No abstract

No abstract

Lactobaciflus buchneri ST2A vigorously decarboxylates histidine to the biogenic amine histamine, which is excreted into the medium. Cells grown in the presence of histidine generate both a transmembrane pH gradient, inside alkaline, and an electrical potential (A*), inside negative, upon addition of histidine. Studies of the mechanism of histidine uptake and histamine excretion in membrane vesicles and proteoliposomes devoid of cytosolic histidine decarboxylase activity demonstrate that histidine uptake, histamine efflux, and histidine/ histamine exchange are electrogenic processes. Histidine/histamine exchange is much faster than the unidirectional fluxes of these substrates, is inhibited by an inside-negative A* and is stimulated by an inside positive A*. These data suggest that the generation of metabolic energy from histidine decarboxylation results from an electrogenic histidine/histamine exchange and indirect proton extrusion due to the combined action of the decarboxylase and carrier-mediated exchange. The abundance of amino acid decarboxylation reactions among bacteria suggests that this mechanism of metabolic energy generation and/or pH regulation is widespread.Several precursor/product antiport mechanisms have been shown or were anticipated to be involved in generation of metabolic energy (16). Metabolic energy can be obtained from these mechanisms by substrate level phosphorylation (5,17) or by the formation of transmembrane ion gradients (ion and proton motive force). Characteristic for all of these precursor/product antiport mechanisms is that the product is structurally similar to the precursor. Transport of precursor and product can be catalyzed by one membrane transport protein which binds both precursor and product with high affinity. The usual mode in which these carrier proteins function is that of exchange: i.e., reorientation of the substrate binding site to either side of the membrane takes place while a substrate is bound. When the product is more positively charged than the precursor, this antiport generates an inside-negative electrical potential gradient across the membrane. Two examples of such carriers which have been studied in detail are the oxalate/formate antiporter of Oxalobacter formigenes (22) and the malate/lactate carrier of Lactococcus lactis (18). Because the metabolic conversion from oxalate to formate or from malate to lactate is a decarboxylation whereby the carboxylic group leaves the cytoplasm as neutral carbon dioxide or dihydrogen carbonate, the net effect of metabolism and electrogenic precursor/ product antiport is the extrusion of one proton. Consequently, a proton motive force is generated from the free energy of decarboxylation.A different mechanism by which metabolic energy is generated from a decarboxylation reaction is found in the Na'-translocating oxaloacetate decarboxylase (3). In this system, decarboxylation is catalyzed by a membrane-bound protein which uses the free energy directly to translocate sodium ions across the membrane. In this study, it will b...

Approximately 1000 lactobacillus strains were isolated and screened for the production of antimicrobial activity, using a target panel of spoilage organisms and pathogens. Only eight positive strains were found; two of these were studied in more detail. Lactobacillus salivarius M7 produces the new broad spectrum bacteriocin salivaricin B which inhibits the growth of Listeria monocytogenes, Bacillus cereus, Brochothrix thermosphacta, Enterococcus faecalis and many lactobacilli. A new atypical bacteriocin produced by Lact. acidophilus M46, acidocin B, combines the inhibition of Clostridium sporogenes with a very narrow activity spectrum within the genus Lactobacillus and was selected for further characterization. Acidocin B is sensitive to trypsin, heat-stable (80 degrees C for 20 min) and can be extracted from the culture supernatant fluid with butanol. Native acidocin B occurs as a large molecular weight complex (100 kDa), while with SDS-PAGE the partly purified activity migrates as a peptide of 2.4 kDa. Optimization of the cultivation conditions resulted in an eightfold increase of the amount of acidocin B produced during growth. Growth is not necessary for acidocin B production; washed producer cells can synthesize the bacteriocin in a chemically defined production medium. The application potential of acidocin B is discussed.

A total of 182 Lactobacillus strains were screened for production of extracellular polysaccharides (EPS) by a new method: growth in liquid media with high sugar concentrations. Sixty EPS-positive strains were identi®ed; 17 strains produced more than 100 mg/l soluble EPS. Sucrose was an excellent substrate for abundant EPS synthesis. The ability to produce glucans appears to be widespread in the genus Lactobacillus. The monosaccharide composition of EPS produced by Lactobacillus reuteri strain LB 121 varied with the growth conditions (solid compared to liquid medium) and the sugar substrates (sucrose or ranose) supplied in the medium. Strain LB 121 produced both a glucan and a fructan on sucrose, but only a fructan on ranose. This is the ®rst report of fructan production by a Lactobacillus species. EPS production increased with increasing sucrose concentrations and involved extracellular sucrase-type enzymes.