Boundaries in M-Theory

We formulate boundary conditions for an open membrane that ends on the fivebrane of {\cal M}-theory. We show that the dynamics of the eleven-dimensional fivebrane can be obtained from the quantization of a ``small membrane'' that is confined to a single fivebrane and which moves with the speed of light. This shows that the eleven-dimensional fivebrane has an interpretation as a $D$-brane of an open supermembrane as has recently been proposed by Strominger and Townsend. We briefly discuss the boundary dynamics of an infinitely extended planar membrane that is stretched between two parallel fivebranesComment: 16 pages, phyzz

Testing M(atrix)-Theory at Two Loops

I discuss the relation between M-theory and M(atrix)-theory in flat space by considering the effective potential for the scattering of two groups of D0-branes in both theories. An explicit calculation of this potential up to two loop order in M(atrix)-theory reveals a fascinating agreement. Lecture given at Strings '97; June 17, 1997.Comment: 7 pages, latex, 1 figure. Lecture given at STRINGS9

A Two-Loop Test of M(atrix) Theory

We consider the scattering of two Dirichlet zero-branes in M(atrix) theory. Using the formulation of M(atrix) theory in terms of ten-dimensional super Yang-Mills theory dimensionally reduced to $(0+1)$-dimensions, we obtain the effective (velocity dependent) potential describing these particles. At one-loop we obtain the well known result for the leading order of the effective potential $V_{eff}\sim v^4/r^7$, where $v$ and $r$ are the relative velocity and distance between the two zero-branes respectively. A calculation of the effective potential at two-loops shows that no renormalizations of the $v^4$-term of the effective potential occur at this order.Comment: 17 pages, tex, 1 figure, a few misprints corrected. Version to appear in Nuclear Physics

Plasmid-Chromosome Crosstalk in Staphylococcus aureus: A Horizontally Acquired Transcription Regulator Controls Polysaccharide Intercellular Adhesin-Mediated Biofilm Formation

Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) of clonal complex CC398 typically carry various antimicrobial resistance genes, many of them located on plasmids. In the bovine LA-MRSA isolate Rd11, we previously identified plasmid pAFS11 in which resistance genes are co-localized with a novel ica-like gene cluster, harboring genes required for polysaccharide intercellular adhesin (PIA)-mediated biofilm formation. The ica genes on pAFS11 were acquired in addition to a pre-existing ica locus on the S. aureus Rd11 chromosomal DNA. Both loci consist of an icaADBC operon and icaR, encoding a corresponding icaADBC repressor. Despite carrying two biofilm gene copies, strain Rd11 did not produce PIA and transformation of pAFS11 into another S. aureus strain even slightly diminished PIA-mediated biofilm formation. By focusing on the molecular background of the biofilm-negative phenotype of pAFS11-carrying S. aureus, we identified the pAFS11-borne ica locus copy as functionally fully active. However, transcription of both plasmid- and core genome-derived icaADBC operons were efficiently suppressed involving IcaR. Surprisingly, although being different on the amino acid sequence level, the two IcaR repressor proteins are mutually replaceable and are able to interact with the icaA promoter region of the other copy. We speculate that this regulatory crosstalk causes the biofilm-negative phenotype in S. aureus Rd11. The data shed light on an unexpected regulatory interplay between pre-existing and newly acquired DNA traits in S. aureus. This also raises interesting general questions regarding functional consequences of gene transfer events and their putative implications for the adaptation and evolution of bacterial pathogens

A Note on Fluxes in Six-Dimensional String Theory Backgrounds

We study the structure of warped compactifications of type IIB string theory to six space-time dimensions. We find that the most general four-manifold describing the internal dimensions is conformal to a Kahler manifold, in contrast with the heterotic case where the four-manifold must be conformally Calabi-Yau.Comment: 22 pages, phyzz

Nonequilibrium dynamics: a renormalized computation scheme

We present a regularized and renormalized version of the one-loop nonlinear relaxation equations that determine the non-equilibrium time evolution of a classical (constant) field coupled to its quantum fluctuations. We obtain a computational method in which the evaluation of divergent fluctuation integrals and the evaluation of the exact finite parts are cleanly separated so as to allow for a wide freedom in the choice of regularization and renormalization schemes. We use dimensional regularization here. Within the same formalism we analyze also the regularization and renormalization of the energy-momentum tensor. The energy density serves to monitor the reliability of our numerical computation. The method is applied to the simple case of a scalar phi^4 theory; the results are similar to the ones found previously by other groups.Comment: 15 pages, 9 postscript figures, revtex; version published in Phys. Rev, with minor corrections; improves the first version of 1996 by including the discussion of energy momentum tenso

Improving gene transfer in Clostridium pasteurianum through the isolation of rare hypertransformable variants

Effective microbial metabolic engineering is reliant on efficient gene transfer. Here we present a simple screening strategy that may be deployed to isolate rare, hypertransformable variants. The procedure was used to increase the frequency of transformation of the solvent producing organism Clostridium pasteurianum by three to four orders of magnitude