Summary Mycobacterium is often isolated from polycyclic aromatic hydrocarbon (PAH)-contaminated soil as degraders of PAHs. In model systems, Mycobacterium shows attachment to the PAH substrate source, which is considered to be a particular adaptation to low bioavailability as it results into increased substrate flux to the degraders. To examine whether PAH-degrading Mycobacterium in real PAH-contaminated soils, in analogy with model systems, are preferentially associated with PAH-enriched soil particles, the distribution of PAHs, of the PAH-mineralizing capacity and of Mycobacterium over different fractions of a soil with an aged PAH contamination was investigated. The clay fraction contained the majority of the PAHs and showed immediate pyrene- and phenanthrene-mineralizing activity upon addition of (14)C-labelled pyrene or phenanthrene. In contrast, the sand and silt fractions showed a lag time of 15-26 h for phenanthrene and 3-6 days for pyrene mineralization. The maximum pyrene and phenanthrene mineralization rates of the clay fraction expressed per gram fraction were three to six times higher than those of the sand and silt fractions. Most-probable-number (MPN)-polymerase chain reaction demonstrated that Mycobacterium represented about 10% of the eubacteria in the clay fraction, while this was only about 0.1% in the sand and silt fractions, indicating accumulation of Mycobacterium in the PAH-enriched clay fraction. The Mycobacterium community composition in the clay fraction represented all dominant Mycobacterium populations of the bulk soil and included especially species related to Mycobacterium pyrenivorans, which was also recovered as one of the dominant species in the eubacterial communities of the bulk soil and the clay fraction. Moreover, Mycobacterium could be identified among the major culturable PAH-degrading populations in both the bulk soil and the clay fraction. The results demonstrate that PAH-degrading mycobacteria are mainly associated with the PAH-enriched clay fraction of the examined PAH-contaminated soil and hence, that also in the environmental setting of a PAH-contaminated soil, Mycobacterium might experience advantages connected to substrate source attachment.

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In food technology, organic acids (e.g., lactic acid, acetic acid, and citric acid) are popular preservatives. The purpose of this study was to separate the individual effects of the influencing factors pH and undissociated lactic acid on Listeria innocua inactivation. Therefore, the inactivation process was investigated under controlled, initial conditions of pH (pH 0 ) and undissociated lactic acid ([LaH] 0 ). The resulting inactivation curves consisted of a (sometimes negligible) shoulder period followed by a descent phase. In a few cases, a tailing phase was observed. Depending on the conditions, the descent phase contained one or two log-linear parts or had a convex or concave shape. In addition, the inactivation process was characterized by a certain variability, dependent on the severity of the conditions. Furthermore, in the neighborhood of the growth/no growth interface sometimes contradictory observations occurred. Overall, the individual effects of the influencing factors pH and undissociated lactic acid could clearly be distinguished and were also apparent based on fluorescence microscopy. Appropriate model types were developed and enabled prediction of which conditions of pH 0 and [LaH] 0 are necessary to obtain a predetermined inactivation (number of decimal reductions) within a predetermined time range.Favored by their activity against a broad spectrum of pathogenic and spoilage microorganisms, even at low concentrations, organic acids (e.g., lactic acid, acetic acid, and citric acid) are popular preservatives in the food industry, where a tendency towards minimal processing is observed. In addition, lactic acid bacteria are used in food production processes to produce fermented foods. Secondary to the fermentative effect, the lactic acid metabolism has a preservative effect due to the production of other antimicrobial substances such as hydrogen peroxide and organic acids (34).The organic acid antimicrobial activity is twofold: (i) release of protons at dissociation lowers the extracellular pH, and (ii) the undissociated form of the acid is able to diffuse into the cell, affecting the cell metabolism. The latter effect is reported to result in a global inhibition due to acidification of the cytoplasm and a more specific inhibition of several metabolic and anabolic functions (1,23,35). The said antimicrobial activity can reveal itself in (i) inhibition, i.e., an early induction of the stationary phase, and (afterwards) (ii) inactivation, i.e., a decrease in cell concentration to values below the detection limit of the so-called target (pathogenic or spoilage) organism.In predictive microbiology, the knowledge of microbial growth or death responses to environmental factors is translated into mathematical equations that enable prediction of microbial proliferation or inactivation in foods. Final application lies in the quantitative assessment of the microbial safety and quality of food products, e.g., in defining the critical control points within a hazard analysis and critical control point framewo...

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