RESUMEN
Twenty minced beef samples collected from Cairo and Giza markets were examined for aerobic count, psychrotrophic count and incidence of Staphylococcus aureus and Listeria monocytogenes were determined to throw the light on their microbiological status. The inhibitory activity of nisin [500 IU /gm and 750 IU /gm] on Gram-positive bacteria [Staphylococcus aureus and Listeria monocytogenes] was also determined. Results showed that the mean log values of aerobes and psychrotrophic counts were 7.48 and 7.52/ gm respectively while incidence of Staphylococcus aureus and Listeria monocytogenes were 35% and 10% respectively. Application of nisin [500 IU /gm and 750 IU /gm] reduce Staphylococcus aureus count by 2.1, 2.6 and Listeria monocytogenes by 3.1, 3.6 log/gm respectively. Dramatic decrease was noticed within the first two days of refrigerating storage for both Staphylococcus aureus and Listeria monocytogenes