Sensitivity of Listeria monocytogenes Scott A to Nisin and Diacetyl after Starvation in Sodium Phosphate Buffered Saline

Abstract

This study determined the effectiveness of nisin and diacetyl to inhibit the growth of Listeria monocytogenes (Lm) under normal cell cultivation and starvation conditions in sodium phosphate buffered saline (SPBS). Inhibitory effects of nisin at 320 or 1000 AU/mL or diacetyl at 0.25%, 0.50%, or 1.0% and combinations on Lm in brain heart infusion (BHI), SPBS, and potassium phosphate buffered saline (KPBS) were determined on BHIA (nonselective medium) and PALCAM (selective medium) agar at 0 and 2 h posttreatment after 0, 7, 14, and 21 d of starvation. Two-hour exposure to 1000 AU/mL nisin and 1% diacetyl gave 8 to 9 log CFU/mL reductions of nonstarved control cells regardless of plating medium after suspension in BHI, SPBS, or KPBS with inhibitors, but with 7 d starved cells a 2-h exposure reduced Lm levels to less than the detection limit (20 CFU/mL). Cells starved in SPBS for 14 or 21 d then suspended in BHI plus inhibitors were reduced 5 to 6 log CFU/mL. SPBS suspensions on days 14 and 21 were reduced 4 log CFU/mL and 2 to 3 log CFU/mL, respectively, on BHI media and 5 to 6 log CFU/mL and 2 to 3 log CFU/mL on PALCAM. Recovery was the same regardless of plating medium, indicating treated cells were killed and not merely injured by the nisin and diacetyl treatments. This study showed that nisin and diacetyl combinations were more effective on Lm than when either chemical was used separately in BHI, SPBS, or KPBS. Lm cells starved for 14 or more days were much more resistant to the nisin and diacetyl combinations than were nonstarved control cells.