Lactic acid bacteria in yogurt production

Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus are both lactic acid bacteria, which are frequently used for the production of yogurt. The organisms differ in their lactose degradation and their fermentation end products. In this tutorial, genome-scale metabolic models will be reconstructed using gapseq starting from the organisms’ genomes in multi-protein sequences fasta file (translated sequences of protein-coding genes).

Required gapseq version: 1.2 0d0123e (or later)

Input files

  • Genome assemblies:

    • Lactobacillus delbrueckii subsp. bulgaricus ATCC 11842 = JCM 1002. RefSeq: GCF_000056065.1

    • Streptococcus thermophilus ATCC 19258. RefSeq: GCF_010120595.1

  • Growth media (milk) file: milk.csv This growth media is based on the main ingredients described for whole milk (without fatty acids), as listed here.

NOTE: All intermediate files produced by the commands below are stored at the github repository gapseq.tutorial.data, which you can download/clone if you wish to start not at the beginning but at a later step of this tutorial or want to cross-check your results.

Preparations

Download required files and rename assembly filed for the ease of this tutorial.

mkdir yoghurt
cd yoghurt

# Download genome assemblies from NCBI (RefSeq) in protein sequences
wget ftp://ftp.ncbi.nlm.nih.gov/genomes/all/GCF/000/056/065/GCF_000056065.1_ASM5606v1/GCF_000056065.1_ASM5606v1_protein.faa.gz
wget ftp://ftp.ncbi.nlm.nih.gov/genomes/all/GCF/010/120/595/GCF_010120595.1_ASM1012059v1/GCF_010120595.1_ASM1012059v1_protein.faa.gz

# Download gapfill-medium file from github
wget https://github.com/Waschina/gapseq.tutorial.data/raw/master/yogurt/milk.csv

# Rename genomes
mv GCF_000056065.1_ASM5606v1_protein.faa.gz ldel.faa.gz
mv GCF_010120595.1_ASM1012059v1_protein.faa.gz sthe.faa.gz

gapseq reconstruction pipeline

(1) Reaction & pathway prediction (2) Transporter prediction (3) Draft model reconstruction (4) Gapfilling

modelA="ldel"
modelB="sthe"

# (1) Reaction & Pathway prediction
gapseq find -p all -b 200 -m auto -t auto $modelA.faa.gz
gapseq find -p all -b 200 -m auto -t auto $modelB.faa.gz

# (2) Transporter prediction
gapseq find-transport -b 200 $modelA.faa.gz 
gapseq find-transport -b 200 $modelB.faa.gz

# (3) Building Draft Model - based on Reaction-, Pathway-, and Transporter prediction
gapseq draft -r $modelA-all-Reactions.tbl -t $modelA-Transporter.tbl -p $modelA-all-Pathways.tbl -u 200 -l 100 -c $modelA.faa.gz
gapseq draft -r $modelB-all-Reactions.tbl -t $modelB-Transporter.tbl -p $modelB-all-Pathways.tbl -u 200 -l 100 -c $modelB.faa.gz

# (4) Gapfilling
gapseq fill -m $modelA-draft.RDS -n milk.csv -c $modelA-rxnWeights.RDS -g $modelA-rxnXgenes.RDS -b 100
gapseq fill -m $modelB-draft.RDS -n milk.csv -c $modelB-rxnWeights.RDS -g $modelB-rxnXgenes.RDS -b 100

FBA and FVA prediction of metabolic by-products

Here, we will use the R-Package cobrar to perform Flux-Balance-Analysis (FBA) and Flux-Variability-Analysis (FVA) with the two reconstructed network models.

First, we define a function, that automatically performs FBA with the minimization of total flux (MTF) as secondary objective and FVA for all exchange reactions. The function also summarizes the results in a sorted data.table.

getMetaboliteProduction <- function(mod) {
  require(cobrar)
  require(data.table)
  
  # MTF
  sol.mtf <- pfba(mod)
  dt.mtf  <- data.table(ex = mod@react_id,
                        name = mod@react_name,
                        mtf.flux = sol.mtf@fluxes)
  
  # FVA
  sol.fv <- fva(mod, react = mod@react_id[grep("^EX_cpd[0-9]+_e0", mod@react_id)])
  sol.fv$growth.fraction <- NULL
  
  dt <- merge(dt.mtf, sol.fv, by.x = "ex", by.y = "react")
  dt <- dt[mtf.flux > 1e-6]
  
  return(dt[order(-mtf.flux)])
}

Now, we can apply this function to the network models of L. delbrueckii and S. thermophilus to predict the top 10 produced metabolic by-products.

library(cobrar)

ld <- readRDS("ldel.RDS") # for L. delbrueckii
st <- readRDS("sthe.RDS") # for S. thermophilus

getMetaboliteProduction(ld)[1:10]
getMetaboliteProduction(st)[1:10]

Output for L. delbrueckii (ld):

                ex               rxn.name           l           u    mtf.flux
 1: EX_cpd00221_e0  D-Lactate-e0 Exchange 0.000000000 4.700648111 4.630126137
 2: EX_cpd00067_e0         H+-e0 Exchange 4.489895516 4.581828957 4.546847983
 3: EX_cpd00108_e0  Galactose-e0 Exchange 2.499994262 2.500000000 2.500000000
 4: EX_cpd00011_e0        CO2-e0 Exchange 0.245315421 0.337271815 0.280318515
 5: EX_cpd00239_e0        H2S-e0 Exchange 0.093096142 0.093119095 0.093119080
 6: EX_cpd00130_e0   L-Malate-e0 Exchange 0.017063114 0.044365180 0.026857932
 7: EX_cpd00029_e0    Acetate-e0 Exchange 0.004859881 0.035769093 0.022378268
 8: EX_cpd00141_e0 Propionate-e0 Exchange 0.007424155 0.007443787 0.007441387
 9: EX_cpd00036_e0  Succinate-e0 Exchange 0.005246866 0.005287035 0.005269831
10: EX_cpd00047_e0    Formate-e0 Exchange 0.000000000 0.003589257 0.003586849

Output for S. thermophilus (st):

                ex                          rxn.name           l            u     mtf.flux
 1: EX_cpd00221_e0             D-Lactate-e0 Exchange 0.000000000 9.405942e+00 7.2350156625
 2: EX_cpd00011_e0                   CO2-e0 Exchange 0.000000000 1.001951e+01 0.4805519366
 3: EX_cpd00029_e0               Acetate-e0 Exchange 0.000000000 8.060152e-01 0.1185652022
 4: EX_cpd00239_e0                   H2S-e0 Exchange 0.088819140 9.015105e-02 0.0888196466
 5: EX_cpd00036_e0             Succinate-e0 Exchange 0.011037101 8.630227e-01 0.0476673792
 6: EX_cpd00363_e0               Ethanol-e0 Exchange 0.000000000 9.405942e+00 0.0019970957
 7: EX_cpd01981_e0 5-Methylthio-D-ribose-e0 Exchange 0.000665698 6.656986e-04 0.0006656986
 8: EX_cpd00020_e0              Pyruvate-e0 Exchange 0.000000000 1.209023e+00 0.0000000000
 9: EX_cpd00071_e0          Acetaldehyde-e0 Exchange 0.000000000 1.209023e+00 0.0000000000
10: EX_cpd00100_e0              Glycerol-e0 Exchange 0.000000000 3.359532e-03 0.0000000000

As expected, both organisms produce Lactate in the FBA+MTF solution. The FVA further predicted a lower bound for L-Lactate or D-Lactate production of zero. This is due to the fact, that the models harbour also the capability to produce the respective other Lactate enantiomer. In contrast to S. thermophilus, the FBA simulation predicted a release of Galactose by L. debrueckii. In fact, L. debrueckii is usually reported to be Galactose negative; i.e. does not produce acid from this hexose (https://bacdive.dsmz.de/strain/6449) and utilized only the glucose part of lactose, while S. thermophilus has been reported to be Galactose positive (https://bacdive.dsmz.de/strain/14786).