Microbial attached cells

Sulfide-mineral oxidation by microbial populations has been postulated to proceed via direct or indirect mechanisms (Tributsch and Bennett, 1981a,b Boon and Heijnen, 2001 Fowler, 2001 Sand et al., 2001 Tributsch, 2001). In the direct mechanism, it is assumed that the action taken by the attached cell or bacterium on a metal sulfide will solubilize the mineral surface through direct enzymatic oxidation reactions. The sulfur moiety on the mineral surface is oxidized to sulfate without the production of any detectable intermediates. The indirect mechanism assumes that the cell or bacteria do not act directly on the sulfide-mineral surface, but catalyze reactions proximal to the mineral surface. The products of these bacterially catalyzed reactions act on the mineral surfaces to promote oxidation of the dissolved Fe(II) and S° that are generated via chemical oxidative processes. Ferrous iron and S°, present at the mineral surface, are biologically oxidized to Fe(III) and sulfate. Physical attachment is not required for the bacterial catalysis to occur. The resulting catalysis promotes chemical oxidation of the sulfide-mineral surface, perpetuating the sulfide oxidation process (Figure 1). 

Microbial attachment to rocks and sand particles forms the basis for the largest and most extensive utilization of immobilized microbial cells in the world today trickling filters. 

E. Influence of Interfacial Films on Microbial Attachment. Air-water and solid-water interfaces accumulate surface-activeor hydrophobic organics present in the aqueous phase. Both the increased concentration of organic foodstuffs and the potentially increased adhesiveness of the interfacial film tend to increase the relative concentration of microbes at interfaces. In terms of cell immobilization, it may be possible to increase the strength of adhesion and the durability of the immobilized wholecell system by pre-coating the solid substrate with an appropriate interfacial film. 

Plant and other lectins have the ability to adhere strongly to microbial cell surfaces and lectins are responsible for the adhesion of Rhizobim trifotii to root hair cells of clover. Discovery of lectins which will bind other microbes may lead to the use of lectins as bridging agents to bind cells to inert support materials. Fletcher (83) studied the effects of proteins on the adhesion of a marine Pseudomonas sp. to polystyrene Petri dishes. Bovine serum albumin, gelatin, fibrinogen and pepsin (pKj — 5.8) all inhibit attachment at pH 7.6 when present either prior to or concurrently with the microbial cells. Bovine serum albumin decreased the adhesion of previously attached cells. 

To avoid this problem plastics are now used rather extensively as carriers for fixed-bed reactors (9,10,12), including denitrification (8). The shape and pore size of plastics can be controlled to give desirable flow characteristics, and the surface can be modified for better microbial attachment, e.g. by plasma treatment. In earlier applications no rigorous conditions or requirements existed for true immobilization of the microbes. Consequently, many cells were detached from the carrier to the effluent causing an increase in the BOD and COD of the effluent water (. ... 

The increased resistance of bacteria to antibiotic therapy is a growing concern for doctors and medical officials worldwide. In the last two decades bacteria have developed resistance to almost all the commercially available antibiotics and the number of new antibiotics expected to enter the market is limited. One of the modes by which bacteria exert this resistance is their ability to develop biofilms. Biofilms are bacterial communities encased in a hydrated polymeric matrix. Biofilm development is known to follow a series of complex but discrete and well-regulated steps (Fig. 4.1) (1) microbial attachment to the surface, (2) growth and aggregation of cells into microcolonies, (3) maturation, and (4) dissemination of progeny cells for new colony formalion (87,88). 

Bioelectrochemical Hydrogen Production, Fig.1 Schematic layout of a microbial electrolysis cell (MEC) showing the anode and cathode chamber, the electrodes with attached biocatalysts, the membrane separator and the power supply, as well as the anodic and cathodic half reactions. AEM anion exchange membrane, CEM cation exchange membrane... 

Hoepelman, A. I. M. and Tuomanen, E. I., Consequences of Microbial Attachment Directing Host cell functions with adhesins. Infect Immun, 1992, 60 1729-1733. 

It is necessary to estabUsh a criterion for microbial death when considering a sterilization process. With respect to the individual cell, the irreversible cessation of all vital functions such as growth, reproduction, and in the case of vimses, inabiUty to attach and infect, is a most suitable criterion. On a practical level, it is necessary to estabUsh test criteria that permit a conclusion without having to observe individual microbial cells. The failure to reproduce in a suitable medium after incubation at optimum conditions for some acceptable time period is traditionally accepted as satisfactory proof of microbial death and, consequentiy, stetihty. The appHcation of such a testing method is, for practical purposes, however, not considered possible. The cultured article caimot be retrieved for subsequent use and the size of many items totally precludes practical culturing techniques. In order to design acceptable test procedures, the kinetics and thermodynamics of the sterilization process must be understood. 

Because enzymes can be intraceUularly associated with cell membranes, whole microbial cells, viable or nonviable, can be used to exploit the activity of one or more types of enzyme and cofactor regeneration, eg, alcohol production from sugar with yeast cells. Viable cells may be further stabilized by entrapment in aqueous gel beads or attached to the surface of spherical particles. Otherwise cells are usually homogenized and cross-linked with glutaraldehyde [111-30-8] to form an insoluble yet penetrable matrix. This is the method upon which the principal industrial appHcations of immobilized enzymes is based. 

Although exopolysaccharides do not normally have a structural role, they do form structures that can be detected by either light or electron microscopy. Exopolysaccharides may form part of a capsule closely attached to the microbial cell surface, or appear as loose slime secreted by the cell but not directly attached to it mucoid Exopolysaccharide producing cells usually form mucoid colonies on solid media and colonies liquid cultures of these cells may become very viscous. However, growth conditions can... 

Microbial cells transported with the stream of fluid above the surface interact with conditioning films. Immediately after attachment, microorganisms initiate production of slimy adhesive substances, predominantly exopolysaccharides (EPS) that assist the formation of microcolonies and microbial films. EPS create bridges for microbial cells to the substratum and permit negatively charged bacteria to adhere to both negatively and positively charged surfaces. EPS may also control interfacial chemistry at the mineral/biofilm interface. 

The initial step in the sequence of events leading to influenza virus infections in mammalian hosts is mediated by the multiple attachment of virus particles to host sialoside receptors in the nasopharynx [41]. These receptors consist largely of cell surface sialylated glycoproteins and gangliosides. The subsequent steps involve receptor-mediated endocytosis with ensuing release of the viral nucleo-plasmid. The first event responsible for the receptor-virus interaction is therefore an attractive target for both antiviral and related microbial intervention. 

It has also been demonstrated that the transmembrane potentials of prokaryotes are typically 50% greater than their eukaryotic counterparts. This chemiosmotic potential has been proposed to act electrophoretically on host defense peptides attached to the microbial surface. It should also be noted that although increased cationicity of the host defense peptides is generally associated with increased antimicrobial activity there is a threshold by which selectivity between host and microbial cells is lost at the behest of increased cationicity. 

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