4 for coliforms, and 3.3 to 6.2 for E. coli. Eight samples (3.5%) were positive for Salmonella. Listeria spp. and L. monocytogenes were detected in 31 (13.8%) and 18 (8.0%) of 225 samples, respectively. Listeria innocua, Listeria welshimeri, and Listeria grayi were isolated from 7.6, 1.3, and 0.9% of samples. These results indicate that avocados may carry countable levels of microorganisms and could be a vehicle for transmission of Salmonella and L. monocytogenes. HIGHLIGHTS
Listeria monocytogenes is an important pathogen that has been implicated in foodborne illnesses and the recall of products such as fruit and vegetables. This study determines the prevalence of virulence-associated genes and serogroups and evaluates the effects of different growth media and environmental conditions on biofilm formation by L. monocytogenes. Eighteen L. monocytogenes isolates from Hass avocados sold at markets in Guadalajara, Mexico, were characterized by virulence-associated genes and serogroup detection with PCR. All isolates harbored 88.8% actA, 88.8% plcA, 83.3% mpl, 77.7% inlB, 77.7% hly, 66.6% prfA, 55.5% plcB, and 33.3% inlA. The results showed that 38.8% of isolates harbored virulence genes belonging to Listeria pathogenicity island 1 (LIPI-1). PCR revealed that the most prevalent serogroup was serogroup III (1/2b, 3b, and 7 (n = 18, 66.65%)), followed by serogroup IV (4b, 4d–4e (n = 5, 27.7%)) and serogroup I (1/2a–3a (n = 1, 5.5%)). The assessment of the ability to develop biofilms using a crystal violet staining method revealed that L. monocytogenes responded to supplement medium TSBA, 1/10 diluted TSBA, and TSB in comparison with 1/10 diluted TSB (p < 0.05) on polystyrene at 240 h (p < 0.05). In particular, the biofilm formation by L. monocytogenes (7.78 ± 0.03–8.82 ± 0.03 log10 CFU/cm2) was significantly different in terms of TSBA on polypropylene type B (PP) (p < 0.05). In addition, visualization by epifluorescence microscopy, scanning electron microscopy (SEM), and treatment (DNase I and proteinase K) revealed the metabolically active cells and extracellular polymeric substances of biofilms on PP. L. monocytogenes has the ability to develop biofilms that harbor virulence-associated genes, which represent a serious threat to human health and food safety.
Tomatoes (Solanum lycopersicum) are one of the most consumed fruits worldwide. The fruit can become contaminated with Salmonella and Listeria monocytogenes at different stages of the production and supply chain, and these pathogens may survive under different storage conditions. The effect of relative humidity, temperature and time of storage on the attachment and survival characteristics of both pathogens on the surface of tomatoes was investigated. Fresh whole Roma tomatoes were inoculated with a cocktail of Salmonella or L. monocytogenes and stored at 5, 12, 25, 30, or 35 ºC for 10 days. Every day during storage, relative humidity and temperature were measured and tomatoes were removed to enumerate cells of these pathogens that were loosely attached (LA, cells detached from the tomato surface by simple rinse) and strongly attached (SA, cells that required sonication to detach. Enumeration was achieved by spread-plate method. Surviving populations of LA and SA cells were obtained and the attachment strength (SR) was calculated to express the proportion of SA cells on the tomato surface. The LA initial counts of Salmonella and L. monocytogenes on the surface of the fruit after inoculation were 6.6 and 6.5 log CFU/tomato, and for SA were 5.1 and 5.6 log CFU/tomato, respectively. For both pathogens, the LA counts were higher (p < 0.05) than the SA counts. Also, the LA and SA counts varied significantly as a function of temperature, relative humidity and time of storage. The S R for Salmonella was affected by the time but not the temperature of storage, while the S R for L. monocytogenes was affected by the temperature, relative humidity and time of storage (p < 0.05). Understanding the attachment and survival of Salmonella and L. monocytogenes on tomatoes stored at different temperature conditions may be useful in determining ways to prevent/reduce the establishment of pathogens and designing improved decontamination methods for tomatoes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.