In Swiss-type cheese, Propionibacterium freudenreichii produces the sweetish, nutty flavor and typical eyes by metabolizing mainly lactate to propionate, acetate and CO2. In the presence of both lactate isomers, P. freudenreichii metabolizes preferably L-lactate. The high intracellular pyruvate concentration in P. freudenreichii is thought to inhibit the D-lactate dehydrogenase (D-Ldh) more than the L-Ldh (Crow, V. L. 1986, Appl. Environ. Microbiol. 52, 352-358). We checked 48 P. freudenreichii strains from our strain collection for the presence of Ldh genes. All of the tested strains harbored two L-Ldh genes (EC 1.1.1.27), but only 30 strains carried the D-Ldh gene (EC 1.1.1.28). We hypothesized that strains without D-Ldh metabolize less lactate, grow less and thus may improve the ripening stability of long-ripened Swiss-type cheeses. The correlation between Ldh genes and lactate catabolism was examined measuring growth (OD650) after 72h, at 30°C and aerobic incubation in three different broths: casein-peptone & yeast (CPY), L-lactate & CPY and D-lactate & CPY. Compared to CPY no significant growth in L- and D-lactate broth was found for 14 out of the 48 strains. Six out of the 18 strains lacking the D-Ldh gene grew exclusively in the presence of L-lactate, whereas three strains grew in D- and L-lactate broth. Out of the 30 strains harboring the D-Ldh gene 12 strains grew only in D-lactate broth, 12 strains grew in D- and L-lactate broth, and one strain grew only in L-lactate broth. The anaerobic growth and the use of D-lactate by 14 selected strains (with and without D-Ldh) were further studied in cheese and in lactate broths. In contrast to the results obtained under aerobic conditions, strains lacking the D-Ldh gene grew under anaerobic conditions in D-lactate broth as well. Further studies are needed to investigate the growth and metabolic activity of these strains in cheese.
