EFFECT OF DILUTION RATE ON BACTERIOPHAGE DEVELOPMENT IN AN IMMOBILIZED CELL SYSTEM USED FOR CONTINUOUS INOCULATION OF LACTOCOCCI IN MILK

The response to bacteriophage contamination of a free cell and an immo bilized cell bioreactor was studied during continuous pH-controlled fe rmentation of milk with Lactococcus lactis ssp. lactis. After phage in fection (1 x 10(5) pfu/ml), the phage population reached 10(10) pfu/ml in a free cell bioreactor operated at a dilution rate of 0.5/h and th en declined to about 10(7) pfu/ml as a phage-resistant cell population became established in the system. In the immobilized cell bioreactor operated at dilution rates of. 0.5 and 3/h, the phage population conti nuously increased until reaching 10(10) pfu/ml where it remained throu ghout the 48 h of continuous culture. Conversely, phage populations de creased during the first 30 min following contamination at dilution ra tes of 10 and 15/h but subsequently increased. For all tested conditio ns in the immobilized cell bioreactor, the phage-resistant population increased to 10(2) to 10(4) cfu/ml, but the effluent milk contained mo stly phage-sensitive cells. Analysis of bead populations showed the im plantation of the phage as well as a limited population of phage-resis tant cells. The effluent biomass from the immobilized cell bioreactor sharply reduced acidifying activity because this biomass was composed mainly of phage-sensitive cells and contained high phage populations.