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In
the dairy industry, exopolysaccharides (EPS) contribute to improving
the texture and viscosity of cheese and yoghurt and also receive
increasing attention because of their beneficial properties for health.
For lactic acid bacteria, the production of EPS is well studied.
However, for dairy propionibacteria the biosynthesis of EPS is poorly
documented. A polysaccharide synthase-encoding gene was identified in
the genome of Propionibacterium freudenreichii subsp. shermanii TL 34 (CIP 103027). This gene best aligns with Tts, the polysaccharide synthase gene of Streptococcus pneumoniae
type 37 that is responsible for the production of a β-glucan capsular
polysaccharide. PCR amplification showed the presence of an internal
fragment of this gene in twelve strains of P. freudenreichii subsp. shermanii
with a ropy phenotype in YEL+ medium. The gene sequence is highly
conserved, as less than 1% of nucleotides differed among the 10 strains
containing the complete gtf gene. The same primers failed to detect the gene in Propionibacterium acidipropionici strain TL 47, which is known to excrete exopolysaccharides in milk. The presence of (1→3, 1→2)-β-d-glucan capsule was demonstrated for 7 out of 12 strains by agglutination with a S. pneumoniae-type
37-specific antiserum. The presence of mRNA corresponding to the gene
was detected by RT-PCR in three strains at both exponential and
stationary growth phases. This work represents the first identification
of a polysaccharide synthase gene of P. freudenreichii, and further studies will be undertaken to elucidate the role of capsular EPS.
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