Use of Carnobacterium piscicola to limit the growth of Listeria monocytogenes in mussel products : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Microbiology at Massey University, Palmerston North, New Zealand
Bacteria were screened in order to find an organism antagonistic to Listeria monocytogenes which could be applied to mussel products and enhance their safety, especially when temperature-abused. A Listeria monocytogenes isolate from the seafood industry was selected as the target organism. Strains of Lactobacillus reuteri and Enterococcus fecium were screened on plates incubated at 35°C and 10°C for anti-listerial compounds, but none were found. A non-bacteriocinogenic strain of Carnobacterium piscicola, A9b- was selected as the antagonist for detailed examination of growth in broth, agar and mussel systems at 10°C. This temperature was chosen to represent temperature abuse of refrigerated products. To distinguish between the growth of the Carnobacterium piscicola strain and wild-type Listeria monocytogenes a "semi-selective" agar was developed using phenol-red indicator, and mannitol as the sole carbohydrate source. Growth rates of Carnobacterium piscicola and Listeria monocytogenes were compared when grown alone and as a co-culture in agar and broth. Growth rates of Listeria monocytogenes when grown alone, and in the presence of Carnobacterium piscicola, were determined on mussels. Regression analyses were done for the inhibition of Listeria monocytogenes by Carnobacterium piscicola. In all cases Carnobacterium piscicola significantly inhibited the growth of Listeria monocytogenes (P
= 0.018, P
< 0.001). Growth of both organisms was faster in broth, than on mussels or agar. The greatest inhibition of Listeria monocytogenes was observed in broth reaching log₁₀4.8 at 41 hours of incubation, prior to decreasing after this time. In agar and mussels the inhibition lasted longer and had not decreased at the end of the trial. The log₁₀ reduction in growth of Listeria monocytogenes in agar was measured at 3.4 and in mussels measured at 1.6. These results were statistically significant (P<0.001 for all). Inhibition of wild type Listeria monocytogenes was also shown in broth when a much lower concentration of Carnobacterium piscicola was used. These results should be considered as preliminary and further confirmatory work should be done. However, Carnobacterium piscicola A9b- shows promise as an antagonistic organism to assist in the control of Listeria monocytogenes in mussel products along with industry-accepted good hygienic practices.