Host range and specificity of P. acnes phages. Resistance/susceptibility of Propionibacterium strains (in rows) against 15 of the 48 sequenced phages (in columns) is shown. (a) Phage infection outcomes of 67 P. acnes strains. All P. acnes strains in clades IA-1, IA-2, IB-1 and IB-2 were susceptible to the tested phages, while phage resistance was found in strains of clades IB-3, II and III, colored in pink. The dendrogram to the left shows the phylogenetic clades of P. acnes strains (Fitz-Gibbon et al., 2013). Only topology is shown. (b) Phage infection outcomes of three P. humerusii strains, one P. granulosum strain and three P. avidum strains show that P. acnes phages could infect and lyse P. humerusii strains, while P. granulosum and P. avidum were resistant to all tested phages.