Influence of carbohydrate starvation and arginine on culturability and amino acid utilization of lactococcus lactis subsp. lactis

The strain-dependent cytostatic activity of Lactococcus lactis on CRC cell lines is mediated through the release of arginine deiminase

BACKGROUND

Colorectal cancer (CRC) is one of the most commonly diagnosed cancers, posing a serious public health challenge that necessitates the development of new therapeutics, therapies, and prevention methods. Among the various therapeutic approaches, interventions involving lactic acid bacteria (LAB) as probiotics and postbiotics have emerged as promising candidates for treating and preventing CRC. While human-isolated LAB strains are considered highly favorable, those sourced from environmental reservoirs such as dairy and fermented foods are also being recognized as potential sources for future therapeutics.

RESULTS

In this study, we present a novel and therapeutically promising strain, Lactococcus lactis ssp. lactis Lc4, isolated from dairy sources. Lc4 demonstrated the ability to release the cytostatic agent - arginine deiminase (ADI) - into the post-cultivation supernatant when cultured under conditions mimicking the human gut environment. Released arginine deiminase was able to significantly reduce the growth of HT-29 and HCT116 cells due to the depletion of arginine, which led to decreased levels of c-Myc, reduced phosphorylation of p70-S6 kinase, and cell cycle arrest. The ADI release and cytostatic properties were strain-dependent, as was evident from comparison to other L. lactis ssp. lactis strains.

CONCLUSION

For the first time, we unveil the anti-proliferative properties of the L. lactis cell-free supernatant (CFS), which are independent of bacteriocins or other small molecules. We demonstrate that ADI, derived from a dairy-Generally Recognized As Safe (GRAS) strain of L. lactis, exhibits anti-proliferative activity on cell lines with different levels of argininosuccinate synthetase 1 (ASS1) expression. A unique feature of the Lc4 strain is also its capability to release ADI into the extracellular space. Taken together, we showcase L. lactis ADI and the Lc4 strain as promising, potential therapeutic agents with broad applicability.

Metabolomics-based response of Salmonella to desiccation stress and skimmed milk powder storage

The strong survival ability of Salmonella in low-moisture foods (LMFs) has been of public concern, and is considered a threat to people's health. Recently, the development of omics technology has promoted research on the molecular mechanisms of the desiccation stress response of pathogenic bacteria. However, multiple analytical aspects related to their physiological characteristics remain unclear. We explored the physiological metabolism changes of S. enterica Enteritidis exposed to a 24 h-desiccation treatment and a subsequent 3-month desiccation storage in skimmed milk powder (SMP) with an approach of gas chromatography-mass spectrometry (GC-MS) and ultra-performance liquid chromatography-Q Exactive-mass spectrometry (UPLC-QE-MS). A total of 8,292 peaks were extracted, of which 381 were detected by GC-MS and 7,911 peaks were identified by LC-MS/MS, respectively. Through analyses of differentially expressed metabolites (DEMs) and key pathways, a total of 58 DEMs emerged from the 24 h-desiccation treatment, which exhibited the highest relevance for five metabolic pathways, involving glycine, serine, and threonine metabolism, pyrimidine metabolism, purine metabolism, vitamin B6 metabolism, and pentose phosphate pathway. After 3-month SMP storage, 120 DEMs were identified, which were related to several regulatory pathways including arginine and proline metabolism, serine and threonine metabolism, β-alanine metabolism, glycerolipid metabolism, and glycolysis. The analyses of key enzyme activities of XOD, PK, and G6PDH and ATP content provided further evidence that supported the metabolic responses such as nucleic acid degradation, glycolysis, and ATP production played an important role in Salmonella's adaptation to desiccation stress. This study enables a better understanding of metabolomics-based responses of Salmonella at the initial stage of desiccation stress and the following long-term adaptive stage. Meanwhile, the identified discriminative metabolic pathways may serve as potentially useful targets in developing strategies for the control and prevention of desiccation-adapted Salmonella in LMFs.

Stationary Lactococcus cremoris: Energetic State, Protein Synthesis Without Nitrogen and Their Effect on Survival

During storage and ripening of fermented foods, Lactococcus cremoris is predominantly in a non-growing state. L. cremoris can become stationary due to starvation or acidification, and its metabolism in these non-growing states affects the fermented product. Available studies on the response of L. cremoris to acid and starvation stress are based on population level data. We here characterized the energetic state and the protein synthesis capacity of stationary L. cremoris cultures at the single cell level. We show that glucose starved stationary cells are energy-depleted, while acid-induced stationary cells are energized and can maintain a pH gradient over their membrane. In the absence of glucose and arginine, a small pH gradient can still be maintained. Subpopulations of stationary cells can synthesize protein without a nitrogen source, and the subpopulation size decreases with increasing stationary phase length. Protein synthesis capacity during starvation only benefits culturability after 6 days. These results highlight significant differences between glucose starved stationary and acid-induced stationary cells. Furthermore, they show that the physiology of stationary phase L. cremoris cells is multi-facetted and heterogeneous, and the presence of an energy source during stationary phase impacts the cells capacity to adapt to their environment.

In Vitro Screening of Chicken-Derived Lactobacillus Strains that Effectively Inhibit Salmonella Colonization and Adhesion

Inhibition of Salmonella by Lactobacillus has been a popular research topic for decades; however, the inhibition potential of chicken-derived Salmonella by chicken-derived Lactobacillus has not yet been studied. In this study, 89 strains of Lactobacillus from chicken intestines were isolated by national standard method, Gram staining, physiological, and biochemical experiments and molecular sequencing; The inhibition characteristics of 89 strains of chicken derived Lactobacillus against 10 strains Salmonella (S. Enteritidis SE05, SC31, SC21, SC72 SC74, SC79, SC83, SC87; S. bongori SE47; S. Typhimurium, SC85) were detected by agar inhibition zone, The results showed that the inhibition zone of 24 strains of chicken derived Lactobacillus was more than 10 mm, which indicated that the isolated chicken derived Lactobacillus could effectively inhibit the growth of Salmonella; The drug resistance and bile salt tolerance of these 24 strains were analyzed, The results showed that the standard strains LG and L76 were not resistant, and the other 22 Lactobacillus strains showed different degrees of resistance. The strains LAB24, LAB26, LAB53, LAB69, and L76 showed good tolerance at the concentration of 3 g/L bile salt; Caco-2 cell experiment and flow cytometry were used to analyze the inhibitory effect of chicken derived Lactobacillus on the adhesion of Salmonella to Caco-2 cells, The results showed that 16 probiotics could effectively inhibit the adhesion of Salmonella to Caco-2 cells. Twelve probiotics were identified by molecular biology. The results showed that L76 was Enterococcus faecalis, and the other 11 strains were Lactobacillus.

Culture conditions affect Lactobacillus reuteri DSM 17938 ability to perform glycerol bioconversion into 3-hydroxypropionic acid

The platform molecule 3-hydroxypropionic acid (3-HP) can be produced using Lactobacillus reuteri through a two-step bioprocess that involves a growth phase followed by a bioconversion phase. The bioproduction is performed by resting cells that convert glycerol into 3-HP and 1,3-propanediol in fed-batch mode. This work aimed at studying the effect of the growth conditions of L. reuteri DSM 17938 during the first step, on the glycerol bioconversion into 3-HP during the second step. A Plackett and Burman design was carried out to test, in controlled bioreactors, the effect of 11 growth conditions simultaneously, at fixed bioconversion conditions. The supplementation of the growth medium with vitamin B12 and cysteine displayed a negative effect on the 3-HP bioproduction. The addition of glucose, phytone peptone, Tween 80, 1,2-propanediol and betaine in the growth medium, together with a low temperature and an optimal pH of 6.0 during the growth phase increased the bioconversion duration from 56 h to 89 h at a glycerol feeding rate of 0.5 g·h^-1. A validating experiment displayed that the 3-HP titer, 3-HP production yield and 3-HP specific production rate were significantly improved by 25 %, 150 % and 61 %, respectively.

Suppression of lactate production in fed-batch culture of some lactic acid bacteria with sucrose as the carbon source

We report a method for suppression of lactate production by lactic acid bacteria (LAB) in culture. LAB produce lactate to regenerate NAD⁺ that is consumed during glycolysis. Glucose suppresses NAD⁺ regeneration pathways other than lactate dehydrogenase and non-glycolytic ATP production pathways. Therefore, the carbon source was changed to sucrose, and fed-batch culture was performed to limit the glycolytic flux and thus suppress lactate production. As a result, lactate productivity (i.e., the amount of lactate produced per amount of grown cell) in the sucrose/fed-batch culture was decreased compared to that in glucose/batch culture, in all five LAB strains examined. The productivity level decreased to 24% and 46% in Lactobacillus reuteri JCM 1112 and Lactococcus lactis JCM 7638, respectively. Metabolic flux analysis of Lactobacillus reuteri JCM 1112 revealed increased contributions of the mannitol production pathway to NAD⁺ regeneration and the arginine deiminase pathway to ATP production in the sucrose/fed-batch culture.

Suppression of lactate production by using sucrose as a carbon source in lactic acid bacteria

Lactic acid bacteria (LAB) grow by producing lactate from sugar. However, the accumulation of lactate inhibits their growth. Here, the lactate productivity per cell in a semi-solid medium prepared with a chlorella powder in several LAB strains was much lower than that in the conventional MRS medium. Furthermore, the lactate production was suppressed not only in semi-solid medium, but also in chlorella liquid medium. The lactate productivity by Lactococcus lactis subsp. lactis NBRC 12007 in the chlorella liquid medium and MRS medium was 3.0 and 6.9 g-lactate·g-cell^-1, respectively. The productivity of lactate in the chlorella liquid medium decreased to 44% of that in MRS medium. Gas chromatography/mass spectrometry (GC/MS) analysis of the culture supernatants revealed that the utilization of sucrose in the chlorella powder led to the suppression of lactate production. Comparison of the metabolites extracted from the cells indicated that the two ATP generating pathways, the arginine deiminase pathway and the decarboxylation reaction of glutamate and GABA, which are usually repressed by glucose, are activated in chlorella medium. It was considered that these pathways which do not require NAD⁺ for generation of ATP are not repressed when sucrose is used as a carbon source. Thus, the utilization of these pathways results in the suppression of the lactate production.

Ampicillin-treated Lactococcus lactis MG1363 populations contain persisters as well as viable but non-culturable cells

Lactococcus lactis is used as cell-factory and strain selections are regularly performed to improve production processes. When selection regimes only allow desired phenotypes to survive, for instance by using antibiotics to select for cells that do not grow in a specific condition, the presence of more resistant subpopulations with a wildtype genotype severely slows down the procedure. While the food grade organism L. lactis is not often exposed to antibiotics we characterized its response to ampicillin in more detail, to better understand emerging population heterogeneity and how this might affect strain selection procedures. Using growth-dependent viability assays we identified persister subpopulations in stationary and exponential phase. Growth-independent viability assays revealed a 100 times larger subpopulation that did not grow on plates or in liquid medium, but had an intact membrane and could maintain a pH gradient. Over one third of these cells restored their intracellular pH when we induced a temporary collapse, indicating that this subpopulation was metabolically active and in a viable but non-culturable state. Exposure of L. lactis MG1363 to ampicillin therefore results in a heterogeneous population response with different dormancy states. These dormant cells should be considered in survival-based strain selection procedures.

Influence of culture conditions on the technological properties of Carnobacterium maltaromaticum CNCM I-3298 starters

AIM

The aim of this study is to investigate the effect of a broad spectrum of culture conditions on the acidification activity and viability of Carnobacterium maltaromaticum CNCM I-3298, the main technological properties that determine the shelf-life of biological time-temperature integrator (TTI) labels.

METHODS AND RESULTS

Cells were cultivated at different temperatures (20-37°C) and pH (6-9·5) according to a modified central composite design and harvested at increasing times up to 10 h of stationary phase. Acidification activity and viability of freeze-thawed concentrates were assessed in medium mimicking the biological label. Acidification activity was influenced by all three culture conditions, but pH and harvest time were the most influential. Viability was not significantly affected by the tested range of culture conditions.

CONCLUSIONS

Carnobacterium maltaromaticum CNCM I-3298 must be cultivated at 20°C, pH 6 and harvested at the beginning of stationary phase to exhibit fastest acidification activities. However, if slower acidification activities are pursued, the recommended culture conditions are 30°C, pH 9·5 and a harvest time between 4-6 h of stationary phase.

SIGNIFICANCE AND IMPACT OF THE STUDY

Quantifying the impact of fermentation temperature, pH and harvest time has led to a predictive model for the production of biological TTI covering a broad range of shelf-lives.

Intestinal luminal putrescine is produced by collective biosynthetic pathways of the commensal microbiome

The intestinal microbiome produces various metabolites that may harm or benefit the host. However, the production pathways of these metabolites have not been well characterised. The polyamines putrescine and spermidine required for physiological process are also produced by intestinal microbiome. The production and release of these polyamines by microbiome are poorly understood, though we have confirmed that intestinal bacteria produced putrescine from arginine. In this study, we characterised polyamine synthesis by analysing the collective metabolic functions of the intestinal microbiome. In particular, we analysed polyamines and their intermediates in faecal cultures, as well as the colonic contents of rats injected with isotope-labelled arginine through a colon catheter, using mass spectrometry. Isotope-labelled putrescine was detected in faecal cultures and colonic contents of rats injected with isotope-labelled arginine. Putrescine is produced through multiple pathways, and its extracellular intermediates are exchanged between bacterial species. Additionally, we demonstrated that the collective metabolic pathway depends on a complex exchange of metabolites released into the colonic lumen. This study demonstrates the existence of putrescine biosynthetic pathways based on the collective metabolic functions of the intestinal microbial community. Our findings provide knowledge to manipulate the levels of intestinal microbial products, including polyamines, that may modulate host health.

Genomic resequencing combined with quantitative proteomic analyses elucidate the survival mechanisms of Lactobacillus plantarum P-8 in a long-term glucose-limited experiment

Lactobacillus plantarum, commonly isolated from plant material, is widely used to produce various types of fermented foods. However, nutrient-limiting conditions are often encountered during industrial applications. The present study aimed to investigate the response of L. plantarum P-8 to glucose-limited conditions in a long-term experiment. Genotypic and proteomic changes in L. plantarum P-8 were monitored over 3 years in glucose-limited and glucose-normal media using whole-genome resequencing and tandem mass tag-based quantitative proteomic analysis. Results showed that L. plantarum employed numerous survival mechanisms, including alteration of the cell envelope, activation of the PTS system, accumulation and consumption of amino acids, increase in the metabolism of carbohydrates (via glycolysis, citric acid cycle, and pyruvate metabolism), and increase in the production of ATP in response to glucose starvation. This study demonstrates the feasibility of experimental evolution of L. plantarum P-8, while whole-genome resequencing of adapted isolates provided clues toward bacterial functions involved and a deeper mechanistic understanding of the adaptive response of L. plantarum to glucose-limited conditions.

SIGNIFICANCE

We have conducted a 3-year experiment monitoring genotypic and proteomic changes in Lactobacillus plantarum P-8 in glucose-limited and glucose-normal media. Whole-genome resequencing and tandem mass tag-based quantitative proteomics were performed for analyzing genomic evolution of L. plantarum P-8 in glucose-limited and glucose-normal conditions. In addition, differential expressed proteins in all generations between these two conditions were identified and functions of these proteins specific to L group were predicted. L. plantarum employed numerous survival mechanisms, including alteration of the cell envelope, activation of the PTS system, accumulation and consumption of amino acids, increase in the metabolism of carbohydrates (glycolysis, citric acid cycle, and pyruvate metabolism), and increase in the production of ATP in response to glucose starvation.

Identification, characterization and molecular analysis of the viable but nonculturable Rhodococcus biphenylivorans

Numerous bacteria, including pollutant-degrading bacteria can enter the viable but nonculturable state (VBNC) when they encounter harsh environmental conditions. VBNC bacteria, as a vast majority of potent microbial resource can be of great significance in environmental rehabilitation. It is necessary to study the VBNC state of pollutant-degrading bacteria under various stress conditions. The aim of this study was to determine whether Rhodococcus biphenylivorans could enter the VBNC state under oligotrophic and low temperature conditions, and to examine the changes of morphology, enzymatic activity and gene expressions that might underline such state. The obtained results indicated that R. biphenylivorans TG9(T) could enter into the VBNC state and recover culturability under favorable environmental conditions. Results from Illumina high throughput RNA-sequencing revealed that the up-regulated genes related to ATP accumulation, protein modification, peptidoglycan biosynthesis and RNA polymerase were found in the VBNC cells, and the down-regulated genes mainly encoded hypothetical protein, membrane protein and NADH dehydrogenase subunit, which render VBNC cells more tolerant to survive under inhospitable conditions. This study provides new insights into prevention and control of the VBNC state of pollutant-degrading bacteria for their better capabilities in environmental rehabilitation.

Detection and viability of Lactococcus lactis throughout cheese ripening

Recent evidences highlighted the presence of Lactococcus lactis during late cheese ripening. For this reason, the role of this microorganism, well known as dairy starter, should be reconsidered throughout cheese manufacturing and ripening. Thus, the main objective of this study was to develop a RT-qPCR protocol for the detection, quantification and determination of the viability of L. lactis in ripened cheese samples by direct analysis of microbial nucleic acids. Standard curves were constructed for the specific quantification of L. lactis in cheese matrices and good results in terms of selectivity, correlation coefficient and efficiency were obtained. Thirty-three ripened cheeses were analyzed and, on the basis of RNA analysis, twelve samples showed 106 to 108 CFU of L. lactis per gram of product, thirteen from 103 to 105 CFU/g, and in eight cheeses, L. lactis was not detected. Traditional plating on M17 medium led to loads ranging from 105 to 109 CFU/g, including the cheese samples where no L. lactis was found by RT-qPCR. From these cheeses, none of the colonies isolated on M17 medium was identified as L. lactis species. These data could be interpreted as a lack of selectivity of M17 medium where colony growth is not always related to lactococcal species. At the same time, the absence or low abundance of L. lactis isolates on M17 medium from cheese where L. lactis was detected by RT-qPCR support the hypothesis that L. lactis starter populations are mainly present in viable but not culturable state during ripening and, for this reason, culture-dependent methods have to be supplemented with direct analysis of cheese.

Growth rate-dependent control in Enterococcus faecalis: effects on the transcriptome and proteome, and strong regulation of lactate dehydrogenase

Enterococcus faecalis V583 was grown in a glucose-limited chemostat at three different growth rates (0.05, 0.15, and 0.4 h⁻¹). The fermentation pattern changed with growth rate, from a mostly homolactic profile at a high growth rate to a fermentation dominated by formate, acetate, and ethanol production at a low growth rate. A number of amino acids were consumed at the lower growth rates but not by fast-growing cells. The change in metabolic profile was caused mainly by decreased flux through lactate dehydrogenase. The transcription of ldh-1, encoding the principal lactate dehydrogenase, showed very strong growth rate dependence and differed by three orders of magnitude between the highest and the lowest growth rates. Despite the increase in ldh-1 transcript, the content of the Ldh-1 protein was the same under all conditions. Using microarrays and quantitative PCR, the levels of 227 gene transcripts were found to be affected by the growth rate, and 56 differentially expressed proteins were found by proteomic analyses. Few genes or proteins showed a growth rate-dependent increase or decrease in expression across the whole range of conditions, and many showed a maximum or minimum at the middle growth rate (i.e., 0.15 h⁻¹). For many gene products, a discrepancy between transcriptomic and proteomic data were seen, indicating posttranscriptional regulation of expression.

Effect of arginine on the alcohol dehydrogenase and acetaldehyde dehydrogenase enzymes of alcohol metabolism in Saccharomyces cerevisiae

Arginine possesses advantageous pharmacological properties such as liver injury protection. We have previously shown that the arginine stimulated the activities of commercial alcohol dehydrogenase (ADH, EC 1.1.1.1) and acetaldehyde dehydrogenase (ALDH, EC 1.2.1.10) enzymes in vitro experiment. We therefore examined on the activities, zymogram staining intensity, and protein expression of alcohol metabolizing ADH and ALDH in Saccharomyces cerevisiae cultured in a medium supplemented with different concentrations of arginine. The enhanced activity, zymogram staining intensity, and protein expression of ADH in the cell-free extracts of S. cerevisiae showed at 0.01 and 0.05% (w/v) arginine supplementation. These parameters of ALDH in the cell-free extracts of S. cerevisiae showed in the 0.005-0.05% arginine treatment concentration, but these parameters were shown to be decreased at a concentration of 0.1% (w/v) arginine, which was the highest supplementation. These results indicate that arginine can be used to enhance the enzyme activities, staining intensity for the protein activity in the zymogram analysis, and increased protein expression of ADH and ALDH in S. cerevisiae. These results also indicate that arginine can be used to the protection of alcoholic liver injury and hangover by strong activation of alcohol metabolizing ADH and ALDH.

Arginine metabolism in sugar deprived Lactococcus lactis enhances survival and cellular activity, while supporting flavour production

Flavour development in cheese is affected by the integrity of Lactococcus lactis cells. Disintegrated cells enhance for instance the enzymatic degradation of casein to free amino acids, while integer cells are needed to produce specific flavour compounds from amino acids. The impact of the cellular activity of these integer cells on flavour production remains to be elucidated. In this study we investigated whether lactose-deprived L. lactis cells that use arginine as an alternative energy source can extend cellular activity and produce more specific flavours. In cheese experiments we demonstrated that arginine metabolising cells survived about 3 times longer than non-arginine metabolising cells, which suggests prolonged cellular activity. Cellular activity and flavour production of L. lactis was further studied in vitro to enable controlled arginine supplementation. Comparable with the results found in cheese, the survival rates of in vitro incubated cells improved when arginine was metabolised. Furthermore, elongated cellular activity was reflected in 3-4-fold increased activity of flavour generating enzymes. The observed prolonged cellular activity resulted in about 2-fold higher concentrations of typical Gouda cheese flavours. These findings provide new leads for composing starter cultures that will produce specific flavour compounds.

Effect of tannic acid on Lactobacillus plantarum wine strain during starvation: A proteomic study

The molecular mechanisms involved in tannic-acid (TA)-mediated cell growth retardation and viability prolongation of Lactobacillus plantarum VP08 strain were evaluated by a proteomic analysis of starved cells grown in the presence of TA or glucose as carbon source. The tannase activity and the cell growth retardation as well as viability prolongation were confirmed by enzymatic assay and growing tests, respectively. In order to gain information about the effect triggered at the molecular level by TA, total proteins (extracted from starved cells grown in 250 mg/L TA, or 2 g/L glucose) were analyzed by a 2D-PAGE/MS approach to detect differentially expressed proteins. A total of 15 spots were found to be down-regulated and 21 up-regulated in TA-grown cells. The results indicate an overall impact of TA on proteins involved in some cellular and metabolic pathways: glycolysis, amino acid metabolism, translation and protein folding. The modulation of specific proteins correlates with the positive effect of TA on the survival of tannase-positive L. plantarum.

Quantitative approach to determining the contribution of viable-but-nonculturable subpopulations to malolactic fermentation processes

Different sizes of viable-but-nonculturable cell subpopulations of a lactic acid bacterium strain were induced by adding increasing amounts of SO(2). The experimental data obtained here were fitted to a segregated kinetic model developed previously. This procedure allowed us to determine in quantitative terms the contribution of this physiological state to malolactic fermentation.

Assessment of real-time RT-PCR for quantification of Lactobacillus plantarum gene expression during stationary phase and nutrient starvation

AIMS

In this study, we evaluated the impact of different real-time reverse-transcription PCR (RT-PCR) data normalization methods on the interpretation of stationary-phase and nutrient-starved Lactobacillus plantarum WCFS1 gene expression levels.

METHODS AND RESULTS

Lactobacillus plantarum WCFS1 culture characteristics and housekeeping gene transcripts were measured during stationary phase in standard growth medium and during extreme nutrient starvation. These conditions differentially affected L. plantarum viability and RNA/DNA ratios. Real-time RT-PCR gene expression data were normalized according to three different methods: (i) total RNA amounts added to the reactions; (ii) the comparative 2(-Delta Delta Ct) method using recA as a reference; and (iii) the geNorm approach based on the average expression values of several housekeeping genes. Each of these methods revealed differences in the abundance of housekeeping gene transcripts between L. plantarum in the exponential phase of growth and in stationary phase or undergoing nutrient starvation.

CONCLUSIONS

Real-time RT-PCR data analysis with a normalization factor comprised of several of the most stably expressed housekeeping genes best accounted for the expected activity levels of the cells contained in the different cultures.

SIGNIFICANCE AND IMPACT OF THE STUDY

The relative normalization of real-time RT-PCR data using multiple housekeeping reference genes should be useful for the quantification of bacterial gene expression levels in nonoptimal growth conditions in situ.

Carbohydrate starvation causes a metabolically active but nonculturable state in Lactococcus lactis

This study characterized the ability of lactococci to become nonculturable under carbohydrate starvation while maintaining metabolic activity. We determined the changes in physiological parameters and extracellular substrate levels of multiple lactococcal strains under a number of environmental conditions along with whole-genome expression profiles. Three distinct phases were observed, logarithmic growth, sugar exhaustion, and nonculturability. Shortly after carbohydrate starvation, each lactococcal strain lost the ability to form colonies on solid media but maintained an intact cell membrane and metabolic activity for over 3.5 years. ML3, a strain that metabolized lactose rapidly, reached nonculturability within 1 week. Strains that metabolized lactose slowly (SK11) or not at all (IL1403) required 1 to 3 months to become nonculturable. In all cases, the cells contained at least 100 pM of intracellular ATP after 6 months of starvation and remained at that level for the remainder of the study. Aminopeptidase and lipase/esterase activities decreased below detection limits during the nonculturable phase. During sugar exhaustion and entry into nonculturability, serine and methionine were produced, while glutamine and arginine were depleted from the medium. The cells retained the ability to transport amino acids via proton motive force and peptides via ATP-driven translocation. The addition of branched-chain amino acids to the culture medium resulted in increased intracellular ATP levels and new metabolic products, indicating that branched-chain amino acid catabolism resulted in energy and metabolic products to support survival during starvation. Gene expression analysis showed that the genes responsible for sugar metabolism were repressed as the cells entered nonculturability. The genes responsible for cell division were repressed, while autolysis and cell wall metabolism genes were induced neither at starvation nor during nonculturability. Taken together, these observations verify that carbohydrate-starved lactococci attain a nonculturable state wherein sugar metabolism, cell division, and autolysis are repressed, allowing the cells to maintain transcription, metabolic activity, and energy production during a state that produces new metabolites not associated with logarithmic growth.

Identification of the leucine-to-2-methylbutyric acid catabolic pathway of Lactococcus lactis

Nutrient starvation and nonculturability in bacteria lead to changes in metabolism not found during the logarithmic phase. Substrates alternate to those used during growth are metabolized in these physiological states, yielding secondary metabolites. In firmicutes and actinobacteria, amino acid catabolic pathways are induced during starvation and nonculturability. Examination of lactococci showed that the population entered a nonculturable state after carbohydrate depletion and was incapable of growth on solid media; however, the cells gained the ability to produce branched-chain fatty acids from amino acids. Gene expression profiling and in silico pathway analysis coupled with nuclear magnetic resonance spectroscopy were used to delineate the leucine catabolic pathway. Lactococci produced acetic and propionic acid during logarithmic growth and starvation. At the onset of nonculturability, 2-methylbutyric acid was produced via hydroxymethyl-glutaryl-coenzyme A (CoA) and acetyl-CoA, along with ATP and oxidation/reduction precursors. Gene expression profiling and genome sequence analysis showed that lactococci contained redundant genes for branched-chain fatty acid production that were regulated by an unknown mechanism linked to carbon metabolism. This work demonstrated the ability of a firmicute to induce new metabolic capabilities in the nonculturable state for producing energy and intermediates needed for transcription and translation. Phylogenetic analyses showed that homologues of these enzymes and their functional motifs were widespread across the domains of life.

Differential expression of proteins and genes in the lag phase of Lactococcus lactis subsp. lactis grown in synthetic medium and reconstituted skim milk

We investigated protein and gene expression in the lag phase of Lactococcus lactis subsp. lactis CNRZ 157 and compared it to the exponential and stationary phases. By means of two-dimensional polyacrylamide gel electrophoresis, 28 highly expressed lag-phase proteins, implicated in nucleotide metabolism, glycolysis, stress response, translation, transcription, cell division, amino acid metabolism, and coenzyme synthesis, were identified. Among the identified proteins, >2-fold induction and down-regulation in the lag phase were determined for 12 proteins in respect to the exponential phase and for 18 proteins in respect to the stationary phase. Transcriptional changes of the lag-phase proteins in L. lactis were studied by oligonucleotide microarrays. Good correlation between protein and gene expression studies was demonstrated for several differentially expressed proteins, including nucleotide biosynthetic enzymes, adenylosuccinate synthase (PurA), IMP dehydrogenase (GuaB), and aspartate carbamoyl transferase (PyrB); heat-shock protein DnaK; serine hydroxymethyl transferase (GlyA); carbon catabolite control protein (CcpA); elongation factor G (FusA); and cell division protein (FtsZ).

Influence of ethanol and low pH on arginine and citrulline metabolism in lactic acid bacteria from wine

The aim of this work was to study the effects of ethanol on cell growth and arginine and citrulline metabolism in two heterofermentative lactic acid bacteria from wine, and to determine their possible association with the formation of ethyl carbamate (EC), a carcinogenic compound. Lactobacillus hilgardii X1B is able to utilize arginine and citrulline, while Oenococcus oeni m can only use citrulline, a precursor of EC. Growth of both microorganisms was partially inhibited by 10 and 15% (v/v) ethanol. Specific arginine consumption by L. hilgardii increased when the pH value diminished from 6.5 to 3.8, but was not affected by an increasing ethanol concentration. However, the ethanol concentration affected the specific citrulline consumption of both microorganisms. Arginine metabolism by L. hilgardii X1B increased the amount of citrulline, thus allowing production of EC in the medium. Citrulline utilization by both microorganisms, at all pH values studied, indirectly inhibited the formation of EC; indeed, one of the precursors had practically disappeared after 48 h of incubation. Due to its ability to form precursors, L. hilgardii X1B has the potential to contribute to EC formation, whereas citrulline utilization by O. oeni m in the presence of ethanol may contribute to diminishing the formation of EC. Rapid degradation of citrulline in the presence of ethanol by O. oeni m is important from a toxicological point of view, because it is important to keep the EC levels as low as possible.

Cheddar cheese cooking temperature induces differential lactococcal cell permeabilization and autolytic responses as detected by flow cytometry: implications for intracellular enzyme accessibility

AIMS

To determine the influence of cheese cooking temperature on autolysis and permeabilization of two lactococcal starter strains in broth and in Cheddar cheese juice during ripening.

METHODS AND RESULTS

Flow cytometry (FCM) was used to identify and enumerate intact and permeabilized cells in broth and in Cheddar cheese juice. Levels of intracellular enzyme activities were quantified concurrently. Permeabilized cell numbers increased for both strains in broth following a temperature shift from 32 to 38 degrees C and was accompanied by an increase in the level of accessible intracellular enzyme activities. The relative proportions of intact and permeabilized cell populations, as detected by FCM in cheese juice, changed during 42-day ripening. Permeabilized cell populations increased during ripening for both strains; however, an increase in accessible intracellular enzyme activity was observed only for the highly autolytic strain Lactococcus lactis AM2.

CONCLUSIONS

Differences in the autolytic and permeabilization response induced by cooking temperature in two lactococcal strains affects intracellular enzyme accessibility in Cheddar cheese.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights the importance of the autolytic and permeabilization properties of lactic acid bacteria starter strains and their impact on cheese ripening.

Persistence of Streptococcus mutans in stationary-phase batch cultures and biofilms

Streptococcus mutans is a member of oral plaque biofilms and is considered the major etiological agent of dental caries. We have characterized the survival of S. mutans strain UA159 in both batch cultures and biofilms. Bacteria grown in batch cultures in a chemically defined medium, FMC, containing an excess of glucose or sucrose caused the pH to decrease to 4.0 at the entry into stationary phase, and they survived for about 3 days. Survival was extended up to 11 days when the medium contained a limiting concentration of glucose or sucrose that was depleted by the time the bacteria reached stationary phase. Sugar-limited cultures maintained a pH of 7.0 throughout stationary phase. Their survival was shortened to 3 days by the addition of exogenous lactic acid at the entry into stationary phase. Sugar starvation did not lead to comparable survival in biofilms. Although the pH remained at 7.0, bacteria could no longer be cultured from biofilms 4 days after the imposition of glucose or sucrose starvation; BacLight staining results did not agree with survival results based on culturability. In both batch cultures and biofilms, survival could be extended by the addition of 0.5% mucin to the medium. Batch survival increased to an average of 26 (+/-8) days, and an average of 2.7 x 10(5) CFU per chamber were still present in biofilms that were starved of sucrose for 12 days.

Identification and inactivation of genetic loci involved with Lactobacillus acidophilus acid tolerance

Amino acid decarboxylation-antiporter reactions are one of the most important systems for maintaining intracellular pH between physiological limits under acid stress. We analyzed the Lactobacillus acidophilus NCFM complete genome sequence and selected four open reading frames with similarities to genes involved with decarboxylation reactions involved in acid tolerance in several microorganisms. Putative genes encoding an ornithine decarboxylase, an amino acid permease, a glutamate gamma-aminobutyrate antiporter, and a transcriptional regulator were disrupted by insertional inactivation. The ability of L. acidophilus to survive low-pH conditions, such as those encountered in the stomach or fermented dairy foods, was investigated and compared to the abilities of early- and late-stationary-phase cells of the mutants by challenging them with a variety of acidic conditions. All of the integrants were more sensitive to low pH than the parental strain. Interestingly, each integrant also exhibited an adaptive acid response during logarithmic growth, indicating that multiple mechanisms are present and orchestrated in L. acidophilus in response to acid challenge.

Environmental stress responses in Lactobacillus: a review

Environmental stress responses in Lactobacillus, which have been investigated mainly by proteomics approaches, are reviewed. The physiological and molecular mechanisms of responses to heat, cold, acid, osmotic, oxygen, high pressure and starvation stresses are described. Specific examples of the repercussions of these effects in food processing are given. Molecular mechanisms of stress responses in lactobacilli and other bacteria are compared.