Inhibition of bacterial growth (Listeria monocytogenes & Staphylococcus aureus) by Nisin Z, NaCl and their combination during fish storage at refrigerated temperature

Abstract

To identify and quantify the effects of nisin Z and its combinations with different percentages of NaCl on the storage and shelflife of olive flounder (Paralichthys olivaceus) through antimicrobial activities against Listeria monocytogenes and Staphylococcus aureus. Nisin Z was isolated and purified from our laboratory identified Lactococcus lactis subsp. Lactis I2. The combinations of treatments are as follows: nisin Z, nisin Z + 3, 5, & 7% NaCl, and 0.02 M HCl (- control). In agar disk diffusion method, individual nisin Z and NaCl presented very little antimicrobial activities. However, antimicrobial effects of the combinations of two antimicrobials were significantly higher (P < 0.05) than control and individual components. Moreover, formation of clear zone by nisin Z with 3, 5, and 7% NaCl were significantly higher than control, whereas nisin Z + 7% NaCl demonstrated the maximum clear zone (9-11 mm) against these two foodborne pathogens. Minced olive flounder inoculated with log 5.57 and 4.94 CFU/g of L. monocytogenes and S. aureus respectively and treated with nisin Z + 7% NaCl reduced the bacterial load below log 3 and 2 respectively. After inoculating the bacteria in minced fish and treated with nisin Z and NaCl, the pH value of all treatments shows significantly different at the end of 12 days. But combined effect of nisin Z with 7% NaCl showed the pH value significantly lower (P < 0.05) compare to control and other treatments. The results revealed that these good combinations of nisin Z and NaCl play a vigorous antimicrobial role to increase the fish storage time in addition to shelflife of refrigerated fish products.

Keywords: nisin Z; NaCl; Lactococcus lactis; Listeria monocytogenes; Staphylococcus aureus.