Bacterial Adhesion to Animal Cells and Tissues

The study of mechanisms of adhesion vary from simple methods providing limited information to more complex methods that are difficult to perform and analyze but provide more meaningful information. This chapter discusses some of the most important methods and emphasizes the types of knowledge gained from the use of a specific method. Bacteria that exhibit increased hydrophobicity, as determined by the contact angle technique, are more readily engulfed by phagocytes, consistent with the notion that hydrophobicity is important in adhesion. Adhesion to microspheres can be determined by light microscopy, among many other techniques (enzyme-linked immunosorbent assay (ELISA) and use of radiolabeled bacteria). Hemagglutination reactions have been responsible for the initial identification of many lectin adhesins and were important in the characterization of adhesin-saccharide specificities, through the use of carbohydrate inhibitors of the reactions. In approaching important subjects such as affinity, the chapter mentions two basic types of experiments that can be performed. The usefulness of the kinetic approach is extended when experimental variables are manipulated. Methods have been developed to easily differentiate between extracellularly bound and internalized bacteria. One of simplest tests to determine whether an adhesin is expressed in vivo during a natural infection is to assay for antibodies against the adhesin in patient sera. Another relatively easy assay is to screen isolates from an infection for mRNA for an adhesin by reverse transcription-PCR.

This chapter presents concise reviews of the general architectural plan of gram-positive and gram-negative cell surfaces. Within this context, a few specific examples are used to present basic concepts of the genetics of adhesin expression and adhesive organelle biogenesis. The adhesins of the gram-positive bacteria are anchored on the surface by four different mechanisms that include cell wall-anchoring mechanism, transmembrane mechanism, association of adhesins with surface proteins and the association of adhesins with surface glycolipids. The best described and probably the most prominent mechanism for anchoring adhesins is the cell wall-anchoring mechanism. The adhesins of the gram-negative bacteria are anchored on the outer membrane surface by four different mechanisms. The most widely used mechanism for fimbrial biogenesis appears to be the chaperone-usher pathway. This pathway is shared by many enterobacteria as well as some respiratory pathogens. Expression of type 1 fimbriae, as well as several other virulence factors, can also be affected in certain strains of E. coli by the excision of pathogenicity islands. Short peptides that inhibit quorumsensing mechanisms were found to inhibit S. aureus adhesion to epithelial cells as well as biofilm formation on medical device plastics. This is an interesting and promising new direction in the efforts to develop antiadhesion therapies for infectious diseases.

This chapter discusses concept and presents examples of the various mechanisms for receptor specificity diversification. The best-known examples of this type of variation are the pathogenicity islands (PAIs), which carry genes that encode adhesins with distinct receptor specificity, as well as genes that confer other specific aspects of pathogenicity. The changes in receptor specificity emerging from either acquisition or deletion of PAIs are usually much more pronounced than with allelic variation. The organisms express at least two families of adhesins, each of which can vary in its receptor specificity. The mechanism that controls the variation of the pilus shaft is gene rearrangement. Genomic DNA encodes one complete pilin gene and multiple copies of incomplete pilin genes. To better understand the biological consequences of the rapid change in receptor specificity, the chapter addresses how changes in two major adhesins expressed by E. coli, type 1 and P fimbrial bacterial adhesins, affect host and tissue tropism of the organism. The intraspecies diversities in the receptor specificities of FimH and PapG are due to allelic variations in the fimH and papG genes. In conclusion, the remarkable diversity in the fine sugar specificity of the type 1 and P fimbrial lectins clearly illustrates the concept that functional diversity of fimbrial lectins plays an important role in host tropism, tissue tropism, and infectivity.

This chapter focuses on contact-induced signaling of nonphagocytic cells (NPCs) as an immediate postadhesion event, and discusses two major events, one involving an immediate activation of cytoskeletal responses and the other involving activation of transcriptional responses. The former is needed for completion of internalization, while the latter results in the production of inflammatory mediators that can have a great systemic influence on the course of infection. While the cytoskeletal response is associated mainly with actual or abortive entry of bacteria into the cell, the transcriptional response may result either from adherent or from internalized bacteria. An adherent bacterium can transmit a transcriptional signal to nonphagocytic cells (NPCs) in two ways. One way is by using a specific secretory system to create a channel through which effector molecules can be passed into the host cell cytosol, and the second is by secreting effector molecules into the space between the bacterium and the cell, resulting in a relatively high concentration at the cell surface. In summary, considering the four types of activation mechanisms that include the high concentration mechanism, the association mechanism, the adhesin-dependent mechanism and the cooperativity mechanism, and the vast number of modulins produced by pathogenic bacteria, it appears that adhesion per se is only one prerequisite for a wide variety of potential subsequent reactions between the bacterium and target cell that influence the infectious process.

As bacterial adhesion is an indispensable component of biofilm formation, the factors involved in bacterium-substratum interactions are an important basis for understanding biofilm physiology. In fact, biofilm formation may be regarded as one of the most important sequelae of bacterial adhesion. This chapter discusses the advantages gained by bacteria from participation in a biofilm community, the mechanisms of biofilm formation, and attempts to block biofilm development in an effort to prevent infections. The chapter focuses on two types of medically important biofilms which have been extensively studied: hard tissue biofilms on tooth surfaces (i.e., dental plaque) and biomedical implant-associated biofilms. It is likely that there is an inverse relationship between the initial number of adherent bacteria required for biofilm formation and the avidity of the adhesion. The presence of this polysaccharide seems to be required for biofilm formation by strains of these organisms. This material appears to be the major constituent of the so-called slime, previously shown to be essential for biofilm formation by Staphylococcus epidermidis. The organisms produce another polysaccharide rich in galactose, which also contributes to biofilm formation. A type of quorum sensing used by many gram-negative bacterial species is the system involving acyl homoserine lactones. The dental biofilm is based on many bacterium-bacterium interactions, especially metal ion-requiring lectin-carbohydrate interactions. Infectious diseases associated with indwelling medical devices are now considered a major problem in health care. Some bacteria bind best to uncoated biomedical devices, and coating with various compounds can prevent bacterial adhesion.

The study of adhesion during the 1960s, 1970s, and 1980s focused in large part on establishing the importance of this field of biological research. The molecules involved in adhesion have become clearly linked to infections, and information regarding their structure and molecular function is constantly growing. A section contains a tabulation of all of the studies of adhesion between January 1992 and December 2002 in which adhesins, receptors, and target substrata involved in the adhesion of pathogenic bacteria to host cells and tissues were performed. The species studied number almost 300, but it remains true that studies of the adhesion of Escherichia coli outnumber those of many of the other species combined. It is interesting that the number of adhesion studies performed for some of the pathogens does not reflect the clinical significance of the infections in each case. For many of those organisms which are being studied extensively, the numbers of putative adhesins and receptors identified have increased dramatically over the last 10 years compared to the previous decade. The studies cited here were essentially limited to those which involved adhesion tests of host cells or tissue components, materials of prosthetic devices, or other indwelling medical equipment. One table in this chapter includes only a very few studies of adhesion to foods (e.g., lettuce leaves) and essentially no studies of adhesion to food processing components (e.g., stainless steel tables) or to environmental materials (e.g., pipes in water-processing plants).