ABSTRACTBacteriophages infectingTetragenococcus halophilus, a halophilic lactic acid bacterium, have been a major industrial concern due to their detrimental effects on the quality of food products. Previously characterized tetragenococcal phages displayed narrow host ranges, but there is little information on these mechanisms. Here, we revealed the host’s determinant factors for phage susceptibility using two virulent phages, phiYA5_2 and phiYG2_4, that infectT. halophilusYA5 and YG2, respectively. Phage-resistant derivatives were obtained from these host strains, and mutations were found at the capsular polysaccharide (CPS) synthesis (cps) loci. Quantification analysis verified that capsular polysaccharide production by thecpsderivatives from YG2 was impaired. Transmission electron microscopy observation confirmed the presence of filamentous structures outside the cell walls of YG2 and their absence in thecpsderivatives of YG2. Phage adsorption assays revealed that phiYG2_4 adsorbed to YG2 but not itscpsderivatives, which suggests that the capsular polysaccharide of YG2 is the specific receptor for phiYG2_4. Interestingly, phiYA5_2 adsorbed and infectedcpsderivatives of YG2, although neither adsorption to nor infection of the parental strain YG2 by phiYA5_2 was observed. The plaque-surrounding halos formed by phiYA5_2 implied the presence of the virion-associated depolymerase that degrades the capsular polysaccharide of YA5. These results indicated that the capsular polysaccharide is a physical barrier rather than a binding receptor for phiYA5_2 and that phiYA5_2 specifically overcomes the capsular polysaccharide of YA5. Thus, it is suggested that tetragenococcal phages utilize CPSs as binding receptors and/or degrade CPSs to approach host cells.IMPORTANCET. halophilusis a halophilic lactic acid bacterium that contributes to the fermentation processes for various salted foods. Bacteriophage infections ofT.halophilushave been a major industrial problem causing fermentation failures. Here, we identified thecpsloci inT.halophilusas genetic determinants of phage susceptibility. The structural diversity of the capsular polysaccharide is responsible for the narrow host ranges of tetragenococcal phages. The information provided here could facilitate future studies on tetragenococcal phages and the development of efficient methods to prevent bacteriophage infections.