Biofilm formation is critical for the infection cycle of Vibrio cholerae. Vibrio exopolysaccharides (VPS) and the matrix proteins RbmA, Bap1 and RbmC are required for the development of biofilm architecture. We demonstrate that RbmA binds VPS directly and uses a binary structural switch within its first fibronectin type III (FnIII-1) domain to control RbmA structural dynamics and the formation of VPS-dependent higher-order structures. The structural switch in FnIII-1 regulates interactions in trans with the FnIII-2 domain, leading to open (monomeric) or closed (dimeric) interfaces. The ability of RbmA to switch between open and closed states is important for V. cholerae biofilm formation, as RbmA variants with switches that are locked in either of the two states lead to biofilms with altered architecture and structural integrity.

cAs biofilms grow, resident cells inevitably face the challenge of resource limitation. In the opportunistic pathogen Pseudomonas aeruginosa PA14, electron acceptor availability affects matrix production and, as a result, biofilm morphogenesis. The secreted matrix polysaccharide Pel is required for pellicle formation and for colony wrinkling, two activities that promote access to O 2 . We examined the exploitability and evolvability of Pel production at the air-liquid interface (during pellicle formation) and on solid surfaces (during colony formation). Although Pel contributes to the developmental response to electron acceptor limitation in both biofilm formation regimes, we found variation in the exploitability of its production and necessity for competitive fitness between the two systems. The wild type showed a competitive advantage against a non-Pel-producing mutant in pellicles but no advantage in colonies. Adaptation to the pellicle environment selected for mutants with a competitive advantage against the wild type in pellicles but also caused a severe disadvantage in colonies, even in wrinkled colony centers. Evolution in the colony center produced divergent phenotypes, while adaptation to the colony edge produced mutants with clear competitive advantages against the wild type in this O 2 -replete niche. In general, the structurally heterogeneous colony environment promoted more diversification than the more homogeneous pellicle. These results suggest that the role of Pel in community structure formation in response to electron acceptor limitation is unique to specific biofilm models and that the facultative control of Pel production is required for PA14 to maintain optimum benefit in different types of communities. Most bacteria form multicellular communities called biofilms by producing and encasing themselves in matrices of secreted polymers (1). The National Institutes of Health has estimated that more than half of all bacterial infections involve such biofilm formation, a feature that complicates treatment due to a variety of associated mechanisms that confer increased antibiotic resistance and tolerance in these communities (2). Steep chemical gradients that form within cellular aggregates give rise to microenvironmental heterogeneity; consequently, cells in biofilms exist in diverse physiological states, at least some of which are unique to this lifestyle. For example, in the opportunistic pathogen Pseudomonas aeruginosa PA14, some genes expressed specifically in biofilms are critical for the establishment of lung infections in a mouse model (3). A better understanding of the physiology of biofilm development is required for rational approaches to new therapies for many types of bacterial infections.Using a colony biofilm model, we have found that a primary factor influencing P. aeruginosa PA14 community morphology is the availability of electron acceptors (4-6). As colonies increase in thickness, the formation of O 2 gradients renders the community anoxic at depth and leads to an increase in intr...

Extracellular electron transfer (EET), the reduction of compounds that shuttle electrons to distal oxidants, can support bacterial survival when preferred oxidants are not directly accessible. EET has been shown to contribute to virulence in some pathogenic organisms and is required for current generation in mediator-based fuel cells. In several species, components of the electron transport chain (ETC) have been implicated in electron shuttle reduction, raising the question of how shuttling-based metabolism is integrated with primary routes of metabolic electron flow. The clinically relevant bacterium Pseudomonas aeruginosa can utilize carbon sources (i.e., electron donors) covering a broad range of reducing potentials and possesses a branched ETC that can be modulated to optimize respiratory efficiency. It also produces electron shuttles called phenazines that facilitate intracellular redox balancing, increasing the complexity of its metabolic potential. In this study, we investigated the reciprocal influence of respiratory metabolism and phenazine-associated physiology in P. aeruginosa PA14. We found that phenazine production affects respiratory activity and terminal oxidase gene expression and that carbon source identity influences the mechanisms enabling phenazine reduction. Furthermore, we found that growth in biofilms, a condition for which phenazine metabolism is critical to normal development and redox balancing, affects the composition of the P. aeruginosa phenazine pool. Together, these findings can aid interpretation of P. aeruginosa behavior during host infection and provide inroads to understanding the cross talk between primary metabolism and shuttling-based physiology in the diverse bacteria that carry out EET. IMPORTANCE The clinically relevant pathogen Pseudomonas aeruginosa uses diverse organic compounds as electron donors and possesses multiple enzymes that transfer electrons from central metabolism to O2. These pathways support a balanced intracellular redox state and produce cellular energy. P. aeruginosa also reduces secondary metabolites called phenazines to promote redox homeostasis and virulence. In this study, we examined the reciprocal relationship between these primary and secondary routes of electron flow. We found that phenazines affect respiratory function and that the complement of phenazines produced is strongly affected by growth in assemblages called biofilms. These results provide a more nuanced understanding of P. aeruginosa redox metabolism and may inform strategies for treating persistent infections caused by this bacterium.

Lactate is thought to serve as a carbon and energy source during chronic infections. Sites of bacterial colonization can contain two enantiomers of lactate: the l-form, generally produced by the host, and the d-form, which is usually produced by bacteria, including the pulmonary pathogen Pseudomonas aeruginosa. Here, we characterize P. aeruginosa’s set of four enzymes that it can use to interconvert pyruvate and lactate, the functions of which depend on the availability of oxygen and specific enantiomers of lactate. We also show that anaerobic pyruvate fermentation triggers production of the aerobic d-lactate dehydrogenase in both liquid cultures and biofilms, thereby enabling metabolic cross-feeding of lactate over time and space between subpopulations of cells. These metabolic pathways might contribute to P. aeruginosa growth and survival in the lung.

Microbes in biofilms face the challenge of substrate limitation. In particular, cells in biofilms growing in the laboratory or during host colonization often become limited for oxygen. Previously we found that phenazines, antibiotics produced by, balance the intracellular redox state for cells in biofilms. Here, we show that genes involved in denitrification are induced in phenazine-null (Δ) mutant biofilms grown under an aerobic atmosphere, even in the absence of nitrate. This finding suggests that resident cells employ a bet-hedging strategy to anticipate the potential availability of nitrate and counterbalance their highly reduced redox state. Consistent with our previous characterization of aerobically-grown colonies supplemented with nitrate, we find that the pathway that is induced in Δ colonies combines the nitrate reductase activity of the periplasmic enzyme Nap with downstream reduction of nitrite to nitrogen gas catalyzed by the enzymes Nir, Nor, and Nos. This regulatory relationship differs from the denitrification pathway that functions under anaerobic growth with nitrate as the terminal electron acceptor, which depends on the membrane-associated nitrate reductase Nar. We identify sequences in the promoter regions of the and operons that are required for the effects of phenazines on expression. We also show that specific phenazines have differential effects on gene expression. Finally, we provide evidence that individual steps of the denitrification pathway are catalyzed at different depths within aerobically grown biofilms, suggesting metabolic cross-feeding between community subpopulations. An understanding of the unique physiology of cells in biofilms is critical to our ability to treat fungal and bacterial infections. Colony biofilms of the opportunistic pathogen , grown under an aerobic atmosphere but without nitrate, express a denitrification pathway that differs from that used for anaerobic growth. We report that the components of this pathway are induced by electron acceptor limitation and that they are differentially expressed over biofilm depth. These observations suggest that (i) exhibits "bet hedging" in that it expends energy and resources to prepare for nitrate availability when other electron acceptors are absent; and (ii) that cells in distinct biofilm microniches may be able to exchange substrates to catalyze full denitrification.

Sectioning via paraffin embedding is a broadly established technique in eukaryotic systems. Here we provide a method for the fixation, embedding, and sectioning of intact microbial colony biofilms using perfused paraffin wax. To adapt this method for use on colony biofilms, we developed techniques for maintaining each sample on its growth substrate and laminating it with an agar overlayer, and added lysine to the fixative solution. These optimizations improve sample retention and preservation of micromorphological features. Samples prepared in this manner are amenable to thin sectioning and imaging by light, fluorescence, and transmission electron microscopy. We have applied this technique to colony biofilms of Pseudomonas aeruginosa, Pseudomonas synxantha, Bacillus subtilis, and Vibrio cholerae. The high level of detail visible in samples generated by this method, combined with reporter strain engineering or the use of specific dyes, can provide exciting insights into the physiology and development of microbial communities.

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240/250 words) 25 Extracellular electron transfer (EET), the reduction of compounds that shuttle electrons to distal oxidants, can support bacterial survival when preferred oxidants are not directly accessible. EET has been shown to contribute to virulence in some pathogenic organisms and is required for current generation in mediator-based fuel cells. In several species, components of the electron transport chain (ETC) have been implicated in electron shuttle 30 reduction, raising the question of how shuttling-based metabolism is integrated with primary routes of metabolic electron flow. The clinically relevant bacterium Pseudomonas aeruginosa can utilize carbon sources (i.e., electron donors) covering a broad range of reducing potentials and possesses a branched ETC that can be modulated to optimize respiratory efficiency. It also produces electron shuttles called phenazines that facilitate 35 intracellular redox balancing, increasing the complexity of its metabolic potential. In this study, we investigated the reciprocal influence of respiratory metabolism and phenazineassociated physiology in Pseudomonas aeruginosa PA14. We found that phenazine production affects respiratory activity and terminal oxidase gene expression, and that carbon source identity influences the mechanisms enabling phenazine reduction. 40 Furthermore, we found that growth in biofilms, a condition for which phenazine metabolism is critical to normal development and redox balancing, dramatically affects the composition of the P. aeruginosa phenazine pool. Together, these findings can aid interpretation of P. aeruginosa behavior during host infection and provide inroads to understanding the crosstalk between primary metabolism and shuttling-based physiology 45 in the diverse bacteria that carry out EET. 3 IMPORTANCE (147/150 words)Pseudomonas aeruginosa is a major cause of healthcare-associated infections and long-50 term lung infections in people with cystic fibrosis. It can use diverse organic compounds as electron donors and possesses multiple enzymes that can transfer electrons from central metabolism to O2. These pathways support a balanced intracellular redox state and the production of cellular energy. Under hypoxic conditions, P. aeruginosa can reduce phenazines, secondary metabolites that also promote redox homeostasis and that 55 contribute to virulence. We asked how these primary and secondary routes of electron flow influence each other. We found that phenazines affect respiratory function, that the roles of respiratory enzymes in phenazine reduction are highly condition-dependent, and that the complement of phenazines produced is strongly affected by growth in assemblages called biofilms. These results provide a more nuanced understanding of P. 60 aeruginosa redox metabolism and may inform strategies for treating persistent infections caused by this bacterium. 4 Members of the domain Bacteria exhibit a vast variety of pathways and substrates that can be used to generate energy (1). This metabolic diversity allows...