EDITORIAL article Front. Microbiol., 10 November 2022Sec. Food Microbiology Volume 13 - 2022 | https://doi.org/10.3389/fmicb.2022.1056796

Screening of 10 environmental samples (mainly of rhizospheric origin) for lytic activity against two bacterial phytopathogens, Pseudomonas syringae pv. tomato DC3000 (CFBP2212) and Xanthomonas hortorum pv. vitians (CFBP3979), revealed that four samples harboured phages that were active against one strain. Only one sample, composed of an artisanal nettle liquid manure, contained phages able to lyse both strains. Electron microscopy revealed the presence of tailed bacteriophages, with all phages isolated on the Xanthomonas strain displaying a contractile tail typical of members of the family Myoviridae, whereas phages isolated on the Pseudomonas strain were related to members of the family Siphoviridae and short-tailed members of the family Podoviridae. Sequence analysis of the two Podoviridae-like bacteriophages isolated on Pseudomonas syringae pv. tomato, Pst_GM1 isolated from nettle manure and Pst_GIL1 isolated from infected lettuce leaves, revealed (i) strong homology between the two isolated phages, (ii) a high degree of sequence similarity to various phages isolated from various environments and from different geographical locations, and (iii) similarity of these phages to members of the family Autographiviridae, and more precisely, the genus Ghunavirus. Further investigation of the potential of nettle manure to host phages that could be active against a wider range of strains revealed that it contained phages active against 10 phytopathogens (out of 16 tested). Thus, nettle manure (and likely other plant manures) could represent a valuable source of phages, especially those targeting bacterial phytopathogens, in the same way that anthropized environments such as sewage are widely used as sources of phages active against opportunistic or acute pathogens of humans.

Cold-smoked salmon is a widely consumed ready-to-eat seafood product that is a fragile commodity with a long shelf-life. The microbial ecology of cold-smoked salmon during its shelf-life is well known. However, to our knowledge, no study on the microbial ecology of cold-smoked salmon using next-generation sequencing has yet been undertaken. In this study, cold-smoked salmon microbiotas were investigated using a polyphasic approach composed of cultivable methods, V3—V4 16S rRNA gene metabarcoding and chemical analyses. Forty-five cold-smoked salmon products processed in three different factories were analyzed. The metabarcoding approach highlighted 12 dominant genera previously reported as fish spoilers: Firmicutes Staphylococcus, Carnobacterium, Lactobacillus, β-Proteobacteria Photobacterium, Vibrio, Aliivibrio, Salinivibrio, Enterobacteriaceae Serratia,Pantoea, γ-Proteobacteria Psychrobacter, Shewanella and Pseudomonas. Specific operational taxonomic units were identified during the 28-day storage study period. Operational taxonomic units specific to the processing environment were also identified. Although the 45 cold-smoked salmon products shared a core microbiota, a processing plant signature was found. This suggest that the bacterial communities of cold-smoked salmon products are impacted by the processing environment, and this environment could have a negative effect on product quality. The use of a polyphasic approach for seafood products and food processing environments could provide better insights into residential bacteria dynamics and their impact on food safety and quality.

Amplicon sequencing approaches have been widely used in food bacterial ecology. However, choices regarding the methodology can bias results. In this study, bacterial communities associated with cold-smoked salmon products and their processing plant surfaces were monitored via sequencing of the V3-V4 region of the 16S rRNA gene. The impact of DNA extraction protocols, sampling methods (swabbing or sponging) and surface materials on bacterial communities were investigated. alpha and beta diversity analyses revealed that DNA extraction methods mainly influence the observed cold-smoked salmon microbiota composition. Moreover, different DNA extraction methods revealed significant differences in observed community richness and evenness. beta-Proteobacteria: Photobacterium, Serrano and Firmicutes: Brochothrix, Carnobacterium and Staphylococcus were identified as the dominant genera. Surface microbiota richness, diversity and composition were mainly affected by cleaning and disinfection procedures but not by DNA extraction methods. Surface community richness and evenness appeared higher when sampled by sponging compared to swabbing. beta-diversity analyses highlighted that surface topology, cleaning and disinfection and sampling devices seemed to affect the bacterial community composition. The dominant surface bacteria identified were mainly Flayobacteriaceae, beta-Proteobacteria and gamma-Proteobacteria described as fish spoilers such as Acinetobacter, Pseudomonas and Shewanella. DNA extraction and sampling methods can have an impact on sequencing results and the ecological analysis of bacterial community structures. This study confirmed the importance of methodology standardization and the need for analytical validation before 16S rDNA metabarcoding surveys.

Omics databases have exploded, opening the avenue to take strain diversity or physiological variability into consideration in microbiological risk assessment (MRA). However, one obstacle to the integration of omics data in MRA is the production of quantitative data that may be used to build mathematical models. Gene expression is recognized as relevant biomarker to describe bacterial behavior and reverse transcription quantitative PCR (RT-qPCR) is considered as the gold standard for accurate, sensitive, and fast measurement of gene expression. However, numerous critical points may arise throughout the entire workflow of RT-qPCR data acquisition influencing accuracy of the results and reliability of the conclusions. Although recommendations about the minimum information that should be found in publications about quantitative real-time PCR experiments, heterogeneity in the reporting of RT-qPCR quality controls in publications remains. Herein, the step-by step RT-qPCR quality controls established for the selection of Bacillus weihenstephanensis resistance biomarkers were described. Throughout this example, appropriate quality procedures and quality controls that shall be set up and carefully assessed to ensure reliable interpretations in RT-qPCR were depicted.

Microbial analyses of fermented milk products require selective methods to discriminate between close species simultaneously present in high amounts. A culture-based method combining novel chromogenic agar media and appropriate incubation conditions was developed to enumerate lactic acid bacteria (LAB) strains in fermented milk. M1 agar, containing two chromogenic substrates, allowed selective enumeration of Lactobacillus rhamnosus, two strains of Lactobacillus paracasei subsp. paracasei and Streptococcus salivarius subsp. thermophilus based on differential β-galactosidase and β-glucosidase activities. Depending on the presence of some or all of the above strains, M1 agar was supplemented with L-rhamnose or vancomycin and incubations were carried out at 37 °C or 44 °C to increase selectivity. A second agar medium, M2, containing one chromogenic substrates was used to selectively enumerate β-galactosidase producing Lactobacillus delbrueckii subsp. bulgaricus at 47 °C. By contrast with the usual culture media, the chromogenic method allowed unambiguous enumeration of each species, including discrimination between the two L. paracasei, up to 10(9) CFU/g of fermented milk. In addition, the relevance of the method was approved by enumerating reference ATCC strains in pure cultures and fermented milk product. The method could also be used for enumerations on non-Danone commercial fermented milk products containing strains different from those used in this study, showing versatility of the method. To our knowledge, this is the first description of a chromogenic culture method applied to selective enumeration of LAB.

Traditionally, culture-based methods have been used to enumerate microbial populations in dairy products. Recent developments in molecular methods now enable faster and more sensitive analyses than classical microbiology procedures. These molecular tools allow a detailed characterization of cell physiological states and bacterial fitness and thus, offer new perspectives to integration of microbial physiology monitoring to improve industrial processes. This review summarizes the methods described to enumerate and characterize physiological states of technological microbiota in dairy products, and discusses the current deficiencies in relation to the industry's needs. Recent studies show that Polymerase chain reaction-based methods can successfully be applied to quantify fermenting microbes and probiotics in dairy products. Flow cytometry and omics technologies also show interesting analytical potentialities. However, they still suffer from a lack of validation and standardization for quality control analyses, as reflected by the absence of performance studies and official international standards.

A rapid and specific analytical method to enumerate viable Bifidobacteria in dairy products containing a mix of lactic acid bacteria was developed to proof of concept. A polyclonal antibody against Bifidobacterium animalis ssp. lactis (B. lactis) was used for specific detection by flow cytometry (FCM). The antibody, which targeted the cell wall of B. lactis, had a high labelling efficiency of 95.5 +/- 1.9% of the population. A viability probe, 5,6-carboxyfluorescein diacetate, was combined with the antibody for real-time quantification of viable B. lactis. Enumeration results from FCM on four different commercial dairy products that contain B. lactis showed a good correlation with the standard culture-based method. Moreover, it provided a species-specific method to enumerate viable B. lactis within 2 h; the standard method counts the total cultivable Bifidobacteria genus on plates after 72 h. This study demonstrated that FCM could be a rapid solution for probiotic analysis in dairy products. (C) 2014 Elsevier Ltd. All rights reserved.

Recent developments in molecular methods aid in detecting and quantifying food microorganisms through faster, more sensitive and more specific procedures than classical microbiology techniques. Molecular tools were widely used to detect pathogens in food but yet remain insufficient to directly quantify them in food products. Nevertheless, recent studies show that quantitative PCR (qPCR)-based methods are applied with success in enumerating fermenting microbes and health promoting bacteria in dairy products. The increasing number of sequenced genomes is now a crucial support to designing specific primers and probes. Technological advances in qPCR-based methods including viable quantitative PCR are also highlighted and could be promising strategies for quantifying merely viable cells, detecting multiple microorganisms in a single reaction and constructing high throughput PCR workflows. Application of real-time PCR in the food industry has proven its reliability nevertheless its widespread use in the dairy industry will need technical recommendations and standardisation methods.

Molecular methods are being increasingly applied to detect, quantify and study microbial populations in food or during food processes. Among these methods, PCR-based techniques have been the subject of considerable focus and ISO guidelines have been established for the detection of food-borne pathogens. More particularly, real-time quantitative PCR (qPCR) is considered as a method of choice for the detection and quantification of microorganisms. One of its major advantages is to be faster than conventional culture-based methods. It is also highly sensitive, specific and enables simultaneous detection of different microorganisms. Application of reverse-transcription-qPCR (RT-qPCR) to study population dynamics and activities through quantification of gene expression in food, by contrast with the use of qPCR, is just beginning. Provided that appropriate controls are included in the analyses, qPCR and RT-qPCR appear to be highly accurate and reliable for quantification of genes and gene expression. This review addresses some important technical aspects to be considered when using these techniques. Recent applications of qPCR and RT-qPCR in food microbiology are given. Some interesting applications such as risk analysis or studying the influence of industrial processes on gene expression and microbial activity are reported.