Infective cycle of dairy bacteriophages

Abstract

Phages follow two strategies to survive: they can either infect and lyse the host cell (virulent/lytic phages) or insert its genome into the host chromosome (prophages). In either case, the first step is the adsorption of virions to specific attachment sites (receptors) on the cell wall surface. Adsorption velocity and efficiency depend upon a series of external factors, such as the presence of divalent ions (Ca2+, Mg2+, Mn2+), pH, temperature, inorganic salts, and the physiological state of bacterial cells. In general, both divalent cations and physiological state of the host show little or no influence on the rate of adsorption, whereas maximum adsorption is in general attained at the optimal cell growth temperature and pH close to neutrality. Adsorption interference is regarded as a significant mechanism of phage resistance in lactic acid bacteria (LAB), and thus research on this topic is very valuable. These studies are directed to characterize the chemical nature of phage receptors and encompass the use of diverse chemical compounds and enzymes. When receptors are carbohydrate in nature, the use of lectins and either simple or complex saccharides in competition and desorption assays could be very useful. Injection of phage DNA into the cytoplasm comes after irreversible attachment step of adsorption. Unlike adsorption, the DNA injection process was found to be highly dependent on Ca2+. Besides, temperature and physiological state of the host had an important influence as well. During the latent phase of a lytic cycle, phage DNA is replicated and phage genes are transcribed in strict order. Proteins of tailed phages are assembled through separate pathways into heads, tails and fibers. Once phage virions are fully assembled, they must escape from the host cell so as to persist in nature. To do this, lytic phages must degrade or at least compromise the peptidoglycan layer of the cell wall.Likewise the rest of tailed bacteriophages, those infecting LAB lyse the host using two phage-encoded factors: a small hydrophobic protein that permeabilizes the membrane (holin), allowing the second one (lysin) to gain access to the cell wall. Genes encoding for either holin or lysin were found to be highly conserved among phages infecting LAB.