Slime layer

Abstract: Slime layers are being studied in our laboratories in an attempt to understand their functions in the control of pollution in natural streams. A method for fixing, staining, and embedding microorganisms in the intact slime has been developed. In this method, epoxy resin discs are placed in a holder and are introduced into a simulated stream. After various periods of time the discs are punched out of the holder into the fixative. The disc with the attached slime is fixed, stained (4% osmium tetroxide plus ruthenium red), dehydrated, and embedded in epoxy resin so that thin sections can be cut through the vertical plane of the slime mass. Such thin sections permit detailed examination of the attached layer, the surface-slime interface, the spatial relationships between cells in the vertical slime structure, and the strands of extracellular material between and around cells. No special attachment structures were noted as the cells appeared to be attached to the surface by extracellular material alone. This material was observed in strands and netlike forms between cells which are positioned 1 to 4 mum apart in the slime.

Abstract: Five strains of type III pneumococcus have been shown to possess wide capsular slime layers during the logarithmic phase of growth in serum broth. The slime layer stains metachromatically with methylene blue and can be visualized under the electron microscope as a fuzzy halo which extends well beyond the surace of the capsule proper and causes centrifugates of the organism to be of extremely large volume. This outer capsular structure is most readily demonstrated in vivo and in nutrient broth containing glucose and serum. It disappears from the surface of the cell with aging of the culture, and is easily removed by dilute alkali, alcohol, and heat. Exposure of slime-covered type III pneumococci to homologous antibody and to type III polysaccharidase reveals that the slime layer contains the same type-specific polysaccharide that is present in the rest of the capsule. From a type III strain producing a prominent slime layer an intermediate mutant has been isolated which forms small non-mucoid colonies on blood agar and possesses a relatively small capsule with a barely discernible slime layer. The wide slime layer protects virulent type III pneumococci from surface phagocytosis. Whenever the type III cells lose their broad slime layer, whether from aging of the culture, from mutation, from exposure to injurious chemicals, or from the action of type III polysaccharidase, they become susceptible to phagocytosis by the surface mechanism. Once phagocyted the type III pneumococci are promptly destroyed, even in the absence of antibodies.

Abstract: Macromolecular material from the slime layer of the cyanobacterium Microcystis flos-aquae C3-40 was defined as material that adhered to cells during centrifugation in growth medium but was dislodged by washing with deionized water and retained within dialysis tubing with a molecular-weight cutoff of 3,500. At each step of this isolation procedure, the slime was observed microscopically. Cells in the centrifugal pellet were surrounded by large amounts of slime that excluded negative stain, whereas cells that had been washed with water lacked visible slime. Two independently isolated lots of slime contained no detectable protein (<1%, wt/wt) and consisted predominantly of anthrone-reacting polysaccharide. Sugars in a hydrolysate of slime polysaccharide were derivatized with trimethylsilylimidazole and examined by gas chromatography-mass spectrometry. The composition of the slime polysaccharide was 1.5% (wt/wt) galactose, 2.0% glucose, 3.0% xylose, 5.0% mannose, 5.5% rhamnose, and 83% galacturonic acid. This composition resembles that of the plant polysaccharide pectin, which was treated in parallel as a control. Consistent with earlier indications that M. flos-aquae slime preferentially binds certain cations, the ratio of Fe to Na in the dialyzed slime was 10 times that in the growth medium. The composition of the slime is discussed with respect to possible mechanisms of cation binding in comparison with other cyanobacterial exopolysaccharides and pectin.