Dairy product safety is a global public health issue that demands new approaches and technologies to control foodborne pathogenic microorganisms. Natural antimicrobial agents such as nisin can be added to control the growth of pathogens of concern in dairy foods, namely Listeria monocytogenes and Staphylococcus aureus. However, several factors affect the antimicrobial efficacy of nisin when directly added into the food matrix such as lack of stability at neutral pH, interaction with fat globules, casein, and divalent cations. To overcome these limitations, new and advanced strategies are discussed including nisin encapsulation technology, addition to active packaging, bioengineering, and combination with other antimicrobials. This review highlights advanced technologies with potential to expand and improve the use of nisin as a dairy preservative.

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The rise in consumption of Hispanic-style cheeses (HSC), due in large part to the increasing Hispanic population in the United States, has not been met with advances in food safety sufficient to prevent the numerous outbreaks and recalls due to Listeria monocytogenes. Hispanic-style cheeses are typically high moisture and have low salt content and low acidity from being subjected to little to no ripening. These conditions necessitate refrigeration to maintain safety and quality, as the majority of traditional extrinsic preservation methods are either ineffective or disrupt the mild sensory attributes of HSC. Unfortunately, the cold-growth of L. monocytogenes presents significant problems from post-pasteurization contamination or insufficient pasteurization. In this review, we discuss the factors affecting listerial contamination and growth in HSC, and present current knowledge of L. monocytogenes incidence in manufacturing settings and commercial prevalence. Furthermore, we differentiate HSC types by processing methods to aid with interpretation of works involving nonstandardized varieties and, finally, summarize research on intervention methods for eliminating listerial contaminants in HSC.

Hispanic-style fresh cheeses, such as queso fresco, have relatively low salt content, high water activity, and near neutral pH, which predisposes them to growth of Listeria monocytogenes. Biosafety constraints limit the incorporation of L. monocytogenes into cheeses manufactured via traditional methods in challenge studies, so few have focused on in situ testing of novel antimicrobials in fresh cheeses. We have developed a modular, miniaturized laboratory-scale queso fresco model for testing the incorporation of novel antilisterials. We have demonstrated the assessment of the antilisterials nisin and ferulic acid, alone and in combination, at various levels. Our results support the inhibitory effects of ferulic acid in cheese, against both L. monocytogenes and its common surrogate Listeria innocua, and we provide preliminary evaluation of its consumer acceptability.

Our objective was to assess the antimicrobial efficacy of a Listeria bacteriophage endolysin that may address limitations of current antilisterial processes for fresh cheeses. Listeria monocytogenes is highly problematic in the manufacture and processing of ready-to-eat foods due to its environmental persistence and its ability to grow under refrigerated storage. Special care must be taken to prevent listerial contamination during the production of fresh cheeses, as their delicate flavor and texture are incompatible with many of the antimicrobial processes and additives commonly used for other foods. Bacteriophage-derived cell wall hydrolytic enzymes, known as endolysins, comprise one possible intervention that may not suffer from the high strain specificity of their parent bacteriophages or the development of resistant strains. We recombinantly expressed endolysin PlyP100 and compared its lytic activity in vitro across several environmental parameters and target organisms, then incorporated it into a fresh cheese model challenged with a cocktail of L. monocytogenes. We show that PlyP100 demonstrates optimal activity under pH and salt concentrations consistent with a low-acid food matrix such as fresh cheese. Furthermore, we show that PlyP100 exhibits target specificity for gram-positive organisms with directly crosslinked peptidoglycan and displays considerable inhibitory activity against L. monocytogenes in fresh cheese for at least 4 wk under refrigerated storage. As PlyP100 demonstrates considerable promise for preventing the propagation of L. monocytogenes in fresh cheeses, this novel preservation method could help safeguard consumer health and the market expansion of an otherwise high-risk food with few other viable preservatives.

Queso Fresco (QF), a fresh Hispanic-style cheese, is often associated with Listeria monocytogenes outbreaks and recalls. Queso Fresco's susceptibility to bacterial contamination is partially due to its high pH and moisture content as well as Listeria's tolerance for the salt content typical for QF. Nine different brands of US QF, 2 packages from 4 different lots (to account for temporal variability), were sampled. The pH, salt content, and moisture content were analyzed in addition to microbial testing including yeasts and molds, coliforms, lactic acid bacteria enumeration, and L. monocytogenes counts. The cheeses were also inoculated with a cocktail of 5 food and human isolates of food-borne outbreak-associated Listeria monocytogenes strains to evaluate how the differences between brands influenced Listeria growth. Three of the cheeses underwent additional genus-level microbial analysis using extracted 16S rDNA, allowing for phylogenetic analysis between bacterial taxa including diversity and relative abundance. We found little variation between the sampled QF pH (range = 6.62-6.86), salt content (1.53-2.01%), and moisture content (43.90-54.50%). Yeasts and molds were below the detection limit of enumeration in all of the cheeses and coliforms were below the detection limit across the first 3 lots, but were detected at varying levels in the fourth lot (>3.0 most probable number/g) for 3 of the brands. Listeria monocytogenes was not isolated after enrichment in any of the samples. All cheeses tested positive for the presence of lactic acid bacteria, with only 1 of the cheeses being labeled as produced with added cultures having substantial counts. Fourteen days after inoculation with L. monocytogenes, at least 2.5 log10 cfu/g of growth was found for all QF brands stored at 4°C. Microbial genus analysis showed that, among the 3 brands, the microbial community was more similar within brand than when compared with the other 2 brands. Thermus, Anoxybacillus, and Streptococcus accounted for the dominant genera of brands A, B, and C, respectively. These variations within the microbial community may account for sensory differences and help manufacturers determine quality control consistency more readily than culture-based methods.

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