Abstract
Lethal and sublethal injury of two Gram-positive (Staphylococcus aureus and Listeria monocytogenes) and one Gram-negative (Escherichia coli) bacteria in milk by pulsed electric fields (PEF) were determined using non-selective and selective media. PEF inactivation kinetics including lethal and sublethal injury fractions was also studied. The proportion of the sublethally injured microbial cells depended on the microorganisms, electric field strength and treatment time. The proportion of sublethally injured microbial cells reached maximum after a specific PEF treatment, and it kept constant or progressively decreased at greater electric field strengths and with longer PEF treatments. For the strain of L. monocytogenes, the proportion of sublethally injured cells increased from 18.98% to 43.64% with the increasing electric field strength from15 to 30 kV/cm. While for the strains of E. coli and S. aureus, the proportion of sublethally injured cells achieved the maximum (40.74% and 36.51%, respectively) at 25 kV/cm and then decreased. The proportion of the sublethally injured microbial cells reached maximum at 400 μs (S. aureus and L. monocytogenes) or 500 μs (E. coli), and decreased at longer treatments at 30 kV/cm. The PEF inactivation kinetics including lethal and sublethally injured fractions was analyzed by the Hülsheger model, and the model parameters (EC, tC, kE, bt) for lethal and sublethal injury were also calculated.
Abstract
A previously developed multiplex PCR targeting gyrB of Vibrios at genus level and pntA genes for specific detection of Vibrio cholerae, Vibrio parahaemolyticus and Vibrio vulnificus was evaluated. The sensitivity of the multiplex PCR on spiked seafood was 1.5 × 103 CFU g−1. One hundred and fifty seafood samples were collected from retail stores and hypermarkets in different locations in Kuala Lumpur, Petaling Jaya and Seri Kembangan. The prevalence of V. parahaemolyticus was 29% (43/150). The pntA primers for V. parahaemolyticus detection were 100% specific and comparable to the toxR gene-based PCR. Six (12%) and 2 (4%) isolates contained trh and tdh genes, respectively. Repetitive Extragenic Palindromic PCR (REP-PCR) was used to genetically characterize the V. parahaemolyticus isolates in which 41 REP profiles were observed and all the isolates were categorized into 11 distinct clusters at the similarity of 80%. tdh-positive isolates shared a low level of similarity with trh-positive isolates. The prevalence of V. parahaemolyticus and particularly the presence of virulent gene such as trh and tdh among the isolates reiterate a high risk of contamination for seafood consumers in Malaysia. DNA fingerprinting of V. parahaemolyticus in this study indicates a high genetic diversity among the isolates and REP-PCR was able to distinguish the isolates with different virulotypes.
Abstract
The objectives of this study were to determine the prevalence of Listeria spp., specifically Listeria monocytogenes in ready-to-eat (RTE) foods and ascertain the efficiency of detecting L. monocytogenes with different selective culture media. A total of 396 RTE food samples were purchased from hypermarkets and streetside hawker stalls to examine the presence of Listeria spp. and L. monocytogenes. The presumptive isolates were characterized biochemically and were further confirmed by Polymerase Chain Reaction (PCR). Out of 396 samples, Listeria spp. was detected in 71 (17.9%) samples in which 45 (11.4%) were positive for L. monocytogenes. Among the studied RTE foods, salads and vegetables had the highest prevalence (14.7%) of L. monocytogenes, followed by chicken and chicken products (13.2%), beverages (10%), eggs and egg products (9.5%), beef and beef products (6.7%), lunch boxes (6.7%) and seafood and seafood products (6.7%). Both Listeria selective agar and PALCAM agar displayed a low sensitivity and specificity in L. monocytogenes detection compared to CHROMagar™ Listeria which demonstrated 96.9% of sensitivity and 99.1% of specificity in L. monocytogenes detection in naturally-contaminated foods. In conclusion, this work revealed consumption of RTE foods as a potential risk of listeriosis in this region. The high contamination rate of L. monocytogenes in salads and vegetables from hypermarkets and streetside hawker stalls was of great concern due to emerging fresh produce-borne L.monocytogenes globally. The scenario warrants further surveillance and action by the local authority to control the incidence of L. monocytogenes contamination in RTE foods.
