Influence of pH and temperature on the growth of Enterococcus faecium and Enterococcus faecalis

Abstract

-A total of 70 enterococcal strains isolated from traditional Italian raw milk cheeses were identified by API 20 Strep and Biolog GP. All the identifications were confirmed by species-specific PCR. Four species of genus Enterococcus were found: E. faecalis, E. faecium, E. durans and E. hirae. E. faecalis and E. faecium were the dominant enterococcal species isolated in artisanal cheeses. According to the most representative species and the origin of isolation, 21 strains (11 E. faecalis and 10 E. faecium) were selected for the conductimetric analysis. The influence of pH (5.0; 5.5; 6.0 and 6.5) and temperature (25 °C and 37 °C) on the metabolism and the development of enterococci was evaluated by determination of Generation Time (GT), Detection Time (DT) and the maximum conductance value (∆µS) of the curve during a period of 48 hours. Conductance trials showed that both species were able to adapt to adverse cultural conditions (low values of pH and temperature) during cheese-making and ripening. E. faecalis appeared to be less sensitive on the whole. In the worst conditions (pH 5.0, 25 °C), after an adaptation phase (about 10 h), these microorganisms showed GT values almost double those at 37 °C. The same behavior can occur in cheese, where enterococci can reach numbers of up to 10 5 -10 8 cfu·g -1 during ripening. This could explain the fact that enterococci (in particular, E. faecalis and E. faecium) represent the typical and important microflora in raw milk cheese. E. faecalis / E. faecium / conductimetric analysis / pH / temperature / growthRésumé -Influence du pH et de la température sur la croissance d'Enterococcus faecalis et Enterococcus faecium. Un total de 70 souches d'entérocoques, isolées de certains fromages italiens traditionnels, a été identifié à l'aide des galeries API 20 Strep et Biolog GP. Toutes les identifications ont été confirmées par PCR espèce-spécifique. Quatre espèces de ce genre ont été isolées : E. faecalis, E. faecium, E. durans et E. hirae. E. faecalis et E. faecium étaient les espèces dominantes isolées des fromages artisanaux. Un total de 21 souches (11 E. faecalis et 10 E. faecium) a été sélec-tionné pour les analyses de conductimétrie selon les espèces les plus représentées et selon l'origine de l'isolement. L'influence du pH (5,0 ; 5,5 ; 6,0 et 6,5) et de la température (25 °C et 37 °C) sur le métabolisme et sur le développement des entérocoques a été évaluée par détermination du temps de génération (GT), du temps de détection (DT) et de la valeur conductimétrique maximale (∆µS) de la courbe au cours d'une période de 48 h. Les mesures de conductance ont démontré que les deux espèces ont la capacité de s'adapter aux conditions de culture défavorables (valeurs faibles du pH et de la température) aussi bien pendant la fabrication du fromage que l'affinage. Les deux espèces ont un comportement différent : E. faecalis s'est avéré être moins sensible. Dans les plus mauvaises conditions (pH 5,0-25 °C), après la phase d'adaptation (environ 10 h), ces micro-organismes montrent...

Technological, phenotypic and genotypic characterisation of wild lactic acid bacteria involved in the production of Bitto PDO Italian cheese

“…Differently from the curd, the most numerous group was mesophilic lactobacilli (mean value, 8.02 log 10 CFU.g −1 ). In agreement with previous studies on other raw milk cheeses, the enterococci levels increased from curd to cheese in three log units (Morandi et al 2005).…”

“…Indeed, enterococci comprise a major part of the fresh cheese curd microflora, and in some cases they are the predominant microorganisms in the fully ripened product (Giraffa 2003;Jokovic et al 2008). The isolation of enterococci from curd and ripened cheeses can be ascribed to their tolerance to high salt content and low pH, as well as to their adaptability to a wide range of different substances and growth conditions (Morandi et al 2005). Moreover, the peculiar cheese-making technique used for the manufacture of Bitto, which includes heating the curd to 48-52°C, could contribute to the selection of these microorganisms.…”

Raw milk hygiene at farms, processing units and local markets in Burkina Faso

“…Milk is an excellent medium for bacteria growth and the population can double every half hour at +25°C when pH is in the range of 6.0-6.5 (International Dairy Federation, 1990;Marandi, Brasca, Alfieri, Lodi, & Tamburini, 2005). There are immunoactive substances in milk, for example lysozyme, lactoperoxidase, lactoferrins and immunoglobulins, and these have anti-microbial flora properties (Harding, 1999).…”

Thermal inactivation of Enterococcus faecium

“…At the same time, the colonisation of the infant gut with non-pathogenic microflora is a prerequisite for the health of the individual during adolescence and maturity (Lodinova-Zadnikova et al 2003). E. faecium is also commonly present in cheese (Morandi et al 2005). The presence of E. faecium is undesirable mainly because the bacterium easily develops resistance to antibiotics (vancomycin, tetracycline), see e.g.…”