Loss of phage resistance encoded by plasmid pSK112 in chemostat cultures of Lactococcus lactis ssp. cremoris SK110.

In cultures of L. lactis ssp. cremoris SK110, phage SK11G-resistant through the presence of pSK112, phage-sensitive variants segregated spontaneously that lacked the plasmid. In overnight batch culture these comprised up to 1% of the total population. Upon prolonged incubation in chemostat culture, a further loss of resistance was observed after a lag period. At high growth rates (0.7 h-1) this period amounted to approximately 35 generations, whereas cultures grown at rates of 0.4 and 0.1 h-1 remained resistant for 55 and 70 generations, respectively. At average-to-high growth rate, characteristics of the partially mixed populations that evolved were comparable to those of pure cultures of L. lactis ssp. cremoris SK110. However, in the culture fluid of the mixed populations that occurred at growth rate 0.1 h-1, higher acetate and formate concentrations were found than in the fluid of pure cultures of L. lactis ssp. cremoris SK110. This indicated that the former metabolized lactose more efficiently. Competition experiments between the resistant strain and a cured, sensitive derivative, L. lactis ssp. cremoris SK112, gave stable mixed populations. It is concluded that at average-to-high growth rates, loss of resistance from cultures of L. lactis ssp. cremoris SK110 had occurred due to instability of the plasmid and not to a competitive disadvantage of the resistant strain towards emerging sensitive variants.