Rapid selection of phage-resistant mutants in Streptococcus thermophilus by immunoselection and cell sorting

Immunoselection and flow cytometry allowed the isolation from Streptococcus thermophilus strain Str31 of double mutants displaying resistance to the phage φ31 and good acid production. Strain Str31 is very sensitive to phage φ31. This phage–host system seemed therefore particularly suitable to test the validity of the selection method adopted in this study. Mutants were stable with respect to both characters. The isolation of the double mutants required 4 to 5 days. The approach does not involve genetic manipulations and can therefore be an alternative to genetic engineering when this technology cannot be applied.