Modelling the growth kinetics of Listeria as a function of temperature, pH and organic acid concentration

The combined effects of temperature, pH and organic acids (lactic, acetic and propionic) on the growth kinetics of Listeria innocua ATCC 33090 were studied. First, a multiplicative model was built assuming independent effects of all environmental factors. Thus, the model was expanded by the inclusion of a novel term describing the effects of interactions on the growth/no growth limits. The proposed approach allows an accurate description of the boundary between growth and no growth of Listeria.

Modelling Bacillus cereus growth

The aim of this study was to model the growth, in a model system, of toxigenic strains of Bacillus cereus as a function of temperature, pH and water activity. Optical density (OD) values were transformed into numbers of colony-forming units (CFU) by the use of a 'calibrating' relation. The growth curves were then fitted to the Gompertz function which allowed the estimation of the lag-time (lambda) and the growth rate (mu). These two parameters were then modelled according to the controlling factors which were pH and water activity at 20 and 30 degrees C.

Characterization of diacetin B, a bacteriocin from Lactococcus lactis subsp. lactis bv. diacetylactis UL720

Fourteen Lactococcus lactis strains showing inhibitory activity against Listeria innocua SICC 4202 were isolated from different French raw milks and raw milk cheeses and screened for bacteriocin production by the triple layer method under conditions that eliminate the effects of lactic acid and hydrogen peroxide. Three bacteriocinogenic strains (two Lactococcus lactis subsp. lactis bv. diacetylactis UL719 and UL720 and one Lactococcus lactis subsp. lactis UL730) were selected for their high capacity to inhibit the growth of various food pathogens, including Listeria monocytogenes, Staphylococcus aureus, and clostridial strains. The inhibitory compounds from these three strains are inactivated by selected proteases, indicating their protein nature. They retained their antibacterial activity after heat treatments of 100 degrees C for 60 min and 121 degrees C for 20 min, and in the pH range from 2 to 11. The bacteriocin diacetin B produced by strain UL720 has been purified by a pH-dependent adsorption-desorption procedure, followed by reverse-phase high performance liquid chromatography, with a yield of 1.25% of the original activity. Mass spectrometry analysis indicates that the pure peptide has a molecular mass of 4292.32 or 4490.28 Da, while amino acid sequencing allowed the identification of the primary structure of the bacteriocin composed of 37 amino acid residues. The structure of the peptide did not show similarity with other known bacteriocins from lactic acid bacteria.

Cloning, expression, and nucleotide sequence of genes involved in production of lactococcin DR, a bacteriocin from lactococcus lactis subsp. lactis

The partial nucleotide sequence of a Lactococcus lactis subsp. lactis ADRIA 85LO30 bacteriocin-producing operon was determined. The first two open reading frames of the operon are necessary to get bacteriocin expression in L. lactis IL1403R.

Development of a Method for Detection of Lactic Acid Bacteria Producing Exclusively the l -(+)- Isomer of Lactic Acid

A method was developed for the detection and isolation, within a population of lactic acid bacteria, of strains producing exclusively the l -(+)- isomer of lactic acid; the visual detection of colonies of these particular strains can be carried out directly on agar plates (50 to 70 colonies per plate). The method is based on an enzymatic stereospecific reaction involving d -(−)-lactate dehydrogenase and linked to a staining reaction; the diffusion area of the d -(−)- isomer stains red around the d -(−)- and the dl -lactic acid-producing colonies, while the colonies producing exclusively l -(+)-lactic acid are detected by the absence of the colored halo. The intensity of staining was increased when cellulose powder and Tween 20 were added to the agar medium.

Plasmid-encoded determinants for bacteriocin production and immunity in a Lactococcus lactis strain and purification of the inhibitory peptide

Lactococcin, a bacteriocin produced by Lactococcus lactis subsp. lactis ADRIA 85LO30, was purified as a 2.3-2.4 kDa peptide. Six non-bacteriocin-producing (Bac-) and non-immune (Imm-) strains were isolated after curing experiments. These strains had in common the loss or modification of two plasmids: pOS4 (32 kb) and pOS5 (70 kb). By comparing pOS5 and several modified plasmids, a DNA region from pOS5 of about 10 kb, which was necessary for wild-type bacteriocin production and immunity, was identified.

Inhibition of Clostridium tyrobutyricum by bacteriocin-like substances produced by lactic acid bacteria

Lactic acid bacteria were selected for their inhibitory activity against Clostridium tyrobutyricum under conditions that eliminate the effects of lactic acid and hydrogen peroxide. Four strains were isolated belonging to the species Lactococcus lactis ssp. lactis. The sensitivity of the inhibitory substances to pronase and trypsine indicates that they are proteins or peptides different from nisin. Their resistance to phospholipase D indicates that they are also different from lactostrepcin. The inhibitory substances are produced during the exponential phase of growth. Their activity is bactericidal and directed toward some strains of Clostridium tyrobutyricum, Lactobacillus helveticus, and Streptococcus thermophilus, but strains used as dairy starters, Lactobacillus lactis, Streptococcus thermophilus, and Propionibacterium shermanii, are not all affected by the inhibition.

[Growth kinetics of Escherichia coli and Saccharomyces cerevisiae cultured on membrane filters]

The growth kinetics of Escherichia coli and Saccharomyces cerevisiae cultivated on Millipore membrane filters (MF) was studied. Exponential growth kinetics was observed for both microorganisms, but in the case of E. coli cultivated on 0.8 micron and 1.2 micron MF, a decrease in the growth rate compared to that observed in liquid medium was noted. On the other hand, S. cerevisiae was unable to grow on a MF belonging to some batches, whereas E. coli showed normal behavior towards these MF. These data underline the importance of the effect upon growth kinetics of the relative cell size proportionally to the size of the MF pores.

[The inhibition of post-exponential growth in "Saccharomyces cerevisiae" by L-lysine (author's transl)]

The inhibitory effect of L-lysine upon the post-exponential growth of Saccharomyces cerevisiae has been previously demonstrated and the present report extends the observation. During the post exponential phase, the cells do not divide but the cell mass normally increases two or three-fold; in the presence of exogenous lysine this increase is considerably reduced, bud growth is blocked and the cytological features of the post-exponential cells are considerably modified. The effect is apparent in liquid as well as on solid media. L-alpha-aminoadipate, being a precursor of L-lysine, exerts a similar effect since the exponential phase. Mutants resistant to the effect of lysine proved to be permeability mutants. The phenomenon is not restricted to any one strain of S. cerevisiae but characterized all strains of the species which are unable to catabolize lysine.