Short communication: impact of pH and temperature on the acidifying activity of Carnobacterium maltaromaticum.

The acidifying activity of Carnobacterium maltaromaticum LMA28, a strain isolated from French soft cheese, was studied in trypticase soy broth with yeast extract (TSB-YE) medium and in milk. In TSB-YE supplemented with lactose, glucose, or galactose, lactose and glucose were metabolized with a maximum growth rate of 0.32 h(-1) and galactose was not metabolized. During hydrolysis of lactose, the galactose moiety was not excreted. The major product was l(+) lactic acid, with no significant difference in the lactic acid yield. Glucose was not completely metabolized because cell growth stopped when pH values reached an average of 5.0. In sterilized UHT milk, the addition of 1 g/L of YE enhanced its coagulation. Compared with commercial starter lactic acid bacteria such as Lactococcus lactis DSMZ 20481 or Streptococcus thermophilus INRA 302, Carnobacterium maltaromaticum LMA 28 was shown to be a slow acidifying strain. However, in spite of this weak acidifying ability, C. maltaromaticum LMA 28 can sustain low pH values in coculture with Lc. lactis DSMZ 20481 or S. thermophilus INRA 302. The individual and interactive effects of initial pH values (5.2 to 8.0) and incubation temperatures (23 to 37 degrees C) on acidifying activity were studied by response surface methodology. The 3 strains displayed different behaviors depending on pH and temperature. The psychrotrophic lactic acid strain C. maltaromaticum LMA 28 was able to grow at alkaline pH values and during storage conditions. It could be used as a potential ripening flora in soft cheese.

Intracellular pH determination of pristinamycin-producing Streptomyces pristinaespiralis by image analysis.

Intracellular pH (pH(i)) is an essential parameter in the regulation of intracellular processes. Thus, its measurement might provide clues regarding the physiological state of cells cultivated in vitro. pH(i) of the filamentous, pristinamycin-producing Streptomyces pristinaespiralis was determined by epifluorescence microscopy and image analysis using the pH-sensitive fluorescent probe BCECF-AM [2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, acetoxymethyl ester]. Staining cell culture samples (OD(660)=1) of S. pristinaespiralis with 20 microM BCECF at 28 degrees C for 30 min yielded a green/red fluorescence ratio (R:(527/600)) that correlated with the pH(i) of the cells for values ranging from 6.5 to 8.5. When S. pristinaespiralis was cultivated in pristinamycin-producing conditions (in batch mode, with a constant external pH of 6.8), the measured pH(i) varied between 6.3 and 8.7. In fact, pH(i) correlated with the excretion of pristinamycins and glucose consumption during the production process.

Diacetyl production mechanism by a strain of Lactococcus lactis spp. lactis bv. diacetylactis. Study of alpha-acetolactic acid extracellular accumulation under anaerobiosis.

Diacetyl production via alpha-acetolactic acid (ALA) extracellular decarboxylation in Lactococcus lactis spp. lactis bv. diacetylactis SD 933 cultures has been assessed under anaerobiosis both in batch and continuous fermentations at pH 5.5 and 8.0 by studying the effects of alpha-acetolactate decarboxylase (ADC) addition in the culture broth. This enzyme, favoring the formation of acetoin instead of diacetyl, was added extracellularly and did not disturb diacetyl production. Moreover, oxidation experiments on extracellular culture media did not reveal any increase in diacetyl amount caused by extracellular ALA oxidative decarboxylation. These observations confirm previous assertions concerning the mechanism and localization of diacetyl synthesis by the SD 933 strain.