RNA arbitrarily primed PCR and fourier transform infrared spectroscopy reveal plasticity in the acid tolerance response of Streptococcus macedonicus

Investigation and Rapid Discrimination of Food-Related Bacteria under Stress Treatments Using IR Microspectroscopy

Because the robust and rapid determination of spoilage microorganisms is becoming increasingly important in industry, the use of IR microspectroscopy, and the establishment of robust and versatile chemometric models for data processing and classification, is gaining importance. To further improve the chemometric models, bacterial stress responses were induced, to study the effect on the IR spectra and to improve the chemometric model. Thus, in this work, nine important food-relevant microorganisms were subjected to eight stress conditions, besides the regular culturing as a reference. Spectral changes compared to normal growth conditions without stressors were found in the spectral regions of 900-1500 cm^-1 and 1500-1700 cm^-1. These differences might stem from changes in the protein secondary structure, exopolymer production, and concentration of nucleic acids, lipids, and polysaccharides. As a result, a model for the discrimination of the studied microorganisms at the genus, species and strain level was established, with an accuracy of 96.6%. This was achieved despite the inclusion of various stress conditions and times after incubation of the bacteria. In addition, a model was developed for each individual microorganism, to separate each stress condition or regular treatment with 100% accuracy.

Lactic Acid Bacteria Isolation from Spontaneous Sourdough and Their Characterization Including Antimicrobial and Antifungal Properties Evaluation

This research effort aimed at isolating and phenotypically characterizing lactic acid bacteria (LAB) isolates from a spontaneous rye sourdough manufactured following traditional protocols, as well as at evaluating their antimicrobial and antifungal properties as key features for future industrial applications. Thirteen LAB strains of potential industrial interest were isolated and identified to species-level via PCR. Most of the sourdough isolates showed versatile carbohydrate metabolisms. The Leuconostoc mesenteroides No. 242 and Lactobacillus brevis No. 173 demonstrated to be gas producers; thus, revealing their heterofermenter or facultative homofermenter features. Viable counts higher than 7.0 log10 (CFU/mL) were observed for Lactobacillus paracasei No. 244, Lactobacillus casei No. 210, L. brevis No. 173, Lactobacillus farraginis No. 206, Pediococcus pentosaceus No. 183, Lactobacillus uvarum No. 245 and Lactobacillus plantarum No. 135 strains, after exposure at pH 2.5 for 2 h. Moreover, L. plantarum No. 122, L. casei No. 210, Lactobacillus curvatus No. 51, L. paracasei No. 244, and L. coryniformins No. 71 showed growth inhibition properties against all the tested fifteen pathogenic strains. Finally, all LAB isolates showed antifungal activities against Aspergillus nidulans, Penicillium funiculosum, and Fusarium poae. These results unveiled the exceptionality of spontaneous sourdough as a source of LAB with effective potential to be considered in the design of novel commercial microbial single/mixed starter cultures, intended for application in a wide range of agri-food industries, where the antimicrobial and antifungal properties are often sought and necessary. In addition, metabolites therefrom may also be considered as important functional and bioactive compounds with high potential to be employed in food and feed, as well as cosmetic and pharmaceutical applications.

Mechanism of Antibacterial Activity of Choline-Based Ionic Liquids (CAGE)

The continued emergence of antibiotic-resistant organisms has severely depleted our arsenal of effective antimicrobials. Ionic liquids (ILs) show great promise as antibacterial agents but understanding the mechanism of attack on bacterial cells is key to ensuring that design of IL-based biocides impart maximum efficacy with minimal toxicity, while also avoiding the potential for the target organisms to become resistant. Here we report the antibacterial attributes of a set of choline and geranate (CAGE)-based ILs and identify the mechanism by which they interact with the Gram-negative cell wall of Escherichia coli. CAGE is envisaged as an antimicrobial agent to treat topical infections in skin. Our earlier work has shown that CAGE is highly effective across a breadth of bacterial, fungal, and viral species and is benign to human cells. This combination makes CAGE an ideal antimicrobial for human use. Four CAGE variants with varying ratios of choline and geranic acid were synthesized and tested for their antibacterial activity (1:4, 1:2, 1:1, and 2:1 choline:geranic acid). The minimum bactericidal concentration required to kill E. coli correlated with the geranic acid content. Using molecular dynamics (MD) simulations, we identified the mechanism of CAGE action on the E. coli membrane, namely that choline is attracted to the negatively charged cell membrane and consequently inserts geranic acid into the lipid bilayer. The disruption of the cell membrane was confirmed with propidium iodide staining via flow cytometry and scanning electron microscopy. Fourier Transform infrared spectroscopic analysis of treated cells showed an altered lipid profile similar to phase transition, indicating the disruption of the lipid bilayer conformation. E. coli cells repeatedly exposed to CAGE did not exhibit resistance. This study provides the fundamental mechanism of the action of choline-based ILs on Gram-negative bacteria and demonstrates the promise of CAGE as a powerful antimicrobial agent to treat infections.

Lactobacillus casei Encapsulated in Soy Protein Isolate and Alginate Microparticles Prepared by Spray Drying

This article presents a novel formulation for preparation of Lactobacillus casei 01 encapsulated in soy protein isolate and alginate microparticles using spray drying method. A response surface methodology was used to optimise the formulation and the central composite face-centered design was applied to study the effects of critical material attributes and process parameters on viability of the probiotic after microencapsulation and in simulated gastrointestinal conditions. Spherical microparticles were produced in high yield (64%), narrow size distribution (d₅₀=9.7 µm, span=0.47) and favourable mucoadhesive properties, with viability of the probiotic of 11.67, 10.05, 9.47 and 9.20 log CFU/g after microencapsulation, 3 h in simulated gastric and intestinal conditions and four-month cold storage, respectively. Fourier-transform infrared spectroscopy confirmed the probiotic stability after microencapsulation, while differential scanning calorimetry and thermogravimetry pointed to high thermal stability of the soy protein isolate-alginate microparticles with encapsulated probiotic. These favourable properties of the probiotic microparticles make them suitable for incorporation into functional food or pharmaceutical products.

Stress Physiology of Lactic Acid Bacteria

Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the "stressome" of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

Incidence of Bacteriocins Produced by Food-Related Lactic Acid Bacteria Active towards Oral Pathogens

In the present study we investigated the incidence of bacteriocins produced by 236 lactic acid bacteria (LAB) food isolates against pathogenic or opportunistic pathogenic oral bacteria. This set of LAB contained several strains (≥17%) producing bacteriocins active against food-related bacteria. Interestingly only Streptococcus macedonicus ACA-DC 198 was able to inhibit the growth of Streptococcus oralis, Streptococcus sanguinis and Streptococcus gordonii, while Lactobacillus fermentum ACA-DC 179 and Lactobacillus plantarun ACA-DC 269 produced bacteriocins solely against Streptococcus oralis. Thus, the percentage of strains that were found to produce bacteriocins against oral bacteria was ~1.3%. The rarity of bacteriocins active against oral LAB pathogens produced by food-related LAB was unexpected given their close phylogenetic relationship. Nevertheless, when tested in inhibition assays, the potency of the bacteriocin(s) of S. macedonicus ACA-DC 198 against the three oral streptococci was high. Fourier-transform infrared spectroscopy combined with principal component analysis revealed that exposure of the target cells to the antimicrobial compounds caused major alterations of key cellular constituents. Our findings indicate that bacteriocins produced by food-related LAB against oral LAB may be rare, but deserve further investigation since, when discovered, they can be effective antimicrobials.

RNA fingerprinting analysis of Oenococcus oeni strains under wine conditions

Oenococcus oeni is a lactic acid bacterium of economic interest used in winemaking. This bacterium is the preferred species for malolactic fermentation (MLF) due its adaptability to the chemically harsh wine environment. MLF enhances the organoleptic properties and ensures deacidification of wines. The aim of this work was the transcriptional characterization of six O. oeni strains, four of them selected from distinct winemaking regions of Portugal, as candidates to malolactic starters, and two commercial malolactic starters. Using crossed assays with wines from different Portuguese winemaking regions, strain characteristic transcriptional patterns induced by each wine were analyzed based on Random Arbitrarily Primed PCR (RAP-PCR). The obtained results suggest that the starter strains showed more constrained and limited transcription profiles, whereas a high variation on the distribution of the transcription profiles was observed for the regional strains in each wine. According with our results, RAP-PCR is a useful technique for a preliminary investigation of strain behavior under different wine environmental conditions, which can be applied in field studies to monitor differential patterns of global gene expression and to select markers for the surveillance of malolactic starters performance in winemaking, as well as for quality and safety control.

Aga1, the first alpha-Galactosidase from the human bacteria Ruminococcus gnavus E1, efficiently transcribed in gut conditions

Differential gene expression analysis was performed in monoxenic mice colonized with Ruminococcus gnavus strain E1, a major endogenous member of the gut microbiota. RNA arbitrarily primed-PCR fingerprinting assays allowed to specifically detect the in vivo expression of the aga1 gene, which was further confirmed by RT-PCR. The aga1 gene encoded a protein of 744 residues with calculated molecular mass of 85,207 Da. Aga1 exhibited significant similarity with previously characterized α-Galactosidases of the GH 36 family. Purified recombinant protein demonstrated high catalytic activity (104 ± 7 U mg(-1)) and efficient p-nitrophenyl-α-d-galactopyranoside hydrolysis [k(cat)/K(m) = 35.115 ± 8.82 s(-1) mM(-1) at 55 °C and k(cat)/K(m) = 17.48 ± 4.25 s(-1) mM(-1) at 37 °C].

Fourier transform infrared spectroscopy as a tool to characterize molecular composition and stress response in foodborne pathogenic bacteria

Vibrational spectroscopy techniques have shown capacity to provide non-destructive, rapid, relevant information on microbial systematics, useful for classification and identification. Infrared spectroscopy enables the biochemical signatures from microbiological structures to be extracted and analyzed, in conjunction with advanced chemometrics. In addition, a number of recent studies have shown that Fourier Transform Infrared (FT-IR) spectroscopy can help understand the molecular basis of events such as the adaptive tolerance responses expressed by bacteria when exposed to stress conditions in the environment (e.g. those that cells confront in food and during food processing). The current review gives an overview of the published experimental techniques, data-processing algorithms and approaches used in FT-IR spectroscopy to assess the mechanisms of bacterial inactivation by food processing technologies and antimicrobial compounds, to monitor the spore and membrane properties of foodborne pathogens in changing environments, to detect stress-injured microorganisms in food-related environments, to assess dynamic changes in bacterial populations, and to study bacterial tolerance responses.