Combined antimicrobial effect of nisin and a listeriophage against Listeria monocytogenes in broth but not in buffer or on raw beef.

The effect of nisin and listeriophage LH7, alone and in combination, on the growth and survival of two strains of Listeria monocytogenes in broth and two model food systems, with appropriate controls, was determined. Growth curves for both bacterial strains in tryptic soy broth incubated at 7 or 30 degrees C, and with the addition of nisin and/or listeriophage at lag, mid-exponential or early stationary phase, were obtained by measuring absorbance at 550 nm. Numbers of mixed populations of both L. monocytogenes strains in phosphate buffered saline (pH 5.5) and on vacuum-packaged fresh beef, both stored for 4 weeks at 4 degrees C, and with the addition of nisin and/or listeriophage, were determined. This was achieved by plating appropriately diluted samples on both Tryptic Soy Agar and Modified Oxford Agar to determine both L. monocytogenes numbers and the presence of sub-lethal injury. In broth nisin alone, reduced levels or prevented growth of the two strains under the conditions studied, but regrowth to levels equivalent to those of untreated cells, occurred. Listeriophage LH7 alone, on the other hand, had no effect in broth under the conditions studied. Notably, however, a mixture of nisin and listeriophage displayed a combined effect in broth and reduced levels of cells substantially without regrowth under the conditions studied. In both model food systems only nisin appeared to be active, in a manner consistent with existing literature, and no combined action was apparent. The use of nisin and listeriophage has potential to control L. monocytogenes in foods but a further understanding of the interactions in this complex system needs to be achieved before it could be applied practically.