Potentiel inhibiteur de la souche de Brevibacterium linens productrice de la linenscine OC2, vis-à-vis des listeria et de Staphylococcus aureus

Abstract - Inhibitory potential against Listeria spp and Staphylococcus aureus of the strain of Brevibacterium linens producing linenscin OC2
Twenty-four strains of the genus listeria (14 of L monocytogenes and 10 of L innocua) along with 24 strains of Staphylococcus aureus were tested for their sensitivity to the culture filtrate of Brevibacterium linens OC2, an orange cheese coryneform bacterium producing linenscin OC2, an inhibitory peptidic compound. AH strains were inhibited, sta-phylococci being slightly more sensitive than listeria. The inhibitory potential of the filtrate towards L monocytogenes and L innocua (one strain of each), and two strains of S aureus was investigated using three concentrations of the filtrate (0.1, 1 and 10%). A viability loss or a bacteristatic effect was observed, depending on the strain and the amount of filtrate in the culture medium. The length of the phase of inhibition was correlated with filtrate concentration, after which all strains resumed growth at a rate decreasing as the percentage of filtrate increased. Regrowth of the survivors was also observed in mixed cultures of B linens OC2 (10⁷ cfu / mL in the inoculum) and a cocktail of four L monocytogenes strains (10⁴ cfu / mL of each in the inoculum). As it has been shown that the inhibitory activity of the filtrate was stable over time, the presence of very low proportions of resistant cells in the sensitive population was investigated. Their level was predicted from the analysis of regrowth curves and confirmed using the Most Probable Numbers (MPN) technique of bacterial enumeration in liquid medium containing the filtrate in which five tubes were inoculated from each of three successive ten-fold dilutions. The proportion of listeria resistant to 10% of filtrate in the medium was in the range of 10^-4 of the total population. Therefore, it can be predicted that if the contamination level of listeria is above 10⁴ listeria on the surface of a piece of smear cheese at the time B linens reaches a sufficient level to produce enough inhibitory substance, then linenscin-resistant survivors might survive and proliferate. Washing and brushing the surface of this kind of cheese during curing would contribute to the contamination of other cheeses and subsequently the entire factory environment by linenscin-resistant listeria. This would eventually abolish the possibility of controlling listeria and other undesirable bacteria on the surface of this kind of cheese by use of the strain of B linens producing linenscin OC2. It is concluded that this strain must be used with care and only after full assessment of its efficacy on cheese surfaces in experiments carried out under strict containment conditions.

Résumé - Vingt-quatre souches du genre listeria (14 de l'espèce L monocytogenes et 10 de l'espèce L innocua) et 24 souches de Staphylococcus aureus ont été testées pour leur sensibilité vis-à-vis du filtrat de culture de la souche de Brevibacterium linens OC2 dans un milieu de laboratoire. Toutes les souches ont été inhibées, celles de S aureus étant un peu plus sensibles que celles de listeria. Ensuite, le potentiel inhibiteur de ce filtrat vis-à-vis de L monocytogenes (une souche), de L innocua (une souche) et de S aureus (deux souches) a été testé à diverses concentrations. Selon la souche cible et la concentration du milieu de culture en filtrat, on a observé une chute de population ou seulement un effet bactériostatique, suivi, pour toutes les souches, d'une reprise de croissance. Le même phénomène de reprise de croissance a été observé dans le cas de culture en association de la souche inhibitrice avec un mélange de quatre souches de L monocytogenes. On a montré par ailleurs que le filtrat cru ou chauffé 5 minutes à 100 °C restait stable pendant près d'un mois d'incubation à 30 °C. L'hypothèse d'une sous-population résistante dans les cultures en phase stationnaire des souches cibles a été vérifiée, et le niveau de cette sous-population a été déterminé à l'aide de la méthode de dénombrement microbien en milieu liquide et l'utilisation des nombres les plus probables (NPP).