The present invention is directed to a method of producing a Streptococcus thermophilus strain comprising the steps of

a) Providing a mother strain in the form of Streptococcus thermophilus DSM32502,
b) Growing a culture of the mother strain in the presence of a bacteriophage, to which the mother strain is not resistant, to obtain a number of mutant strains, which are resistant to the said bacteriophage,
c) Measuring the acidification time of the bacteriophage-resistant mutant strains and the mother strain in a milk base and selecting at least one mutant strain with a reduced acidification time as compared to the mother strain to obtain a fast-acidifying mutant strain.