Dairy practices select for thermostable endolysins in phages

Endolysins are phage lytic enzymes that degrade the bacterial cell wall to release phage progeny. We found that the diversity of endolysins is unsuspectedly low and dominated by a single structural type in distinct phages that infect Streptococcus thermophilus . Through X-ray crystallography, we discovered that this type of endolysin contains a highly conserved calcium-binding motif in its cell wall binding domain. Inactivation of the motif in the purified endolysin or in the phage genome revealed its key role in stabilizing the enzyme under conditions that mimic the cheesemaking process, including at elevated temperatures. Fermented dairy products such as yogurt and cheeses are incubated or cooked at higher temperatures and require the addition of thermophilic lactic acid bacteria such as S. thermophilus to drive milk fermentation. It appears that dairy practices have influenced the genetic signatures of streptococcal phages by selecting for thermostable endolysins, which in turn have limited their diversity.