The Response and Survival Mechanisms of Staphylococcus aureus under High Salinity Stress in Salted Foods

Intracellular and Extracellular Metabolic Response of the Lactic Acid Bacterium Weissella confusa Under Salt Stress

BACKGROUND

Weissella confusa is a member of the lactic acid bacterium group commonly found in many salt-fermented foods. Strains of W. confusa isolated from high-salinity environments have been shown to tolerate salt stress to some extent. However, the specific responses and mechanisms of W. confusa under salt stress are not fully understood.

METHODS

To study the effect of NaCl stress on W. confusa, growth performance and metabolite profiles of the strains were compared between a NaCl-free group and a 35% NaCl-treated group. Growth performance was assessed by measuring viable cell counts and examining the cells using scanning electron microscopy (SEM). Intracellular and extracellular metabolites were analyzed by non-targeted metabolomics based on liquid chromatography-mass spectrometry (LC-MS).

RESULTS

It was found that the viable cell count of W. confusa decreased with increasing salinity, and cells could survive even in saturated saline (35%) medium for 24 h. When exposed to 35% NaCl, W. confusa cells exhibited surface pores and protein leakage. Based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, 42 different metabolites were identified in the cells and 18 different metabolites in the culture medium. These different metabolites were mainly involved in amino acid metabolism, carbohydrate metabolism, and nucleotide metabolism. In addition, salt-exposed cells exhibited higher levels of intracellular ectoine and lactose, whose precursors, such as aspartate, L-2,4-diaminobutanoate, and galactinol, were reduced in the culture medium.

CONCLUSIONS

This study provides insight into the metabolic responses of W. confusa under salt stress, revealing its ability to maintain viability and alter metabolism in response to high NaCl concentrations. Key metabolites such as ectoine and lactose, as well as changes in amino acid and nucleotide metabolism, may contribute to its tolerance to salt. These findings may improve our understanding of the bacterium's survival mechanisms and have potential applications in food fermentation and biotechnology.

Antibiofilm Effects of Modifying Polyvinylidene Fluoride Membranes with Polyethylenimine, Poly(acrylic acid) and Graphene Oxide

Biofouling in membrane filtration systems poses significant operational challenges, leading to decreased permeate flux. The aim of this work was to study the anti-biofilm properties of new nanofiltration membranes produced via layer-by-layer, LBL, assembly by coating a polyvinylidene fluoride (PVDF) support with a polyethylenimine (PEI) and poly(acrylic acid)/graphene oxide (PAA-GO) mixture. The membranes were characterized according to contact angle, scanning electron microscopy (SEM), atomic force microscopy and their Z-potential. Biofilm quantification and characterization were carried out using crystal violet staining and SEM, while bacterial viability was assessed by using colony-forming units. The membrane with three bilayers ((PAA-PEI)3/PVDF) showed a roughness of 77.78 nm. The incorporation of GO ((GO/PAA-PEI)3/PVDF) produced a membrane with a smoother surface (roughness of 26.92 nm) and showed salt rejections of 16% and 68% for NaCl and Na2SO4, respectively. A significant reduction, ranging from 82.37 to 77.30%, in biofilm formation produced by S. aureus and E. coli were observed on modified membranes. Additionally, the bacterial viability on the modified membranes was markedly reduced (67.42-99.98%). Our results show that the modified membranes exhibited both antibiofilm and antimicrobial capacities, suggesting that these properties mainly depend on the properties of the modifying agents, as the initial adherence on the membrane surface was not totally suppressed, but the proliferation and formation of EPSs were prevented.

Identification of novel genetic factors that regulate c-di-AMP production in Staphylococcus aureus using a riboswitch-based biosensor

Nucleotide secondary messengers regulate various processes in bacteria allowing them to rapidly respond to changes in environmental conditions. c-di-AMP is an essential second messenger required for the growth of the human pathogen Staphylococcus aureus, regulating potassium, osmolyte uptake, and beta-lactam resistance. Cellular concentrations of c-di-AMP are regulated by the activities of two enzymes, DacA and GdpP, which synthesize and hydrolyze c-di-AMP, respectively. Besides these, only a limited number of other factors are known to regulate c-di-AMP levels. Using a c-di-AMP biosensor consisting of the Bacillus subtilis c-di-AMP-binding kimA riboswitch and yfp, we were able to efficiently detect differences in cellular c-di-AMP levels in S. aureus. To identify novel factors that regulate c-di-AMP levels, we introduced the biosensor into a library of S. aureus transposon mutants. In this manner, we obtained mutants with increased c-di-AMP levels that contained insertions in gdpP coding for the c-di-AMP hydrolase and ybbR (cdaR) coding for a c-di-AMP cyclase regulator, thus validating our screen. We also identified two high c-di-AMP mutants with insertions upstream of the nrdIEF operon coding for the ribonucleotide reductase enzyme. Further analysis revealed that the insertion down-regulated nrdIEF expression, indicating that the enzyme is a negative regulator of c-di-AMP production. This negative regulation was dependent on rsh, encoding for the synthase of the endogenous GdpP inhibitor (p)ppGpp. The methods established in this work can be readily adapted for use in other bacteria to uncover genetic or environmental factors regulating c-di-AMP levels.IMPORTANCEc-di-AMP is an important secondary messenger, produced by many bacterial species including the opportunistic pathogen Staphylococcus aureus. In this bacterium, c-di-AMP controls cell wall homeostasis, cell size, and osmotic balance. In addition, it has been shown that strains with high c-di-AMP levels exhibit increased resistance to beta-lactam antibiotics. Here, we developed a biosensor-based method for the rapid detection of c-di-AMP levels in S. aureus. We utilized the biosensor in a genetic screen for the identification of novel factors that impact cellular c-di-AMP. In this manner, we identified the ribonucleotide reductase as a novel factor altering cellular c-di-AMP levels and showed that reducing its expression leads to increased cellular c-di-AMP levels. As methicillin-resistant S. aureus strains are considered as a global health threat, it is important to study processes that dictate cellular c-di-AMP levels, which are associated with antibiotic resistance.

Thiourea-based rotaxanes: anion transport across synthetic lipid bilayers and antibacterial activity against Staphylococcus aureus

We report the synthesis of two rotaxanes (1 and 2) whose rings have appended thiourea units for the selective recognition of Cl- anions. Rotaxane 1 transports Cl- across synthetic lipid bilayers more efficiently than 2, exhibiting EC50 values of 0.243 mol% versus 0.736 mol%, respectively. A control rotaxane (3) without the thiourea units and the individual axle (4) also showed Cl- transport, although with much lower efficiency (EC50 values of 4.044 mol% and 4.986 mol%). The unthreaded ring (5) showed the lowest transport activity. This trend highlights the advantage of the interlocked system with a ring containing thiourea units. We also investigated how the membrane composition of liposomes influences the transport activity of 1 and 2, observing higher Cl- transport in membranes with higher fluidity. Additionally, we demonstrated that rotaxane 1 can kill drug-resistant and osmotolerant Staphylococcus aureus when used in combination with NaCl or arachidonic acid. The latter is known to increase the fluidity of the membrane in S. aureus, highlighting cooperative behavior. This work provides new insights into how various structural features and the membrane environment influence the anion transport activity of rotaxanes, offering important design principles for optimizing future rotaxanes for biomedical and other applications.

The unusual mode of action of the polyketide glycoside antibiotic cervimycin C

Cervimycins A-D are bis-glycosylated polyketide antibiotics produced by Streptomyces tendae HKI 0179 with bactericidal activity against Gram-positive bacteria. In this study, cervimycin C (CmC) treatment caused a spaghetti-like phenotype in Bacillus subtilis 168, with elongated curved cells, which stayed joined after cell division, and exhibited a chromosome segregation defect, resulting in ghost cells without DNA. Electron microscopy of CmC-treated Staphylococcus aureus (3 × MIC) revealed swollen cells, misshapen septa, cell wall thickening, and a rough cell wall surface. Incorporation tests in B. subtilis indicated an effect on DNA biosynthesis at high cervimycin concentrations. Indeed, artificial downregulation of the DNA gyrase subunit B gene (gyrB) increased the activity of cervimycin in agar diffusion tests, and, in high concentrations (starting at 62.5 × MIC), the antibiotic inhibited S. aureus DNA gyrase supercoiling activity in vitro. To obtain a more global view on the mode of action of CmC, transcriptomics and proteomics of cervimycin treated versus untreated S. aureus cells were performed. Interestingly, 3 × MIC of cervimycin did not induce characteristic responses, which would indicate disturbance of the DNA gyrase activity in vivo. Instead, cervimycin induced the expression of the CtsR/HrcA heat shock operon and the expression of autolysins, exhibiting similarity to the ribosome-targeting antibiotic gentamicin. In summary, we identified the DNA gyrase as a target, but at low concentrations, electron microscopy and omics data revealed a more complex mode of action of cervimycin, which comprised induction of the heat shock response, indicating protein stress in the cell.IMPORTANCEAntibiotic resistance of Gram-positive bacteria is an emerging problem in modern medicine, and new antibiotics with novel modes of action are urgently needed. Secondary metabolites from Streptomyces species are an important source of antibiotics, like the cervimycin complex produced by Streptomyces tendae HKI 0179. The phenotypic response of Bacillus subtilis and Staphylococcus aureus toward cervimycin C indicated a chromosome segregation and septum formation defect. This effect was at first attributed to an interaction between cervimycin C and the DNA gyrase. However, omics data of cervimycin treated versus untreated S. aureus cells indicated a different mode of action, because the stress response did not include the SOS response but resembled the response toward antibiotics that induce mistranslation or premature chain termination and cause protein stress. In summary, these results point toward a possibly novel mechanism that generates protein stress in the cells and subsequently leads to defects in cell and chromosome segregation.

Interplay of xenobiotic-degrading and antibiotic-resistant microorganisms among the microbiome found in the air, handrail, and floor of the subway station

Investigating the quality of the subway environment, especially regarding antibiotic resistance genes (ARGs) and xenobiotics, conveys ecological and health impacts. In this study, compositions and relations of microorganisms harboring ARGs and xenobiotic degradation and metabolism genes (XDGs) in the Sukhumvit subway station (MRT-SKV) in Bangkok was assessed by analyzing the taxonomic and genetic diversity of the microbiome in the air and on the surfaces of floor and handrail. The major bacteria in the MRT-SKV (including Moraxella, which was abundant in the bioaerosol and handrail samples, and Staphylococcus, which was abundant in the bioaerosol samples) were found to contain both ARGs and XDGs. The co-abundance correlation network revealed notable relationships among bacteria harboring antibiotic resistance genes (ARGs) and xenobiotic degradation genes (XDGs). Significant associations were observed between ARGs linked to glycopeptide and fluoroquinolone resistance and genes associated with benzoate, styrene, and atrazine degradation pathways, as well as between ARGs related to cephamycin, cephalosporin, and MLS resistance and XDGs associated with the cytochrome P450-dependent drug metabolism pathway. These correlations suggested that selective pressure exerted by certain xenobiotics and antibiotics can simultaneously affect both ARGs and XDGs in the environment and should favor correlations and co-survival among ARG- and XDG-containing bacteria in the environments. The correlations may occur via shared mechanisms of resistance to both xenobiotics and antibiotics. Finally, different correlation pairs were seen in different niches (air, handrail, floor) of the subway environment or different geolocations. Thus, the relationship between ARG and XDG pairs most likely depends on the unique characteristics of the niches and on the prominent types of xenobiotics and antibiotics in the subway environment. The results indicated that interactions and connections between microbial communities can impact how they function. These microorganisms can have profound effects on accumulation of xenobiotics and ARGs in the MRT-SKV.

Motility behavior and physiological response mechanisms of aerobic denitrifier, Enterobacter cloacae strain HNR under high salt stress: Insights from individual cells to populations

The motility behaviors at the individual-cell level and the collective physiological responsive behaviors of aerobic denitrifier, Enterobacter cloacae strain HNR under high salt stress were investigated. The results revealed that as salinity increased, electron transport activity and adenosine triphosphate content decreased from 15.75 μg O2/g/min and 593.51 mM/L to 3.27 μg O2/g/min and 5.34 mM/L, respectively, at 40 g/L, leading to a reduction in the rotation velocity and vibration amplitude of strain HNR. High salinity stress (40 g/L) down-regulated genes involved in ABC transporters (amino acids, sugars, metal ions, and inorganic ions) and activated the biofilm-related motility regulation mechanism in strain HNR, resulting in a further decrease in flagellar motility capacity and an increase in extracellular polymeric substances secretion (4.08 mg/g cell of PS and 40.03 mg/g cell of PN at 40 g/L). These responses facilitated biofilm formation and proved effective in countering elevated salt stress in strain HNR. Moreover, the genetic diversity associated with biofilm-related motility regulation in strain HNR enhanced the adaptability and stability of the strain HNR populations to salinity stress. This study enables a deeper understanding of the response mechanism of aerobic denitrifiers to high salt stress.

Environmental processes and health implications potentially mediated by dust-borne bacteria

Understanding microbial migration and survival mechanisms in dust events (DEs) can elucidate genetic and metabolic exchange between environments and help predict the atmospheric pathways of ecological and health-related microbial stressors. Dust-borne microbial communities have been previously characterized, but the impact and interactions between potentially active bacteria within transported communities remain limited. Here, we analysed samples collected during DEs in Israel, using amplicon sequencing of the 16S rRNA genes and transcripts. Different air trajectories and wind speeds were associated not only with the genomic microbial community composition variations but also with specific 16S rRNA bacterial transcripts. Potentially active dust-borne bacteria exhibited positive interactions, including carbon and nitrogen cycling, biotransformation of heavy metals, degradation of organic compounds, biofilm formation, and the presence of pathogenic taxa. This study provides insights into the potential interactive relationships and survival strategies of microorganisms within the extreme dust environment.

Pseudogenomic insights into the evolution of Mycobacterium ulcerans

BACKGROUND

Buruli ulcer (BU) disease, caused by Mycobacterium ulcerans (MU), and characterized by necrotic ulcers is still a health problem in Africa and Australia. The genome of the bacterium has several pseudogenes due to recent evolutionary events and environmental pressures. Pseudogenes are genetic elements regarded as nonessential in bacteria, however, they are less studied due to limited available tools to provide understanding of their evolution and roles in MU pathogenicity.

RESULTS

This study developed a bioinformatic pipeline to profile the pseudogenomes of sequenced MU clinical isolates from different countries. One hundred and seventy-two MU genomes analyzed revealed that pseudogenomes of African strains corresponded to the two African lineages 1 and 2. Pseudogenomes were lineage and location specific and African lineage 1 was further divided into A and B. Lineage 2 had less relaxation in positive selection than lineage 1 which may signify different evolutionary points. Based on the Gil-Latorre model, African MU strains may be in the latter stages of evolutionary adaption and are adapting to an environment rich in metabolic resources with a lower temperature and decreased UV radiation. The environment fosters oxidative metabolism and MU may be less reliant on some secondary metabolites. In-house pseudogenomes from Ghana and Cote d'Ivoire were different from other African strains, however, they were identified as African strains.

CONCLUSION

Our bioinformatic pipeline provides pseudogenomic insights to complement other whole genome analyses, providing a better view of the evolution of the genome of MU and suggest an adaptation model which is important in understanding transmission. MU pseudogene profiles vary based on lineage and country, and an apparent reduction in insertion sequences used for the detection of MU which may adversely affect the sensitivity of diagnosis.

"Antibacterial effect and possible mechanism of action of 1,3,4-oxadiazole in Staphylococcus aureus"

Staphylococcus aureus is one of the main etiological agents causing foodborne diseases, and the development of new antibacterial agents is urgent. This study evaluated the antibacterial activity and the possible mechanism of action of the 1,3,4-oxadiazole LMM6 against S. aureus. The minimum inhibitory concentration (MIC) of LMM6 ranged from 1.95 to 7.81 µg ml-1. The time-kill assay showed that 48-h treatment at 1× to 8× MIC reduced S. aureus by 4 log colony forming unit (CFU), indicating a bacteriostatic effect. Regarding the possible mechanism of action of LMM6, there was accumulation of reactive oxygen species (ROS) and an increase in the absorption of crystal violet (∼50%) by the cells treated with LMM6 at 1× and 2× MIC for 6-12 h. In addition, there was increased propidium iodide uptake (∼84%) after exposure to LMM6 for 12 h at 2× MIC. After 48 h of treatment, 100% of bacteria had been injured. Scanning electron microscopy observations demonstrated that LMM6-treated cells were smaller compared with the untreated group. LMM6 exhibited bacteriostatic activity and its mechanism of action involves increase of intracellular ROS and disturbance of the cell membrane, which can be considered a key target for controlling the growth of S. aureus.

A South African Perspective on the Microbiological and Chemical Quality of Meat: Plausible Public Health Implications

Meat comprises proteins, fats, vitamins, and trace elements, essential nutrients for the growth and development of the body. The increased demand for meat necessitates the use of antibiotics in intensive farming to sustain and raise productivity. However, the high water activity, the neutral pH, and the high protein content of meat create a favourable milieu for the growth and the persistence of bacteria. Meat serves as a portal for the spread of foodborne diseases. This occurs because of contamination. This review presents information on animal farming in South Africa, the microbial and chemical contamination of meat, and the consequential effects on public health. In South Africa, the sales of meat can be operated both formally and informally. Meat becomes exposed to contamination with different categories of microbes, originating from varying sources during preparation, processing, packaging, storage, and serving to consumers. Apparently, meat harbours diverse pathogenic microorganisms and antibiotic residues alongside the occurrence of drug resistance in zoonotic pathogens, due to the improper use of antibiotics during farming. Different findings obtained across the country showed variations in prevalence of bacteria and multidrug-resistant bacteria studied, which could be explained by the differences in the manufacturer practices, handling processes from producers to consumers, and the success of the hygienic measures employed during production. Furthermore, variation in the socioeconomic and political factors and differences in bacterial strains, geographical area, time, climatic factors, etc. could be responsible for the discrepancy in the level of antibiotic resistance between the provinces. Bacteria identified in meat including Escherichia coli, Listeria monocytogenes, Staphylococcus aureus, Campylobacter spp., Salmonella spp., etc. are incriminated as pathogenic agents causing serious infections in human and their drug-resistant counterparts can cause prolonged infection plus long hospital stays, increased mortality and morbidity as well as huge socioeconomic burden and even death. Therefore, uncooked meat or improperly cooked meat consumed by the population serves as a risk to human health.

CodY: An Essential Transcriptional Regulator Involved in Environmental Stress Tolerance in Foodborne Staphylococcus aureus RMSA24

Staphylococcus aureus (S. aureus), as the main pathogen in milk and dairy products, usually causes intoxication with vomiting and various kinds of inflammation after entering the human body. CodY, an important transcriptional regulator in S. aureus, plays an important role in regulating metabolism, growth, and virulence. However, little is known about the role of CodY on environmental stress tolerance. In this research, we revealed the role of CodY in environmental stress tolerance in foodborne S. aureus RMSA24. codY mutation significantly reduced the tolerance of S. aureus to desiccation and oxidative, salt, and high-temperature stresses. However, S. aureus was more tolerant to low temperature stress due to mutation of codY. We found that the expressions of two important heat shock proteins-GroEL and DanJ-were significantly down-regulated in the mutant codY. This suggests that CodY may indirectly regulate the high- and low-temperature tolerance of S. aureus by regulating the expressions of groEL and danJ. This study reveals a new mechanism of environmental stress tolerance in S. aureus and provides new insights into controlling the contamination and harm caused by S. aureus in the food industry.

Impact of the Food-Related Stress Conditions on the Expression of Enterotoxin Genes among Staphylococcus aureus

Staphylococcus aureus is one of the most important foodborne pathogens. S. aureus has the capability to produce a variety of toxins, including staphylococcal enterotoxins (SEs). The aim of this study was to evaluate the survival rate of S. aureus cells and analyze enterotoxins gene expression after exposure to osmotic stress and acidic/alkaline stress. To determine survival rates, the traditional plate counting method and flow cytometry were used. The expression levels of the enterotoxin genes were performed by quantitative reverse transcription PCR (RT-qPCR). Expression changes differed depending on the stressors chosen. The obtained results in this study showed the effect of critical food-related stress conditions on SE gene expression in S. aureus. The study showed different expression levels of the tested enterotoxins genes depending on the stress. The most tested enterotoxin genes (seg, sei, and selo) after exposure to pH = 4.5 stress have similar expression as in the optimal condition. After alkaline treatment (pH = 9.6), a similar expression gene value as for the optimal condition was observed. The analysis of gene expression in response to stress caused by NaCl, showed that the expression of selp decreased, whereas selu, selm, and selo genes increased. A significantly decreased expression of the sea gene was observed.

Shelf Life and Safety of Vacuum Packed HPP-Treated Soaked Cod Fillets: Effects of Salt Content and Multilayer Plastic Film

High microbiological quality standards, food safety, and environmental sustainability represent crucial topics in food production chains. For this reason, fish industries, which import salted and seasoned fish products from supplier countries, i.e., Norway, Denmark, USA (Alaska State), etc., have tried to reduce the salt content of each carton during transportation (reducing carbon emissions and the weight of major quantities of transported fish). In the present study, 360 differently processed fish fillet samples, belonging to the species Gadus macrocephalus caught in FAO zone 67, were microbiologically and chemically screened. This study aimed to provide original data concerning the applicability of sustainable solutions investigating the combined effects of salt content reduction combined with new recyclable multilayer plastic film packaging (vacuum skin packaging with two different oxygen transmission rate values). The microbiological results showed no substantial changes comparing the two differently salted products, highlighting their high hygienic characteristics which were also observed in their chemical analysis. The shelf life evolutions (comparing the two different studied plastic films) highlighted that, after 35 days from HPP treatments, bacterial loads gained high values, over 6 log cfu/g. This study highlights that, compared to the currently used plastic films, the results of the new and sustainable multilayer plastic films show that they can provide safe food matrices in combination with HPP technologies. Therefore, this preliminary investigation brings closer attention to alternative and environmentally sustainable production systems with their designs based on the multidisciplinary approach of food production systems.

Bactericidal Efficacy of Nanostructured Surfaces Increases under Flow Conditions

Bacterial colonization on solid surfaces creates enormous problems across various industries causing billions of dollars' worth of economic damages and costing human lives. Biomimicking nanostructured surfaces have demonstrated a promising future in mitigating bacterial colonization and related issues. The importance of this non-chemical method has been elevated due to bacterial evolvement into antibiotic and antiseptic-resistant strains. However, bacterial attachment and viability on nanostructured surfaces under fluid flow conditions has not been investigated thoroughly. In this study, attachment and viability of Pseudomonas aeruginosa (P. aeruginosa) and Staphylococcus aureus (S. aureus) on a model nanostructured surface were studied under fluid flow conditions. A wide range of flow rates resulting in a broad spectrum of fluid wall shear stress on a nanostructured surface representing various application conditions were experimentally investigated. The bacterial suspension was pumped through a custom-designed microfluidic device (MFD) that contains a sterile Ti-6Al-4V substrate. The surface of the titanium substrate was modified using a hydrothermal synthesis process to fabricate the nanowire structure on the surface. The results of the current study show that the fluid flow significantly reduces bacterial adhesion onto nanostructured surfaces and significantly reduces the viability of adherent cells. Interestingly, the bactericidal efficacy of the nanostructured surface was increased under the flow by ∼1.5-fold against P. aeruginosa and ∼3-fold against S. aureus under static conditions. The bactericidal efficacy had no dependency on the fluid wall shear stress level. However, trends in the dead-cell count with the fluid wall shear were slightly different between the two species. These findings will be highly useful in developing and optimizing nanostructures in the laboratory as well as translating them into successful industrial applications. These findings may be used to develop antibacterial surfaces on biomedical equipment such as catheters and vascular stents or industrial applications such as ship hulls and pipelines where bacterial colonization is a great challenge.