Transfection of Lactobacillus bulgaricus protoplasts by bacteriophage DNA.

A protoplast transfection system has been developed for Lactobacillus bulgaricus. The procedure involves a polyethylene glycol-mediated fusion of bacteriophage DNA encapsulated in liposomes into mutanolysin-treated cells. With L. bulgaricus B004 and DNA isolated from the phage phi c5004, transfection reached a maximum when at least 95% of the cells were osmotically fragile. The incorporation of phage DNA into liposomes was essential; no transfectants were detected in the absence of liposomes. The largest number of transfectants was observed after longer periods (20 min) of fusion of mutanolysin-treated cells and liposomes with polyethylene glycol. The maximum efficiency of 5 x 10(7) PFU/microgram of DNA was reached after a 24-h incubation in growth media prior to plating transfected cells in an agar overlay to detect the appearance of plaques. A minimum of 4 h of incubation in growth medium after fusion was required to detect the production and release of virions. The possibility that the high frequencies observed were due to bursting of transfected cells and subsequent infection of additional cells was found not to be a factor. The number of transfectants observed was directly proportional to the quantity of DNA added. These results define conditions appropriate for the introduction of DNA into L. bulgaricus.