Atypical citrate‐fermenting Lactococcus lactis strains isolated from dromedary’s milk

Aim: The aim was to isolate and characterize Lactococcus strains with new properties compared to those of usual Lactococcus dairy starters derived from cow’s milk. Methods and Results: Algerian dromedary’s milk was screened for proteolytic isolates able to grow rapidly on agar milk medium. PCR experiments revealed that 74 proteolytic isolates belonged to the genus Lactococcus and harboured the prtP gene encoding the lactococcal cell‐surface proteinase. Among these, 85% were able to ferment citrate (Cit⁺ phenotype) and were classified as Lactococcus lactis ssp. lactis biovar. diacetylactis. This classification was confirmed after sequencing of the 16S rDNA gene of five Cit⁺ isolates. In contrast to dairy lactococci described in the literature, several Cit⁺ isolates exhibited a tolerance to 50°C (Ther⁺) and alkaline pH. Two genetic approaches allowed to show the presence of four independent plasmids (so‐called pTher, pPrt, pLac, pCit) associated with the four respective phenotypes: Ther⁺, cell‐surface proteinase activity PrtP (PrtP⁺), lactose catabolism (Lac⁺) and citrate utilization (Cit⁺). Two types of pCit plasmid were amplified by inverse PCR: class 1 was characterized by a 9‐kb plasmid harbouring the expected lactococcal citQRP operon and class 2 by a 23‐kb plasmid harbouring the Leuconostoc cit cluster (citI‐CitMCDEFGRP). Conclusions: This work enlarges knowledge of the biovariety diacetylactis by far mainly limited to the citrate‐fermenting ability and suggests that the cit plasmid system of some lactococcal strains could have been acquired from another lactic acid bacteria (Leuconostoc spp.). Significance and Impact of the Study: This study reveals new potential dairy lactococci starters of the biovariety diacetylactis able to grow rapidly in milk at a higher temperature in addition to their casein, lactose and citrate‐utilizing abilities.