Search

Aims: To assess the structure and stability of a dominant lactic acid bacteria (LAB) population during the propagation of rye sourdough in an industrial semi-fluid production over a period of 7 months., Methods and Results: The sourdough was started from a 6-year-old freeze-dried sourdough originating from the same bakery. A unique microbial consortium consisting mainly of bacteria belonging to species Lactobacillus helveticus, Lactobacillus panis and Lactobacillus pontis was identified based on culture-dependent (Rep-PCR) and culture-independent [denaturing gradient gel electrophoresis (DGGE)] methods. Three of the isolated Lact. helveticus strains showed remarkable adaptation to the sourdough conditions. They differed from the type strain by the ability to ferment compounds specific to plant material, like salicin, cellobiose and sucrose, but did not ferment lactose., Conclusion: We showed remarkable stability of a LAB consortium in rye sourdough started from lyophilized sourdough and propagated in a large bakery for 7 months. Lactobacillus helveticus was detected as the dominant species in the consortium and was shown to be metabolically adapted to the sourdough environment., Significance and Impact of the Study: The use of an established and adapted microbial consortium as a starter is a good alternative to commercial starter strains., (© 2012 The Society for Applied Microbiology.)

Aims: The present work tests the feasibility of the isothermal microcalorimetry method to study the performance of individual lactic acid bacteria during solid-state fermentation in rye sourdough. Another aim was to elucidate the key factors leading to the formation of different microbial consortia in laboratory and industrial sourdough during continuous backslopping propagation., Methods and Results: Strains of the individual LAB isolated from industrial and laboratory sourdough cycle were grown in 10 kGy irradiated rye dough in vials of an isothermal calorimeter and the power-time curves were obtained. Sugars, organic acids and free amino acids in the sourdough were measured. The OD-time curves of the LAB strains during growth in flour extract or MRS (De Man, Rogosa and Sharpe) broth were also determined. The maximum specific growth rates of Lactobacillus sakei, Lactobacillus brevis, Lactobacillus curvatus and Leuconostoc citreum strains that dominated in backslopped laboratory sourdough were higher than those of Lactobacillus helveticus, Lactobacillus panis, Lactobacillus vaginalis, Lactobacillus casei and Lactobacillus pontis strains originating from industrial sourdough. Industrial strains had higher specific growth rates below pH 4·8. It was supposed that during long-run industrial backslopping processes, the oxygen sensitive species start to dominate because of the O(2) protective effect of rye sourdough., Conclusions: Measurements of the power-time curves revealed that the LAB strains dominating in the industrial sourdough cycle had better acid tolerance but lower maximum growth rate and oxygen tolerance than species isolated from a laboratory sourdough cycle., Significance and Impact of the Study: Isothermal microcalorimetry combined with chemical analysis is a powerful method for characterization of sourdough fermentation process and determination of growth characteristics of individual bacteria in sourdough., (© 2010 The Authors. Journal of Applied Microbiology © 2010 The Society for Applied Microbiology.)

The purpose of this study was to evaluate the influence of introduced bacteria containing a contaminant degrading plasmid on the growth and survival of pine seedlings and mycorrhizosphere microbial flora in contaminated soil. The Pseudomonas fluorescens strain OS81, originally isolated from fungal hyphae in contaminated soil, was supplied with the TOL plasmid pWW0::Km (to generate OS81(pWW0::Km)) and inoculated in humus-soil microcosms with and without pine seedlings mycorrhized with Suillus bovinus. After 3 months of regular treatment with m-toluate (mTA) solutions, the introduced catabolic plasmid was found to be disseminated in the indigenous bacterial population of both mycorrhizosphere and soil uncolonized by the fungus. Transconjugants were represented by bacteria of the genera Pseudomonas and Burkholderia and their number correlated positively with the concentration of mTA applied. Indigenous mTA degrading bacteria with low similarity to Burkholderia species were also enriched in microcosms. They were mostly associated with mycorrhizal soil or fungal structures and virtually absent in microcosms without pines. The total number of Tol(+) bacteria was higher in mycorrhizospheric soil compared with bulk soil. Inoculation with P. fluorescens OS81(pWW0::Km) had a positive effect on the development of roots and fungus in contaminated soil. Both inoculation with the P. fluorescens OS81(pWW0::Km) and mTA contamination as well as the presence of mycorrhized pine roots and fungal hyphae had an effect on the microbial community structure of soil as measured by carbon source oxidation patterns. However, the impact of mTA on the microbial community was more prominent. The study indicates that an effect on plant and fungal development can be obtained by manipulating the mycorrhizosphere. Both introduction of the bacterium carrying the degradative plasmid and the plasmid itself are likely to have a positive effect not only on the organisms involved, but also on bioremediation of contaminated soil, a factor that was not directly monitored here.

The tolerance to, and degradation of m-toluate by Scots pine (Pinus sylvestris), a symbiotic mycorrhizal fungus (Suillus bovinus) and Pseudomonas fluorescens strains, with or without m-toluate-degrading capacity, was determined individually and in all symbiotic/associative plant-microbe combinations. Fungal survival on medium with m-toluate was increased in co-culture with the degradative bacterial strains on agar plates (up to 0.02%, w/v). When fungi were grown in mycorrhizal association with Scots pine seedlings in test-tube microcosms containing expanded clay pellets and growth media, the fungus was able to withstand m-toluate concentrations up to 2.0%, w/v in all treatments. The seedling tolerance remained unaltered regardless of the presence or absence of mycorrhizal fungi or biodegradative bacteria. Reduction in m-toluate levels was only detected in treatments inoculated with bacterial strains harbouring TOL catabolic plasmids. The plant and fungus, alone or in mycorrhizal symbiosis, were unable to cleave m-toluate. The presence of easily available plant-derived carbon sources did not impede m-toluate degradation by the bacteria in the mycorrhizosphere.