A Quorum-Sensing-Induced Bacteriophage Defense Mechanism. mBio

ABSTRACT One of the key determinants of the size, composition, structure, and development of a microbial community is the predation pressure by bacteriophages. Accordingly, bacteria have evolved a battery of antiphage defense strategies. Since main-taining constantly elevated defenses is costly, we hypothesize that some bacteria have additionally evolved the abilities to esti-mate the risk of phage infection and to adjust their strategies accordingly. One risk parameter is the density of the bacterial pop-ulation. Hence, quorum sensing, i.e., the ability to regulate gene expression according to population density, may be an important determinant of phage-host interactions. This hypothesis was investigated in the model system of Escherichia coli and phage . We found that, indeed, quorum sensing constitutes a significant, but so far overlooked, determinant of host susceptibil-ity to phage attack. Specifically, E. coli reduces the numbers of receptors on the cell surface in response toN-acyl-L-homoserine lactone (AHL) quorum-sensing signals, causing a 2-fold reduction in the phage adsorption rate. The modest reduction in phage adsorption rate leads to a dramatic increase in the frequency of uninfected survivor cells after a potent attack by virulent phages. Notably, this mechanismmay apply to a broader range of phages, as AHLs also reduce the risk of phage infection through a different receptor. IMPORTANCE To enable the successful manipulation of bacterial populations, a comprehensive understanding of the factors that naturally shape microbial communities is required. One of the key factors in this context is the interactions between bacteria and the most abundant biological entities on Earth, namely, the bacteriophages that prey on bacteria. This proof-of-principle stud