CAMBRIDGE PRIZE LECTURE. STUDIES ON THE SENSITIVITY OF LACTIC ACID BACTERIA TO HOP BITTER ACIDS

Humulus lupulus aqueous extract and hydrolate as a potential ingredient for cosmetics: chemical characterization and in vitro antimicrobial, cytotoxicity, antioxidant and anti-inflammatory assessment

Humulus lupulus extracts have in their composition different molecules, such as polyphenols, α-acids, β-acids, and hydrocarbons, which contribute to the plant's medicinal properties. These molecules are associated with antimicrobial, antioxidant and anti-inflammatory activities.

OBJECTIVE

This work focuses on the evaluation of H. lupulus biological activities, with the aim of evaluating its potential for inclusion in cosmetic formulations.

METHODS

Two distinct aqueous extracts and two hydrolates obtained via hydrodistillation were evaluated. These include the flower parts (FE, FH) and the mix of aboveground parts (ME, MH). The chemical profiles for both aqueous extracts and hydrolates were identified by high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS). Antimicrobial, antioxidant, cytotoxicity, and anti-inflammatory activity were tested in vitro using standard methods.

RESULTS

Rutin was the major compound found in FE (40.041 μg mg-1 of extract) and ME (2.909 μg mg-1 of extract), while humulenol II was the most abundant compound in hydrolates (FH: 20.83%; MH: 46.80%). Furthermore, FE was able to inhibit the growth of Staphylococcus aureus and Staphylococcus epidermis with MIC values of 50% and 25% (v/v), respectively. FH showed the same effect in Staphylococcus aureus (50% v/v). FH evidenced poor antioxidant potential in DPPH scavenging test and demonstrated significant antioxidant and anti-inflammatory effects by reducing (***p < 0.001) intracellular reactive oxygen species (ROS), NO (nitric oxide) levels (***p < 0.001) and cyclooxygenase-2 (COX-2) protein expression (***p < 0.001) in lipopolysaccharide (LPS)-stimulated macrophages. Nevertheless, it is important to note that FH exhibited cytotoxicity at high concentrations in 3T3 fibroblasts and RAW 264.7 macrophages.

CONCLUSION

The studied H. lupulus aqueous extracts and hydrolates revealed that FH stands out as the most promising bioactive source for cosmetic formulations. However, future research addressing antimicrobial activity is necessary to confirm its potential incorporation into dermatological and cosmetic formulations.

The two faces of microorganisms in traditional brewing and the implications for no- and low-alcohol beers

The production of alcoholic beverages is intrinsically linked to microbial activity. This is because microbes such as yeast are associated with the production of ethanol and key sensorial compounds that produce desirable qualities in fermented products. However, the brewing industry and other related sectors face a step-change in practice, primarily due to the growth in sales of no- and low-alcohol (NoLo) alternatives to traditional alcoholic products. Here we review the involvement of microbes across the brewing process, including both their positive contributions and their negative (spoilage) effects. We also discuss the opportunities for exploiting microbes for NoLo beer production, as well as the spoilage risks associated with these products. For the latter, we highlight differences in composition and process conditions between traditional and NoLo beers and discuss how these may impact the microbial ecosystem of each product stream in relation to microbiological stability and final beer quality.

Humulus lupulus and microbes: Exploring biotic causes for hop creep

BACKGROUND

Hop creep continues to present an unresolved issue for the brewing industry, specifically stemming from those hops added to beer during fermentation. Hops have been found to contain four dextrin-degrading enzymes: alpha amylase, beta amylase, limit dextrinase, and an amyloglucosidase. One recent hypothesis predicts that these dextrin-degrading enzymes could originate from microbes rather than the hop plant itself.

SCOPE AND APPROACH

This review begins by describing how hops are processed and used in the brewing industry. It will then discuss hop creep's origins with a new beer style, antimicrobial factors from hops and resistance mechanisms that bacteria use to counter them, and finally microbial communities that inhabit hops, focusing on whether they can produce the starch degrading enzymes which drive hop creep. After initial identification, microbes with possible links to hop creep were then run through several databases to search the genomes (if available) and for those specific enzymes.

KEY FINDINGS AND CONCLUSIONS

Several bacteria and fungi contain alpha amylase as well as unspecified glycosyl hydrolases, but only one contains beta amylase. Finally, this paper closes with a short summary of how abundant these organisms typically are in other flowers.

Cell Envelope Modifications Generating Resistance to Hop Beta Acids and Collateral Sensitivity to Cationic Antimicrobials in Listeria monocytogenes

Hop beta acids (HBAs) are characteristic compounds from the hop plant that are of interest for their strong antimicrobial activity. In this work, we report a resistance mechanism against HBA in the foodborne pathogen Listeria monocytogenes. Using an evolution experiment, we isolated two HBA-resistant mutants with mutations in the mprF gene, which codes for the Multiple Peptide Resistance Factor, an enzyme that confers resistance to cationic peptides and antibiotics in several Gram-positive bacteria by lysinylating membrane phospholipids. Besides the deletion of mprF, the deletion of dltA, which mediates the alanylation of teichoic acids, resulted in increased HBA resistance, suggesting that resistance may be caused by a reduction in positive charges on the cell surface. Additionally, we found that this resistance is maintained at low pH, indicating that the resistance mechanism is not solely based on electrostatic interactions of HBA with the cell surface. Finally, we showed that the HBA-resistant mutants display collateral sensitivity to the cationic antimicrobials polymyxin B and nisin, which may open perspectives for combining antimicrobials to prevent resistance development.

Effect of Czech Hop Varieties on Aroma of Dry-Hopped Lager Beer

The hoppy aroma in beer is characterized by an overall pleasant profile. The impacts of five Czech hop cultivars, Rubin, Saaz, Vital, Harmonie, and Kazbek, were tested on the hop aroma of the resulting beers, compared with a control beer sample, which was hopped with a commercial hop extract during wort boiling. GC-MS analysis was used for the identification and quantification of aroma-active compounds in the hops and beer. Dry hopping was successful in terms of improving the hoppy aroma in beer. Odorants such as β-myrcene, linalool, geraniol, β-citronellol, humulene epoxide I, and 2-methylbutyl-2-methylpropanoate were found at higher concentrations than the control in all dry-hopped beers. To quantify the success of dry hopping, the transfer rates of hop odorants were calculated. Those of linalool, geraniol and humulene epoxide I were adequate, whereas the transfer rates of polar compounds, e.g., β-myrcene, were relatively low, mostly due to their polarity. Changes in hop oil constituents were clearly notable, with the generation of β-citronellol, the release of other terpene alcohols from their glycosides or oxidation of α-humulene. Yeast metabolism also played an important role in these changes.

Antibacterial effects of biologically active ingredients in hop provide promising options to fight infections by pathogens including multi-drug resistant bacteria

Antibiotic resistance constitutes a global threat to the health care systems. The number of infections due to multidrug-resistant (MDR) bacteria increases progressively resulting in an estimated annual number of 750,000 fatal cases worldwide. Additionally, the lack of novel antibiotic compounds worsens the dilemma. Hence, there is an urgent need for alternative ways to fight antibiotic resistance. One option may be natural compounds with antibacterial properties such as hop and its biologically active ingredients which are used in traditional medicine since ancient times. This prompted us to perform an actual literature survey regarding the antibacterial properties of biologically active ingredients in hop including humulone, lupulone and xanthohumol. The 20 included studies revealed that lupulone and xanthohumol do in fact inhibit the growth of Gram-positive bacteria in vitro. In combination with distinct antibiotic compounds the hop ingredients can even exert synergistic effects resulting in enhanced antibiotic activities against defined Gram-positive and Gram-negative bacteria. In conclusion, biologically active ingredients in hop including lupulone and xanthohumol may be potential antibiotic compounds which either alone or in combination with other antibacterial substances open novel avenues in the combat of infections caused by pathogenic including MDR bacteria.

Biopreservation of beer: Potential and constraints

The biopreservation of beer, using only antimicrobial agents of natural origin to ensure microbiological stability, is of great scientific and commercial interest. This review article highlights progress in the biological preservation of beer. It describes the antimicrobial properties of beer components and microbiological spoilage risks. It discusses novel biological methods for enhancing beer stability, using natural antimicrobials from microorganisms, plants, and animals to preserve beer, including legal restrictions. The future of beer preservation will involve the skilled knowledge-based exploitation of naturally occurring components in beer, supplementation with generally regarded as safe antimicrobial additives, and mild physical treatments.

Natural Anti-Microbials for Enhanced Microbial Safety and Shelf-Life of Processed Packaged Meat

Microbial food contamination is a major concern for consumers and food industries. Consumers desire nutritious, safe and “clean label” products, free of synthetic preservatives and food industries and food scientists try to meet their demands by finding natural effective alternatives for food preservation. One of the alternatives to synthetic preservatives is the use of natural anti-microbial agents in the food products and/or in the packaging materials. Meat and processed meat products are characteristic examples of products that are highly perishable; hence natural anti-microbials can be used for extending their shelf-life and enhancing their safety. Despite several examples of the successful application of natural anti-microbial agents in meat products reported in research studies, their commercial use remains limited. This review objective is to present an extensive overview of recent research in the field of natural anti-microbials, covering essential oils, plant extracts, flavonoids, animal-derived compounds, organic acids, bacteriocins and nanoparticles. The anti-microbial mode of action of the agents, in situ studies involving meat products, regulations and, limitations for usage and future perspectives are described. The review concludes that naturally derived anti-microbials can potentially support the meat industry to provide “clean label”, nutritious and safe meat products for consumers.

Co-fermentation Involving Saccharomyces cerevisiae and Lactobacillus Species Tolerant to Brewing-Related Stress Factors for Controlled and Rapid Production of Sour Beer

Increasing popularity of sour beer urges the development of novel solutions for controlled fermentations both for fast acidification and consistency in product flavor and quality. One possible approach is the use of Saccharomyces cerevisiae in co-fermentation with Lactobacillus species, which produce lactic acid as a major end-product of carbohydrate catabolism. The ability of lactobacilli to ferment beer is determined by their capacity to sustain brewing-related stresses, including hop iso-α acids, low pH and ethanol. Here, we evaluated the tolerance of Lactobacillus brevis BSO464 and Lactobacillus buchneri CD034 to beer conditions and different fermentation strategies as well as their use in the brewing process in mixed fermentation with a brewer's yeast, S. cerevisiae US-05. Results were compared with those obtained with a commercial Lactobacillus plantarum (WildBrewTM Sour Pitch), a strain commonly used for kettle souring. In pure cultures, the three strains showed varying susceptibility to stresses, with L. brevis being the most resistant and L. plantarum displaying the lowest stress tolerance. When in co-fermentation with S. cerevisiae, both L. plantarum and L. brevis were able to generate sour beer in as little as 21 days, and their presence positively influenced the composition of flavor-active compounds. Both sour beers were sensorially different from each other and from a reference beer fermented by S. cerevisiae alone. While the beer produced with L. plantarum had an increased intensity in fruity odor and dried fruit odor, the L. brevis beer had a higher total flavor intensity, acidic taste and astringency. Remarkably, the beer generated with L. brevis was perceived as comparable to a commercial sour beer in multiple sensory attributes. Taken together, this study demonstrates the feasibility of using L. brevis BSO464 and L. plantarum in co-fermentation with S. cerevisiae for controlled sour beer production with shortened production time.

Anti-Contamination Strategies for Yeast Fermentations

Yeasts are very useful microorganisms that are used in many industrial fermentation processes such as food and alcohol production. Microbial contamination of such processes is inevitable, since most of the fermentation substrates are not sterile. Contamination can cause a reduction of the final product concentration and render industrial yeast strains unable to be reused. Alternative approaches to controlling contamination, including the use of antibiotics, have been developed and proposed as solutions. However, more efficient and industry-friendly approaches are needed for use in industrial applications. This review covers: (i) general information about industrial uses of yeast fermentation, (ii) microbial contamination and its effects on yeast fermentation, and (iii) currently used and suggested approaches/strategies for controlling microbial contamination at the industrial and/or laboratory scale.

Hop Extract Acts as an Antioxidant with Antimicrobial Effects against Propionibacterium Acnes and Staphylococcus Aureus

Acne is associated with hyperkeratosis, elevated levels of skin sebum and growth of Propionibacterium acnes (P. acnes) and Staphylococcus aureus (S. aureus). Furthermore, P. acnes promotes inflammation by inducing IL-6 production and oxidative stress. The aim of this study was to assess the antioxidant, anti-inflammatory and antibacterial potential of a hop-CO₂-extract with 50% humulone and lupulone. The susceptibility of P. acnes and S. aureus to the hop extract was tested by using the broth microdilution technique. The minimal inhibitory concentrations (MIC) for P. acnes and S. aureus were 3.1 and 9.4 µg/mL, respectively. In addition, the hop extract showed an antioxidative effect with a half maximal inhibitory concentration (IC₅₀) of 29.43 µg/mL as well as additional anti-inflammatory effects by reducing the IL-6 expression (IC₅₀: 0.8 µg/mL). In addition, a gel formulation with 0.3% hop extract (w/w) had antibacterial activity against P. acnes and S. aureus (inhibition zone value: 5.5 mm and 3 mm, respectively) which was significantly superior to the placebo gel. The positive control (a gel with the antibiotic clindamycin) showed an inhibition zone of 9 mm. Due to its antioxidant, anti-inflammatory and antibacterial effects hop extract might be a treatment option for acne-prone skin.

Overview of craft brewing specificities and potentially associated microbiota

The brewing process differs slightly in craft breweries as compared to industrial breweries, as there are fewer control points. This affects the microbiota of the final product. Beer contains several antimicrobial properties that protect it from pathogens, such as low pH, low oxygen and high carbon dioxide content, and the addition of hops. However, these hurdles have limited power controlling spoilage organisms. Contamination by these organisms can originate in the raw materials, persist in the environment, and be introduced by using flavoring ingredients later in the process. Spoilage is a prominent issue in brewing, and can cause quality degradation resulting in consumer rejection and product waste. For example, lactic acid bacteria are predominately associated with producing a ropy texture and haze, along with producing diacetyl which gives the beer butter flavor notes. Other microorganisms may not affect flavor or aroma, but can retard fermentation by consuming nutrients needed by fermentation yeast. Quality control in craft breweries today relies on culturing methods to detect specific spoilage organisms. Using media can be beneficial for detecting the most common beer spoilers, such as Lactobacillus and Pediococci. However, these methods are time consuming with long incubation periods. Molecular methods such as community profiling or high throughput sequencing are better used for identifying entire populations of beer. These methods allow for detection, differentiation, and identification of taxa.

Heterogeneity between and within Strains of Lactobacillus brevis Exposed to Beer Compounds

This study attempted to investigate the physiological response of six Lactobacillus brevis strains to hop stress, with and without the addition of Mn²⁺ or ethanol. Based on the use of different fluorescent probes, cell viability and intracellular pH (pHi) were assessed by fluorescence microscopy combined with flow cytometry, at the single cell level. The combined approach was faster than the traditional colony based method, but also provided additional information about population heterogeneity with regard to membrane damage and cell size reduction, when exposed to hop compounds. Different physiological subpopulations were detected under hop stress in both hop tolerant and sensitive strains. A large proportion of cells were killed in all the tested strains, but a small subpopulation from the hop tolerant strains eventually recovered as revealed by pHi measurements. Furthermore, a short term protection against hop compounds was obtained for both hop tolerant and sensitive strains, by addition of high concentration of Mn²⁺. Addition of ethanol in combination with hop compounds caused an additional short term increase in damaged subpopulation, but the subsequent growth suggested that the presence of ethanol provides a slight cross resistance toward hop compounds.

Biologically Active Compounds from Hops and Prospects for Their Use

Although female cones of the hop plant (Humulus lupulus) are known primarily as raw material supplying characteristic bitterness and aroma to beer, their equally significant health-promoting effects have been known to mankind for several thousand years and hop is a plant traditionally utilized in folk medicine. This paper summarizes the scientific knowledge on the effects of all 3 major groups of secondary metabolites of hops; polyphenols, essential oils, and resins. Because of their chemical diversity, it is no coincidence that these compounds exhibit a wide range of pharmacologically important properties. In addition to antioxidant, anti-inflammatory, and anticancer-related properties, particular attention is being paid to prenylflavonoids that occur almost exclusively in hops and are considered to be some of the most active phytoestrogens known. Hop oils and resins are well known for their sedative and other neuropharmacological properties, but in addition, these compounds exhibit antibacterial and antifungal effects. Recently, alpha bitter acids have been shown to block the development of a number of complex lifestyle diseases that are referred to by the collective name "metabolic syndrome." Information presented in this review confirms the significant potential for the use of hops in the pharmaceutical industry and provides an understanding of beer as a natural drink that, although moderately consumed, may become a source of many health-promoting compounds.

Role of plasmids in Lactobacillus brevis BSO 464 hop tolerance and beer spoilage

Specific isolates of lactic acid bacteria (LAB) can grow in the harsh beer environment, thus posing a threat to brew quality and the economic success of breweries worldwide. Plasmid-localized genes, such as horA, horC, and hitA, have been suggested to confer hop tolerance, a trait required for LAB survival in beer. The presence and expression of these genes among LAB, however, do not universally correlate with the ability to grow in beer. Genome sequencing of the virulent beer spoilage organism Lactobacillus brevis BSO 464 revealed the presence of eight plasmids, with plasmids 1, 2, and 3 containing horA, horC, and hitA, respectively. To investigate the roles that these and the other five plasmids play in L. brevis BSO 464 growth in beer, plasmid curing with novobiocin was used to derive 10 plasmid variants. Multiplex PCRs were utilized to determine the presence or absence of each plasmid, and how plasmid loss affected hop tolerance and growth in degassed (noncarbonated) beer was assessed. Loss of three of the eight plasmids was found to affect hop tolerance and growth in beer. Loss of plasmid 2 (horC and 28 other genes) had the most dramatic effect, with loss of plasmid 4 (120 genes) and plasmid 8 (47 genes) having significant, but smaller, impacts. These results support the contention that genes on mobile genetic elements are essential for bacterial growth in beer and that beer spoilage ability is not dependent solely on the three previously described hop tolerance genes or on the chromosome of a beer spoilage LAB isolate.

Mapping microbial ecosystems and spoilage-gene flow in breweries highlights patterns of contamination and resistance

Distinct microbial ecosystems have evolved to meet the challenges of indoor environments, shaping the microbial communities that interact most with modern human activities. Microbial transmission in food-processing facilities has an enormous impact on the qualities and healthfulness of foods, beneficially or detrimentally interacting with food products. To explore modes of microbial transmission and spoilage-gene frequency in a commercial food-production scenario, we profiled hop-resistance gene frequencies and bacterial and fungal communities in a brewery. We employed a Bayesian approach for predicting routes of contamination, revealing critical control points for microbial management. Physically mapping microbial populations over time illustrates patterns of dispersal and identifies potential contaminant reservoirs within this environment. Habitual exposure to beer is associated with increased abundance of spoilage genes, predicting greater contamination risk. Elucidating the genetic landscapes of indoor environments poses important practical implications for food-production systems and these concepts are translatable to other built environments.

DOI:http://dx.doi.org/10.7554/eLife.04634.001

Many microbes—including bacteria and fungi—can affect the food and drink we consume, for better and for worse. Some spoil food, making it less tasty or even harmful to health. However, microbes can also be important ingredients: for example, yeast ferments malted barley sugars to make the alcohol and flavor of beer.

Nowadays, many beers are made under carefully controlled conditions, where the only microbes in the beer should be the strain of yeast added to the barley sugars. A more traditional ‘coolship’ method can be used to make sour beers; the barley sugars cool in an open-topped vessel and are fermented by the yeast and bacteria found naturally on the raw ingredients and in the surrounding environment.

Relatively little was known about how microbes spread around and adapt to living inside buildings. Now, Bokulich et al. have used a range of molecular and statistical techniques to examine how bacteria and fungi are dispersed throughout a North American brewery that produces beer using both conventional and coolship brewing techniques. Most of the microbes found in the building originated from the raw ingredients used to make the beer, with different parts of the brewery containing different species. Over the course of a year, some species spread to new parts of the building; a statistical method predicted the sources of these microbes, and revealed some key areas and features of the brewery that affect microbial transfer.

Bokulich et al. also looked at the spread of genes that enable their bacterial hosts to spoil beer, including those that protect bacteria from the antimicrobial action of the hops that flavor many beers. Lactic acid bacteria are the main cause of beer spoilage and so are usually to be avoided in breweries, but are also a normal ingredient in sour beer. In the brewery Bokulich et al. investigated, beer-spoilage and hop-resistance genes were found throughout the brewery, even in areas not used to produce sour beer. However, little beer spoilage occurred.

The techniques used by Bokulich et al. to track the spread of microbes and their detrimental genes could be used in the future to understand how microbes adapt to other indoor environments. Indeed, Bokulich et al. suggest that breweries could be used as models to safely understand the factors that influence microbial movement in any food-production facility as well as other building environments.

DOI:http://dx.doi.org/10.7554/eLife.04634.002

Molecular mechanisms behind the antimicrobial activity of hop iso-α-acids in Lactobacillus brevis

The main bittering component in beer, hop iso-α-acids, have been characterised as weak acids, which act as ionophores impairing microbial cells' function under acidic conditions as present in beer. Besides medium pH, divalent cations play a central role regarding the efficacy of the antimicrobial effect. The iso-α-acids' non-bitter derivatives humulinic acids can be found in isomerised hop extracts and can be generated during hop storage. Therefore, they have been under investigation concerning their influence on beer sensory properties. This study sketches the molecular mechanism behind iso-α-acids' antimicrobial activity in Lactobacillus (L.) brevis regarding their ionophore activity versus the dependence of the inhibitory potential on manganese binding, and suggests humulinic acids as novel tasteless food preservatives. We designed and synthesised chemically modified iso-α-acids to enhance the basic understanding of the molecular mechanism of antimicrobial iso-α-acids. It could be observed that a manganese-binding dependent transmembrane redox reaction (oxidative stress) plays a crucial role in inhibition. Privation of an acidic hydroxyl group neither erased ionophore activity, nor did it entirely abolish antimicrobial activity. Humulinic acids proved to be highly inhibitory, even outperforming iso-α-acids.

The microbiology of malting and brewing

Brewing beer involves microbial activity at every stage, from raw material production and malting to stability in the package. Most of these activities are desirable, as beer is the result of a traditional food fermentation, but others represent threats to the quality of the final product and must be controlled actively through careful management, the daily task of maltsters and brewers globally. This review collates current knowledge relevant to the biology of brewing yeast, fermentation management, and the microbial ecology of beer and brewing.