Effect of Dilution Rate on Bacteriophage Development in an Immobilized Cell System Used for Continuous Inoculation of Lactococci in Milk

Abstract

The response to bacteriophage contamination of a free cell and an immobilized cell bioreactor was studied during continuous pH-controlled fermentation of milk with Lactococcus lactis ssp. lactis. After phage infection (1 × 105pfu/ml), the phage population reached 1010pfu/ml in a free cell bioreactor operated at a dilution rate of 0 .5/h and then declined to about 107pfu/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 continuously increased until reaching 1010pfu/ml where it remained throughout the 48h of continuous culture. Conversely, phage populations decreased during the first 30min following contamination at dilution rates of 10 and 15/h but subsequently increased. For all tested conditions in the immobilized cell bioreactor, the phage-resistant population increased to 102 to 104cfu/ml, but the effluent milk contained mostly phage-sensitive cells. Analysis of bead populations showed the implantation of the phage as well as a limited population of phage-resistant 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.