Environmental stress responses in Lactococcus lactis

Bacteria can encounter a variety of physical conditions during their life. Bacterial cells are able to survive these (often adverse) conditions by the induction of specific or general protection mechanisms. The lactic acid bacterium Lactococcus lactis is widely used for the production of cheese. Before and during this process as well as in its natural habitats, it is subjected to several stressful conditions. Such conditions include oxidation, heating and cooling, acid, high osmolarity/dehydration and starvation. In many environments combinations of these parameters occur. Understanding the stress response behaviour of L. lactis is important to optimize its application in industrial fermentations and is of fundamental interest as L. lactis is a non-differentiating Gram-positive bacterium. The stress response mechanisms of L. lactis have drawn increasing attention in recent years. The presence in L. lactis of a number of the conserved systems (e.g. the heat shock proteins) has been confirmed. Some of the regulatory mechanisms responding to an environmental stress condition are related to those found in other Gram-positive bacteria. Other stress response systems are conserved at the protein level but are under control of mechanisms unique for L. lactis. In a number of cases exposure to a single type of stress provides resistance to other adverse conditions. The unravelling of the underlying regulatory systems gives insight into the development of such cross resistance. Taken together, L. lactis has a unique set of stress response mechanisms, most of which have been identified on the basis of homology with proteins known from other bacteria. A number of the regulatory elements may provide attractive tools for the development of food grade inducible gene expression systems. Here an overview of the growth limits of L. lactis and the molecular characterization of its stress resistance mechanisms is presented.

Characterization of knot complements in the 4-sphere

AbstractKnot complements in S4 are characterized as follows: A connected open set W ⊂ S4 is homeomorphic to the complement of some locally flat 2-sphere in S4 if and only if H1(W) is infinite cyclic, W has one end, and the fundamental group of that end is infinite cyclic. Applications include a characterization of weakly flat 2-spheres in S4 and a complement theorem for 2-spheres in S4

Representing homology classes of simply connected 4-manifolds

AbstractThe main theorem asserts that every 2-dimensional homology class of a compact simply connected PL 4-manifold can be represented by a codimension-0 submanifold consisting of a contractible manifold with a single 2-handle attached. One consequence of the theorem is the fact that every map of S2 into a simply connected, compact PL 4-manifold is homotopic to an embedding if and only if the same is true for every homotopy equivalence. The theorem is also the main ingredient in the proof of the following result: If W is a compact, simply connected, PL submanifold of S4, then each element of H2(W;Z) can be represented by a locally flat topological embedding of S2