Comparative study of the growth of Listeria monocytogenes in defined media and demonstration of growth in continuous culture

A defined, glucose-limited mineral medium for the cultivation of Listeria spp

Members of the genus Listeria are fastidious bacteria with respect to their nutritional requirements, and several minimal media described in the literature fail to support growth of all Listeria spp. Furthermore, strict limitation by a single nutrient, e.g., the carbon source, has not been demonstrated for any of the published minimal media. This is an important prerequisite for defined studies of growth and physiology, including "omics." Based on a theoretical analysis of previously published mineral media for Listeria, an improved, well-balanced growth medium was designed. It supports the growth, not only of all tested Listeria monocytogenes strains, but of all other Listeria species, with the exception of L. ivanovii. The growth performance of L. monocytogenes strain Scott A was tested in the newly designed medium; glucose served as the only carbon and energy source for growth, whereas neither the supplied amino acids nor the buffering and complexing components (MOPS [morpholinepropanesulfonic acid] and EDTA) supported growth. Omission of amino acids, trace elements, or vitamins, alone or in combination, resulted in considerably reduced biomass yields. Furthermore, we monitored the specific growth rates of various Listeria strains cultivated in the designed mineral medium and compared them to growth in complex medium (brain heart infusion broth [BHI]). The novel mineral medium was optimized for the commonly used strain L. monocytogenes Scott A to achieve optimum cell yields and maximum specific growth rates. This mineral medium is the first published synthetic medium for Listeria that has been shown to be strictly carbon (glucose) limited.

Antibacterial and antioxidant activities of essential oils isolated from Thymbra capitata L. (Cav.) andOriganum vulgare L

Antilisterial activities of Thymbra capitata and Origanum vulgare essential oils were tested against 41 strains of Listeria monocytogenes. The oil of T. capitata was mainly constituted by one component, carvacrol (79%), whereas for O. vulgare three components constituted 70% of the oil, namely, thymol (33%), gamma-terpinene (26%), and p-cymene (11%). T. capitata essential oil had a significantly higher antilisterial activity in comparison to O. vulgare oil and chloramphenicol. No significant differences in L. monocytogenes susceptibilities to the essential oils tested were registered. The minimum inhibitory concentration values of T. capitata essential oil and of carvacrol were quite similar, ranging between 0.05 and 0.2 microL/mL. Antioxidant activity was also tested, the essential oil of T. capitata showing significantly higher antioxidant activity than that of O. vulgare. Use of T. capitata and O. vulgare essential oils can constitute a powerful tool in the control of L. monocytogenes in food and other industries.

Development of a synthetic minimal medium for Listeria monocytogenes

A defined solid and liquid minimal medium, HTM, which contained methionine and cysteine as the sole amino acids, was developed for Listeria monocytogenes. Complex broth-grown L. monocytogenes had to adapt to HTM by inducing amino acid biosyntheis. HTM is the simplest minimal medium available for growth of L. monocytogenes.

Stress response of Listeria monocytogenes isolated from cheese and other foods

The responses to pH and sodium chloride of four strains of Listeria monocytogenes isolated from Portuguese cheese, with a sodium chloride concentration of about 2% (w/v) and a pH value from 5.1 to 6.2, were studied. Two isolates from meat and two clinical isolates related to food-borne listeriosis, in which the implicated food product had about 2-3.5% (w/v) sodium chloride, also were studied. The effect of temperature on pH and sodium chloride sensitivity was also determined. The results show that natural isolates vary in response to these stresses and the data were often at variance with previously published data. Strains varied in sensitivity to low pH and to high sodium chloride concentration but the cheese isolates tended to be more resistant. A lower temperature was associated with a decrease in resistance to low pH and to sodium chloride. All strains showed an acid tolerance response induction when grown at pH 5.5 and although the time required for maximum induction of the response varied between strains, 2 h of acid adaptation, at least, was necessary which is longer than previously reported. Some strains showed an osmotolerance response after incubation in 3.5% (w/v) sodium chloride. Osmoadaptation, in addition to inducing an osmotolerance response, also induced cross-protection against acid shock conditions (pH 3.5). The acid tolerance response also induced a cross-protection against osmotic shock conditions (20% (w/v) sodium chloride). In some cases there was a relationship between the degree of resistance and adaptation, but usually the behaviour of a particular strain was independent of the conditions from which it was isolated.

Mathematical modelling of the growth rate and lag time for Listeria monocytogenes

Growth data for Listeria monocytogenes were collected from the literature and a global model built with existing secondary models describing independently the effects of environmental factors on the growth rate and lag time was based on these data. The growth rates calculated with this model were consistent with the published ones but the fit was poor near the limits of growth of the micro-organism. The model was also less accurate to describe the lag time. It seems then that reliable predictions of the growth rate of L. monocytogenes could be obtained in a wide range of growth conditions, but models should take into account interactions between environmental factors. Furthermore, it is necessary to better model the lag phase duration and particularly to model the effect of the history of the inoculum on the subsequent lag time.

Estimation of temperature dependent growth rate and lag time of Listeria monocytogenes by optical density measurements

An automated turbidimetric system, Bioscreen C, was used to monitor growth of ten strains of Listeria monocytogenes at different temperatures. Several methods for estimation of maximum specific growth rate (mu(max)) and lag time (lag) from turbidimetric data were compared to values estimated from viable count data. By using a calibration factor, reliable estimations of mu(max) could be obtained from turbidimetric measurements. On the other hand, accurate estimations of lag required some viable count data.