Genetic response to bacteriophage infection in Lactococcus lactis reveals a four-strand approach involving induction of membrane stress proteins, D-alanylation of the cell wall, maintenance of proton motive force, and energy conservation

Abstract

In this study, whole-genome microarrays were used to gain insights into the global molecular response of Lactococcus lactis subsp. lactis IL1403 at an early stage of infection with the lytic phage c2. The bacterium differentially regulated the expression of 61 genes belonging to 14 functional categories, including cell envelope processes (12 genes), regulatory functions (11 genes), and carbohydrate metabolism (7 genes). The nature of these genes suggests a complex response involving four main mechanisms: (i) induction of membrane stress proteins, (ii) d-alanylation of cell wall lipoteichoic acids (LTAs), (iii) maintenance of the proton motive force (PMF), and (iv) energy conservation. The phage presence is sensed as a membrane stress in L. lactis subsp. lactis IL1403, which activated a cell wall-targeted response probably orchestrated by the concerted action of membrane phage shock protein C-like homologues, the global regulator SpxB, and the two-component system CesSR. The bacterium upregulated genes (ddl and dltABCD) responsible for incorporation of d-alanine esters into LTAs, an event associated with increased resistance to phage attack in Gram-positive bacteria. The expression of genes (yshC, citE, citF) affecting both PMF components was also regulated to restore the physiological PMF, which was disrupted following phage infection. While mobilizing the response to the phage-mediated stress, the bacterium activated an energy-saving program by repressing growth-related functions and switching to anaerobic respiration, probably to sustain the PMF and the overall cell response to phage. To our knowledge, this represents the first detailed description in L. lactis of the molecular mechanisms involved in the host response to the membrane perturbations mediated by phage infection.

Fig. 1.

Phage-host challenges to determine the MOI necessary to lyse completely a bacterial population within 40 min. Effects of the addition of phage c2 (indicated by an arrow) at MOIs of 0 (▪), 200 (▴), 400 (•), 600 (⋄), and 800 (□) on the viability of L. lactis IL1403 cells growing in log phase.

Fig. 2.

Flow cytometric analysis of membrane potential in rhodamine 123-stained cells of L. lactis IL1403 before (a) and 10 min (b), 15 min (c), and 25 min (d) after addition of phage c2.

Fig. 3.

Putative working model of the cell wall stressosome of L. lactis IL1403 in response to phage infection.