SummaryAdherence to mammalian host tissues is an important virulence trait in microbial pathogenesis, yet little is known about the adherence mechanisms of mycobacteria. Here, we show that binding ofmycobacteria to epithelial cells but not to macrophages can be specifically inhibited by sulfated carbohydrates. Using heparin-Sepharose chromatography, a 28-kD heparin-binding protein was purified from culture supematants and cell extracts ofMycobacterium bovis and Mycobacterium tuberculosis. This protein, designated heparin-binding hemagglutinin (HBHA), promotes the agglutination of rabbit erythrocytes, which is specifically inhibited by sulfated carbohydrates. HBHA also induces mycobacterial aggregation, suggesting that it can mediate bacteria-bacteria interactions as well. Hemagglutination, mycobacterial aggregation, as well as attachment to epithelial cells are specifically inhibited in the presence of anti-HBHA antibodies. Immunoelectron microscopy using anti-HBHA monoclonal antibodies revealed that the protein is surface exposed, consistent with a role in adherence. Immunoblot analyses using antigen-specific antibodies indicated that HBHA is different from the fibronectin-binding proteins of the antigen 85 complex and p55, and comparison of the NH2-terminal amino acid sequence of purified HBHA with the protein sequence data bases did not reveal any significant similarity with other known proteins. Sera from tuberculosis patients but not from healthy individuals were found to recognize HBHA, indicating its immunogenicity in humans during mycobacterial infections. Identification of putative mycobacterial adhesins, such as the one described in this report, may provide the basis for the development of new therapeutic and prophylactic strategies against mycobacterial diseases.

The elucidation of the genomic sequence of Mycobacterium tuberculosis revealed the presence of a novel multigene family designated PE/PE_PGRS that encodes numerous, highly related proteins of unknown function. In this study, we demonstrate that a transposon insertion in a PE_PGRS gene (1818 PE_PGRS ) found in Mycobacterium bovis BCG Pasteur, which is the BCG homologue of the M. tuberculosis H37Rv gene Rv1818c, introduces new phenotypic properties to this BCG strain. These properties include dispersed growth in liquid medium and reduced infection of macrophages. Complementation of the 1818 PE_PGRS ::Tn5367 mutant with the wild-type gene restores both aggregative growth (clumping) in liquid medium and reestablishes infectivity of macrophages to levels equivalent to those for the parent BCG strain. Western blot analysis using antisera raised against the 1818 PE_PGRS protein shows that PE_PGRS proteins are found in cell lysates of BCG and M. tuberculosis H37Ra and in the cell wall fraction of M. tuberculosis H37Rv. Moreover, immunofluorescent labeling of mycobacteria indicates that certain PE_PGRS proteins are localized at the cell surface of BCG and M. tuberculosis. Together these results suggest that certain PE_PGRS proteins may be found at the surface of mycobacteria and influence both cell surface interactions among mycobacteria as well as the interactions of mycobacteria with macrophages.It is well recognized that an important first step in bacterial infection of the host is the manifestation of specific surface components by the pathogen to facilitate its entry of host cells and tissues. Many organisms have evolved novel adhesins that can bind to molecules commonly found on the eukaryotic cell surface, such as integrins (17, 38), glycosaminoglycans (16,22,41), or specific sugar residues (8, 10). A number of complex surface structures are also expressed by bacteria to seek out the appropriate host cells; these include the formation of fimbriae (pili) (2) and complex cell wall structures exemplified by the type III secretion systems (14,19,26). Following entry into the host, Mycobacterium tuberculosis shows a tropism for macrophages but can also infect epithelial cells (30,33,43). M. tuberculosis has also been shown to have ligands that bind to extracellular matrix proteins like fibronectin (1, 37, 47) and proteoglycans (16, 33). Schlesinger et al. (42) have described complement and mannose receptors on macrophages that promote the phagocytosis of mycobacteria. Genetic studies of M. tuberculosis have identified numerous genes, such as mce (3), eis (45), and erp (7), encoding proteins that enhance mycobacterial entry and survival within macrophages. Although progress has been made, the molecular mechanisms of mycobacterial infection of host cells remains unexplained.Transposon mutagenesis has been successfully used to identify novel genes that encode for bacterial virulence factors and surface components (6, 27). In the past few years, transposon mutagenesis systems specific for mycobacteria have been developed (...

Dinoflagellates (Eukaryota; Alveolata; Dinophyceae) are single-cell eukaryotic microorganisms implicated in many toxic outbreaks in the marine and estuarine environment. Co-existing with dinoflagellate communities are bacterial assemblages that undergo changes in species composition, compete for nutrients and produce bioactive compounds, including toxins. As part of an investigation to understand the role of the bacteria in dinoflagellate physiology and toxigenesis, we have characterized the bacterial community associated with laboratory cultures of four 'Pfiesteria-like' dinoflagellates isolated from 1997 fish killing events in Chesapeake Bay. A polymerase chain reaction with oligonucleotide primers specific to prokaryotic 16S rDNA gene sequences was used to characterize the total bacterial population, including culturable and non-culturable species, as well as possible endosymbiotic bacteria. The results indicate a diverse group of over 30 bacteria species co-existing in the dinoflagellate cultures. The broad phylogenetic types of dinoflagellate-associated bacteria were generally similar, although not identical, to those bacterial types found in association with other harmful algal species. Dinoflagellates were made axenic, and the culturable bacteria were added back to determine the contribution of the bacteria to dinoflagellate growth. Confocal scanning laser fluorescence microscopy with 16S rDNA probes was used to demonstrate a physical association of a subset of the bacteria and the dinoflagellate cells. These data point to a key component in the bacterial community being species in the marine alpha-proteobacteria group, most closely associated with the alpha-3 or SAR83 cluster.