Multiple control of the acetate pathway in Lactococcus lactis under aeration by catabolite repression and metabolites

Strategies to enhance stress tolerance in lactic acid bacteria across diverse stress conditions

Lactic acid bacteria (LAB) hold significant importance in diverse fields, including food technology, industrial biotechnology, and medicine. As basic components of starter cultures, probiotics, immunomodulators, and live vaccines, LAB cells resist a variety of stressors, including temperature fluctuations, osmotic and pH shocks, exposure to oxidants and ultraviolet radiation, substrate deprivation, mechanical damage, and more. To stay alive in these adversities, LAB employ a wide range of stress response strategies supported by various mechanisms, for example rearrangement of metabolism, expression of specialized biomolecules (e.g., chaperones and antioxidants), exopolysaccharide synthesis, and complex repair and regulatory systems. LAB can coordinate responses to various stressors using global regulators. In this review, we summarize current knowledge about stress response strategies used by LAB and consider mechanisms of response to specific stressful factors, supported by illustrative examples. In addition, we discuss technical approaches to increase the stress resistance of LAB, including pre-adaptation, genetic modification of strains, and adjustment of cultivation conditions. A critical analysis of the recent findings in this field augments comprehension of stress tolerance mechanisms in LAB, paving the way for prospective research directions with implications in fundamental and practical areas.

Transcriptome-phenotype matching analysis of how nitrogen sources influence Lacticaseibacillus rhamnosus tolerance to heat stress and oxidative stress

BACKGROUND

Spray drying is the most cost-effective production method for lactic acid bacteria starters, but heat and oxidative stresses result in low survival rates. The heat stress and oxidative stress tolerance of Lacticaseibacillus rhamnosus cultured in tryptone-free MRS (NP-MRS) broth was much stronger than that in MRS or tryptone-free MRS broth supplemented with phenylalanine (Phe-MRS). Here, multiple transcriptome-phenotype matching was performed on cells cultured in NP-MRS, MRS and Phe-MRS broths to reveal the mechanism by which nitrogen sources influence L. rhamnosus tolerance to heat stress and oxidative stress.

RESULTS

Compared with cells cultured in NP-MRS broth, 83 overlapping differentially expressed genes (DEGs) were downregulated by either tryptone or phenylalanine. The overlapping DEGs were mainly classified into carbohydrate metabolism and membrane transport pathways, which are often repressed by glucose during carbon catabolite repression (CCR). In the presence of glucose, the heat stress or oxidative stress tolerance of L. rhamnosus hsryfm 1301 was not strengthened by supplementation with secondary carbohydrates. Replacing glucose with mannose, fructose or ribose improved the heat stress and oxidative stress tolerance of L. rhamnosus hsryfm 1301 (5 to 46-fold).

CONCLUSIONS

Alleviation of CCR might be a reason for the resistance of L. rhamnosus hsryfm 1301 to heat stress and oxidative stress in a low-nitrogen environment. The survival rate of L. rhamnosus during spray drying will hopefully be improved by relieving CCR. It is a new discovery that nitrogen sources influence CCR in L. rhamnosus.

Current trends in the development of soy-based foods containing probiotics and paving the path for soy-synbiotics

In the world of highly processed foods, special attention is drawn to the nutrient composition and safety of consumed food products. Foods fortified with probiotic bacteria confer beneficial effects on human health and are categorized as functional foods. The salubrious activities of probiotics include the synthesis of vital bioactives, prevention of inflammatory diseases, anticancerous, hypocholesterolemic, and antidiarrheal effects. Soy foods are exemplary delivery vehicles for probiotics and prebiotics and there are diverse strategies to enhance their functionality like employing mixed culture fermentation, engineering probiotics, and incorporating prebiotics in fermented soy foods. High potential is ascribed to the concurrent use of probiotics and prebiotics in one product, termed as "synbiotics," which implicates synergy, in which a prebiotic ingredient particularly favors the growth and activity of a probiotic micro-organism. The insights on emended bioactive profile, metabolic role, and potential health benefits of advanced soy-based probiotic and synbiotic hold a promise which can be profitably implemented to meet consumer needs. This article reviews the available knowledge about strategies to enhance the nutraceutical potential, mechanisms, and health-promoting effects of advanced soy-based probiotics. Traditional fermentation merged with diverse strategies to improve the efficiency and health benefits of probiotics considered vital, are also discussed.

Lifestyle, metabolism and environmental adaptation in Lactococcus lactis

Lactococcus lactis serves as a paradigm organism for the lactic acid bacteria (LAB). Extensive research into the molecular biology, metabolism and physiology of several model strains of this species has been fundamental for our understanding of the LAB. Genomic studies have provided new insights into the species L. lactis, including the resolution of the genetic basis of its subspecies division, as well as the control mechanisms involved in the fine-tuning of growth rate and energy metabolism. In addition, it has enabled novel approaches to study lactococcal lifestyle adaptations to the dairy application environment, including its adjustment to near-zero growth rates that are particularly relevant in the context of cheese ripening. This review highlights various insights in these areas and exemplifies the strength of combining experimental evolution with functional genomics and bacterial physiology research to expand our fundamental understanding of the L. lactis lifestyle under different environmental conditions.

Respiratory Physiology of Lactococcus lactis in Chemostat Cultures and Its Effect on Cellular Robustness in Frozen and Freeze-Dried Starter Cultures

Lactococcus lactis is used in large quantities by the food and biotechnology industries. L. lactis can use oxygen for respiration if heme is supplied in the growth medium. This has been extensively studied in batch cultures using various mutants, but quantitative studies of how the cell growth affects respiratory metabolism, energetics, and cell quality are surprisingly scarce. Our results demonstrate that the respiratory metabolism of L. lactis is remarkably flexible and can be modulated by controlling the specific growth rate. We also link the physiological state of cells during cultivation to the quality of frozen or freeze-dried cells, which is relevant to the industry that may lack understanding of such relationships. This study extends our knowledge of respiratory metabolism in L. lactis and its impact on frozen and freeze-dried starter culture products, and it illustrates the influence of cultivation conditions and microbial physiology on the quality of starter cultures.

In this study, we used chemostat cultures to analyze the quantitative effects of the specific growth rate and respiration on the metabolism in Lactococcus lactis CHCC2862 and on the downstream robustness of cells after freezing or freeze-drying. Under anaerobic conditions, metabolism remained homofermentative, although biomass yields varied with the dilution rate (D). In contrast, metabolism shifted with the dilution rate under respiration-permissive conditions. At D = 0.1 h⁻¹, no lactate was produced, while lactate formation increased with higher dilution rates. Thus, a clear metabolic shift was observed, from flavor-forming respiratory metabolism at low specific growth rates to mixed-acid respiro-fermentative metabolism at higher specific growth rates. Quantitative analysis of the respiratory activity, lactose uptake rate, and metabolite production rates showed that aerobic acetoin formation provided most of the NADH consumed in respiration. Moreover, the maintenance-associated lactose consumption under respiration-permissive conditions was only 10% of the anaerobic value, either due to higher respiratory yield of ATP on consumed lactose or due to lower maintenance-related ATP demand. The cultivation conditions also affected the quality of the starter cultures produced. Cells harvested under respiration-permissive conditions at D = 0.1 h⁻¹ were less robust after freeze-drying and had lower acidification activity for subsequent milk acidification, whereas respiration-permissive conditions at the higher dilution rates led to robust cells that performed equally well or better than anaerobic cells.

IMPORTANCELactococcus lactis is used in large quantities by the food and biotechnology industries. L. lactis can use oxygen for respiration if heme is supplied in the growth medium. This has been extensively studied in batch cultures using various mutants, but quantitative studies of how the cell growth affects respiratory metabolism, energetics, and cell quality are surprisingly scarce. Our results demonstrate that the respiratory metabolism of L. lactis is remarkably flexible and can be modulated by controlling the specific growth rate. We also link the physiological state of cells during cultivation to the quality of frozen or freeze-dried cells, which is relevant to the industry that may lack understanding of such relationships. This study extends our knowledge of respiratory metabolism in L. lactis and its impact on frozen and freeze-dried starter culture products, and it illustrates the influence of cultivation conditions and microbial physiology on the quality of starter cultures.

Effects of ccpA gene deficiency in Lactobacillus delbrueckii subsp. bulgaricus under aerobic conditions as assessed by proteomic analysis

Background

Aerobic growth provides benefits in biomass yield and stress tolerance of Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus). Catabolite control protein A (CcpA) is a master regulator involved in the aerobic and anaerobic growth, metabolic production and stress response in L. bulgaricus, but its potential molecular mechanisms remains unclear. The aim of this study is to elucidate the role of CcpA in L. bulgaricus in aerobic growth at the proteomic perspective.

Results

The differential proteomic analysis was performed on the L. bulgaricus ATCC11842 and its ccpA inactivated mutant strain using iTRAQ technology. A total of 132 differentially expressed proteins were obtained, among which 58 were up-regulated and 74 were down-regulated. These proteins were mainly involved in the cellular stress response, carbohydrate and energy metabolism, amino acid transport and protein synthesis, genetic information processing. Moreover, inactivation of ccpA negatively affected the expression of key enzymes involved in glycolysis pathway, while it enhanced the expression of proteins related to the pyruvate pathway, supporting the conclusion that CcpA mediated the shift from homolactic fermentation to mixed acid fermentation in L. bulgaricus.

Conclusions

Overall, these results showed that the role of CcpA in L. bulgaricus as a pleiotropic regulator in aerobic metabolism and stress response. This proteomic analysis also provide new insights into the CcpA-mediated regulatory network of L. bulgaricus and potential strategies to improve the production of starter and probiotic cultures based on the metabolic engineering of global regulators.

Role of Glucose and 2-Oxoglutarate/Malate Translocator (OMT1) in the Production of Phenyllactic Acid and p-Hydroxyphenyllactic Acid, Two Food-Borne Pathogen Inhibitors

This work aims to uncover how glucose affected the production of phenyllactic acid (PLA) and p-hydroxyphenyllactic acid ( p-OH-PLA). The highest yields of PLA (68.53 mg/L) and p-OH-PLA (50.39 mg/L) were observed after Lactobacillus plantarum strain YM-4-3 fermentation in media containing 30 and 10 g/L glucose, respectively. Additionally, the antimicrobial activity of YM-4-3 against food-borne pathogens and the NADH/NAD⁺ ratio were positively correlated with the production of PLA and p-OH-PLA, respectively. In addition, a 2-oxoglutarate/malate translocator coding gene ( Omt1) was selected based on the qPCR results, and its knockout mutant, compared with the wild-type strain YM-4-3, showed that the PLA and p-OH-PLA production was decreased by 1.37-6.99 and 1.53-1.59 times, respectively. This result indicated that OMT1 was involved in the biosynthesis of PLA and p-OH-PLA. To conclude, this study suggests that glucose, NADH/NAD⁺ ratio and/or the Omt1 gene, PLA, and p-OH-PLA production, and antimicrobial activity contribute to a cause-and-effect relationship.

Impact of aerobic and respirative life-style on Lactobacillus casei N87 proteome

Lactic acid bacteria (LAB) are used as starter, adjunct and/or probiotic cultures in fermented foods. Several species are recognized as oxygen-tolerant anaerobes, and aerobic and respiratory cultivations may provide them with physiological and technological benefits. In this light, mechanisms involved in the adaptation to aerobic and respiratory (supplementation with heme and menaquinone) growth conditions of the O₂-tolerant strain Lactobacillus casei N87 were investigated by proteomics. In fact, in this bacterial strain, respiration induced an increase in biomass yield and robustness to oxidative, long-term starvation and freeze-drying stresses, while high concentrations of dissolved O₂ (dO₂ 60%) negatively affected its growth and cell survival. Proteomic results well paralleled with physiological and metabolic features and clearly showed that aerobic life-style led to a higher abundance of several proteins involved in carbohydrate metabolism and stress response mechanisms and, concurrently, impaired the biosynthesis of proteins involved in nucleic acid formation and translation processes, thus providing evidence at molecular level of the significant damage to L.casei N87 fitness. On the contrary, the activation of respiratory pathways due to heme and menaquinone supplementation, led to a decreased amount of chaperones and other stress related proteins. These findings confirmed that respiration reduced oxidative stress condition, allowing to positively modulate the central carbohydrate and energy metabolism and improve growth and stress tolerance features. Results of this study could be potentially functional to develop competitive adjunct and probiotic cultures effectively focused on the improvement of quality of fermented foods and the promotion of human health.

Oxygen-Inducible Conversion of Lactate to Acetate in Heterofermentative Lactobacillus brevis ATCC 367

Lactobacillus brevis is an obligatory heterofermentative lactic acid bacterium that produces high levels of acetate, which improve the aerobic stability of silages against deterioration caused by yeasts and molds. However, the mechanism involved in acetate accumulation has yet to be elucidated. Here, experimental evidence indicated that aerobiosis resulted in the conversion of lactate to acetate after glucose exhaustion in L. brevis ATCC 367 (GenBank accession number NC_008497). To elucidate the conversion pathway, in silico analysis showed that lactate was first converted to pyruvate by the reverse catalytic reaction of lactate dehydrogenase (LDH); subsequently, pyruvate conversion to acetate might be mediated by pyruvate dehydrogenase (PDH) or pyruvate oxidase (POX). Transcriptional analysis indicated that the pdh and pox genes of L. brevis ATCC 367 were upregulated 37.92- and 18.32-fold, respectively, by oxygen and glucose exhaustion, corresponding to 5.32- and 2.35-fold increases in the respective enzyme activities. Compared with the wild-type strain, the transcription and enzymatic activity of PDH remained stable in the Δpox mutant, while those of POX increased significantly in the Δpdh mutant. More lactate but less acetate was produced in the Δpdh mutant than in the wild-type and Δpox mutant strains, and more H₂O₂ (a product of the POX pathway) was produced in the Δpdh mutant. We speculated that the high levels of aerobic acetate accumulation in L. brevis ATCC 367 originated mainly from the reuse of lactate to produce pyruvate, which was further converted to acetate by the predominant and secondary functions of PDH and POX, respectively.IMPORTANCE PDH and POX are two possible key enzymes involved in aerobic acetate accumulation in lactic acid bacteria (LAB). It is currently thought that POX plays the major role in aerobic growth in homofermentative LAB and some heterofermentative LAB, while the impact of PDH remains unclear. In this study, we reported that both PDH and POX worked in the aerobic conversion of lactate to acetate in L. brevis ATCC 367, in dominant and secondary roles, respectively. Our findings will further develop the theory of aerobic metabolism by LAB.

Investigation of Factors Affecting Aerobic and Respiratory Growth in the Oxygen-Tolerant Strain Lactobacillus casei N87

Aerobic and respiratory cultivations provide benefits for some lactic acid bacteria (LAB). Growth, metabolites, enzymatic activities (lactate dehydrogenase; pyruvate and NADH oxidases, NADH peroxidase; catalase), antioxidant capability and stress tolerance of Lactobacillus casei N87 were evaluated in anaerobic, aerobic and respiratory (aerobiosis with heme and menaquinone supplementation) batch cultivations with different dissolved oxygen (DO) concentrations. The expression of pox (pyruvate oxidase) and cydABCD operon (cytochrome bd oxidase complex) was quantified by quantitative Real Time polymerase chain reaction. Respiration increased biomass production compared to anaerobiosis and unsupplemented aerobiosis, and altered the central metabolism rerouting pyruvate away from lactate accumulation. All enzymatic activities, except lactate dehydrogenase, were higher in respiratory cultures, while unsupplemented aerobiosis with 60% of DO promoted H2O2 and free radical accumulation. Respiration improved the survival to oxidative and freeze-drying stresses, while significant numbers of dead, damaged and viable but not cultivable cells were found in unsupplemented aerobic cultures (60% DO). Analysis of gene expression suggested that the activation of aerobic and respiratory pathways occurred during the exponential growth phase, and that O2 and hemin induced, respectively, the transcription of pox and cydABCD genes. Respiratory cultivation might be a natural strategy to improve functional and technological properties of L. casei.

Effect of the absence of the CcpA gene on growth, metabolic production, and stress tolerance in Lactobacillus delbrueckii ssp. bulgaricus

The catabolite control protein A (CcpA) is a kind of multi-effect regulatory protein. In the study, the effect of the inactivation of CcpA and aerobic conditions on the growth, metabolic production, and stress tolerance to heat, oxidative, and cold stresses in Lactobacillus delbrueckii ssp. bulgaricus was investigated. Results showed that inactivation of CcpA distinctly hindered growth. Total lactic acid concentration was significantly lower in aerobiosis for both strains and was lower for the mutant strain than L. bulgaricus. Acetic acid production from the mutant strain was higher than L. bulgaricus in aerobiosis compared with anaerobiosis. Enzyme activities, lactate dehydrogenase (LDH), phosphate fructose kinase (PFK), pyruvate kinase (PK), and pyruvic dehydrogenase (PDH), were significantly lower in the mutant strain than L. bulgaricus. The diameters of inhibition zone were 13.59 ± 0.02 mm and 9.76 ± 0.02 mm for L. bulgaricus in anaerobiosis and aerobiosis, respectively; and 8.12 ± 0.02 mm and 7.38 ± 0.02 mm for the mutant in anaerobiosis and aerobiosis, respectively. For both strains, cells grown under aerobic environment possess more stress tolerance. This is the first study in which the CcpA-negative mutant of L. bulgaricus is constructed and the effect of aerobic growth on stress tolerance of L. bulgaricus is evaluated. Although aerobic cultivation does not significantly improve growth, it does improve stress tolerance.

Acetate kinase isozymes confer robustness in acetate metabolism

Acetate kinase (ACK) (EC no: 2.7.2.1) interconverts acetyl-phosphate and acetate to either catabolize or synthesize acetyl-CoA dependent on the metabolic requirement. Among all ACK entries available in UniProt, we found that around 45% are multiple ACKs in some organisms including more than 300 species but surprisingly, little work has been done to clarify whether this has any significance. In an attempt to gain further insight we have studied the two ACKs (AckA1, AckA2) encoded by two neighboring genes conserved in Lactococcus lactis (L. lactis) by analyzing protein sequences, characterizing transcription structure, determining enzyme characteristics and effect on growth physiology. The results show that the two ACKs are most likely individually transcribed. AckA1 has a much higher turnover number and AckA2 has a much higher affinity for acetate in vitro. Consistently, growth experiments of mutant strains reveal that AckA1 has a higher capacity for acetate production which allows faster growth in an environment with high acetate concentration. Meanwhile, AckA2 is important for fast acetate-dependent growth at low concentration of acetate. The results demonstrate that the two ACKs have complementary physiological roles in L. lactis to maintain a robust acetate metabolism for fast growth at different extracellular acetate concentrations. The existence of ACK isozymes may reflect a common evolutionary strategy in bacteria in an environment with varying concentrations of acetate.

Regulation of acetate kinase isozymes and its importance for mixed-acid fermentation in Lactococcus lactis

Acetate kinase (ACK) converts acetyl phosphate to acetate along with the generation of ATP in the pathway for mixed-acid fermentation in Lactococcus lactis. The reverse reaction yields acetyl phosphate for assimilation purposes. Remarkably, L. lactis has two ACK isozymes, and the corresponding genes are present in an operon. We purified both enzymes (AckA1 and AckA2) from L. lactis MG1363 and determined their oligomeric state, specific activities, and allosteric regulation. Both proteins form homodimeric complexes, as shown by size exclusion chromatography and static light-scattering measurements. The turnover number of AckA1 is about an order of magnitude higher than that of AckA2 for the reaction in either direction. The Km values for acetyl phosphate, ATP, and ADP are similar for both enzymes. However, AckA2 has a higher affinity for acetate than does AckA1, suggesting an important role under acetate-limiting conditions despite the lower activity. Fructose-1,6-bisphosphate, glyceraldehyde-3-phosphate, and phospho-enol-pyruvate inhibit the activities of AckA1 and AckA2 to different extents. The allosteric regulation of AckA1 and AckA2 and the pool sizes of the glycolytic intermediates are consistent with a switch from homolactic to mixed-acid fermentation upon slowing of the growth rate.

Temperature and respiration affect the growth and stress resistance of Lactobacillus plantarum C17

AIMS

The aim of the study is to gain further insight on the respiratory behaviour of Lactobacillus plantarum and its consequences on stress tolerance.

METHODS AND RESULTS

We investigated the effect of temperature and respiration on the growth and stress (heat, oxidative, freezing, freeze-drying) response of Lact. plantarum C17 during batch cultivations. Temperature as well as respiration clearly affected the physiological state of cells, and generally, cultures grown under respiratory conditions exhibited improved tolerance of some stresses (heat, oxidative, freezing) compared to those obtained in anaerobiosis. Our results revealed that the activities in cell-free extracts of the main enzymes related to aerobic metabolism, POX (pyruvate oxidase) and NPR (NADH peroxidase), were significantly affected by temperature. POX was completely inhibited at 37°C, while the activity of NPR slightly increased at 25°C, indicating that in Lact. plantarum, the temperature of growth may be involved in the activation and modulation of aerobic/respiratory metabolism.

CONCLUSIONS

We confirmed that respiration confers robustness to Lact. plantarum cells, allowing a greater stress tolerance and advantages in the production of starter and probiotic cultures.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study on respiratory metabolism on a strain other than the model strains WCFS1; novel information on the role of temperature in the modulation of aerobic/respiratory metabolism in Lact. plantarum is presented.

Effect of inactivation of ccpA and aerobic growth in Lactobacillus plantarum: A proteomic perspective

Lactobacillus plantarum is a facultative heterofermentative lactic acid bacterium widely used in the production of most fermented food due to its ability to thrive in several environmental niches, including the human gut. In order to cope with different growth conditions, it has developed complex molecular response mechanisms, characterized by the induction of a large set of proteins mainly regulated by HrcA and CtsR repressors as well as by global regulators such as carbon catabolite control protein A (CcpA). In this study, the role of CcpA in the regulation of growth under anaerobiosis and aerobiosis, and the adaptation to aeration in L. plantarum WCFS1 were comprehensively investigated by differential proteomics. The inactivation of ccpA, in both growth conditions, significantly changed the expression level of 76 proteins, mainly associated with carbohydrate and energy metabolism, membrane transport, nucleotide metabolism, protein biosynthesis and folding. The role of CcpA as pleiotropic regulator was particularly evident at the shift from homolactic fermentation to mixed fermentation. Proteomic results also indicated that the mutant strain was more responsive to aerobic growth condition.

Inactivation of ccpA and aeration affect growth, metabolite production and stress tolerance in Lactobacillus plantarum WCFS1

The growth of Lactobacillus plantarum WCFS1 and of its ΔccpA ery mutant, WCFS1-2, was compared in batch fermentations in a complex medium at controlled pH (6.5) and temperature (30°C) with or without aeration, in order to evaluate the effect of ccpA inactivation and aeration on growth, metabolism and stress resistance. Inactivation of ccpA and, to a lesser extent, aeration, significantly affected growth, expression of proteins related to pyruvate metabolism and stress, and tolerance to heat, oxidative and cold/starvation stresses. The specific growth rate of the mutant was ca. 60% of that of the wild type strain. Inactivation of ccpA and aerobic growth significantly affected yield and production of lactic and acetic acid. Stationary phase cells were more stress tolerant than exponential phase cells with little or no effect of inactivation of ccpA or aeration. On the other hand, for exponential phase cells inactivation of ccpA impaired both heat stress and cold/starvation stress, but increased oxidative stress tolerance. For both strains, aerobically grown cells were more tolerant of stresses. Evidence for entry in a viable but non-culturable status upon prolonged exposure to cold and starvation was found. Preliminary results of a differential proteomic study further confirmed the role of ccpA in the regulation of carbohydrate catabolism and class I stress response genes and allow to gain further insight on the role of this pleiotropic regulator in metabolism and stress. This is the first study in which the impact of aerobic growth on stress tolerance of L. plantarum is evaluated. Although aerobic cultivation in batch fermentations does not improve growth it does improve stress tolerance, and may have significant technological relevance for the preservation of starter and probiotic cultures.

Standardized assay medium to measure Lactococcus lactis enzyme activities while mimicking intracellular conditions

Knowledge of how the activity of enzymes is affected under in vivo conditions is essential for analyzing their regulation and constructing models that yield an integrated understanding of cell behavior. Current kinetic parameters for Lactococcus lactis are scattered through different studies and performed under different assay conditions. Furthermore, assay conditions often diverge from conditions prevailing in the intracellular environment. To establish uniform assay conditions that resemble intracellular conditions, we analyzed the intracellular composition of anaerobic glucose-limited chemostat cultures of L. lactis subsp. cremoris MG 1363. Based on this, we designed a new assay medium for enzyme activity measurements of growing cells of L. lactis, mimicking as closely as practically possible its intracellular environment. Procedures were optimized to be carried out in 96-well plates, and the reproducibility and dynamic range were checked for all enzyme activity measurements. The effects of freezing and the carryover of ammonium sulfate from the addition of coupling enzymes were also established. Activities of all 10 glycolytic and 4 fermentative enzymes were measured. Remarkably, most in vivo-like activities were lower than previously published data. Yet, the ratios of V(max) over measured in vivo fluxes were above 1. With this work, we have developed and extensively validated standard protocols for enzyme activity measurements for L. lactis.