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Endospores formed by Bacillus, Clostridia, and related genera are encased in a protein shell called the coat. In many species, including B. subtilis, the coat is the outermost spore structure, and in other species, such as the pathogenic organisms B. anthracis and B. cereus, the spore is encased in an additional layer called the exosporium. Both the coat and the exosporium have roles in protection of the spore and in its environmental interactions. Assembly of both structures is a function of the mother cell, one of two cellular compartments of the developing sporangium. Studies in B. subtilis have revealed that the timing of coat protein production, the guiding role of a small group of morphogenetic proteins, and several types of posttranslational modifications are essential for the fidelity of the assembly process. Assembly of the exosporium requires a set of novel proteins as well as homologues of proteins found in the outermost layers of the coat and of some of the coat morphogenetic factors, suggesting that the exosporium is a more specialized structure of a multifunctional coat. These and other insights into the molecular details of spore surface morphogenesis provide avenues for exploitation of the spore surface layers in applications for biotechnology and medicine.

Curli are the major proteinaceous component of a complex extracellular matrix produced by many Enterobacteriaceae. Curli were first discovered in the late 1980s on Escherichia coli strains that caused bovine mastitis, and have since been implicated in many physiological and pathogenic processes of E. coli and Salmonella spp. Curli fibers are involved in adhesion to surfaces, cell aggregation, and biofilm formation. Curli also mediate host cell adhesion and invasion, and they are potent inducers of the host inflammatory response. The structure and biogenesis of curli are unique among bacterial fibers that have been described to date. Structurally and biochemically, curli belong to a growing class of fibers known as amyloids. Amyloid fiber formation is responsible for several human diseases including Alzheimer's, Huntington's, and prion diseases, although the process of in vivo amyloid formation is not well understood. Curli provide a unique system to study macromolecular assembly in bacteria and in vivo amyloid fiber formation. Here, we review curli biogenesis, regulation, role in biofilm formation, and role in pathogenesis.

The assembly of bacterial pili as exemplified here by P and type 1 pili of E. coli is a complex process involving specific molecular interactions between structural and chaperone proteins. The assembly process occurs postsecretionally, i.e. after the subunits are translocated across the cytoplasmic membrane. In a single cell, hundreds of thousands of interactive subunits are typically surface localized and assembled into pili. Periplasmic chaperones are generally required to bind to the interactive subunits and partition them into assembly-competent complexes. The binding of the chaperone to the subunits apparently protects the interactive surfaces and prevents them from aggregating at the wrong time and place within the cell. Pili are most likely assembled into linear polymers that package into right-handed helices after their translocation through specific outer-membrane channels. Each pilus filament is a quaternary assembly of the structural subunit and several minor subunits including the adhesin moiety. Although the assembly and organization of P and type 1 pili are very similar, there are some notable differences. For example, the P pilus adhesin is located exclusively at the tips of the pilus filament and forms part of a morphologically distinct structure. In contrast, the adhesion moiety of type 1 pili is inserted into the pilus filament at intervals, but only the adhesin molecule exposed at the pilus tip is functional. The variability in isoreceptor recognition amongst P pili has been solely ascribed to structural differences in the respective adhesin molecules, whereas in type 1 pili, variability in binding specificity has been attributed to the pilus filament that influences the conformation of the adhesin moiety. Less is known about the structure or assembly of type 4 pili, which are a unique class of pili expressed by several different species of gram-negative bacteria. The phase variation of the pilC assembly gene in N. gonorrheae to the off state results in the accumulation of unassembled subunits toxic to the cells. This process exerts a strong selection pressure on the cells that triggers alterations in the pilin structural gene. Thus, antigenic variation of pili in this organism may be regulated at the level of assembly. Finally, the concept of periplasmic chaperones in postsecretional assembly is most likely a general phenomenon in the biology of gram-negative bacteria. The investigations of pilus assembly will continue to provide insight into the details of how macromolecular assembly reactions are coordinated in the bacterial cell and how the regulation of assembly genes can profoundly affect biological processes.