S. Condon

Spatial and temporal distribution of non-starter lactic acid bacteria in Cheddar cheese

Aims: The aim of this work was to investigate the spatial and temporal distribution of species and strains of non‐starter lactic acid bacteria (NSLAB) within Cheddar cheese.

Improving the quality of European hard-cheeses by controlling of interactions between lactic acid bacteria and propionibacteria

Abstract This work aimed to improve the knowledge and the control of interactions between lactic acid bacteria (LAB) and propionibacteria (PAB) in order to control the propionic acid fermentation in European hard cheeses, and therefore their quality. Among more than 50 couples of strains LAB/PAB used in cheese making, some pairs of strains were chosen in function of quality results on cheeses (sensorial analysis). A whey model system and an alternative conductimetric method, with good reproducibility were used to study interactions. The lipolytic activity of PAB strains varied by a factor 4 to 5. The aroma of ripened Emmental cheese was linked to the level and the type of lipolysis in the cheese, correlated with lipolytic activity of PAB. The main pathways of the utilisation of lactate by PAB, studied by 13 C RMN, were useful to understand the balance between final products: acetic, propionic acid, CO 2 .

Effect of fermentation sugar on hydrogen peroxide accumulation by Streptococcus lactis C10

Hydrogen peroxide accumulated in aerated cultures of Streptococcus lactis C10, the amount depending on the sugar being utilized. When the energy source was galactose or (to a lesser extent) lactose or maltose, H 2 O 2 reached autoinhibitory levels. With glucose, H 2 O 2 reached levels which did not cause obvious inhibition, and then declined. Washed cell suspensions utilized several media ingredients including glucose, galactose and yeast extract as substrates for O 2 uptake, but cell-free extracts utilized none of these. The only substrate which supported O 2 uptake by cell-free extracts was NADH. The activity of NADH oxidase was not affected by several intermediates of carbohydrate metabolism. Evidence was obtained to show that its synthesis was regulated via induction by O 2 with an overriding catabolite repression type control, when cells were grown at the expense of glucose.

Growth of Lactococcus lactis strains at low water activity: correlation with the ability to accumulate glycine betaine.

Lactococcus lactis strains were divided into two groups based on their ability to grow in the presence of an upper limit of either 2% w/v NaCl (sensitive) or 4% w/v NaCl (tolerant). Growth inhibition of NaCl tolerant strains was substantially relieved by glycine betaine which was accumulated in significant amounts when growing at low water activities (a(w)). Very little accumulation of glycine betaine occurred during growth of the NaCl sensitive strains. The NaCl tolerant strains had substantial levels of glycine betaine transport activity in vitro, whereas the NaCl sensitive strains had little or no such activity. A low a(w) sensitive mutant of L. lactis subsp. cremoris MG1363 (NaCl tolerant) was isolated following ISS1 insertional mutagenesis. This mutant was inhibited at an a(w) of 0.988 produced by addition of 2% w/v NaCl or the equivalent glucose concentration (0.58 M). The mutant did not accumulate glycine betaine when growing at low a(w), and did not transport glycine betaine when assayed in vitro.

Synergistic lethal combination of nitrite and acid pH on a verotoxin-negative strain of Escherichia coli O157

This study was concerned with the possible consequences of reducing the nitrite concentration of a fermented sausage environment on the survival of the pathogen E. coli O157:H45, a verotoxin-negative relative of E. coli O157:H7. A liquid medium, FM, was constructed with a liquid phase, a(w) and pH similar to fermented sausage. Survival of E. coli O157:H45 in FM depended on both pH and nitrite concentration. In trials in which the pH was decreased by growing Pediococcus acidilactici in FM, survival of E. coli O157:H45 was clearly dependent on nitrite concentration; at least 100 ppm nitrite was required to inhibit growth and the number of survivors after 2 days with 200 ppm nitrite was 1000-fold less than in the absence of nitrite. In laboratory-scale sausage fermented with P. acidilactici, E. coli O157:H45 failed to grow in the absence of nitrite and the numbers slowly declined over 14 days. However, the rate of decline was much faster with nitrite present even at 50 ppm; at 200 ppm nitrite, the E. coli O157:H45 population declined 100 times faster than in the absence of nitrite.

Inability of dairy propionibacteria to grow in milk from low inocula.

Growth of propionibacteria in complex media was independent of the initial number of cells; in contrast, growth of propionibacteria in milk and whey did not occur if the initial level of cells was < 10(6) cfu/ml. Addition of vitamins, minerals or complex nitrogen sources to the milk or whey, or incubation under anaerobic conditions had no effect on the lack of growth. Addition of freeze-dried whey, prepared from skim milk reconstituted from powder, to a complex medium prevented growth from low inocula in the complex medium, demonstrating the presence of an inhibitor or inhibitors in the whey. The inhibitor(s) was heat stable, had a low molecular mass and retained its activity for at least 4 weeks at 20 degrees C. Pregrowth of some lactic acid bacteria, used as starter cultures in Swiss-type cheese manufacture, in milk for 2 weeks at 20 degrees C removed the inhibition, which explains how propionibacteria develop in Swiss-type cheese from low numbers even though they are inhibited in milk.

Changes in acid tolerance of Lactococcus lactis during growth at constant pH

Cells of Lactococcus lactis MG1363 growing in batch culture in TYG (tryptone, yeast extract, glucose) medium at constant pH 7.0 became gradually more acid sensitive shortly after inoculation until a point of maximum sensitivity was reached in early log-phase. The acid tolerance then gradually increased in the mid- and late-log phase until maximum tolerance was reached at the onset of stationary phase. This pattern has been termed the growth-phase acid tolerance. The variation in acid tolerance seen in pH 7.0 grown cells of L. lactis MG1363 did not result from changes in internal pH or membrane H+ ATPase activity levels. Neither the amount of glucose present during mid-log phase nor the amount of lactate produced by the cells correlated with the pattern of the log-phase acid tolerance. Cells grown in partially spent TYG medium showed a reduced growth rate and increased acid tolerance compared to cells grown in fresh TYG medium. Supplementing the spent medium with tryptone or yeast extract or both restored the growth rate and cells became more sensitive to acid. Fractionation of tryptone yielded a fraction which stimulated the growth of MG1363 in partially spent medium and delayed the acquisition of acid tolerance. The active compound(s) has a putative molecular weight of about 1 kDa and was partially degraded by papain and trypsin.