Iron is an essential element for life but also serves as an environmental signal for biofilm development in the opportunistic human pathogen Pseudomonas aeruginosa. Under iron-limiting conditions, P. aeruginosa displays enhanced twitching motility and forms flat unstructured biofilms. In this study, we present evidence suggesting that iron-regulated production of the biosurfactant rhamnolipid is important to facilitate the formation of flat unstructured biofilms. We show that under iron limitation the timing of rhamnolipid expression is shifted to the initial stages of biofilm formation (versus later in biofilm development under iron-replete conditions) and results in increased bacterial surface motility. In support of this observation, an rhlAB mutant defective in biosurfactant production showed less surface motility under iron-restricted conditions and developed structured biofilms similar to those developed by the wild type under iron-replete conditions. These results highlight the importance of biosurfactant production in determining the mature structure of P. aeruginosa biofilms under iron-limiting conditions.The biofilm mode of bacterial growth is a surface-attached state in which cells are closely packed and encased in an extracellular polymeric matrix (10, 27). Biofilms are abundant in nature and are of clinical, environmental, and industrial importance. Biofilm development is known to follow a series of complex but discrete and tightly regulated steps (18, 27), including (i) microbial attachment to the surface, (ii) growth and aggregation of cells into microcolonies, (iii) maturation, and (iv) dissemination of progeny cells that can colonize new niches. Over the last decade, several key processes important for biofilm formation have been identified, including quorum sensing (12) and surface motility (28).One of the best-studied model organisms for biofilm development is the bacterium Pseudomonas aeruginosa (10), a notorious opportunistic pathogen which causes many types of infections, including biofilm-associated chronic lung infections in individuals with cystic fibrosis (10, 24, 41). Like most organisms, P. aeruginosa requires iron for growth, as iron serves as a cofactor for enzymes that are involved in many basic cellular functions and metabolic pathways. Recent work has shown that at iron concentrations that are not limiting for growth, this metal serves as a signal for biofilm development (40). Iron limitation imposed, for example, by the mammalian iron chelator lactoferrin blocks the ability of P. aeruginosa biofilms to mature from thin layers of cells attached to a surface into large multicellular mushroom-like biofilm structures (40). By chelating iron, lactoferrin induces twitching motility (a specialized form of surface motility), which causes the cells to move across the surface instead of settling down to form structured communities (39,40). In a recent paper, Berlutti et al. (5) provided further support for the role of iron in cell aggregation and biofilm formation. They reported that in t...

Bdellovibrio bacteriovorus is an obligate predator of bacteria that grows and divides within the periplasm of its prey. Functions involved in the early steps of predation have been identified and characterized, but mediators of prey invasion are still poorly detailed. By combining omics data available for Bdellovibrio and like organisms (BALO’s), we identified 43 genes expressed in B. bacteriovorus during the early interaction with prey. These included genes in a tight adherence (TAD) operon encoding for two type IVb fimbriae-like pilin proteins (flp1 and flp2), and their processing and export machinery. Two additional flp genes (flp3 and flp4) were computationally identified at other locations along the chromosome, defining the largest and most diverse type IVb complement known in bacteria to date. Only flp1, flp2 and flp4 were expressed; their respective gene knock-outs resulted in a complete loss of the predatory ability without losing the ability to adhere to prey cells. Additionally, we further demonstrate differential regulation of the flp genes as the TAD operon of BALOs with different predatory strategies is controlled by a flagellar sigma factor FliA, while flp4 is not. Finally, we show that FliA, a known flagellar transcriptional regulator in other bacteria, is an essential Bdellovibrio gene.

The outer membrane proteins of Desulfovibrio piger and Bilophila wadsworthia (Omp-DP and Omp-BW, respectively) and the genes encoding them (omp-DP and omp-BW) were isolated and characterized. Native Omp-DP and Omp-BW form a trimeric structure of approximately 120 kDa. These proteins disaggregated into monomers with a molecular weight of approximately 53 kDa after heating at 95 degrees C for 10 min. The pore-forming abilities of these oligomeric proteins demonstrated that they form small nonspecific channels with an exclusion limit of 260-300 Da. The omp-DP and omp-BW genes were cloned and sequenced. Sequence analyses revealed an open reading frame of 1,512 bp for omp-DP and 1,440 bp for omp-BW. The mature Omp-DP protein consisted of 480 amino acids and had a calculated MW of 53,290 Da. The mature Omp-BW protein consisted of 456 amino acids and had a calculated MW of 50.050 Da. Alignment of Omp-DP with Omp-BW revealed 54% homology, whereas alignment with other known porins showed a low level of homology. Analysis of the secondary structures indicated that both proteins span the outer membrane 18 times with amphipathic beta-strands. This research presents porins which were isolated and characterized for the first time from bacteria belonging to the Desulfovibrionaceae family.

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