Abstract
Lactococcus lactis is a lactic acid bacterium of major importance for the dairy industry and for human health. Recent sequencing surveys of this species have provided evidence that all lactococcal genomes contain prophages and prophage-like elements. The prophage-related sequences encompass up to 10% of the bacterial chromosomes and thus contribute significantly to the genetic diversity of lactococci. However, the impact of these resident prophages on the physiology of L. lactis is presently unknown. The genome of the first sequenced prototype strain, L. lactis ssp. lactis IL1403, contains six prophage-like elements which together represent 6.7% of the IL1403 chromosome. Diverse prophage genes other than those encoding phage repressors have been shown to be expressed in lysogenic conditions, suggesting that prophage genes are indeed able to modulate the physiology of their host. To elucidate the effect of resident prophages on the behavior of L. lactis in different growth conditions, we constructed and characterized, for the first time, a derivative strain of IL1403 that is prophage-free. This strain provides unique experimental opportunities for the study of different aspects of lactococcal physiology using the well-defined genetic background of IL1403. Here, we show that resident prophages modify the growth and survival of the host strain to a considerable extent in different conditions, including in the gastrointestinal environment. They also may affect cellular autolytic properties and the host cells’ susceptibility to virulent bacteriophages and antimicrobial agents. It thus appears that prophages contribute significantly to lactococcal cell physiology and might play an important role in the adaptation of L. lactis to cultivation and environmental conditions.
Highlights
Lactococci (Lactococcus lactis species) are widely used in the industrial manufacturing of fermented dairy products (Leroy and De Vuyst, 2004; Smit et al, 2005)
We examined the impact of IL1403 prophages on aspects of cell physiology that are of particular relevance for the industrial and health applications of L. lactis
The bIL285 is integrated between the ykhD gene and the radC gene, which are oriented in the opposite direction to the prophage lysin and integrase genes, respectively
Summary
Lactococci (Lactococcus lactis species) are widely used in the industrial manufacturing of fermented dairy products (Leroy and De Vuyst, 2004; Smit et al, 2005). Prophage-related sequences encompass from 3 to 10% of the total genome, and represent a significant part of the observed genomic differences among L. lactis strains (Kelleher et al, 2018). Lactis IL1403 strain is the first completely sequenced lactococcal strain (Bolotin et al, 2001) It has been widely used for both fundamental and applied research, including diverse studies of lactococcal genetics and physiology, as well as lactococcal phage biology (Chopin et al, 2005; Douillard and De Vos, 2014; Kok et al, 2017). The genome of IL1403 contains six prophage-like elements, which represent in total 6.7% of the IL1403 chromosome (Chopin et al, 2001). We examined the impact of IL1403 prophages on aspects of cell physiology that are of particular relevance for the industrial and health applications of L. lactis. Our results reinforce previous findings of the complex effect of prophages on the behavior of the bacterial cell, and demonstrate the significant contributions of prophages to the adaptive phenotypic plasticity and fitness of L. lactis
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