Genetic transfer systems for delivery of plasmid deoxyribonucleic acid to Lactobacillus acidophilus ADH: conjugation, electroporation, and transduction

Lactobacillus acidophilus ADH is a bacteriocin-producing human isolate that adheres to human fetal intestinal cells and human ileal cells. We have employed both electroporation and conjugation methodologies to transfer various plasmids to L. acidophilus ADH. Furthermore, we have demonstrated transduction of plasmid DNA within this strain by a temperate bacteriophage (phi adh) harbored by L. acidophilus ADH. Plasmid pGK12 was introduced into strain ADH by electroporation at frequencies as high as 3.3 X 10(5) transformants/micrograms of plasmid DNA. Transconjugants of strain ADH were recovered at frequencies of 10(-2) (pAMB1), 10(-4) (pVA797::Tn917), and 10(-4) (pVA797) per donor cell after filter-mating with Lactococcus lactis ssp. lactis. Plasmid pGK12 was transduced from a phage phi adh lysogen into a recipient strain of L. acidophilus ADH at an average frequency of 3.4 X 10(-8) transductants/pfu. Transformants, transconjugants, or transductants were verified by both phenotype and plasmid profile for acquisition of plasmid DNA. The ability to transfer plasmids and mobilize DNA sequences by electroporation, conjugation, and transduction will augment our efforts to define and characterize the activities of L. acidophilus in the intestinal tract.     

Probing Local Force Propagation in Tensed Fibrous Gels

Fibrous hydrogels are a key component of soft animal tissues. They support cellular functions and facilitate efficient mechanical communication between cells. Due to their nonlinear mechanical properties, fibrous materials display non-trivial force propagation at the microscale, that is enhanced compared to that of linear-elastic materials. In the body, tissues are constantly subjected to external loads that tense or compress them, modifying their micro-mechanical properties into an anisotropic state. However, it is unknown how force propagation is modified by this isotropic-to-anisotropic transition. Here, force propagation in tensed fibrin hydrogels is directly measured. Local perturbations are induced by oscillating microspheres using optical tweezers. 1-point and 2-point microrheology are combined to simultaneously measure the shear modulus and force propagation. A mathematical framework to quantify anisotropic force propagation trends is suggested. Results show that force propagation becomes anisotropic in tensed gels, with, surprisingly, stronger response to perturbations perpendicular to the axis of tension. Importantly, external tension can also increase the range of force transmission. Possible implications and future directions for research are discussed. These results suggest a mechanism for favored directions of mechanical communication between cells in a tissue under external loads.