Coagulase-negative Staphylococci favor conversion of arginine into ornithine despite a widespread genetic potential for nitric oxide synthase activity

Enhancement of colour formation of fermented sausages by overexpression of nitric oxide synthase in Staphylococcus vitulinus under hydrogen peroxide stress

This study used hydrogen peroxide (H2O2) treatment to overexpress the gene of nitric oxide synthase (nos) in Staphylococcus vitulinus, which was then inoculated into fermented sausages to observe its effect on colour development. The results showed that a low concentration of H2O2 (50 mM) could up-regulate the expression of nos by increasing the oxidative stress level of S. vitulinus. At 2 h after treatment, the expression of nos in S. vitulinus was the highest (P < 0.05), and the relative enzyme activity was increased to about 1.5 times that of the untreated. The growth of S. vitulinus was not substantially affected by 50-mM H2O2 treatment (P > 0.05). When H2O2-treated S. vitulinus was inoculated into fermented sausages, the content of nitrosomyoglobin was increased, and the a*-value (indicating redness) was not significantly different from that in the group treated with nitrite (P > 0.05). This study provides a potential method to enhance the ability of S. vitulinus for colourising fermented sausage by inducing the overexpression of nos.

Replacement of nitrates and nitrites in meat-derived foods through the utilization of coagulase-negative staphylococci: A review

Nitrates and nitrites, which are synthetic additives, are traditionally used as curing agents in meat-based products. These synthetic additives are employed in the preparation of fermented meat foods to improve quality characteristics and microbiological safety, develop distinct flavours and red-colour stability, and counteract lipid oxidation. Nitrites also display significant bacteriostatic and bactericidal action against spoilage microorganisms and foodborne pathogens (such as Clostridium botulinum and Listeria monocytogenes). However, meat curing is currently under scrutiny because of its links to cardiovascular diseases and colorectal cancer. Based on the current literature, this review provides recent scientific evidence on the potential utilisation of coagulase-negative staphylococci (CNS) as nitrate and nitrite substitutes in meat-based foods. Indeed, CNS are reported to reproduce the characteristic red pigmentation and maintain the typical high-quality traits of cured-meats, thanks to their arginine degradation pathway, thus providing the nitrite-related desirable attributes in cured meat. The alternative strategy, still based on the NOS pathway, consisting of supplementing meat with arginine to release nitric oxide (NO) and obtain a meat characterised by the desired pinkish-red colour, is also reviewed. Exploiting NOS-positive CNS strains seems particularly challenging because of CNS technological adaptation and the oxygen dependency of the NOS reaction; however, this exploitation could represent a turning point in replacing nitrates and nitrites in meat foods.

The mechanism of Leuconostoc mesenteroides subsp. IMAU:80679 in improving meat color: Myoglobin oxidation inhibition and myoglobin derivatives formation based on multi enzyme-like activities

The Leuconostoc mesenteroides subsp. IMAU:80679 (LM) was chosen for its superior capability in enhancing redness, and was incubated in a broth system containing metmyoglobin (MetMb) to investigate its mechanisms for color improvement. The a* value of LM group reached its highest level of 52.75 ± 1.04 at 24 h, significantly higher than control of 19.75 ± 0.6 (p < 0.05). The addition of LM could inhibit myoglobin oxidation to some extent. Meanwhile, higher content of nitrosylmyoglobin (NOMb) and Zn-protoporphyrin (Znpp) were observed in LM samples during the whole incubation period. Furthermore, enzymatic activity and encoded genes related to MetMb reduction and pigment formation were determined to explain its possible mechanism on color enhancement. Finally, by extracting crude enzymes and adding them to meat batters, the redness of crude enzyme group was comparable to that achieved with 20 ppm nitrite, providing a potential method on compensating for nitrite/nitrate substitution in meat products.

Bacterial nitric oxide synthase in colorizing meat products: Current development and future directions

Nitrite has been widely used in meat products for its abilities including color formation, antimicrobial properties, flavor formation and preventing lipid oxidation. However, the possible generation of N-nitrosamines through reaction of nitrite with secondary amines arises many concerns in the usage of nitrite. For a long time, nitrite substitution is unsettled issue in the meat industry. Many attempts have been tried, however, the alternative solutions are often ephemeral and palliative. In recent years, bacterial nitric oxide synthase (bNOS) has received attention for its critical roles, especially in reddening meat products. This comprehensive background study summarizes the application of bNOS in colorizing meat products, its functions in bacteria, and methods of regulating the bNOS pathway. Based on this information, some strategies for promoting the nitric oxide yield for effectively substituting nitrite are presented, such as changing the environmental conditions for bacterial survival and adding substrate. Thus, bNOS is a promising nitrite substitute for color formation, and further research on its other roles in meat needs to be carried out to obtain the complete picture.

Metabolic framework of spontaneous and synthetic sourdough metacommunities to reveal microbial players responsible for resilience and performance

BACKGROUND

In nature, microbial communities undergo changes in composition that threaten their resiliency. Here, we interrogated sourdough, a natural cereal-fermenting metacommunity, as a dynamic ecosystem in which players are subjected to continuous environmental and spatiotemporal stimuli.

RESULTS

The inspection of spontaneous sourdough metagenomes and transcriptomes revealed dominant, subdominant and satellite players that are engaged in different functional pathways. The highest microbial richness was associated with the highest number of gene copies per pathway. Based on meta-omics data collected from 8 spontaneous sourdoughs and their identified microbiota, we de novo reconstructed a synthetic microbial community SDG. We also reconstructed SMC-SD43 from scratch using the microbial composition of its spontaneous sourdough equivalent for comparison. The KEGG number of dominant players in the SDG was not affected by depletion of a single player, whereas the subdominant and satellite species fluctuated, revealing unique contributions. Compared to SMC-SD43, SDG exhibited broader transcriptome redundancy. The invariant volatilome profile of SDG after in situ long-term back slopping revealed its stability. In contrast, SMC-SD43 lost many taxon members. Dominant, subdominant and satellite players together ensured gene and transcript redundancy.

CONCLUSIONS

Our study demonstrates how, by starting from spontaneous sourdoughs and reconstructing these communities synthetically, it was possible to unravel the metabolic contributions of individual players. For resilience and good performance, the sourdough metacommunity must include dominant, subdominant and satellite players, which together ensure gene and transcript redundancy. Overall, our study changes the paradigm and introduces theoretical foundations for directing food fermentations. Video Abstract.

Potential of Bacteria from Alternative Fermented Foods as Starter Cultures for the Production of Wheat Sourdoughs

Microbial strains for starter culture-initiated sourdough productions are commonly isolated from a fermenting flour-water mixture. Yet, starter culture strains isolated from matrices other than sourdoughs could provide the dough with interesting metabolic properties and hence change the organoleptic properties of the concomitant breads. Furthermore, the selection of sourdough starter cultures does not need to be limited to lactic acid bacteria (LAB), as other food-grade microorganisms are sometimes found in sourdoughs. Therefore, different strains belonging to LAB, acetic acid bacteria (AAB), and coagulase-negative staphylococci (CNS) that originated from different fermented food matrices (fermenting cocoa pulp-bean mass, fermented sausage, and water kefir), were examined as to their prevalence in a wheat sourdough ecosystem during 72-h fermentations. Limosilactobacillus fermentum IMDO 222 (fermented cocoa pulp-bean mass isolate) and Latilactobacillus sakei CTC 494 (fermented sausage isolate) seemed to be promising candidates as sourdough starter culture strains, as were the AAB strains Acetobacter pasteurianus IMDO 386B and Gluconobacter oxydans IMDO A845 (both isolated from fermented cocoa pulp-bean mass), due to their competitiveness in the wheat flour-water mixtures. Wheat breads made with G. oxydans IMDO A845 sourdoughs were significantly darker than reference wheat breads.

Nitrosylmyoglobin formation in meat by Lactobacillus fermentum AS1.1880 is due to its nitric oxide synthase activity

The objective of this study was to clarify whether formation of nitrosylmyoglobin (MbFe^IINO) by Lactobacillus fermentum AS1.1880 in meat is due to nitric oxide synthase (NOS) activity. Confocal laser scanning microscopy exhibited strong green fluorescence in the L. fermentum sample treated with a nitric oxide (NO)-specific probe, directly indicating that NO was produced. Furthermore, determination of NOS activity based on the presence of NO metabolites indicated the existence of NOS in L.fermentum. A NOS inhibitor, N^G-nitro-L-arginine methyl ester, significantly inhibited the activity of NOS in L.fermentum (P < 0.05). Futhermore, NOS protein was detected in L.fermentum by Western blot analysis. L-arginine addition largely increased the NOS activity of L.fermentum (P < 0.05). In meat batters, the redness of a sample inoculated with L.fermentum was higher than that of the control and colour was significantly improved with the addition of L-arginine (P < 0.05), indicating that more MbFe^IINO was formed.

Exploring the Ambiguous Status of Coagulase-Negative Staphylococci in the Biosafety of Fermented Meats: The Case of Antibacterial Activity Versus Biogenic Amine Formation

A total of 332 staphylococcal strains, mainly isolated from meat, were screened for antibacterial activity. Eighteen strains exhibited antibacterial activity towards species within the same genus. These antibacterial strains were further screened against Clostridium botulinum, to assess their potential as anticlostridial starter cultures for the development of fermented meat products without added nitrate or nitrite. Only Staphylococcus sciuri IMDO-S72 had the ability to inhibit all clostridial strains tested, whilst displaying additional activity against Bacillus cereus, Listeria monocytogenes and Staphylococcus aureus. Apart from their potential as bioprotective cultures, the staphylococcal collection was also screened for biogenic amine production, as these compounds may compromise food quality. To this end, ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS) was applied. A low incidence of biogenic amine production was found, with tyramine and β-phenylethylamine being the most prevalent ones. Concentrations remained relatively low (< 52 mg/L) after a prolonged incubation period, posing no or little threat towards food safety. Taken together, S. sciuri IMDO-S72 could serve as an interesting candidate for the bioprotection of fermented meats as it showed promising antibacterial activity as well as absence of biogenic amine production.

Theoretical basis of nitrosomyoglobin formation in a dry sausage model by coagulase-negative staphylococci: Behavior and expression of nitric oxide synthase

Three coagulase-negative staphylococci (CNS) species were investigated for gene expression of nitric oxide synthase (NOS) and the ability of nitrosomyoglobin (NO-Mb) formation in a dry sausage model without nitrite addition. The expression of nos gene was systematically proven from DNA to RNA to protein, and nitric oxide (NO) generation was also directly detected. In the dry sausage model system, the redness (a*-values) of samples inoculated with the three CNS species were higher than those inoculated with Pediococcus pentosaceus and the control (P < 0.05). The results from UV-vis and electron spin resonance spectroscopies revealed that pentacoordinate NO-Mb was formed in the sausages with either CNS or nitrite added. The sausage inoculated with Staphylococcus vitulinus had the highest NO-Mb content among the CNS-treated sausages. Dimer interface residues and phosphorylation sites of NOS in . itulinus differ from the other two CNS species as revealed by amino acid sequences, which may be responsible for the different catalytic activities.

Sucrose enhances colour formation in dry sausages by up-regulating gene expression of nitric oxide synthase in Staphylococcus vitulinus

The effects of glucose and sucrose on the gene expression of nitric oxide synthase (NOS) in Staphylococcus vitulinus and colour formation in dry sausages were investigated. The results showed that sucrose addition promoted nitric oxide (NO) production in media when compared with glucose. In addition, sucrose could up-regulate nos (encoding NOS) and katA (encoding catalase KatA) gene expression by enhancing oxidative stress levels. In the sausages inoculated with S. vitulinus, a*-values (indicating redness) of the sausages with added sucrose were higher than those of samples with added glucose (P < 0.05) but did not differ from those in the nitrite treatment group (P > 0.05). The UV-vis spectra results showed that nitrosylmyoglobin (NO-Mb) was formed in the sausages with either S. vitulinus or nitrite added. In the S. vitulinus-inoculated sausages, sucrose addition led to a higher NO-Mb content than that after glucose addition, which was attributed to up-regulation of the nos gene. This study provides a potential method to enhance NO yield in S. vitulinus and colour formation in dry sausages without nitrite addition.

Proteome Response of Staphylococcus xylosus DSM 20266T to Anaerobiosis and Nitrite Exposure

The viability and competitiveness of Staphylococcus xylosus in meat mostly depend on the ability to adapt itself to rapid oxygen and nutrients depletion during meat fermentation. The utilization of nitrite instead of oxygen becomes a successful strategy for this strain to improve its performance in anaerobiosis; however, metabolic pathways of this strain underlying this adaptation, are partially known. The aim of this study was to provide an overview on proteomic changes of S. xylosus DSM 20266T cultured under anaerobiosis and nitrite exposure. Thus, two different cultures of this strain, supplemented or not with nitrite, were in vitro incubated in aerobiosis and anaerobiosis monitoring cell viability, pH, oxidation reduction potential and nitrite content. Protein extracts, obtained from cells, collected as nitrite content was depleted, were analyzed by 2DE/MALDI-TOF/TOF-MS. Results showed that DSM 20266T growth was significantly sustained by nitrite in anaerobiosis, whereas no differences were found in aerobiosis. Accordingly, nitrite content was depleted after 13 h only in anaerobiosis. At this time of sampling, a comparative proteomic analysis showed 45 differentially expressed proteins. Most differences were found between aerobic and anaerobic cultures without nitrite; the induction of glycolytic enzymes and glyoxylate cycle, the reduction of TCA enzymes, and acetate fermentation were found in anaerobiosis to produce ATP and maintain the cell redox balance. In anaerobic cultures the nitrite supplementation partially restored TCA cycle, and reduced the amount of glycolytic enzymes. These results were confirmed by phenotypic microarray that, for the first time, was carried out on cell previously adapted at the different growth conditions. Overall, metabolic changes were similar between aerobiosis and anaerobiosis NO₂-adapted cells, whilst cells grown under anaerobiosis showed different assimilation profiles by confirming proteomic data; indeed, these latter extensively assimilated substrates addressed at both supplying glucose for glycolysis or fueling alternative pathways to TCA cycle. In conclusion, metabolic pathways underlying the ability of S. xylosus to adapt itself to oxygen starvation were revealed; the addition of nitrite allowed S. xylosus to take advantage of nitrite to this condition, restoring some metabolic pathway underlying aerobic behavior of the strain.

Mapping the dominant microbial species diversity at expiration date of raw meat and processed meats from equine origin, an underexplored meat ecosystem, in the Belgian retail

Although equine meats and their derived smoked or fermented products are popular in some regions of the world, they only form a minor fraction of the global meat consumption. The latter may explain why their associated bacterial communities have not received much attention. In the present study, 69 different samples of equine meats and meat products were investigated. The samples consisted of raw meat from horses (17 samples) and zebra (7), as well as non-fermented but smoked (24) and fermented (21) horse meat products. After purchase, all samples were stored at 4 °C and analysed at expiration date. Besides an estimation of the total microbial counts, specific attention was paid to the identification of lactic acid bacteria (LAB) and catalase-positive cocci, in particular the group of coagulase-negative staphylococci (CNS), involved, due to their technological relevance in view of the elaboration of meat products. Samples that were loosely wrapped in butcher paper instead of vacuum- or modified-atmosphere packages were also screened for pseudomonads and enterobacterial species. In total, 1567 bacterial isolates were collected, subjected to (GTG)₅-PCR fingerprinting of genomic DNA, and identified by multiple gene sequencing (based on the 16S rRNA, pheS, rpoA, rpoB, and/or tuf genes). Overall, the bacterial species diversity consisted mostly of LAB but was contingent on the type of product. Raw meat was dominated by Carnobacterium divergens, Lactobacillus sakei, Lactococcus piscium, and Leuconostoc gelidum, with zebra meat being particularly rich in lactococci. Smoked and fermented horse meat products contained mostly Lb. sakei and, to a lesser degree, Lactobacillus curvatus. In addition, several catalase-positive cocci (mostly Staphylococcus equorum), Anoxybacillus sp., Brevibacterium sp., Brochothrix thermosphacta, and the enterobacterial species Hafnia alvei were found.

Nonconventional starter cultures of coagulase-negative staphylococci to produce animal-derived fermented foods, a SWOT analysis

Coagulase-negative staphylococci (CNS) are ubiquitous micro-organisms that are commonly present on animal skin and animal-derived foods. They are members of the beneficial microbial consortia of several fermented food products where they contribute to quality. Currently, only a few CNS species are included in commercial starter cultures, although many other ones with promising properties have been isolated from diverse food ecosystems. In the present study, a Strengths-Weaknesses-Opportunities-Threats (SWOT) analysis of the potential use of unconventional CNS starter cultures for the fermentation of animal-derived foods is carried out. An overview of both their desirable and worrisome metabolic traits is given. In general, the application of innovative CNS-based starter cultures offers opportunities to modulate flavour, improve the safety and health aspects and develop novel colour development strategies for clean label products. Yet, their implementation is often not straightforward as nontrivial obstacles or threats are encountered, which relate to technological, food safety and legal concerns. As most of the desirable and undesirable characteristics of CNS species are strain dependent, a case-by-case evaluation is needed when evaluating specific strains for their potential use as novel starter cultures.

Nitric oxide synthase: What is its potential role in the physiology of staphylococci in meat products?

Coagulase-negative staphylococci are frequently isolated from meat products and two species are used as starter cultures in dry fermented sausages. In these products, they face various environmental conditions such as variation of redox potential and oxygen levels that can lead to oxidative stress. Furthermore, when nitrate and nitrite are added as curing salts, staphylococci also experience nitrosative stress. A nos gene encoding a nitric oxide synthase (NOS) is present in the genome of all staphylococci. NOS produces nitric oxide (NO) and citrulline from arginine, but its activity is still poorly characterized, particularly in coagulase-negative staphylococci. NO is highly reactive with a broad spectrum of activity resulting from targeting metal centres (heme and non-heme) and protein thiols. At low concentration, NO acts as a signalling molecule, while at higher concentration it generates stress. Thus, it was initially suggested that staphylococcal NOS counteract oxidative stress in relation to PerR and Fur regulators. In the physiology of staphylococci, it has recently been highlighted that NO controls the rate of aerobic respiration and regulates the transition from aerobic to nitrate respiration and also helps maintain the membrane potential in relation to the two-component systems SrrAB and AirRS. As NO interacts with heme centres, it binds the heme iron atom of myoglobin to form nitrosomyglobin, which is the typical red pigment of cured meat. However, the contribution of NOS to this reaction in meat products has yet to be evaluated.

Effect of temperature and pH on the community dynamics of coagulase-negative staphylococci during spontaneous meat fermentation in a model system

Coagulase-negative staphylococci (CNS) contribute to the product quality of fermented meats. In spontaneously fermented meats, CNS communities are variable and difficult to predict, as their compositions depend on a superposed combination of different processing factors. To partially disentangle this superposition, a meat model system was used to study the influence of temperature and pH on the CNS community dynamics. Therefore, cured pork mince was prepared that was divided into three batches of different initial acidity levels, namely pH 5.7, pH 5.5, and pH 5.3. These three batches were incubated at three different temperatures, namely 23 °C, 30 °C, and 37 °C. Hence, the experimental set-up resulted in nine combinations of different temperature and initial pH values. Samples were analysed after 3 and 14 days to monitor pH, colony counts, and species diversity of the CNS communities, based on mannitol-salt-phenol-red agar (MSA) medium. At conditions of mild acidity (pH 5.7) and low temperature (23 °C), as often encountered during artisan-type meat fermentations, a co-prevalence of Staphylococcus xylosus, Staphylococcus equorum, and Staphylococcus saprophyticus occurred. At the same initial pH but higher incubation temperatures (30 °C and 37 °C), Staphylococcus lugdunensis became the prevailing CNS species, besides S. saprophyticus (30 °C) and the coagulase-positive species Staphylococcus aureus (37 °C). When the initial pH was set at 5.5, S. saprophyticus was the prevailing CNS species at both 23 °C and 30 °C, but it was replaced by Staphylococcus epidermidis and Staphylococcus simulans at 37 °C after 3 and 14 days, respectively. At the most acidic conditions (pH 5.3), CNS counts declined and many of the MSA isolates were of non-staphylococcal nature. Among others, Staphylococcus carnosus (23 °C), Staphylococcus warneri (30 °C), and S. epidermidis (37 °C) were found. Overall, the results of the present study indicated that the processing factors temperature and pH had a clear impact on the shaping of staphylococcal communities during meat fermentation.

The narrowing down of inoculated communities of coagulase-negative staphylococci in fermented meat models is modulated by temperature and pH

Coagulase-negative staphylococci (CNS) are involved in colour and flavour formation of fermented meats. Their communities are established either spontaneously, as in some artisan-type products, or using a starter culture. The latter usually consists of Staphylococcus carnosus and/or Staphylococcus xylosus strains, although strains from other CNS species also have potential for application. However, it is not entirely clear how the fitness of alternative starter cultures within a fermented meat matrix compares to conventional ones and how this may be affected by processing conditions. Therefore, the aim of this study was to assess the influence of two key processing conditions, namely temperature and acidity, on the competitiveness of a cocktail of five different strains of CNS belonging to species that are potentially important for meat fermentation (Staphylococcus xylosus 2S7-2, S. carnosus 833, Staphylococcus epidermidis ATCC 12228, Staphylococcus equorum DFL-S19, and Staphylococcus saprophyticus FPS1). To this end, fermented meat models consisting of cured meat batters with initial pH values of 5.3, 5.5, or 5.7 were inoculated with these strains, stuffed in containers, and incubated at 23, 30, or 37 °C. Both the pH level and the temperature influenced the composition of the CNS communities, giving a competitive advantage to the best adapted species. Staphylococcus xylosus preferred low temperature and mild acidity, whereas an elevated temperature selected for S. epidermidis and a low pH for S. carnosus. Under the conditions tested, S. saprophyticus and S. equorum were outcompeted by the three other CNS species. Hence, CNS communities in fermented meats are not only established based on the initial presence of specific species in the meat batter but also by their subsequent adaptation to the processing conditions during fermentation, potentially overruling the use of starter cultures.

Contribution of nitric oxide synthase from coagulase-negative staphylococci to the development of red myoglobin derivatives

As part of the microbial community of meat or as starter cultures, coagulase-negative staphylococci (CNS) serve several essential technological purposes in meat products, such as color development through the reduction of nitrate to nitrite. As the safety of nitrite as an additive has been questioned, we explored the potential of CNS to develop red myoglobin derivatives such as oxymyoglobin and nitrosomyoglobin. Nitrosoheme was extracted to evaluate NO production. This production could be due to a nitric oxide synthase (NOS) activity. In all CNS strains, a nos gene was identified. The NOS sequences deduced were highly conserved within CNS. A phylogenetic tree based on the NOS sequences revealed that the strains within species were clustered. Ninety-one percent of the strains, whatever the species, were able to form red myoglobin derivatives in aerobic conditions, but a high variability was observed between strains within species. However, NO production was low as nitrosomyoglobin represented 8% to 16% of the red pigments according to the species. Formation of oxymyoglobin, especially under aerobic conditions, was substantial, but varied greatly within species. The mechanism involved in the formation of oxymyoglobin could rely on staphylococcal reductases and remains to be explored.

Insight into the Genome of Staphylococcus xylosus, a Ubiquitous Species Well Adapted to Meat Products

Staphylococcus xylosus belongs to the vast group of coagulase-negative staphylococci. It is frequently isolated from meat products, either fermented or salted and dried, and is commonly used as starter cultures in sausage manufacturing. Analysis of the S. xylosus genome together with expression in situ in a meat model revealed that this bacterium is well adapted to meat substrates, being able to use diverse substrates as sources of carbon and energy and different sources of nitrogen. It is well-equipped with genes involved in osmotic, oxidative/nitrosative, and acidic stress responses. It is responsible for the development of the typical colour of cured meat products via its nitrate reductase activity. It contributes to sensorial properties, mainly by the the catabolism of pyruvate and amino acids resulting in odorous compounds and by the limiting of the oxidation of fatty acids, thereby avoiding rancidity.

Effects of glucose and oxygen on arginine metabolism by coagulase-negative staphylococci

Coagulase-negative staphylococci (CNS) are not only part of the desirable microbiota of fermented meat products but also commonly inhabit skin and flesh wounds. Their proliferation depends on the versatility to use energy sources and the adaptation to fluctuating environmental parameters. In this study, the conversion of the amino acid arginine by two strains with arginine deiminase (ADI) activity (Staphylococcus carnosus 833 and S. pasteuri αs3-13) and a strain with nitric oxide synthase (NOS) activity (S. haemolyticus G110) was modelled as a function of glucose and oxygen availability. Both factors moderately inhibited the ADI-based conversion kinetics, never leading to full repression. However, for NOS-driven conversion of arginine by S. haemolyticus G110, oxygen was an absolute requirement. When changing from microaerobic conditions to aerobiosis, a switch from homolactic fermentation to a combined formation of lactic acid, acetic acid, and acetoin was found in all cases, after which lactic acid and acetic acid were used as substrates. The kinetic model proposed provided a suitable description of the data of glucose and arginine co-metabolism as a function of oxygen levels and may serve as a tool to further analyse the behaviour of staphylococci in different ecosystems or when applying specific food processing conditions.

Evidence for Nitric Oxide Synthase Activity in Staphylococcus xylosus Mediating Nitrosoheme Formation

Staphylococcus xylosus is used as a starter culture in fermented meat products and contributes to color formation by the reduction of nitrate to nitrite. Nitrite is a food additive that is chemically turned to nitric oxide (NO) in meat but its safety has been questioned. The objective of this study was to determine the ability of NO synthase (NOS) of S. xylosus C2a to produce NO. For this purpose, a nos deletion mutant (Δnos) in S. xylosus was constructed and NO production was evaluated in a test based on its ability to form nitrosomyoglobin and nitrosoheme. Production of NO was abrogated in the Δnos mutant under aerobic conditions and reduced about 35-40% comparing to the wild type C2a under limited oxygenation. This mutant was sensitive to oxidative stress. The expression of genes encoding catalase was modulated in the mutant with an up-regulation of katA and a down-regulation of katB and katC. The Δnos mutant displayed high colony pigmentation after prolonged growth on agar medium. Finally, the Δnos mutant showed no growth in minimal medium. Growth was not restored in the minimal medium by complementation with nos, but was restored by either addition of phenylalanine or complementation with pdt, a gene that encodes a prephenate dehydratase involved in phenylalanine biosynthesis and co-transcribed with nos. Our findings clearly demonstrate NOS-mediated NO production in S. xylosus, a meat-associated coagulase-negative Staphylococcus.

Amino acid conversions by coagulase-negative staphylococci in a rich medium: Assessment of inter- and intraspecies heterogeneity

The ability of coagulase-negative staphylococci (CNS) to convert amino acids into volatile compounds and biogenic amines was investigated after 24h and 48 h of incubation in a rich medium (brain heart infusion). Volatile compounds were measured with static-headspace gas chromatography and mass spectrometry (SH-GC-MS); biogenic amine measurements were carried out with a newly developed method based on ultra-performance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS). In total, 56 CNS strains from five different species were used, namely Staphylococcus carnosus, Staphylococcus epidermidis, Staphylococcus equorum, Staphylococcus saprophyticus, and Staphylococcus xylosus. With respect to the production of volatile compounds, the leucine-derived 3-methyl butanol was produced over time by most CNS strains, up to 52 μM for S. xylosus W1-1 after 48 h of incubation. The average production by strains of S. xylosus was significantly higher than for strains of S. carnosus, whereas strains of S. epidermidis turned out to be poor producers. Yet, differences between species were blurred to a large degree because of the high strain variability. A few strains also produced 3-methyl butanal on top of the amount that was already present in the medium background, although most CNS led to a decrease of this compound. Concerning biogenic amines, the average total concentrations per species remained below 100 μM after 48 h of incubation. The most abundant variant was 2-phenylethylamine (PEA), especially within S. carnosus (average of 65 μM after 48 h of incubation). Yet, some individual strains were able to produce higher concentrations, as found for the PEA production of 295 μM by S. epidermidis ATCC 12228 after 48 h of incubation. The insights obtained during this study indicate heterogeneity and are of importance in view of both starter culture development and the evaluation of a spontaneously established CNS microbiota in artisan-type meat fermentations.