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. Author manuscript; available in PMC: 2017 Oct 24.
Published in final edited form as: Acc Chem Res. 2017 Jun 12;50(7):1633–1639. doi: 10.1021/acs.accounts.7b00095

Discovery of Two Bacterial Nitric Oxide-Responsive Proteins and Their Roles in Bacterial Biofilm Regulation

Sajjad Hossain ‡, Lisa-Marie Nisbett †, Elizabeth M Boon †,‡,§,*
PMCID: PMC5654536  NIHMSID: NIHMS912753  PMID: 28605194

CONSPECTUS

Bacterial biofilms form when bacteria adhere to a surface and produce an exopolysaccharide matrix (Costerton et al. Science 1999, 284, 1318; Davies et al. Science 1998, 280, 295; Flemming et al. Nat. Rev. Microbiol. 2010, 8, 623). Because biofilms are resistant to antibiotics, they are problematic in many aspects of human health and welfare, causing, for instance, persistent fouling of medical implants such as catheters and artificial joints (Brunetto et al. Chimia 2008, 62, 249). They are responsible for chronic infections in the lungs of cystic fibrosis patients and in open wounds, such as those associated with burns and diabetes. They are also a major contributor to hospital-acquired infections (Sievert et al. Infec. Control Hosp. Epidemiol. 2013, 34, 1; Tatterson et al. Front. Biosci. 2001, 6, D890).

It has been hypothesized that effective methods of biofilm control will have widespread application (Landini et al. Appl. Microbiol. Biotechnol. 2010, 86, 813). A promising strategy is to target the mechanisms that drive biofilm dispersal, because dispersal results in biofilm removal and in the restoration of antibiotic sensitivity. First documented in Nitrosomonas europaea (Schmidt et al. J. Bacteriol. 2004, 186, 2781) and the cystic fibrosis-associated pathogen Pseudomonas aeruginosa (Barraud et al. J. Bacteriol. 2006, 188, 7344; J. Bacteriol. 2009, 191, 7333), regulation of biofilm formation by nanomolar levels of the diatomic gas nitric oxide (NO) has now been documented in numerous bacteria (Barraud et al. Microb. Biotechnol. 2009, 2, 370; McDougald et al. Nat. Rev. Microbiol. 2012, 10, 39; Arora et al. Biochemistry 2015, 54, 3717; Barraud et al. Curr. Pharm. Des. 2015, 21, 31). NO-mediated pathways are, therefore, promising candidates for biofilm regulation. Characterization of the NO sensors and NO-regulated signaling pathways should allow for rational manipulation of these pathways for therapeutic applications.

Several laboratories, including our own, have shown that a class of NO sensors called H-NOX (heme-nitric oxide or oxygen binding domain) affects biofilm formation by regulating intracellular cyclic di-GMP concentrations and quorum sensing (Arora et al. Biochemistry 2015, 54, 3717; Plate et al. Trends Biochem. Sci. 2013, 38, 566; Nisbett et al. Biochemistry 2016, 55, 4873). Many bacteria that respond to NO do not encode an hnoX gene, however. My laboratory has now discovered an additional family of bacterial NO sensors, called NosP (nitric oxide sensing protein). Importantly, NosP domains are widely conserved in bacteria, especially Gram-negative bacteria, where they are encoded as fusions with or in close chromosomal proximity to histidine kinases or cyclic di-GMP synthesis or phosphodiesterase enzyme, consistent with signaling. In this Account, we briefly review NO and HNOX signaling in bacterial biofilms, describe our discovery of the NosP family, and provide support for its role in biofilm regulation in Pseudomonas aeruginosa, Vibrio cholerae, Legionella pneumophila, and Shewanella oneidensis.

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NITRIC OXIDE SIGNALING IN BACTERIAL BIOFILMS

Bacterial Biofilms

Most bacteria, most of the time, live in a multicellular community called a biofilm.1-5 Biofilms are frequently formed on surfaces (e.g., medical implants, tissues, metals, soil, plastics) at the solid–liquid interface, but they can also form at liquid–air interfaces. Bacteria in a biofilm are encased within a selfsecreted exopolysaccharide matrix.6 In part due to this protective coating, biofilms are extremely persistent, effective, and tolerant.7-13 For example, biofilms coating human intestines contribute to health through the production of vitamins and digestion of food14 but also harbor pathogens that lead to inflammatory diseases.15 Furthermore, biofilm communities are extremely tolerant of antibiotics (~1000-fold more resistant than planktonic cells); thus pathogenic biofilms represent a significant threat to human health and welfare.16,17

Nitric Oxide and Bacterial Biofilms

Nitric oxide (NO) has well documented concentrationdependent functions in eukaryotes. NO is produced by the enzyme NO synthase. When produced at low concentrations, NO acts as a signaling molecule, regulating processes like vasodilation and neurotransmission upon ligation to its sensor, soluble guanylyl cyclase (sGC).18 Eukaryotes also produce NO at high concentrations to, for example, combat invading pathogens.19,20 At these elevated concentrations, eukaryotic cells must detect and respond to NO to prevent host cell damage.19-22

Growing evidence suggests that bacteria also have concentration-dependent responses to NO. Bacteria experience high concentrations of NO during denitrification, a process by which bacteria respire nitrate or nitrite under oxygen-limiting conditions,23 as well as during infection of a eukaryotic host, as suggested above. Therefore, characterization of NOresponsive bacterial proteins such as FNR-like transcription factors (fumarate and nitrate regulatory proteins),24 the NOresponsive transcriptional activator NorR (regulator of NO reductase),25 and the NO-sensitive repressor NsrR (repressor of nitrosative stress)26 have been pursued to understand bacterial responses to elevated levels of NO.

More recently, it has been documented that bacteria also respond to low concentrations of NO. In particular, low levels of NO are linked to regulation of biofilm formation in bacteria.27 NO is implicated in establishing symbiosis between Euprymna scolopes and Vibrio f ischeri,28 and in Nitrosomonas europaea29 and Pseudomonas aeruginosa,30-33 nanomolar concentrations of NO cause biofilm dispersal. Materials that slowly release NO are linked to antibiofilm properties in model systems.34-43 Furthermore, a P. aeruginosa mutant that lacks nitrite reductase (ΔnirS), which generates NO from nitrite in anaerobic conditions, does not disperse from biofilms, whereas a mutant deficient in NO reductase (ΔnorCB) exhibits enhanced dispersal.30 Indeed, NO has been shown to affect biofilm formation in a wide variety of both Gram-negative and Gram-positive bacteria.27,44-46 Therefore, NO-mediated pathways are promising candidates for biofilm regulation.

NITRIC OXIDE AND H-NOX SIGNALING IN BACTERIAL BIOFILMS

H-NOX (heme-nitric oxide or oxygen binding domain) proteins have now been established as NO sensors in bacteria.27,47,48 Our laboratory has been working to characterize the H-NOX family of bacterial NO sensors for 11 years.49-60 H-NOX proteins were discovered based on homology to the mammalian NO sensor, soluble guanylate cyclase,61 and they are found in the genomes of ~200 species of bacteria. In obligate anaerobes, H-NOX proteins appear to primarily function as molecular oxygen sensors62 and are encoded as N- or C-terminal fusions with methyl-accepting chemotaxis proteins.61 In facultative anaerobes, H-NOX proteins primarily function as NO sensors47,48 and are encoded near genes coding for cyclic di-GMP synthases or cyclic di-GMP phosphodiesterases or histidine kinases.61 Because NO regulates biofilm formation in many bacteria and H-NOX genes are encoded in bacterial genomes adjacent to signaling proteins, we hypothesized that two well established biofilm regulatory mechanisms, intracellular cyclic di-GMP signaling and intercellular quorum sensing, might be regulated by NO binding to hnoX-encoded proteins, that is, H-NOXs (Figure 1). Our findings are briefly summarized below.

Figure 1.

Figure 1

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NO ligated H-NOX has been shown to regulate biofilm formation through several biochemical mechanisms. (A) NO-bound H-NOX in V. harveyi inhibits the autokinase activity of a cocistronic hybrid quorum sensing histidine kinase. (B) NO-bound H-NOX in S. woodyi regulates the cyclic di-GMP output of a cocistronic bifunctional cyclic di-GMP synthase and cyclic di-GMP phosphodiesterase. (C) NO-bound H-NOX in S. oneidensis inhibits the autokinase activity of its cocistronic histidine kinase, which results in a change in cyclic di- GMP levels due to a decrease in the phosphorylation levels of the cognate response regulators HnoB, HnoC, and HnoD.

NO-Bound H-NOX Regulated Cyclic Di-GMP Signaling

It is widely recognized that bis(3′-5′)-cyclic-dimeric-guanosine monophosphate (cyclic di-GMP) is a ubiquitous bacterial signaling molecule63,64 that balances motility and virulence in individual cells vs cell adhesion and persistence in biofilms. It appears that, at least in a broad sense, the total concentration of intracellular cyclic di-GMP is tightly correlated with biofilm development: as cyclic di-GMP concentrations go up, bacteria enter biofilm, and as cyclic di-GMP concentrations drop, bacteria leave biofilm. The total concentration of intracellular cyclic di-GMP is tightly regulated by a variety of enzymes that both synthesize and degrade cyclic di-GMP.65 Cyclic di-GMP is synthesized by proteins with diguanylate cyclase (DGC) activity, identified by a GGDEF motif (a five amino acid conserved sequence) and often called a GGDEF domain.66 Proteins with phosphodiesterase (PDE) activity degrade cyclic di-GMP. These are identified by a conserved EAL67 or HDGYP68 amino acid motif and referred to as EAL or HD-GYP domains. Although the details of cyclic di-GMP regulation in bacteria are far from completely understood, the current hypothesis is that the activity of GGDEF, EAL, and HD-GYP domains is regulated by specific stimuli.69-72

In the genomes of many bacteria, especially environmental species, hnoX (heme-nitric oxide or oxygen binding domain) genes are located near genes coding for cyclic di-GMP cyclases or phosphodiesterases, leading to the hypothesis that the NO-ligation of H-NOX could regulate biofilm formation through cyclic di-GMP signaling. Indeed, several of these H-NOX/cyclic di-GMP systems have been biochemically characterized, and to varying extents, also biologically characterized, including those from the bacteria Legionella pneumophila,73 Shewanella woodyi, 57,58,60 Shewanella oneidensis,74 and Agrobacterium vitis.48 Interestingly, in some of these species, NO-bound H-NOX appears to promote biofilm formation, while in other species, it appears to suppress biofilm formation. Specifically, in L. pneumophila and S. oneidensis, NO signaling through H-NOX is reported to enhance biofilm formation, while in S. woodyi and A. vitis, NO signaling through H-NOX disperses biofilms. Although the exact NO phenotype in these species vary, it is very clear that in all cases, H-NOX is able to regulate cyclic di- GMP concentrations in response to NO ligation and that these molecular events ultimately affect biofilm formation, as briefly described below.

In L. pneumophila,73 S. woodyi,57,58,60 and A. vitis,48 NObound H-NOX binds to and directly regulates a bifunctional enzyme that affects cyclic di-GMP synthesis and cyclic di-GMP hydrolysis, but again, the details vary. In L. pneumophila, the cyclic di-GMP phosphodiesterase domain is degenerate and NO-bound H-NOX inhibits cyclic di-GMP synthesis. In S. woodyi and A. vitis, both the cyclic di-GMP cyclase and phosphodiesterase domains are active, and in both organisms, NO-bound H-NOX appears to strongly inhibit cyclic di-GMP cyclase activity and substantially increase phosphodiesterase activity.

Finally, in S. oneidensis,74 as well as Vibrio cholerae74 and Pseudoalteromonas atlantica,49 NO-ligated H-NOX binds to and inhibits the activity of a histidine kinase, which subsequently regulates cyclic di-GMP levels through a phosphotransfer signaling pathway. All of these NO and H-NOX-mediated cyclic di-GMP signaling pathways have been previously reviewed,27,47,48 so we direct the readers to those reviews for more detailed information on the studies described here.

Discovery and Characterization of NO and H-NOX Regulated Quorum Sensing

Quorum sensing regulates community-wide processes like biofilm formation, virulence gene production, and bioluminescence. 75 Vibrio harveyi has three described quorum sensing circuits. Each involves the synthesis, secretion, and detection of a small molecule called an autoinducer. Autoinducer detection regulates phosphorylation of its cognate receptor-hybrid histidine kinase. Each receptor-kinase exchanges phosphate with a common phosphorelay protein, LuxU, which ultimately regulates bioluminescence through transcription factors named LuxO and LuxR. We have demonstrated that NO participates in quorum sensing through LuxU. In essence, NO enters quorum sensing circuits analogously to an autoinducer: it binds its cognate receptor (the H-NOX/HqsK complex; HqsK = HNOX- associated quorum sensing kinase) and contributes to the flux of phosphate to and from LuxO (Figure 1). In a subsequent study, we found that H-NOX and HqsK quorum sensing also regulate biofilm and flagella formation in a NOconcentration dependent manner.55 Furthermore, we have also recently found that a similar NO-regulated quorum sensing circuit is also operational in Vibrio parahaemolyticus.48

Interestingly, similar H-NOX-regulated histidine kinase signaling pathways have been described in Vibrio f ischeri76 and Silicibacter sp. strain Trich4B.77 In these H-NOX signaling pathways, NO-bound H-NOX appears to regulate the activity of a hybrid histidine kinase that exchanges phosphate with LuxU-like phosphorelay proteins to ultimately control, not quorum sensing, but bacterial/eukaryotic symbiosis. These mechanisms of NO and H-NOX-mediated hybrid histidine kinase signaling pathways have also been reviewed previously, 27,47,48 so please refer to those manuscripts for more details.

NITRIC OXIDE AND NosP SIGNALING IN BACTERIAL BIOFILMS

As described above, NO has been found to affect biofilm regulation in many bacteria,27,44-46 and some bacteria respond to NO via ligation with a H-NOX domain.27,47,48 However, there are many examples of organisms that lack an annotated hnoX gene in their genome, yet are able to respond to low levels of NO to disperse their biofilms, suggesting the existence of an alternative NO sensor. My laboratory has now discovered an additional family of putative bacterial NO sensors, called NosP (nitric oxide sensing protein). The discovery and preliminary characterization of NosP signaling pathways is described below.

Discovery of NosP and NO and NosP-Mediated Biofilm Formation in P. aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa has drawn special attention in microbiology because it readily forms biofilms and is the leading cause of death in cystic fibrosis patients as well as a leading cause of hospital-aquired infections.78 NO is well documented as a signaling molecule that directs P. aeruginosa to disperse from biofilms; as low as picomolar concentrations of NO have been shown to cause P. aeruginosa to leave biofilms.30,31,33 Very little is known about the mechanism underlying NO-mediated biofilm dispersal in P. aeruginosa, however. Not surprisingly, it is correlated with increased cyclic di-GMP phosphodiesterase activity, resulting in decreased cyclic di-GMP levels;31 as described above, decreased levels of cyclic di-GMP are tightly correlated with biofilm dispersal in many bacterial species.64,79-81 Prior to our work, several proteins, including BdlA,31,82 GcbA,83 DipA,84 and NbdA85 had been associated with NO-mediated biofilm dispersal, but none of them had been demonstrated to be a primary NO sensor. Therefore, we set out to discover the primary NO sensor in P. aeruginosa.

Using various bioinformatics and biochemical analyses, as well as analysis of published literature,32,74,86-90 we identified a novel family of hemoproteins that we termed NosP.91 NosP domains are widely distributed in bacterial genomes (found in ~800 sequenced genomes) and encoded in putative operons with kinases or cyclic di-GMP synthesis/phosphodiesterase enzymes, suggesting signaling. NosP shares no significant sequence homology with H-NOX.

In P. aeruginosa, NosP (PA1975) is in the same operon as kinase PA1976. Prior to our work, PA1976 had previously been implicated in biofilm regulation,32 although the specific stimulus regulating PA1976 activity was not known. We demonstrated that biofilms of a P. aeruginosa mutant lacking components of the NosP signaling pathway lose the ability to disperse in response to NO (Figure 2).91 Upon cloning, expressing, and purifying NosP, we found it binds heme and ligates to NO with a dissociation rate constant that is comparable to other well-established NO-sensing proteins. Moreover, we showed that NO-bound NosP is able to regulate the phosphorelay activity of a hybrid histidine kinase that was previously shown to be involved in biofilm regulation in P. aeruginosa.32 Thus, we submit that we have discovered a novel NO-responsive pathway that regulates biofilm in P. aeruginosa.

Figure 2.

Figure 2

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NO ligated NosP has been shown to regulate biofilm formation in P. aeruginosa through regulation of a phosphorelay pathway. NO-bound NosP in P. aeruginosa inhibits the autokinase activity of a cocistronic hybrid histidine kinase, which alters the phosphate flux in a downstream signaling pathway, ultimately affecting biofilm formation in this bacterium.

The Role of NO and NosP in Biofilm Formation in Other Organisms

Based on its wide conservation in bacteria, we hypothesize that NosP may be a global biofilm regulator. We have found support of our hypothesis in the literature and preliminary data from our laboratory (Figure 3). We are currently investigating the role of NosP in several organisms in order to test our hypothesis. Below we briefly discuss some of our initial findings.

Figure 3.

Figure 3

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NO ligated NosP has been predicted to regulate biofilm formation through several biochemical mechanisms. (A) NO-bound NosP in V. cholerae inhibits the autokinase activity of its cocistronic histidine kinase. (B) NO-bound NosP in L. pneumophila inhibits the autokinase activity of its cocistronic histidine kinase. (C) NO-bound NosP in S. oneidensis inhibits the autokinase activity of its cocistronic histidine kinase, which also should result in a change in the cyclic di- GMP levels due to a decrease in phosphorylation levels of the cognate response regulators HnoB, HnoC, and HnoD.

In V. cholerae, a stand-alone NosP domain (VC1444) is predicted to be cocistronic with a histidine kinase (VC1445) that has recently been annotated as a quorum sensing hybrid histidine kinase.86,87 Overexpression of this kinase results in a hyperbiofilm phenotype89 and a lack of this hybrid histidine kinase results in a phenotype that is identical to quorum sensing mutants.86 In our laboratory, we have found that, when ligated to NO, NosP from V. cholerae inhibits the autophosphorylation activity of this quorum sensing histidine kinase, suggesting a potential molecular mechanism by which NO may regulate quorum sensing (and thus biofilm formation) in V. cholera.92 Interestingly, this would be analogous to the NO and H-NOX/HqsK mediated quorum sensing pathway we discovered in V. harveyi.54

In S. oneidensis, a NosP domain (SO_2542) is predicted to be upstream of a histidine kinase (SO_2543) and cognate response regulator protein (SO_2541) that has been shown to be involved in NO-mediated biofilm regulation through a multicomponent cyclic di-GMP signaling network.74 As in P. aeruginosa, we have found that NosP from S. oneidensis can undergo heme-dependent NO ligation.93 Further, we have found that S. oneidensis NosP can regulate the autophosphorylation activity of its cocistronic kinase in a NO dependent manner and that mutant S. oneidensis strains that lack either nosP or its associated histidine kinase produce immature biofilms. Consequently, we hypothesize that NO and NosP may regulate biofilm formation in S. oneidensis by modulating the autokinase activity of this histidine kinase (SO_2543), which in turn modulates the previously described cyclic di- GMP signaling network.74

A NosP domain (lpg0279) in Legionella pneumophila is coded for in the same operon as a histidine kinase (lpg0278) and a cyclic di-GMP metabolizing enzyme (lpg0277) with a receiver domain at its N-terminus. In a recent publication, it was demonstrated that deletion of the homologue of lpg0277 in the Legionella pneumophila Lens strain (lpl1054) results in a hyperbiofilm phenotype,88 suggesting involvement of NosP in biofilm regulation.

Similar to the results described above for NosP in P. aeruginosa and V. cholerae, we have found that NO-ligated NosP from L. pneumophila inhibits the autokinase activity of its cocistronic histidine kinase. This kinase normally phosphorylates a bifunctional cyclic di-GMP processing enzyme that is also in the same putative operon. We hypothesize that this change will ultimately affect the intracellular cyclic di-GMP, and thus the biofilm, levels of this bacterium in the presence of NO.

PERSPECTIVES AND FUTURE DIRECTIONS

Our lab is making the case that NosP is a newly discovered NO binding protein involved in bacterial biofilm regulation. To date, we have found that NosP domains exhibit slow NO dissociation kinetics and selectively bind NO and CO, but not molecular oxygen. Additionally, we have also observed that NO bound NosP domains are effective regulators of the activities of their cocistronic partner proteins, thus underscoring a role for these domains as physiological NO sensing hemoproteins.

Interestingly, although the NosP and H-NOX domains are generally not coded for in the same bacteria, there are a few exceptions to this generalization, which raises the questions of if and how they both function as NO sensors. In S. oneidensis, remarkably, not only does this bacterium possesses both HNOX and NosP domains but also the respective H-NOX- and NosP-associated signaling pathways have been demonstrated to participate in the same multicomponent cyclic di-GMP signaling network, which has been shown to regulate biofilm formation in this bacterium.74 We are currently investigating the interaction between NosP and H-NOX in S. oneidensis, as well as in V. cholerae and L. pneumophilia.

Finally, although we have identified two separate families of bacterial NO responsive proteins, H-NOX and NosP, there are still bacteria, specifically many Gram-positive bacteria, for which there are known NO-mediated biofilm regulation phenotypes but neither H-NOX nor NosP domains are encoded in their genomes. Consequently, the identity of the putative primary NO sensor(s) and the associated NO-responsive signaling pathway(s), as well as the molecular mechanism of NO regulation in these bacteria, are all currently unknown. Since the field of NO signaling in bacteria is rapidly expanding, however, it is our hope that these gaps in our understanding of NO signaling in bacteria will be addressed in the near future.

Acknowledgments

This work was supported by the Stony Wold-Herbert Fund, the National Science Foundation, (Grant CHE-1607532 to E.M.B.), and the National Institutes of Health (Grant GM118894-01A1 to E.M.B.). We also thank Dr. Roger Johnson and the Boon Group for helpful discussions.

Biographies

Sajjad Hossain obtained his Bachelor’s in Biology and Master’s in Biotechnology from Hunter College in 2003 and 2004, respectively. He worked as a research assistant on testicular cancer at Lindsley F. Kimball Research Institute New York from 2004 to 2007 and recently received his Ph.D. in Molecular and Cellular Biology from Stony Brook University under the guidance of Elizabeth Boon. He is currently a Postdoctoral Fellow in Dr. Elizabeth Boon’s lab and is working to discover novel bacterial signal transduction pathways that are responsive to nitric oxide.

Lisa-Marie Nisbett received both her B.S. and M.S. degrees in Biology from the LIU Post Campus (formerly known as the C.W. Post Campus) of Long Island University in 2009 and 2012, respectively. For her M.S., she worked on assessing the antiviral efficacy of DABCO-hydrocarbon modified cloths against influenza virus in MDCK cells. Currently, she is a Ph.D. candidate working on the biochemical characterization of NosP signaling pathways in various bacteria.

Elizabeth M. Boon received her A.B. with Highest Honors in Chemistry from Kenyon College in 1997 and her Ph.D. from the California Institute of Technology in 2003. She was a NIH Postdoctoral Fellow at the University of California, Berkeley, before starting at Stony Brook University in 2006. She has received several awards including the Presidential Early Career Award for Scientists and Engineers (PECASE), the NYSTAR Watson Young Investigator Award, the Office of Naval Research Young Investigator Award, and the Rising Star Award from the Research Foundation of the State University of New York. In 2011, the Kavli Foundation and the National Academy of Sciences elected Liz a Kavli Fellow.

Footnotes

Notes

The authors declare no competing financial interest.

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