Lactic acid bacteria
The invention relates to a polynucleotide comprising a lacZ gene (lacZ FS ) encoding a β-galactosidase characterized by a particular profile regarding its efficiency of hydrolysis of lactose. The invention is also directed to a Streptococcus thermophilus strain comprising a lacZ FS allele and bacterial composition thereof, and their use to obtain fermented milk not undergoing post-acidification.
1. A polynucleotide encoding a β-galactosidase FS , which comprises an amino acid sequence which has at least 95% identity with SEQ ID NO:2; and which is defined as a lacZ allele which increases the ratio of the activity of lactose importation of the LacS permease at pH 4.5 over the activity of lactose hydrolysis of the beta-galactosidase at pH 4.5 above 8 in a DGCC715 derivative, said DGCC715 derivative being a strain DGCC715 (deposited at the DSMZ on Feb. 12, 2019 under the accession number DSM33036), into which its lacZ gene was replaced by said polynucleotide encoding a β-galactosidase FS .
2. The polynucleotide according to claim 1 , encoding a β-galactosidase FS comprising an amino acid suppression, an amino acid addition, an amino acid substitution or an amino acid suppression and addition, relative to a β-galactosidase selected from the group consisting of:
a) a β-galactosidase having an amino acid sequence as defined in SEQ ID NO:2; and
b) a β-galactosidase variant comprising an amino acid sequence having at least 95% identity with SEQ ID NO:2, encoded by a lacZ variant allele which does not increase the ratio LacS pH4.5 over LacZ pH4.5 in a DGCC715 derivative to 5 or more than 5, said DGCC715 derivative being a strain DGCC715 into which its lacZ gene was replaced by said lacZ variant allele.
3. The polynucleotide according to claim 1 , wherein the sequence of said β-galactosidase FS does not comprise an arginine at position 354.
4. The polynucleotide according to claim 1 , wherein the sequence of said β-galactosidase FS comprises:
a) an amino acid sequence which is otherwise as defined in SEQ ID NO:2, but which does not comprise an arginine at position 354;
b) an amino acid sequence which has at least 95% identity with SEQ ID NO:2 and does not comprise an arginine at position 354;
c) an amino acid sequence which is otherwise as defined as the one of a β-galactosidase variant protein having at least 95% identity with SEQ ID NO:2, but which does not comprise an arginine at position 354.
5. The polynucleotide according to claim 1 , wherein the sequence of said β-galactosidase FS comprises:
a) an amino acid sequence which is otherwise as defined in SEQ ID NO:2, but which comprises a cysteine or an equivalent amino acid thereof at position 354;
b) an amino acid sequence which has at least 95% identity with SEQ ID NO:2 and comprises a cysteine or an equivalent amino acid thereof at position 354;
c) an amino acid sequence which is otherwise as defined as the one of a β-galactosidase variant protein having at least 95% identity with SEQ ID NO:2, but which comprises a cysteine or an equivalent amino acid thereof at position 354.
6. A polynucleotide comprising a part of at least 100 nucleotides of the polynucleotide according to claim 1 , wherein said nucleotide part encompasses a codon corresponding to the residue 354 of said β-galactosidase FS .
7. A Streptococcus thermophilus strain comprising an allele of the lacZ gene which is a lacZ FS allele encoding a β-galactosidase FS according to claim 1 .
8. A Streptococcus thermophilus strain according to claim 7 , characterized by a difference of efficiency of hydrolysis (ΔEH) of the imported lactose which is less than −0.5 calculated by the following formula (I):
Δ
EH
=
ln
[
LacS
p
H
6
LacZ
p
H
6
]
-
ln
[
LacS
pH
4.5
LacZ
pH
4.5
]
(
I
)
in which formula (I), LacS pH6 and LacS pH4.5 represent the activity of lactose importation of the LacS permease at pH 6 and at pH 4.5 respectively, and LacZ pH6 and LacZ pH4.5 represent the activity of lactose hydrolysis of the beta-galactosidase at pH 6 and at pH 4.5 respectively.
9. A bacterial composition comprising the Streptococcus thermophilus strain of claim 7 , and one or more further lactic acid bacteria selected from the group consisting of Streptococcus, Lactococcus, Lactobacillus, Leuconostoc, Pediococcus, Enterococcus, Oenococcus and Bifidobacterium.
10. A food or feed product comprising the Streptococcus thermophilus strain of claim 7 or a bacterial composition comprising the Streptococcus thermophilus strain of claim 7 , wherein the food or feed product is a dairy, meat or cereal food or feed product.
11. A method for manufacturing a fermented product, comprising:
a) inoculating a substrate with the Streptococcus thermophilus strain of claim 7 or a bacterial composition comprising the Streptococcus thermophilus strain of claim 7 ; and
b) fermenting the inoculated substrate obtained from step a) to obtain a fermented product.
12. A method according to claim 11 , for manufacturing stirred yoghurt, comprising:
a) fermenting a milk substrate inoculated with the Streptococcus thermophilus strain of claim 7 or a bacterial composition comprising the Streptococcus thermophilus strain of claim 7 , to obtain a stirred yoghurt with a pH from 4.2 to 4.7;
b) cooling the stirred yoghurt;
c) packing the stirred yoghurt; and
d) transferring the packages of step c) into a storage cold room;
wherein the temperature of cooling and packing is at least 24° C.
13. A method according to claim 11 , for manufacturing stirred yoghurt, comprising:
a) fermenting a milk substrate inoculated with the Streptococcus thermophilus strain of claim 7 or a bacterial composition comprising the Streptococcus thermophilus strain of claim 7 , to obtain a stirred yoghurt with a pH from 4.2 to 4.7;
b) packing the stirred yoghurt; and
c) transferring the packages of step b) into a storage cold room;
wherein the process does not comprise any cooling step between end of fermentation and packing.
14. A method according to claim 11 , for manufacturing set yoghurt, comprising:
a) packing a milk substrate inoculated with the Streptococcus thermophilus strain of claim 7 or a bacterial composition comprising the Streptococcus thermophilus strain of claim 7 ;
b) fermenting the inoculated milk substrate to obtain a set yoghurt with a pH from 4.2 to 4.7; and
c) directly transferring the packages of step b) into a storage cold room, wherein said process does not comprise a cooling step in a cooling room after the fermentation step b).
15. A method to prepare a Streptococcus thermophilus strain with a full STOP phenotype, comprising:
a) providing a Streptococcus thermophilus strain, having a ratio of the activity of lactose importation of the LacS permease at pH 4.5 over the activity of lactose hydrolysis of the beta-galactosidase at pH 4.5 which is less than 5;
b) replacing the allele of the lacZ gene of said Streptococcus thermophilus strain of step a) with a polynucleotide according to claim 1 , or replacing a part of the allele of the lacZ gene of said Streptococcus thermophilus strain of step a) with a polynucleotide comprising a part of at least 100 nucleotides of the sequence of said β-galactosidase FS comprising or consisting of an amino acid sequence which has at least 95% identity with SEQ ID NO:2, wherein said nucleotide part encompasses the codon corresponding to the residue 354 of said β-galactosidase FS , or modifying the sequence of the lacZ gene of said Streptococcus thermophilus strain of step a) to have a lacZ FS allele with the same sequence as a polynucleotide according to claim 1 ; and
c) recovering the Streptococcus thermophilus strain(s) with a full STOP phenotype when used to ferment milk.
16. A method to identify a lacZ FS allele encoding a β-galactosidase FS , comprising:
a) inserting the lacZ allele to be tested in lieu of the allele of the lacZ gene of the strain DGCC715 (deposited at the DSMZ on Feb. 12, 2019 under the accession number DSM33036), to obtain a DGCC715-derivative; and
b) determining the activity of lactose importation of the LacS permease at pH 4.5 (LacS pH4.5 ) and the activity of lactose hydrolysis of the beta-galactosidase at pH 4.5 (LacZ pH4.5 );
wherein a ratio LacS pH4.5 over LacZ pH4.5 which is more than 8 is indicative of a lacZ allele which is a lacZ FS allele encoding a β-galactosidase FS .