Involvement of a plasmid in production of ropiness (mucoidness) in milk cultures by Streptococcus cremoris MS

Abstract

Curing and genetic transfer experiments showed that lactose-fermenting ability (Lac+) and the ability to produce mucoidness in milk cultures (Muc+) in Streptococcus cremoris MS were coded on plasmids. The Lac+ phenotype was associated with a 75.8-megadalton plasmid, pSRQ2201. The Muc+ phenotype was associated with a 18.5-megadalton plasmid, pSRQ2202. The Lac plasmid, pSRQ2201, was first conjugatively transferred from S. cremoris MS to Lac- S. lactis ML-3/2.2. Later, the Muc plasmid, pSRQ2202, was conjugatively transferred from Lac- Muc+ S. cremoris MS04 to Lac+ nonmucoid S. lactis transconjugant ML-3/2.201. Subsequently, pSRQ2201 and pSRQ2202 were cotransferred from Lac+ Muc+ S. lactis transconjugant ML-3/2.202 to Lac-, nonmucoid, malty S. lactis 4/4.2 and S. lactis subsp. diacetylactis SLA3.25. Transconjugants showing pSRQ2201 were Lac+; those containing pSRQ2202 were Muc+. With the transfer of pSRQ2202, the transconjugants S. lactis ML-3/2.202 and S. lactis subsp. diacetylactis SLA3.2501 not only acquired the Muc+ phenotype but also resistance to bacteriophages, which were lytic to the respective parent strains of S. lactis ML-3/2.201 and S. lactis subsp. diacetylactis SLA3.25.