Control of Salmonella enterica spp. enterica in milk and raw milk cheese using commercial bacteriophage preparations

Abstract

Outbreaks of salmonellosis linked to consumption of raw milk and cheese made from raw milk continue to occur, highlighting the need for additional controls. This study determined the susceptibility of nine selected serovars of Salmonella enterica spp. enterica to commercially available bacteriophage preparations as well as the efficacy of phage application to control Salmonella in pasteurized and raw milk during storage and in the production of raw milk queso fresco and Gouda cheese. Except for serovar Cerro, both SalmoFresh™ and PhageGuard S™ demonstrated high lytic efficiency. Broth microdilution assays identified multiplicities of infection (MOI) ranging from log 1.84–6.69, which varied between serovars and products. Addition of SalmoFresh™ phage product to pasteurized milk at log MOI 6 rapidly reduced Salmonella counts such that mean counts were 2.33–3.22 log CFU/mL lower than control after 7 d at 7 °C. Aside from serovar Cerro, SalmoFresh™ addition was also effective in raw milk with mean counts 1.12–2.27 log CFU/mL lower than control at the end of storage. Despite these observations, counts of Salmonella Montevideo did not differ from control during queso fresco production or storage at 7 °C for 28 d when SalmoFresh™ was added at log MOI 6. Salmonella counts were lower in Gouda before brining when SalmoFresh™ was added to milk at log MOI 6 and was undetectable one week earlier in aged Gouda when added at log MOI 6 or 7. These data demonstrate the potential of commercial phage products to enhance the safety of raw milk, including that used to produce cheese.