Alicyclobacillusspoilage and control - a review

Overview of endospore-forming bacteria in food: The road towards a harmonised method for the enumeration of their spores

Endospore-forming bacteria are an important challenge for the food industry due to their ubiquitous nature, widespread presence in the food chain and sophisticated survival mechanisms. An accurate method is needed that can provide insight into the quality of raw materials, predict spoilage potential and ensure food safety. A plethora of methods exist for the enumeration of spore-forming bacteria which vary among countries, industries and food producers. These methods describe a wide range of values in the key method parameters, such as heat treatment, growth medium, incubation time, and temperature. Consequently the results obtained can vary leading to misalignment and confusion. In addition, many of these methods are empirical and have not been validated. A harmonised international approach for the enumeration of spores is needed to provide consistent and reliable results on which to base food safety and quality decisions. A group of experts associated with the Internal Standardisation Organisation working group undertaking this task has identified the main endospore-forming bacterial species occurring in foods based on a wide selection of publications. Endospores are typically formed by bacteria belonging to twelve families originating from the Negativicutes, Bacilli and Clostridia classes, with the latter two being the most important for the food industry. This review will be used as a first step in method standardisation.

A rapid and sensitive sandwich assay for the detection of Alicyclobacillus acidoterrestris in apple juice based on magnetosome immunomagnetic separation combined with quantum dots immunoassay technology

The contamination of apple juice by Alicyclobacillus acidoterrestris (A. acidoterrestris) can cause significant economic losses. Therefore, developing a rapid and sensitive method for detecting A. acidoterrestris is necessary. To address this issue, this study prepared magnetosome-monoclonal antibodies immunomagnetic microspheres (IMMs) as capture probes to enrich A. acidoterrestris (IMMs-Aa) and monoclonal antibodies-quantum dots (mAb-CdTeQDs) as detection probes for detecting A. acidoterrestris. The obtained IMMs-Aa-mAb-CdTeQDs "sandwich" structure was used to detect A. acidoterrestris based on the fluorescence intensity. The strategy presented a good linear correlation (y = 1048× + 1891, R2 = 0.995) between the fluorescence intensity and the concentration of A. acidoterrestris (101-105 CFU/mL) with a low detection limit of 25.4 CFU/mL within 50 min. The recovery rate of this strategy in spiked apple juice ranged from 88.16 % to 107.03 %. Thus, this study established an efficient and highly sensitive magnetic capture-enrichment-separation-detection method for A. acidoterrestris.

Adaptive responses of Alicyclobacillus acidoterrestris in acidic broth and fruit juices: Focus on the influences of organic acids and temperature conditions

Acid adaptive response (AAR) is a survival mechanism that allows bacteria to develop enhanced stress tolerance. Our previous research identified AAR in Alicyclobacillus acidoterrestris, a thermo-acidophilic bacterium responsible for fruit juice spoilage. However, the roles of specific acidulants, adaptive temperatures, and acidic juice matrices in triggering AAR remain elusive. In this work, acid adaptation of A. acidoterrestris in broth acidified with various organic acids and in fruit juices was investigated, while also considering the ambient temperature. Results revealed that acid adaptation (at pH values of 3.0, 3.2, and 3.5, adjusted with malic, tartaric, or citric acids, and at pH 3.5 adjusted with lactic, succinic, or ascorbic acids, for 1 h) enhanced acid resistance (pH = 2.2, 1 h) of A. acidoterrestris, across all tested temperatures (45 °C, 35 °C, 25 °C, and 10 °C). Moreover, heat tolerance (65 °C, 5 min) was improved, except when using tartaric acid. Among acidulants used during adaptation (pH 3.5, 45 °C), succinic acid induced the highest level of acid resistance, followed by lactic, citric, malic, ascorbic, and tartaric acids, in descending order. For heat resistance, the ranking was succinic, citric, tartaric, lactic, ascorbic, and malic acids. Furthermore, acid adaptation in apple or orange juices enhanced heat resistance (65 °C) of A. acidoterrestris, and the induced resistance increased with extension of adaptation period. Adaptive temperatures of 25 °C and 35 °C were more effective in promoting resistance than 10 °C. These findings highlight the importance of considering adaptive responses of A. acidoterrestris to different preservation stresses and acidic juice environments during juice processing.

Beyond guaiacol and halophenols: Unravelling isobutyric and isovaleric acids as new culprits in off-flavour spoilage by Alicyclobacillus spp

Industries that produce or use fruit-based products have faced several spoilage events, resulting in economic losses caused by product recalls and loss of consumer confidence. Some of these events correlate to the presence of Alicyclobacillus (ACB) in food products since they can produce off-flavours and odours in the final products. Guaiacol (2-methoxyphenol) and halophenols (2,6-dichlorophenol and 2,6-dibromophenol) have been widely explored as the primary culprits of off-flavour spoilage by ACB. However, different compounds might be correlated with these spoilage events. In this work, volatile metabolites produced by distinct ACB species (Alicyclobacillus acidoterrestris, Alicyclobacillus acidocaldarius, and Alicyclobacillus cycloheptanicus) in laboratory medium and fruit juices were identified by HS-SPME-GC-MS and investigated as potential spoilage-related compounds. Isobutyric acid (2-methylpropanoic acid) and isovaleric acid (3-methylbutanoic acid) were revealed to be produced by all three ACB species at concentrations that surpass the odour threshold. These cheesy, sweaty, and sour compounds were responsible for dissonant odours in peach, orange, and tomato juice, harming fruit-based products' quality. More importantly, this work suggests that ACB species previously identified as non-spoilage bacteria, based on a lack of ability to produce guaiacol and halophenols, can also threaten the juice, beverage, and dairy industries. As such, identification methods currently used in industries for ACB control in final products should be revised.

Exploring the extraction, functional properties, and industrial applications of papain from Carica papaya

Papain a protease enzyme naturally present in the Carica papaya has gained significant interest across several industries due to its unique properties and versatility. The unique structure of papain imparts the functionality that assists in elucidating how papain enzyme works and making it beneficial for a variety of purposes. This review highlights recent advancements in papain extraction techniques to enhance production efficiency to meet market demand. The extraction of papain from the Carica papaya plant offers various advantages such as cost-effectiveness, biodegradability, safety, and the ability to withstand a wide range of pH and temperature conditions. Key findings reveal that non-conventional papain extraction techniques offer significant advantages in terms of efficiency, product quality, and environmental sustainability. Furthermore, papain treatment enhances the value of final products due to its anti-bacterial, anti-oxidant, and anti-obesity properties. The ability of papain to hydrolyze a wide range of proteins across various conditions makes it a suitable protease enzyme. While the study emphasizes the advantages of papain, the study also acknowledges limitations such as the continuous research and development to optimize extraction processes which will help unlock papain's potential and meet the growing demand. © 2024 Society of Chemical Industry.

Unlocking dual-mode enzyme activities on bacterial surface: Directional recognition and swift capture of Alicyclobacillus acidoterrestris from fruit juices

The acidophilic and heat-resistant characteristics of Alicyclobacillus acidoterrestris (A. acidoterrestris) pose significant challenges to fruit juice production. Traditional thermal removal methods are often ineffective against this resilient bacterium. To address this issue, we developed a novel adsorbent, magnetic carbonation carbon-lysozyme nanohybrid (MCL), composed of magnetic nanoparticles with a thin carbon shell and covalently grafted lysozyme. The outer lysozyme facilitates binding to the bacterial surface through two modes: electrostatic attraction and chemical interaction, acting as a vital engine for bacterial adhesion. The ultrathin carbon coating enhances dispersion, reduces magnetic loss, provides more adsorption sites for lysozyme grafting, and ensures stable function in acidic environments. Benefiting from the large surface area of MCL and the specific peptidoglycan recognition structure of lysozyme, MCL exhibits rapid adsorption kinetics and can completely remove 104 CFU/mL of A. acidoterrestris from juice within 20 min. The MCL demonstrates excellent capture performance, negligible cytotoxicity, and no significant impact on juice quality, offering a promising non-thermal strategy to improve juice safety.

Influence of physicochemical characteristics on the growth and guaiacol production of Alicyclobacillus acidoterrestris in fruit juices

Alicyclobacillus acidoterrestris is a bacterium known for causing spoilage in the taste and odour of fruit juices due to its thermoacidophilic nature. Its spoilage is attributed to the formation of guaiacol, which requires the presence of suitable precursors in the juices that A. acidoterrestris can metabolize. Therefore, A. acidoterrestris could exhibit different behaviour depending on the physicochemical characteristics the juice. In this study, we aimed to evaluate the behaviour of five A. acidoterrestris strains in seven different fruit juices by monitoring total cell and spore populations and quantifying guaiacol production. Also, physicochemical and phenolic profile, focusing on antimicrobials and guaiacol precursors, were analysed to better understand differences. Results showed growth in orange, apple, and plum juices for all the tested strains, with total cell populations reaching approximately 7 log cfu/mL, except for plum juice. In persimmon juice, growth was only observed in 3 out of 5 strains, for both total cells and spores. In contrast, all strains were inhibited in peach, black grape, and strawberry juices, maintaining a consistent population around 4 log cfu/mL. A strong negative correlation was observed between bacterial population and compounds such as kaempferol (for strains R3, R111, and P1), cyanidin chloride (for strains R111 and P1), and p-coumaric acid (for strain 7094 T). Regarding guaiacol production, orange and persimmon juices exhibited the highest guaiacol levels, with strain P1 (362.3 ± 12.6 ng/mL) and strain EC1 (325.1 ± 1.4 ng/mL) as the top producers, respectively. Plum, black grape, and strawberry juices showed similar guaiacol concentrations (16.9 ± 2.8 to 105.0 ± 33.7 ng/mL). Vanillin was showed positive correlations with guaiacol production in almost all strains (7094 T, R3, R111, and P1), with correlation coefficients of 0.97, 0.99, 0.82, and 0.87, respectively. We have reported different behaviour of A. acidoterrestris strains depending on juice type. Despite growth inhibition observed in some juices, enough guaiacol quantities to spoil the juice can be produced. This highlights the necessity of exploring strategies to prevent guaiacol production, even under growth restriction.

High-pressure carbon dioxide (HPCD): Impact on the quality of fruit juices and inactivation of spores and enzymes

Ensuring microbiological safety in fruit juices while maintaining their nutritional and sensory qualities remains a significant challenge in food processing. Traditional thermal methods, although effective against vegetative pathogens, can degrade important nutrients and are less effective at inactivating bacterial spores. High-pressure carbon dioxide (HPCD) technology has emerged as a promising non-thermal alternative, using CO2 under high pressure to inactivate spores and enzymes. More importantly, HPCD has shown great potential in preserving the quality of fruit juices. This review assesses recent studies on the use of HPCD in fruit juices, focusing on its effectiveness in reducing spore counts and inactivating enzymes like polyphenol oxidase (PPO) and pectin methylesterase (PME). The impact of HPCD on the physicochemical, nutritional, and sensory attributes of fruit juices, such as vitamin retention, color, and cloudiness, is also examined. Despite HPCD's advantages, challenges remain in optimizing process parameters for consistent microbial inactivation, with variations depending on juice composition and microbial strain. Additionally, while initial costs are high, the long-term economic viability of HPCD is favorable due to lower energy consumption and CO2 recyclability. Future research should focus on optimizing equipment design and scaling HPCD technology for industrial applications.

Transcriptomic analysis of the antibacterial mechanism of ε-polylysine-functionalized magnetic composites against Alicyclobacillus acidoterrestris and its application in apple juice

BACKGROUND

Alicyclobacillus acidoterrestris is a common microorganism in fruit juice. It can produce off-odor metabolites and has been considered to be an important factor in juice contamination. Thus, the development of new strategy for the control of A. acidoterrestris has important practical significance. The primary objective of this work was to assess the antibacterial performance of ε-polylysine-functionalized magnetic composites (Fe3O4@MoS2@PAA-EPL) in apple juice and its effect on juice quality. Moreover, the molecular mechanism of Fe3O4@MoS2@PAA-EPL against A. acidoterrestris was explored by RNA sequencing (RNA-Seq).

RESULTS

Experimental results indicated that the synthesized composites possessed the ability to inhibit the viability of A. acidoterrestris vegetative cells and spores. Besides, investigation on the quality of apple juice incubated with Fe3O4@MoS2@PAA-EPL implied that the fabricated composites displayed negligible adverse effects on juice quality. In addition, the results of RNA-Seq demonstrated that 833 differentially expressed genes (DEGs) were identified in Fe3O4@MoS2@PAA-EPL-treated A. acidoterrestris, which were associated with translation, energy metabolism, amino acid metabolism, membrane transport and cell integrity.

CONCLUSION

These results suggested that the treatment of Fe3O4@MoS2@PAA-EPL disrupted energy metabolism, repressed cell wall synthesis and caused membrane transport disorder of bacterial cells. This work provides novel insights into the molecular antibacterial mechanism for ε-polylysine-functionalized magnetic composites against A. acidoterrestris. © 2024 Society of Chemical Industry.

Substrate and nutrient manipulation during continuous cultivation of extremophilic algae, Galdieria spp. RTK 37.1, substantially impacts biomass productivity and composition

The extremophilic nature and metabolic flexibility of Galdieria spp. highlights their potential for biotechnological application. However, limited research into continuous cultivation of Galdieria spp. has slowed progress towards the commercialization of these algae. The objective of this research was to investigate biomass productivity and growth yields during continuous photoautotrophic, mixotrophic and heterotrophic cultivation of Galdieria sp. RTK371; a strain recently isolated from within the Taupō Volcanic Zone in Aotearoa-New Zealand. Results indicate Galdieria sp. RTK371 grows optimally at pH 2.5 under warm white LED illumination. Photosynthetic O2 production was dependent on lighting intensity with a maximal value of (133.5 ± 12.1 nmol O2 mgbiomass -1 h-1) achieved under 100 μmol m-2 s-1 illumination. O2 production rates slowed significantly to 42 ± 1 and <0.01 nmol O2 mgbiomass -1 h-1 during mixotrophic and heterotrophic growth regimes respectively. Stable, long-term chemostat growth of Galdieria sp. RTK371 was achieved during photoautotrophic, mixotrophic and heterotrophic growth regimes. During periods of ammonium limitation, Galdieria sp. RTK371 increased its intracellular carbohydrate content (up to 37% w/w). In contrast, biomass grown in ammonium excess was composed of up to 65% protein (w/w). Results from this study demonstrate that the growth of Galdieria sp. RTK371 can be manipulated during continuous cultivation to obtain desired biomass and product yields over long cultivation periods.

Alicyclobacillus spp. in fruit-based products: Isolation, identification, quantitative assessment (SPME/GC-MS) of spoilage compounds and spore's resistance to thermal shocks

Alicyclobacillus spp. is the cause of great concern for the food industry due to their spores' resistance (thermal and chemical) and the spoilage potential of some species. Despite this, not all Alicyclobacillus strains can spoil fruit juices. Thus, this study aimed to identify Alicyclobacillus spp. strains isolated from fruit-based products produced in Argentina, Brazil, and Italy by DNA sequencing. All Alicyclobacillus isolates were tested for guaiacol production by the peroxidase method. Positive strains for guaiacol production were individually inoculated at concentration of 103 CFU/mL in 10 mL of orange (pH 3.90) and apple (pH 3.50) juices adjusted to 11°Brix, following incubation at 45 °C for at least 5 days to induce the production of the following spoilage compounds: Guaiacol, 2,6-dichlorophenol (2,6-DCP) and 2,6-dibromophenol (2,6-DBP). The techniques of micro-solid phase extraction by headspace (HS-SPME) and gas-chromatography with mass spectrometry (GC-MS) were used to identify and quantify the spoilage compounds. All GC-MS data was analyzed by principal component analysis (PCA). The effects of different thermal shock conditions on the recovery of Alicyclobacillus spores inoculated in orange and apple juice (11°Brix) were also tested. A total of 484 strains were isolated from 48 brands, and the species A. acidocaldarius and A. acidoterrestris were the most found among all samples analyzed. In some samples from Argentina, the species A. vulcanalis and A. mali were also identified. The incidence of these two main species of Alicyclobacillus in this study was mainly in products from pear (n = 108; 22.3 %), peach (n = 99; 20.5 %), apple (n = 86; 17.8 %), and tomato (n = 63; 13 %). The results indicated that from the total isolates from Argentina (n = 414), Brazil (n = 54) and Italy (n = 16) were able to produce guaiacol: 107 (25.8 %), 33 (61.1 %) and 13 (81.2 %) isolates from each country, respectively. The PCA score plot indicated that the Argentina and Brazil isolates correlate with higher production of guaiacol and 2,6-DCP/2,6-DBP, respectively. Heatmaps of cell survival after heat shock demonstrated that strains with different levels of guaiacol production present different resistances according to spoilage ability. None of the Alicyclobacillus isolates survived heat shocks at 120 °C for 3 min. This work provides insights into the incidence, spoilage potential, and thermal shock resistance of Alicyclobacillus strains isolated from fruit-based products.

Pyrococcus furiosus argonaute based Alicyclobacillus acidoterrestrsis detection in fruit juice

Alicyclobacillus acidoterrestris is the major threat to fruit juice for its off-odor producing characteristic. In this study, Pyrococcus furiosus Argonaute (PfAgo), a novel endonuclease with precise DNA cleavage activity, was used for A. acidoterrestrisdetection, termed as PAD. The partially amplified 16 S rRNA gene of A. acidoterrestris can be cleaved by PfAgo activated by a short 5'-phosphorylated single strand DNA, producing a new guide DNA (gDNA). Then, PfAgo was activated by the new gDNA to cut a molecular beacon (MB) with fluorophore-quencher reporter, resulting in the recovery of fluorescence. The fluorescent intensity is positively related with the concentration of A. acidoterrestris. The PAD assay showed excellent specificity and sensitivity as low as 101 CFU/mL, which can be a powerful tool for on-site detection of A. acidoterrestris in fruit juice industry in the future, reducing the economic loss.

Taxonomic Diversity and Functional Traits of Soil Bacterial Communities under Radioactive Contamination: A Review

Studies investigating the taxonomic diversity and structure of soil bacteria in areas with enhanced radioactive backgrounds have been ongoing for three decades. An analysis of data published from 1996 to 2024 reveals changes in the taxonomic structure of radioactively contaminated soils compared to the reference, showing that these changes are not exclusively dependent on contamination rates or pollutant compositions. High levels of radioactive exposure from external irradiation and a high radionuclide content lead to a decrease in the alpha diversity of soil bacterial communities, both in laboratory settings and environmental conditions. The effects of low or moderate exposure are not consistently pronounced or unidirectional. Functional differences among taxonomic groups that dominate in contaminated soil indicate a variety of adaptation strategies. Bacteria identified as multiple-stress tolerant; exhibiting tolerance to metals and antibiotics; producing antioxidant enzymes, low-molecular antioxidants, and radioprotectors; participating in redox reactions; and possessing thermophilic characteristics play a significant role. Changes in the taxonomic and functional structure, resulting from increased soil radionuclide content, are influenced by the combined effects of ionizing radiation, the chemical toxicity of radionuclides and co-contaminants, as well as the physical and chemical properties of the soil and the initial bacterial community composition. Currently, the quantification of the differential contributions of these factors based on the existing published studies presents a challenge.

Inactivation activity and mechanism of pulsed light against Alicyclobacillus acidoterrestris vegetative cells and spores in concentrated apple juice

Alicyclobacillus acidoterrestris has received much attention due to its unique thermo-acidophilic property and implication in the spoilage of pasteurized juices. The objective of this study was to evaluate the sterilization characteristics and mechanisms of pulsed light (PL) against A. acidoterrestris vegetative cells and spores in apple juice. The results indicated that bacteria cells in apple juice (8-20°Brix) can be completely inactivated within the fluence range of 20.25-47.25 J/cm2, which mainly depended on the soluble solids content (SSC) of juice, and the spores in apple juice (12°Brix) can be completely inactivated by PL with the fluence of 54.00 J/cm2. The PL treatment can significantly increase the leakage of reactive oxygen species (ROS) and proteins from cells and spores. Fluorescence studies of bacterial adenosine triphosphate (ATP) indicated that the loss of ATP was evident. Scanning electron microscopy and confocal laser scanning microscope presented that PL-treated cells or spores had serious morphological damage, which reduced the integrity of cell membrane and led to intracellular electrolyte leakage. In addition, there were no significant negative effects on total sugars, total acids, total phenols, pH value, SSC and soluble sugars, and organic acid content decreased slightly during the PL treatment. The contents of esters and acids in aroma components had a certain loss, while that of alcohols, aldehydes and ketones were increased. These results demonstrated that PL treatment can effectively inactivate the bacteria cells and spores in apple juice with little effect on its quality. This study provides an efficient method for the inactivation of A. acidoterrestris in fruit juice.

Antibacterial effects of various molecular weight chitosans against Alicyclobacillus acidoterrestris in orange juice

Alicyclobacillus acidoterrestris has been gaining attention due to its unique thermo-acidophilic properties and being associated with the deterioration of pasteurized beverages. The objective of this study was to evaluate the antibacterial activity of chitosan with various molecular weights (MWs) (164, 85, 29.2, and 7.1 kDa) and concentrations (0-100 μg/mL) against A. acidoterrestris and its effect on guaiacol production. Various chitosan MWs were co-incubated for 7 days, and the bacterial growth, guaiacol, and vanillic acid contents during storage were determined. The chitosans performed antibacterial effects against A. acidoterrestris. Further, 164 kDa chitosan showed excellent results in controlling the growth and guaiacol formation in A. acidoterrestris. These findings demonstrated the efficacy of chitosan antibacterial activity against A. acidoterrestris and mitigating the guaiacol formation. Chitosan's antibacterial properties are attributed to the elimination of cells and suppression of guaiacol production. This study introduces a new approach for reducing A. acidoterrestris contamination in fruit juices, with potential product quality and safety advantages.

Enzyme-Assisted Endogenous Guide DNA Generation-Mediated Pyrococcus furiosus Argonaute for Alicyclobacillus acidoterrestris Detection

Pyrococcus furiosusArgonaute (PfAgo) emerged as a novel endonuclease for the nucleic acid test recently. However, the input of exogenous guide DNA (gDNA) to activate PfAgo has reduced its flexibility. In this work, an enzyme-assisted endogenous gDNA generation-mediated PfAgo for the target detection strategy, termed EGG-PAD, was proposed. With the aid of EcoR Ι, the target double-strand DNA was cut, producing a phosphate group at the 5' end, functioning as gDNA to activate PfAgo for nucleic acid detection. The applicability of this assay was tested in the detection ofAlicyclobacillus acidoterrestris, a bacterium causing the spoilage of fruit juice, showing excellent sensitivity and specificity, ascribed to the "duplex amplification and triple insurance" mechanism. Moreover, EGG-PAD exhibited superior versatility in the identification of common foodborne pathogens. This powerful platform could also be an on-site test tool for detecting nucleic acid-containing organisms such as tumor cell, pathogen, and virus in the future.

Alicyclobacillus sp. SO9, a novel halophilic acidophilic iron-oxidizing bacterium isolated from a tailings-contaminated beach, and its effect on copper extraction from chalcopyrite in the presence of high chloride concentration

Many acidophilic iron-oxidizing bacteria used in the mining industry for the bioleaching of sulfidic minerals are intolerant to high chloride concentrations, resulting in problems where chloride occurs in the deposit at high concentrations or only seawater is available. In search for strains tolerating such conditions a tetrathionate- and iron-oxidizing bacterium was isolated from a tailings-contaminated beach sample at Portman Bay, Cartagena-La Union mining district, Spain, in the presence of 20 g l-1 (0.34 M) sodium chloride. The isolate was able to form spores, did not grow in the absence of NaCl, and oxidized ferrous iron in the presence of up to 1.5 M (∼87 g l-1) NaCl. Genome sequencing based on a combination of Illumina and PacBio reads revealed two contigs, a circular bacterial chromosome of 5.2 Mbp and a plasmid of 90 kbp, respectively. The chromosome comprised seven different 16S rRNA genes. Submission of the chromosome to the Type (Strain) Genome Server (TYGS) without preselection of similar sequences revealed exclusively type strains of the genus Alicyclobacillus. In the TYGS analyses the respective most similar species were dependent on whether the final tree was derived from just 16S rRNA, from the genomes, or from the proteomes. Thus, TYGS analysis clearly showed that isolate SO9 represents a novel species of the genus Alicyclobacillus. In the presence of artificial seawater with almost 0.6 M chloride, the addition of Alicyclobacillus sp. SO9 improved copper dissolution from chalcopyrite (CuFeS2) compared to abiotic leaching without bacteria. The new isolate SO9, therefore, has potential for bioleaching at elevated chloride concentrations.

Exploring the growth characteristics of Alicyclobacillus acidoterrestris for controlling juice spoilage with zero additives

Fruit juice spoilage that caused by contaminated Alicyclobacillus has brought huge losses to beverage industry worldwide. Thus, it is very essential to understand the growth and metabolism processing of Alicyclobacillus acidoterrestris (A. acidoterrestris) in controlling juice spoilage caused by Alicyclobacillus. In this work, simulative models for the growth and metabolism of A. acidoterrestris were systematically conducted in the medium and fruit juice. The results showed that low temperature (4 ℃) and strong acidic environment (pH 3.0-2.0) of medium inhibited the growth and reproduction of A. acidoterrestris. In addition, with decreasing temperature, the color, smell and turbidity of commercially available juice supplemented with A. acidoterrestris significantly improved. This work provided a clear exploration of growth characteristics of A. acidoterrestris by applying theory (medium) to reality (fruit juices), and pave fundamental for exploring the zero additives of controlling juice spoilage.

Computational analysis of carboxylesterase genes and proteins in non-pathogenic food bacterium Alicyclobacillus acidocaldarius: insights from proteogenomics

Over recent years, Alicyclobacillus acidocaldarius, a Gram-positive nonpathogenic rod-shaped thermo-acid-tolerant bacterium, has posed numerous challenges for the fruit juice industry. However, the bacterium's unique characteristics, particularly its nonpathogenic and thermophilic capabilities, offer significant opportunities for genetic exploration by biotechnologists. This study presents the computational proteogenomics report on the carboxylesterase (CE) enzyme in A. acidocaldarius, shedding light on structural and evolutional of CEs from this bacterium. Our analysis revealed that the average molecular weight of CEs in A. acidocaldarius was 41 kDa, with an isoelectric point around 5. The amino acid composition favored negative amino acids over positive ones. The aliphatic index and hydropathicity were approximately 88 and - 0.15, respectively. While the protein sequence showed no disulfide bonds in the CEs' structure, the presence of Cys amino acids was observed in the structure of CEs. Phylogenetic analysis presented more than 99% similarity between CEs, indicating their close evolutionary relationship. By applying homology modeling, the 3-dimensional structural models of the carboxylesterase were constructed, which with the help of structural conservation and solvent accessibility analysis highlighted key residues and regions responsible for enzyme stability and conformation. The specific patterns presented the total solvent accessibility of less than 25 (Å2) was in considerable position as well as Gly residues were noticeably have high accessibility to solvent in all structures. Ala was the more frequent amino acids in the conserved-SASA of carboxylesterases. Furthermore, unsupervised agglomerative hierarchical clustering based on solvent accessibility feature successfully clustered and even distinguished this enzyme from proteases from the same genome. These findings contribute to a deeper understanding of the nonpathogenic A. acidocaldarius carboxylesterase and its potential applications in biotechnology. Additionally, structural analysis of CEs would help to address potential solutions in fruit juice industry with utilization of computational structural biology.

Transcriptomic and Metabolomic Profiling Uncovers Response Mechanisms of Alicyclobacillus acidoterrestris DSM 3922T to Acid Stress

Alicyclobacillus acidoterrestris, which has strong acidophilic and heat-resistant properties, can cause spoilage of pasteurized acidic juice. The current study determined the physiological performance of A. acidoterrestris under acidic stress (pH 3.0) for 1 h. Metabolomic analysis was carried out to investigate the metabolic responses of A. acidoterrestris to acid stress, and integrative analysis with transcriptome data was also performed. Acid stress inhibited the growth of A. acidoterrestris and altered its metabolic profiles. In total, 63 differential metabolites, mainly enriched in amino acid metabolism, nucleotide metabolism, and energy metabolism, were identified between acid-stressed cells and the control. Integrated transcriptomic and metabolomic analysis revealed that A. acidoterrestris maintains intracellular pH (pHi) homeostasis by enhancing amino acids decarboxylation, urea hydrolysis, and energy supply, which was verified using real-time quantitative PCR and pHi measurement. Additionally, two-component systems, ABC transporters, and unsaturated fatty acid synthesis also play crucial roles in resisting acid stress. Finally, a model of the responses of A. acidoterrestris to acid stress was proposed. IMPORTANCE Fruit juice spoilage caused by A. acidoterrestris contamination has become a major concern and challenge in the food industry, and this bacterium has been suggested as a target microbe in the design of the pasteurization process. However, the response mechanisms of A. acidoterrestris to acid stress still remain unknown. In this study, integrative transcriptomic, metabolomic, and physiological approaches were used to uncover the global responses of A. acidoterrestris to acid stress for the first time. The obtained results can provide new insights into the acid stress responses of A. acidoterrestris, which will point out future possible directions for the effective control and application of A. acidoterrestris.

Detection of Alicyclobacillus spp. and Identification of Guaiacol Production Using the GENE-UP® PRO ACB, IFU Method No. 12, and Cosmo Bio Assays

Some species of Alicyclobacillus spoil beverages by producing guaiacol. Current culture-based methods detect the presence of Alicyclobacillus spp. and a subsequent peroxidase assay determines if the isolate can produce guaiacol. However, these methods are time-consuming and can yield false negatives due to differences in growth optima among species. The purpose of this study was to compare a RT-PCR-based method, the GENE-UP® PRO ACB assay, to the IFU Method No. 12 Enumeration and Enrichment methods. Ten species of Alicyclobacillus were detected using the tested RT-PCR assay, while A. dauci and A. kakegewensis were not detected using either IFU protocol. Low concentrations (1-10, 10-100, and 100-1,000 CFU/10 mL) of A. acidoterrestris, A. suci, and A. acidocaldarius were tested in five matrices. The proportion of positive samples identified using the tested RT-PCR assay (62/84) or the IFU Enrichment protocol (62/84) did not differ significantly from the proportion of inoculated samples (63/84). However, the IFU Enumeration method (32/84) detected statistically fewer positives. Additionally, methods identifying guaiacol production were compared. The proportion of correctly identified guaiacol producers using the tested RT-PCR assay (51/63) was not significantly different than those identified using the 3 h Cosmo Bio assay (54/63). Finally, four commercial samples of orange juice and sucrose solution were tested. Alicyclobacillus spp. were identified in all four samples using the IFU Enrichment method and in two samples using the tested RT-PCR assay. However, Alicyclobacillus was not detected in any sample using the IFU Enumeration method. Overall, this study showed consistent detection of Alicyclobacillus spp. using either the IFU Enrichment protocol or the tested RT-PCR assay, which both outperformed the IFU Enumeration protocol. Both the 3 h guaiacol bioassay and the tested RT-PCR assays consistently differentiated guaiacol-producing and nonproducing strains.

Microbial communities in freshwater used for hydraulic fracturing are unable to withstand the high temperatures and pressures characteristic of fractured shales

Natural gas is recovered from shale formations by hydraulic fracturing, a process known to create microbial ecosystems in the deep subsurface. Microbial communities that emerge in fractured shales include organisms known to degrade fracturing fluid additives and contribute to corrosion of well infrastructure. In order to limit these negative microbial processes, it is essential to constrain the source of the responsible micro-organisms. Previous studies have identified a number of potential sources, including fracturing fluids and drilling muds, yet these sources remain largely untested. Here, we apply high-pressure experimental approaches to assess whether the microbial community in synthetic fracturing fluid made from freshwater reservoir water can withstand the temperature and pressure conditions of hydraulic fracturing and the fractured shale environment. Using cell enumerations, DNA extraction and culturing, we show that the community can withstand high pressure or high temperature alone, but the combination of both is fatal. These results suggest that initial freshwater-based fracturing fluids are an unlikely source of micro-organisms in fractured shales. These findings indicate that potentially problematic lineages, such as sulfidogenic strains of Halanaerobium that have been found to dominate fractured shale microbial communities, likely derive from other input sources into the downwell environment, such as drilling muds.

Environmental sporobiota: Occurrence, dissemination, and risks

Spore-forming bacteria known as sporobiota are widespread in diverse environments from terrestrial and aquatic habitats to industrial and healthcare systems. Studies on sporobiota have been mainly focused on food processing and clinical fields, while a large amount of sporobiota exist in natural environments. Due to their persistence and capabilities of transmitting virulence factors and antibiotic resistant genes, environmental sporobiota could pose significant health risks to humans. These risks could increase as global warming and environmental pollution has altered the life cycle of sporobiota. This review summarizes the current knowledge of environmental sporobiota, including their occurrence, characteristics, and functions. An interaction network among clinical-, food-related, and environment-related sporobiota is constructed. Recent and effective methods for detecting and disinfecting environmental sporobiota are also discussed. Key problems and future research needs for better understanding and reducing the risks of environmental sporobiota and sporobiome are proposed.

Response mechanisms to acid stress of acid-resistant bacteria and biotechnological applications in the food industry

Acid-resistant bacteria are more and more widely used in industrial production due to their unique acid-resistant properties. In order to survive in various acidic environments, acid-resistant bacteria have developed diverse protective mechanisms such as sensing acid stress and signal transduction, maintaining intracellular pH homeostasis by controlling the flow of H⁺, protecting and repairing biological macromolecules, metabolic modification, and cross-protection. Acid-resistant bacteria have broad biotechnological application prospects in the food field. The production of fermented foods with high acidity and acidophilic enzymes are the main applications of this kind of bacteria in the food industry. Their acid resistance modules can also be used to construct acid-resistant recombinant engineering strains for special purposes. However, they can also cause negative effects on foods, such as spoilage and toxicity. Herein, the aim of this paper is to summarize the research progress of molecular mechanisms against acid stress of acid-resistant bacteria. Moreover, their effects on the food industry were also discussed. It is useful to lay a foundation for broadening our understanding of the physiological metabolism of acid-resistant bacteria and better serving the food industry.

Antimicrobial activity and mechanism of preservatives against Alicyclobacillus acidoterrestris and its application in apple juice

Alicyclobacillus acidoterrestris has great influence on the quality of apple juice products. In this study, the antibacterial activity of five preservatives (ε-polylysine, propylparaben, monocaprin, octyl gallate and heptylparaben) against A. acidoterrestris and its underlying mechanism were investigated. Results showed that these five preservatives all exerted antibacterial activity through a multiple bactericidal mechanism, and monocaprin and octyl gallate had the highest antibacterial activity, with the minimum inhibitory concentration (MIC) values of 22.5 and 6.25 mg/L, respectively. Five preservatives all changed the permeability of the cell membrane and destroyed the complete cell morphology, with the leakages of the intracellular electrolytes. Moreover, the treatment of ε-polylysine, propylparaben and monocaprin increased the leakage of intracellular protein; propylparaben and octyl gallate reduced the levels of cellular adenosine triphosphate. Also, monocaprin and octyl gallate may stimulate bacteria to release a large amount of reactive oxygen species, so that certain oxidative damage can kill the bacteria. Furthermore, monocaprin and octyl gallate could effectively inactivate the contamination of A. acidoterrestris in apple juices, with the slightly decrease of soluble sugars and organic acids, without significant adverse effects on total sugars and titratable acids. This research highlights the great promise of using monocaprin and octyl gallate as the safe multi-functionalized food additives for food preservations.

Unveiling the complete genome sequence of Alicyclobacillus acidoterrestris DSM 3922T, a taint-producing strain

Several species from the Alicyclobacillus genus have received much attention from the food and beverages industries. Their presence has been co-related with spoilage events of acidic food matrices, namely fruit juices and other fruit-based products, the majority attributed to Alicyclobacillus acidoterrestris. In this work, a combination of short and long reads enabled the assembly of the complete genome of A. acidoterrestris DSM 3922T, perfecting the draft genome already available (AURB00000000), and revealing the presence of one chromosome (4,222,202 bp; GC content 52.3%) as well as one plasmid (124,737 bp; GC content 46.6%). From the 4,288 genes identified, 4,004 sequences were attributed to coding sequences with proteins, with more than 80% being functionally annotated. This allowed the identification of metabolic pathways and networks and the interpretation of high-level functions with significant reliability. Furthermore, the additional genes of interest related to spore germination, off-flavor production, namely the vdc cluster, and CRISPR arrays, were identified. More importantly, this is the first complete and closed genome sequence for a taint-producing Alicyclobacillus species and thus represents a valuable reference for further comparative and functional genomic studies.

Production and Potential Application of Pyroligneous Acids from Rubberwood and Oil Palm Trunk as Wood Preservatives through Vacuum-Pressure Impregnation Treatment

The development of low-environmental-impact technologies for the elimination of biological damage is one of the vital goals of the wood protection industry. The possibility of utilizing pyroligneous acid as a wood preservative can be a great solution to extend the application of the currently fast-growing timber species, which has lower natural durability against biological damage. In this study, the effectiveness of pyroligneous acid as a wood preservative was evaluated by impregnating rubberwood with pyroligneous acid using vacuum-pressure treatment, and the treated woods were exposed to mould fungi, wood-decay fungi and termite attacks under laboratory conditions. Pyroligneous acids produced from rubberwood (RWPA) and oil palm trunk (OPTPA) at different pyrolysis temperatures were evaluated. To fully understand the effectiveness of pyroligneous acids as wood preservatives, different concentrations of pyroligneous acids were impregnated into rubberwood. Concentrations of 50% RWPA and 30% OPTPA were sufficient against mould and decay fungi on rubberwood. Rubberwood impregnated with pyroligneous acid acted as a slow-acting toxic bait to cause a high termite mortality rate due to toxic feeding and does not serve as a good repellent to prevent termites from feeding on the wood. In general, OPTPA has better biological durability compared to RWPA.

Inactivation efficacy and mechanisms of atmospheric cold plasma on Alicyclobacillus acidoterrestris: Insight into the influence of growth temperature on survival

The bactericidal effect of dielectric barrier discharge-atmospheric cold plasma (DBD-ACP, 20, and 30 kV) against Alicyclobacillus acidoterrestris on the saline solution and apple juice was investigated. Results show that DBD-ACP is effective for the inactivation of A. acidoterrestris by causing significant changes in cell membrane permeability and bacterial morphology. The effect of culture temperatures on the resistance of A. acidoterrestris to DBD-ACP was also studied. A. acidoterrestris cells grown at 25°C had the lowest resistance but it was gradually increased as the culture temperature was increased (25-45°C) (p < 0.05). Moreover, results from Fourier transform infrared spectroscopy (FT-IR) and Gas Chromatography-Mass Spectrometer (GC-MS) analysis showed that the increase in the culture temperature can gradually cause the decreased level of cyclohexaneundecanoic acid in the cell membrane of A. acidoterrestris (p < 0.05). In contrast, cyclopentaneundecanoic acid, palmitic acid, and stearic acid showed an increasing trend in which the fluidity of the bacterial cell membrane decreased. This study shows a specific correlation between the resistance of A. acidoterrestris and the fatty acid composition of the cell membrane to DBD-ACP.

Bovicin HC5 and nisin reduce cell viability and the thermal resistance of Alicyclobacillus acidoterrestris endospores in fruit juices

BACKGROUND

Alicyclobacillus acidoterrestris is an important thermoacidophilic spore-forming bacterium in fruit-juice deterioration, and alternative non-thermal methods have been investigated to control fruit juice spoilage. This work aimed to evaluate the capacity of bovicin HC5 and nisin to inhibit the growth of vegetative cells and reduce the thermal resistance of endospores of A. acidoterrestris inoculated (10⁷ CFU mL^-1 ) in different fruit juices. The number of viable cells was determined after 12 h incubation at 43 °C in the presence and absence of nisin or bovicin HC5 (10-100 AU mL^-1 ). The exposure time (min) required to kill 90% of the initial population (reduction of one log factor) at 90 ºC (D_90ºC ) was used to assess the thermal resistance of A. acidoterrestris endospores exposed (80 AU mL^-1 ) or non-exposed to the bacteriocins. Additionally, the effect of bovicin and nisin on the morphology and cell structure of A. acidoterrestris was evaluated by atomic force microscopy (AFM).

RESULTS

Bovicin HC5 and nisin were bactericidal against A. acidoterrestris inoculated in fruit juices and reduced the D_90°C values up to 30-fold. AFM topographical images revealed substantial structural changes in the cellular framework of vegetative cells upon treatment with bovicin HC5 or nisin.

CONCLUSIONS

These results emphasize the potential application of lantibiotics as additional hurdles in food processing to control thermoacidophilic spoilage bacteria in fruit juices. © 2022 Society of Chemical Industry.

The Effect of Temperature-Assisted High Hydrostatic Pressure on the Survival of Alicyclobacillus acidoterrestris Inoculated in Orange Juice throughout Storage at Different Isothermal Conditions

The purpose of this work was to investigate the population dynamics of the spores and vegetative cells of A. acidoterrestris in orange juice treated with temperature-assisted HHP and stored in different isothermal conditions. For this reason, the spores of two A. acidoterrestris strains were inoculated in commercial orange juice and subjected to HHP treatment at 600 MPa/60 °C for 5 and 10 min. Inoculated samples were subsequently stored at 4, 12 and 25 °C for 60 days. During storage, the population of A. acidoterrestris was determined before and after heat shock at 80 °C for 10 min in order to estimate the quantity of spores and any remaining vegetative cells on the Bacillus acidoterrestris medium agar. Results showed that spore populations decreased by 3.0–3.5 log cycles directly after HHP treatment. Subsequently, no significant changes were observed throughout storage regardless of temperature and bacterial strain. However, at 25 °C, an increase of 0.5–1.0 log cycles was noticed. For the remaining vegetative cells, the results illustrated that HHP treatment could eliminate them during storage at 4 and 12 °C, whereas at 25 °C inactivation was strain-dependent. Therefore, temperature-assisted HHP treatment could effectively inactivate A. acidoterrestris spores in orange juice and ensure that the inhibitory effect could be maintained throughout storage at low temperatures.

Fruit Juice Spoilage by Alicyclobacillus: Detection and Control Methods—A Comprehensive Review

Fruit juices have an important place in humans’ healthy diet. They are considered to be shelf stable products due to their low pH that prevents the growth of most bacteria. However thermo-acidophilic endospore forming bacteria of the genus Alicyclobacillus have the potential to cause spoilage of commercially pasteurized fruit juices. The flat sour type spoilage, with absence of gas production but presence of chemical spoilage compounds (mostly guaiacol) and the ability of Alicyclobacillus spores to survive after pasteurization and germinate under favorable conditions make them a major concern for the fruit juice industry worldwide. Their special characteristics and presence in the fruit juice industry has resulted in the development of many isolation and identification methods based on cell detection (plating methods, ELISA, flow cytometry), nucleic acid analysis (PCR, RAPD-PCR, ERIC-PCR, DGGE-PCR, RT-PCR, RFLP-PCR, IMS-PCR, qPCR, and 16S rRNA sequencing) and measurement of their metabolites (HPLC, GC, GC-MS, GC-O, GC-SPME, Electronic nose, and FTIR). Early detection is a big challenge that can reduce economic loss in the industry while the development of control methods targeting the inactivation of Alicyclobacillus is of paramount importance as well. This review includes a discussion of the various chemical (oxidants, natural compounds of microbial, animal and plant origin), physical (thermal pasteurization), and non-thermal (High Hydrostatic Pressure, High Pressure Homogenization, ultrasound, microwaves, UV-C light, irradiation, ohmic heating and Pulse Electric Field) treatments to control Alicyclobacillus growth in order to ensure the quality and the extended shelf life of fruit juices.

Integrated transcriptomic and proteomic analysis reveals the response mechanisms of Alicyclobacillus acidoterrestris to heat stress

Alicyclobacillus acidoterrestris can survive pasteurization and is implicated in pasteurized fruit juice spoilage. However, the mechanisms underlying heat responses remain largely unknown. Herein, gene transcription changes of A. acidoterrestris under heat stress were detected by transcriptome, and an integrated analysis with proteomic and physiological data was conducted. A total of 911 differentially expressed genes (DEGs) was observed. The majority of DEGs and differentially expressed proteins (DEPs) were exclusively regulated at the mRNA and protein level, respectively, whereas only 59 genes were regulated at both levels and had the same change trends. Comparative analysis of the functions of the specifically or commonly regulated DEGs and DEPs revealed that the heat resistance of A. acidoterrestris was primarily based on modulating peptidoglycan and fatty acid composition to maintain cell envelope integrity. Low energy consumption strategies were established with attenuated glycolysis, decreased ribosome de novo synthesis, and activated ribosome hibernation. Terminal oxidases, cytochrome bd and aa₃, in aerobic respiratory chain were upregulated. Meanwhile, the MarR family transcriptional regulator was upregulated, reactive oxygen species (ROS) was discovered, and the concentration of superoxide dismutase (SOD) increased, indicating that the accompanied oxidative stress was induced by high temperature. Additionally, DNA and protein damage repair systems were activated. This study provided a global perspective on the response mechanisms of A. acidoterrestris to heat stress, with implications for better detection and control of its contamination in fruit juice.

Factors That Affect the Microbiological Stability of Chicha Morada during Its Production on an Industrial Scale: A Review

ABSTRACT

Chicha morada, also known as purple corn drink (PCD), is a traditional noncarbonated beverage commonly prepared at homes and restaurants in Peru. However, in recent years, it is being produced at an industrial scale aiming to extend its shelf life, expand its marketing, and make it known worldwide. Traditionally, this beverage, whose main component is purple corn (Zea mays L.), was made and consumed quickly and in some cases, stored under refrigeration until consumption, but never beyond 24 to 48 h. With its industrialization, factories are presented with challenges to design and provide adequate protection of the beverage, assuring its quality and safety. Although its production at an industrial level is similar to that of other noncarbonated drinks containing fruit juice, several processing factors could affect the microbiological stability desired for this beverage, such as the storage of the purple corn drink extract. In this document, a critical review of the production process (raw materials, production stages, and forms of commercialization) that can directly affect the contamination of the beverage is made. Recommendations are made for improving the control points in the industrial process and to avoid potential microbiological problems.

HIGHLIGHTS

Transcriptome-Based Selection and Validation of Reference Genes for Gene Expression Analysis of Alicyclobacillus acidoterrestris Under Acid Stress

Alicyclobacillus acidoterrestris is a major concern in fruit juice industry due to its spoilage potential of acidic fruit juice. Quantifying the expression levels of functional genes by real-time quantitative polymerase chain reaction (RT-qPCR) is necessary to elucidate the response mechanisms of A. acidoterrestris to acid stress. However, appropriate reference genes (RGs) for data normalization are required to obtain reliable RT-qPCR results. In this study, eight novel candidate RGs were screened based on transcriptome datasets of A. acidoterrestris under acid stress. The expression stability of eight new RGs and commonly used RG 16s rRNA was assessed using geNorm, NormFinder, and BestKeeper algorithms. Moreover, the comprehensive analysis using the RefFinder program and the validation using target gene ctsR showed that dnaG and dnaN were the optimal multiple RGs for normalization at pH 4.0; ytvI, dnaG, and 16s rRNA at pH 3.5; icd and dnaG at pH 3.0; and ytvI, dnaG, and spoVE at pH 2.5. This study revealed for the first time that A. acidoterrestris had different suitable RGs under different acid conditions, with implications for further deciphering the acid response mechanisms of this spoilage-causing bacterium.

Integrated analysis of transcriptome and proteome for exploring the mechanism of guaiacol production by Alicyclobacillus acidoterrestris

Alicyclobacillus spp. can cause commercially pasteurized fruit juices/beverages to spoil and the spoilage is characterized by the formation of a distinct medicinal or antiseptic off-odor attributed to guaiacol. The aim of this study was to reveal the mechanism of guaiacol production in A. acidoterrestris by combining transcriptomic and proteomic approaches. RNA-sequencing and iTRAQ analyses were conducted to investigate differences in expression levels of genes and proteins in A. acidoterrestris when producing (with 500 μM vanillic acid) and not producing (without vanillic acid) guaiacol. A total of 225 differentially expressed genes and 77 differentially expressed proteins were identified. The transcription of genes vdcBCD encoding subunits of vanillic acid decarboxylase were 626.47, 185.01 and 52.81-fold up-regulated, respectively; they were the most up-regulated genes involved in guaiacol production. Expressions of the benzoate membrane transport protein, fusaric acid resistance protein, resistance-nodulation- division transporter, some ATP-binding cassette transporters and major facilitator superfamily transporters were increased at either mRNA, protein or both levels, indicating that they participated in the uptake of vanillic acid and extrusion of guaiacol. In the metabolic process of vanillic acid to guaiacol in A. acidoterrestris, genes related to the pathway of tricarboxylic acid cycle and ribosome were up-regulated, while the expression of some genes associated with valine, leucine and isoleucine biosynthesis was decreased. These findings provide novel insight to understand the mechanism of guaiacol production in A. acidoterrestris, which will serve as an important guide for developing strategies for the control of A. acidoterrestris problems in the fruit juice industry.

Viability, Sublethal Injury, and Release of Cellular Components From Alicyclobacillus acidoterrestris Spores and Cells After the Application of Physical Treatments, Natural Extracts, or Their Components

Alicyclobacillus acidoterrestris is a spoiling microorganism regarded as one of the most important causes of spoilage of fruit juices and acidic products. In this paper, four strains of A. acidoterrestris (type strain-DSM 3922; two wild strains isolated from soil-C8 and C24; wild strain isolated from a spoiled pear juice CB1) were treated through natural extracts/active compounds from essential oils (EOs), and physical treatments were used to assess their susceptibility and the presence of sublethal injury. The characterization of damage was also performed. The results suggest that it is possible to control A. acidoterrestris through alternative approaches, although the effect relied upon the age of spores. In addition to the mere antimicrobial effect, some treatments could cause a sublethal injury on spores. Lemon extract was the most effective treatment for both the antimicrobial effect and the sublethal injury, as evidenced by the release of proteins, and calcium dipicolinate [dipicolinic acid (DPA)] by fresh spores and only DPA (with an exception for C8) by old spores. A sublethal injury with protein release was also found for physical treatments [US (ultrasound) or heating]. For the first time, this paper reports on the existence of a sublethal injury for A. acidoterrestris, and this evidence could also be a challenge, because injured microorganisms could restore their metabolism, or an opportunity to design new preserving treatments.

Evaluation of temperature, pH and nutrient conditions in bacterial growth and extracellular hydrolytic activities of two Alicyclobacillus spp. strains

Extremophile bacteria have developed the metabolic machinery for living in extreme temperatures, pH, and high-salt content. Two novel bacterium strains Alicyclobacillus sp. PA1 and Alicyclobacillus sp. PA2, were isolated from crater lake El Chichon in Chiapas, Mexico. Phylogenetic tree analysis based on the 16SrRNA gene sequence revealed that the strain Alicyclobacillus sp. PA1 and Alicyclobacillus sp. PA2 were closely related to Alicyclobacillus species (98% identity and 94.73% identity, respectively). Both strains were Gram variable, and colonies were circular, smooth and creamy. Electron microscopy showed than Alicyclobacillus sp. PA1 has a daisy-like form and Alicyclobacillus sp. PA2 is a regular rod. Both strains can use diverse carbohydrates and triglycerides as carbon source and they also can use organic and inorganic nitrogen source. But, the two strains can grow without any carbon or nitrogen sources in the culture medium. Temperature, pH and nutrition condition affect bacterial growth. Maximum growth was produced at 65 °C for Alicyclobacillus sp. PA1 (0.732 DO₆₀₀) at pH 3 and Alicyclobacillus sp. PA2 (0.725 DO₆₀₀) at pH 5. Inducible extracellular extremozyme activities were determined for β-galactosidase (Alicyclobacillus sp. PA1: 88.07 ± 0.252 U/mg, Alicyclobacillus sp. PA2: 51.57 ± 0.308 U/mg), cellulose (Alicyclobacillus sp. PA1: 141.20 ± 0.585 U/mg, Alicyclobacillus sp. PA2: 51.57 ± 0.308 U/mg), lipase (Alicyclobacillus sp. PA1: 138.25 ± 0.600 U/mg, Alicyclobacillus sp. PA2: 175.75 ± 1.387 U/mg), xylanase (Alicyclobacillus sp. PA1: 174.72 ± 1.746 U/mg, Alicyclobacillus sp. PA2: 172.69 ± 0.855U/mg), and protease (Alicyclobacillus sp. PA1: 15.12 ± 0.121 U/mg, Alicyclobacillus sp. PA2: 15.33 ± 0.284 U/mg). These results provide new insights on extreme enzymatic production on Alicyclobacillus species.

Inactivation Effect of Thymoquinone on Alicyclobacillus acidoterrestris Vegetative Cells, Spores, and Biofilms

Alicyclobacillus acidoterrestris (A. acidoterrestris), a spore-forming bacterium, has become a main challenge and concern for the juices and acid beverage industry across the world due to its thermo-acidophilic characteristic. Thymoquinone (TQ) is one of the active components derived from Nigella sativa seeds. The objective of this study was to investigate antibacterial activity and associated molecular mechanism of TQ against A. acidoterrestris vegetative cells, and to evaluate effects of TQ on A. acidoterrestris spores and biofilms formed on polystyrene and stainless steel surfaces. Minimum inhibitory concentrations of TQ against five tested A. acidoterrestris strains ranged from 32 to 64 μg/mL. TQ could destroy bacterial cell morphology and membrane integrity in a concentration-dependent manner. Field-emission scanning electron microscopy observation showed that TQ caused abnormal morphology of spores and thus exerted a killing effect on spores. Moreover, TQ was effective in inactivating and removing A. acidoterrestris mature biofilms. These findings indicated that TQ is promising as a new alternative to control A. acidoterrestris and thereby reduce associated contamination and deterioration in the juice and acid beverage industry.

One-pot synthesis of magnetic self-assembled carrageenan-ε-polylysine composites: A reusable and effective antibacterial agent against Alicyclobacillus acidoterrestris

Epsilon-polylysine (EPL) is a cationic polymer with broad antibacterial activity. Base on this property, anionic carrageenan (CG) was self-assembled with EPL and introduced into the one-pot coprecipitation process to fabricate the magnetic Fe₃O₄-CG-EPL composites. To demonstrate the successful synthesis of composites, characterization techniques including XRD, FT-IR, TEM, XPS and VSM were employed. The prepared composites exhibited effective antibacterial activity against Alicyclobacillus acidoterrestris with the minimum inhibitory concentration of 10.0 mg mL^-1. Investigations into deciphering the antibacterial mechanism demonstrated that the presence of Fe₃O₄-CG-EPL caused irreversible damage to the cell membrane and serious leakage of intracellular protein, resulting in the inactivation of bacteria. The aim of this work is to develop a new effective control method for A. acidoterrestris in food field.

Bacterial Spore Inactivation in Orange Juice and Orange Peel by Ultraviolet-C Light

Spore-forming bacteria are a great concern for fruit juice processors as they can resist the thermal pasteurization and the high hydrostatic pressure treatments that fruit juices receive during their processing, thus reducing their microbiological quality and safety. In this context, our objective was to evaluate the efficacy of Ultraviolet-C (UV-C) light at 254 nm on reducing bacterial spores of Alicyclobacillus acidoterrestris, Bacillus coagulans and Bacillus cereus at two stages of orange juice production. To simulate fruit disinfection before processing, the orange peel was artificially inoculated with each of the bacterial spores and submitted to UV-C light (97.8-100.1 W/m²) with treatment times between 3 s and 10 min. The obtained product, the orange juice, was also tested by exposing the artificially inoculated juice to UV-C light (100.9-107.9 W/m²) between 5 and 60 min. A three-minute treatment (18.0 kJ/m²) reduced spore numbers on orange peel around 2 log units, while more than 45 min (278.8 kJ/m²) were needed to achieve the same reduction in orange juice for all evaluated bacterial spores. As raw fruits are the main source of bacterial spores in fruit juices, reducing bacterial spores on fruit peels could help fruit juice processors to enhance the microbiological quality and safety of fruit juices.

Genomic Insight of Alicyclobacillus mali FL18 Isolated From an Arsenic-Rich Hot Spring

Extreme environments are excellent places to find microorganisms capable of tolerating extreme temperature, pH, salinity pressure, and elevated concentration of heavy metals and other toxic compounds. In the last decades, extremophilic microorganisms have been extensively studied since they can be applied in several fields of biotechnology along with their enzymes. In this context, the characterization of heavy metal resistance determinants in thermophilic microorganisms is the starting point for the development of new biosystems and bioprocesses for environmental monitoring and remediation. This work focuses on the isolation and the genomic exploration of a new arsenic-tolerant microorganism, classified as Alicyclobacillus mali FL18. The bacterium was isolated from a hot mud pool of the solfataric terrains in Pisciarelli, a well-known hydrothermally active zone of the Campi Flegrei volcano near Naples in Italy. A. mali FL18 showed a good tolerance to arsenite (MIC value of 41 mM), as well as to other metals such as nickel (MIC 30 mM), cobalt, and mercury (MIC 3 mM and 17 μM, respectively). Signatures of arsenic resistance genes (one arsenate reductase, one arsenite methyltransferase, and several arsenite exporters) were found interspersed in the genome as well as several multidrug resistance efflux transporters that could be involved in the export of drugs and heavy metal ions. Moreover, the strain showed a high resistance to bacitracin and ciprofloxacin, suggesting that the extreme environment has positively selected multiple resistances to different toxic compounds. This work provides, for the first time, insights into the heavy metal tolerance and antibiotic susceptibility of an Alicyclobacillus strain and highlights its putative molecular determinants.

Assessment of Spoilage Potential Posed by Alicyclobacillus spp. in Plant-Based Dairy Beverages Mixed with Fruit Juices during Storage

ABSTRACT

The scope of the present study was to assess the spoilage potential of different Alicyclobacillus spp. in commercial pasteurized (ambient-stable) plant-based dairy beverages mixed with fruit juices at different inoculation levels and storage temperatures. Different products (coconut and berry [CB]; almond, mango, and passionfruit [AMP]; and oat, strawberry, and banana [OSB]) were inoculated with 10 or 2 × 103 spores per mL of either Alicyclobacillus acidoterrestris, Alicyclobacillus fastidiosus, or Alicyclobacillus acidocaldarius strain composites, whereas noninoculated samples served as controls. Samples inoculated with A. acidoterrestris and A. fastidiosus were stored at 30 and 45°C, whereas A. acidocaldarius storage took place at 50°C for 240 days. Gas composition, Alicyclobacillus spp. populations, total viable counts, pH, water activity, color, and guaiacol off-taste were monitored. CB and AMP supported growth of A. acidoterrestris and A. fastidiosus, reaching populations of 4.0 to 5.0 log CFU/mL. In OSB, populations of A. fastidiosus remained close to the initial inoculation levels during storage at 30°C, whereas at 45°C, the populations declined <1 CFU/mL. A. acidocaldarius growth was supported in CB samples, but not in AMP and OSB samples, reaching ca. 3.0 log CFU/mL at 50°C, regardless of initial inoculum size. Total color change was increased during storage; however, the instrumentally recorded color changes were not macroscopically visible. Spoilage in terms of guaiacol off-taste was identified only in CB and AMP samples inoculated with A. acidoterrestris after 60 days at 30 and 45°C. The increased popularity of these products along with the scarcity of existing literature related to their spoilage by Alicyclobacillus spp., render the contribution of the findings and data of present study critical for assessing the significance of Alicyclobacillus spp. as a potential spoilage hazard in these products and for assisting in the design and implementation of effective mitigation strategies by the beverage industry.

HIGHLIGHTS

Targeting the vanillic acid decarboxylase gene for Alicyclobacillus acidoterrestris quantification and guaiacol assessment in apple juices using real time PCR

Alicyclobacillus spp. has recently received much attention due to its implication in the spoilage of pasteurized fruit juices, which is characterized by the formation of guaiacol. Previous researches indicate that not all Alicyclobacillus spp. are able to produce guaiacol. The aim of this study was to identify possible differences in the vanillic acid decarboxylase gene involved in guaiacol biosynthesis and then develop specific detection methods for guaiacol producing Alicyclobacillus. Agarose gel electrophoresis results showed that the partial vdcC gene was present in all the guaiacol producing Alicyclobacillus, but absent in non-guaicaol producing strains apart from A. fastidiosus DSM 17978. On the basis of the vdcC gene sequence, a primer pair specific to A. acidoterrestris was designed; then a SYBR Green I real time PCR was established for the direct quantification of A. acidoterrestris in apple juice, and the detection limit was 2.6 × 10¹ CFU/mL. The developed real time PCR system was used to detect A. acidoterrestris in 36 artificially contaminated apple juice samples and guaiacol production in the sample was also analyzed by GC-MS. The Gompertz model was employed to describe the relationship between A. acidoterrestris cell concentration and guaiacol content, and the value of R² was 0.854. This work provides an alternative to conventional methods of guaiacol quantification and A. acidoterrestris detection and could be very useful for the early recognition of A. acidoterrestris contamination in fruit juices.