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The toxicity of homologous series of organic solvents has been investigated for the gram-positive bacteria, Arthrobacter sp. and Nocardia sp., and the gram-negative bacteria, Acinetobacter sp. and Pseudomonas sp. The hydrophobicity of the solvent, expressed by its logP(octanol), proves to be a good measure for the toxicity of solvents in a two-phase system. The transition from toxic to nontoxic solvents occurs between logP(octanol) 3 and 5 and depends on the homologous series. No correlation has been found between the hydrophobicity of the substituent on the alkyl backbone of the solvent and the location of the transition point in toxicity. The logP(octanol), above which all solvents are nontoxic, is used to express the solvent tolerance of the bacteria. In general, the solvent tolerance of gram-negative bacteria is found to be slightly higher than that of gram-positive bacteria, but this does not hold for all homologous series of organic solvents investigated.Because the toxicity effects of organic solvents in a two-phase system can be ascribed to molecular as well as phase toxicity effects, molecular toxicity effects were investigated separately in a one-phase system with subsaturating amounts of organic solvent. The solvent concentration in the aqueous phase, at which 50% of the metabolic activity of the bacteria is lost, is used to express solvent toxicity. This concentration is found to be similar for the gram-positive Arthrobacter and the gram-negative Acinetobacter. Assuming the critical membrane concentration theory (G. J. Osborne et al. Enzyme Microb. Technol. 1990, 12: 281-291) to be valid, it can be concluded that differences in solvent tolerance between these two bacteria, cannot be ascribed to differences in response to molecular toxicity. Prediction of the toxicity of any solvent, using the critical membrane theory, appears to be possible in the case of alkanols or alkyl acetates. However, prediction of the toxicity of ethers appears to be impossible.

The naturally occurring compatible solutes betaine and L-carnitine allow the food-borne pathogen Listeria monocytogenes to adjust to environments of high osmotic strength. Previously, it was demonstrated that L. monocytogenes possesses an ATP-dependent L-carnitine transporter (A. Verheul, F. M. Rombouts, R. R. Beumer, and T. Abee, J. Bacteriol. 177:3205-3212, 1995). The present study reveals that betaine and L-carnitine are taken up by separate highly specific transport systems and support a secondary transport mechanism for betaine uptake in L. monocytogenes. The initial uptake rates of betaine and L-carnitine are not influenced by an osmotic upshock, but the duration of transport of both osmolytes is directly related to the osmotic strength of the medium. Regulation of uptake of both betaine and L-carnitine is subject to inhibition by preaccumulated solute. Internal betaine inhibits not only transport of external betaine but also that of L-carnitine and, similarly, internal L-carnitine inhibits transport of both betaine and L-carnitine. The inhibition is alleviated upon osmotic upshock, which suggests that alterations in membrane structure are transmitted to the allosteric binding sites for betaine and L-carnitine of both transporters at the inner surface of the membrane. Upon osmotic downshock, betaine and L-carnitine are rapidly released by L. monocytogenes as a consequence of activation of a channel-like activity. The osmolyte-sensing mechanism described is new and is consistent with various unexplained observations of osmoregulation in other bacteria.Food-borne listeriosis caused by Listeria monocytogenes has emerged as a topic of considerable public health concern over the past decade. The infection is encountered in neonates, elderly persons, pregnant women, and the immunocompromised, and symptoms may include sepsis, meningitis, infection of the central nervous system, abortion, and stillbirth, with fatality rates of approximately 25%. The ubiquitous distribution of L. monocytogenes in the environment and its relative high tolerance to environmental stresses such as low temperature and high osmotic strength contribute to its status as a hazard in minimally processed ready-to-eat refrigerated products (7).Cold and salt tolerance in L. monocytogenes can be imparted by betaine and L-carnitine. Betaine is present in high concentrations in foods originating from plants, whereas foods of animal origin generally have a high carnitine content (3,15,21,30). The osmoprotective capacity of betaine is well-known among prokaryotic organisms, whereas, so far, L-carnitine has been recognized only as an osmolyte in L. monocytogenes, Lactobacillus plantarum, and Escherichia coli (3,(12)(13)(14). The involvement of betaine and L-carnitine in cold tolerance of the psychrotroph L. monocytogenes has recently been reported (15,25,30). Uptake of L-carnitine in L. monocytogenes is mediated by a constitutively expressed transporter that is driven by ATP. Competition experiments revealed that the L-carnitine transporter has a hig...

Combinations of lactococcal strains from various origins with divers properties were developed as new starters for new dairy products. Flavour formation by such tailor-made cultures was studied. In some cases, a strongly enhanced¯avour was observed. For instance, the combination of B1157 and SK110 strains in milk resulted in a very strong chocolate-like¯avour. B1157 produces only a moderate chocolate-like¯avour, whereas SK110 alone fails to produce this¯avour. Headspace gas chromatography results corroborate the organoleptic evaluations. High levels of branchedchain aldehydes were found when B1157 and SK110 were grown together. The enzyme activities involved in this pathway were studied; both strains contain transaminase activity. Although B1157 had a very high amino acid decarboxylating activity, its release of amino acids from milk protein was limited. SK110 was strongly limited in decarboxylating activity, although this strain is very active in proteolysis. By combining these strains, the substrates released by SK110 can directly be used by the other strain, resulting in the completion of the whole¯avour-formation pathway. This opens new avenues for the preparation of tailor-made cultures.