Bioreactor studies on temperature induction of the Q − mutant of bacteriophage λ in Escherichia coli

Abstract

The parasite-host interactions between bacteriophage λ (denoted as λ) and Escherichia coli bacteria were studied in different bioreactor systems. Although the replicated λ-DNA of Q − mutant remains naked for a longer time to reach a high gene expression, the epidemic of λ-infection and the coevolutionary host-phage relations limit the temperature induction efficiency. The temperature induction is strongly dependent upon the susceptible population density at which λ -infection is activated. Maximum β-galactosidase expression occurs at the threshold of the infection system. According to this concept, the lethal level of parasitic λ to hosts is approx. 5 × 10 6 pfu/ml. Since a higher phage λ burden is exerted upon host cells at a low OD sh, the system moves towards virulence reduction for total survival. Prey-predator isocline analysis is used to consider the stability of the outcome of infection. The host growth has a destabilizing effect at lower population densities and a stabilization effect at higher population. Based upon the predictions, a substrate enrichment enhances bacterial growth and reporter protein production. However, the operations still need to follow the trajectory of threshold tie line to guarantee maximal productivity. Since the washout of infected cells reduces induction performance in continuous cultures, a batch mode of operation is better than continuous stirred tank reactor (CSTR) mode to achieve high gene expression. The threshold cell density regulates induction performances and therefore produces the optimal gene expression by maintaining maximal viable cells that provide sufficient resources for λ expression.