Lactobacilli are of great commercial value because they are widely used as
starters in food fermentation. The species Lactobacillus delbrueckii, which comprises
the three subspecies bulgaricus, lactis and
delbrueckii, is important in dairy products and vegetables. The subspecies
bulgaricus is present in yogurt, and subspecies lactis
is recovered from whey, starter cultures and cheeses (Stiles & Holzapfel, 1997). It is
unusual to recover Lb. delbrueckii subsp. delbrueckii
(Lb. delbrueckii) from dairy
products, from which Lb. delbrueckii subsp. lactis
(Lb. lactis) and subsp. bulgaricus
(Lb. bulgaricus) are typically isolated. However, given the similarity of these two
latter subspecies, a clear identification of isolates on the basis of phenotype criteria
alone is often problematic (Dellaglio, 1989; Millière et al. 1996).
The use of DNA probes and methods based on polymerase chain reaction (PCR)
have greatly facilitated diagnostic identification of bacteria. For the lactobacilli,
DNA probes have been described for Lb. helveticus,
Lb. acidophilus, Lb. fermentum,
Lb. plantarum and most of the other Lactobacillus
species (Pot et al. 1994; Tailliez et
al. 1994; Quere et al. 1997). For Lb. delbrueckii,
an EcoRI DNA fragment of the
plasmid pY85 was used as a probe for this species, although it was not able to
discriminate its three subspecies (Delley et al. 1990).
In our laboratory, a specific amplification of a DNA fragment of ∼1·7 kbp using
universal primers for the amplification of ribosomal RNA (rRNA) genes was found
only for dairy isolates of Lb. lactis and not for Lb. bulgaricus,
Lb. delbrueckii, Lb. helveticus and Lb. acidophilus
(G. Giraffa, P. de Vecchi and L. Rossetti, unpublished
results). This prompted us to test the possible use of this fragment as a specific DNA
probe for Lb. lactis. To this end, Southern and dot blot hybridization experiments
were carried out with total DNA of several strains belonging to different lactic acid
bacteria (LAB) species.