Proteus mirabilis mannose-resistant, Proteus-like fimbriae: MrpG is located at the fimbrial tip and is required for fimbrial assembly

MrpH, a new class of metal-binding adhesin, requires zinc to mediate biofilm formation

Proteus mirabilis, a Gram-negative uropathogen, is a major causative agent in catheter-associated urinary tract infections (CAUTI). Mannose-resistant Proteus-like fimbriae (MR/P) are crucially important for P. mirabilis infectivity and are required for biofilm formation and auto-aggregation, as well as for bladder and kidney colonization. Here, the X-ray crystal structure of the MR/P tip adhesin, MrpH, is reported. The structure has a fold not previously described and contains a transition metal center with Zn²⁺ coordinated by three conserved histidine residues and a ligand. Using biofilm assays, chelation, metal complementation, and site-directed mutagenesis of the three histidines, we show that an intact metal binding site occupied by zinc is essential for MR/P fimbria-mediated biofilm formation, and furthermore, that P. mirabilis biofilm formation is reversible in a zinc-dependent manner. Zinc is also required for MR/P-dependent agglutination of erythrocytes, and mutation of the metal binding site renders P. mirabilis unfit in a mouse model of UTI. The studies presented here provide important clues as to the mechanism of MR/P-mediated biofilm formation and serve as a starting point for identifying the physiological MR/P fimbrial receptor.

Many bacteria use fimbriae to adhere to surfaces, and this function is often essential for pathogens to gain a foothold in the host. In this study, we examine the major virulence-associated fimbrial protein, MrpH, of the bacterial urinary tract pathogen Proteus mirabilis. This species is particularly known for causing catheter-associated urinary tract infections, in which it forms damaging urinary stones and crystalline biofilms that can block the flow of urine through indwelling catheters. MrpH resides at the tip of mannose-resistant Proteus-like (MR/P) fimbriae and is required for MR/P-dependent adherence to surfaces. Although MR/P belongs to a well-known class of adhesive fimbriae encoded by the chaperone-usher pathway, we found that MrpH has a dramatically different structure compared with other tip-located adhesins in this family. Unexpectedly, MrpH was found to bind a zinc cation, which we show is essential for MR/P-mediated biofilm formation and adherence to red blood cells. Furthermore, MR/P-mediated adherence can be modified by controlling zinc levels. These findings have the potential to aid development of better anti-biofilm urinary catheters or other methods to prevent P. mirabilis infection of the urinary tract.

Using Hemagglutination, Surface Shearing, and Acid Treatment to Study Fimbriae in Proteus mirabilis

A critical first step in bacterial virulence and colonization is adherence to mucosal surfaces, often mediated by fimbriae and other protein adhesins. Here are described three short methods for studying these surface proteins and their behaviors, using protocols developed for the opportunistic pathogen Proteus mirabilis. Unlike the mannose-binding type 1 fimbriae produced by Escherichia coli, most P. mirabilis strains produce mannose-resistant/Proteus-like (MR/P) fimbriae. Both types of fimbrial production and adhesion can be easily demonstrated by a simple and economical hemagglutination assay which uses a model system of erythrocytes. The second and third fimbrial methods presented here show how to shear surface-exposed proteins and use acid treatment to separate interlocked fimbrial subunits into monomers.

Pathogenesis of Proteus mirabilis Infection

Proteus mirabilis, a Gram-negative rod-shaped bacterium most noted for its swarming motility and urease activity, frequently causes catheter-associated urinary tract infections (CAUTIs) that are often polymicrobial. These infections may be accompanied by urolithiasis, the development of bladder or kidney stones due to alkalinization of urine from urease-catalyzed urea hydrolysis. Adherence of the bacterium to epithelial and catheter surfaces is mediated by 17 different fimbriae, most notably MR/P fimbriae. Repressors of motility are often encoded by these fimbrial operons. Motility is mediated by flagella encoded on a single contiguous 54-kb chromosomal sequence. On agar plates, P. mirabilis undergoes a morphological conversion to a filamentous swarmer cell expressing hundreds of flagella. When swarms from different strains meet, a line of demarcation, a "Dienes line," develops due to the killing action of each strain's type VI secretion system. During infection, histological damage is caused by cytotoxins including hemolysin and a variety of proteases, some autotransported. The pathogenesis of infection, including assessment of individual genes or global screens for virulence or fitness factors has been assessed in murine models of ascending urinary tract infections or CAUTIs using both single-species and polymicrobial models. Global gene expression studies performed in culture and in the murine model have revealed the unique metabolism of this bacterium. Vaccines, using MR/P fimbria and its adhesin, MrpH, have been shown to be efficacious in the murine model. A comprehensive review of factors associated with urinary tract infection is presented, encompassing both historical perspectives and current advances.

From Catheter to Kidney Stone: The Uropathogenic Lifestyle of Proteus mirabilis

Proteus mirabilis is a model organism for urease-producing uropathogens. These diverse bacteria cause infection stones in the urinary tract and form crystalline biofilms on indwelling urinary catheters, frequently leading to polymicrobial infection. Recent work has elucidated how P. mirabilis causes all of these disease states. Particularly exciting is the discovery that this bacterium forms large clusters in the bladder lumen that are sites for stone formation. These clusters, and other steps of infection, require two virulence factors in particular: urease and MR/P fimbriae. Highlighting the importance of MR/P fimbriae is the cotranscribed regulator, MrpJ, which globally controls virulence. Overall, P. mirabilis exhibits an extraordinary lifestyle, and further probing will answer exciting basic microbiological and clinically relevant questions.

Drug and Vaccine Development for the Treatment and Prevention of Urinary Tract Infections

Urinary tract infections (UTI) are among the most common bacterial infections in humans, affecting millions of people every year. UTI cause significant morbidity in women throughout their lifespan, in infant boys, in older men, in individuals with underlying urinary tract abnormalities, and in those that require long-term urethral catheterization, such as patients with spinal cord injuries or incapacitated individuals living in nursing homes. Serious sequelae include frequent recurrences, pyelonephritis with sepsis, renal damage in young children, pre-term birth, and complications of frequent antimicrobial use including high-level antibiotic resistance and Clostridium difficile colitis. Uropathogenic E. coli (UPEC) cause the vast majority of UTI, but less common pathogens such as Enterococcus faecalis and other enterococci frequently take advantage of an abnormal or catheterized urinary tract to cause opportunistic infections. While antibiotic therapy has historically been very successful in controlling UTI, the high rate of recurrence remains a major problem, and many individuals suffer from chronically recurring UTI, requiring long-term prophylactic antibiotic regimens to prevent recurrent UTI. Furthermore, the global emergence of multi-drug resistant UPEC in the past ten years spotlights the need for alternative therapeutic and preventative strategies to combat UTI, including anti-infective drug therapies and vaccines. In this chapter, we review recent advances in the field of UTI pathogenesis, with an emphasis on the identification of promising drug and vaccine targets. We then discuss the development of new UTI drugs and vaccines, highlighting the challenges these approaches face and the need for a greater understanding of urinary tract mucosal immunity.

Proteus mirabilis and Urinary Tract Infections

Proteus mirabilis is a Gram-negative bacterium and is well known for its ability to robustly swarm across surfaces in a striking bulls'-eye pattern. Clinically, this organism is most frequently a pathogen of the urinary tract, particularly in patients undergoing long-term catheterization. This review covers P. mirabilis with a focus on urinary tract infections (UTI), including disease models, vaccine development efforts, and clinical perspectives. Flagella-mediated motility, both swimming and swarming, is a central facet of this organism. The regulation of this complex process and its contribution to virulence is discussed, along with the type VI-secretion system-dependent intra-strain competition, which occurs during swarming. P. mirabilis uses a diverse set of virulence factors to access and colonize the host urinary tract, including urease and stone formation, fimbriae and other adhesins, iron and zinc acquisition, proteases and toxins, biofilm formation, and regulation of pathogenesis. While significant advances in this field have been made, challenges remain to combatting complicated UTI and deciphering P. mirabilis pathogenesis.

Transcriptional analysis of the MrpJ network: modulation of diverse virulence-associated genes and direct regulation of mrp fimbrial and flhDC flagellar operons in Proteus mirabilis

The enteric bacterium Proteus mirabilis is associated with a significant number of catheter-associated urinary tract infections (UTIs). Strict regulation of the antagonistic processes of adhesion and motility, mediated by fimbriae and flagella, respectively, is essential for disease progression. Previously, the transcriptional regulator MrpJ, which is encoded by the mrp fimbrial operon, has been shown to repress both swimming and swarming motility. Here we show that MrpJ affects an array of cellular processes beyond adherence and motility. Microarray analysis found that expression of mrpJ mimicking levels observed during UTIs leads to differential expression of 217 genes related to, among other functions, bacterial virulence, type VI secretion, and metabolism. We probed the molecular mechanism of transcriptional regulation by MrpJ using transcriptional reporters and chromatin immunoprecipitation (ChIP). Binding of MrpJ to two virulence-associated target gene promoters, the promoters of the flagellar master regulator flhDC and mrp itself, appears to be affected by the condensation state of the native chromosome, although both targets share a direct MrpJ binding site proximal to the transcriptional start. Furthermore, an mrpJ deletion mutant colonized the bladders of mice at significantly lower levels in a transurethral model of infection. Additionally, we observed that mrpJ is widely conserved in a collection of recent clinical isolates. Altogether, these findings support a role of MrpJ as a global regulator of P. mirabilis virulence.

FimA, FimF, and FimH are necessary for assembly of type 1 fimbriae on Salmonella enterica serovar Typhimurium

Salmonella enterica serovar Typhimurium is a Gram-negative member of the family Enterobacteriaceae and is a common cause of bacterial food poisoning in humans. The fimbrial appendages are found on the surface of many enteric bacteria and enable the bacteria to bind to eukaryotic cells. S. Typhimurium type 1 fimbriae are characterized by mannose-sensitive hemagglutination and are assembled via the chaperone/usher pathway. S. Typhimurium type 1 fimbrial proteins are encoded by the fim gene cluster (fimAICDHFZYW), with fimAICDHF expressed as a single transcriptional unit. The structural components of the fimbriae are FimA (major subunit), FimI, FimH (adhesin), and FimF (adaptor). In order to determine which components are required for fimbrial formation in S. Typhimurium, mutations in fimA, fimI, fimH, and fimF were constructed and examined for their ability to produce surface-assembled fimbriae. S. Typhimurium SL1344ΔfimA, -ΔfimH, and -ΔfimF mutants were unable to assemble fimbriae, indicating that these genes are necessary for fimbrial production in S. Typhimurium. However, SL1344ΔfimI was able to assemble fimbriae. In Escherichia coli type 1 and Pap fimbriae, at least two adaptors are expressed in addition to the adhesins. However, E. coli type 1 and Pap fimbriae have been reported to be able to assemble fimbriae in the absence of these proteins. These results suggest differences between the S. Typhimurium type 1 fimbrial system and the E. coli type 1 and Pap fimbrial systems.

Complicated catheter-associated urinary tract infections due to Escherichia coli and Proteus mirabilis

Catheter-associated urinary tract infections (CAUTIs) represent the most common type of nosocomial infection and are a major health concern due to the complications and frequent recurrence. These infections are often caused by Escherichia coli and Proteus mirabilis. Gram-negative bacterial species that cause CAUTIs express a number of virulence factors associated with adhesion, motility, biofilm formation, immunoavoidance, and nutrient acquisition as well as factors that cause damage to the host. These infections can be reduced by limiting catheter usage and ensuring that health care professionals correctly use closed-system Foley catheters. A number of novel approaches such as condom and suprapubic catheters, intermittent catheterization, new surfaces, catheters with antimicrobial agents, and probiotics have thus far met with limited success. While the diagnosis of symptomatic versus asymptomatic CAUTIs may be a contentious issue, it is generally agreed that once a catheterized patient is believed to have a symptomatic urinary tract infection, the catheter is removed if possible due to the high rate of relapse. Research focusing on the pathogenesis of CAUTIs will lead to a better understanding of the disease process and will subsequently lead to the development of new diagnosis, prevention, and treatment options.

Evolution of the chaperone/usher assembly pathway: fimbrial classification goes Greek

Many Proteobacteria use the chaperone/usher pathway to assemble proteinaceous filaments on the bacterial surface. These filaments can curl into fimbrial or nonfimbrial surface structures (e.g., a capsule or spore coat). This article reviews the phylogeny of operons belonging to the chaperone/usher assembly class to explore the utility of establishing a scheme for subdividing them into clades of phylogenetically related gene clusters. Based on usher amino acid sequence comparisons, our analysis shows that the chaperone/usher assembly class is subdivided into six major phylogenetic clades, which we have termed alpha-, beta-, gamma-, kappa-, pi-, and sigma-fimbriae. Members of each clade share related operon structures and encode fimbrial subunits with similar protein domains. The proposed classification system offers a simple and convenient method for assigning newly discovered chaperone/usher systems to one of the six major phylogenetic groups.

Mannose-resistant Proteus-like fimbriae are produced by most Proteus mirabilis strains infecting the urinary tract, dictate the in vivo localization of bacteria, and contribute to biofilm formation

Proteus mirabilis, an etiologic agent of complicated urinary tract infections, expresses mannose-resistant Proteus-like (MR/P) fimbriae whose expression is phase variable. Here we examine the role of these fimbriae in biofilm formation and colonization of the urinary tract. The majority of wild-type P. mirabilis cells in transurethrally infected mice produced MR/P fimbriae. Mutants that were phase-locked for either constitutive expression (MR/P ON) or the inability to express MR/P fimbriae (MR/P OFF) were phenotypically distinct and swarmed at different rates. The number of P. mirabilis cells adhering to bladder tissue did not appear to be affected by MR/P fimbriation. However, the pattern of adherence to the bladder surface was strikingly different. MR/P OFF colonized the lamina propria underlying exfoliated uroepithelium, while MR/P ON colonized the luminal surfaces of bladder umbrella cells and not the exfoliated regions. Wild-type P. mirabilis was usually found colonizing intact uroepithelium, but it occasionally adhered to exfoliated areas. MR/P ON formed significantly more biofilm than either P. mirabilis HI4320 (P = 0.03) or MR/P OFF (P = 0.05). MR/P OFF was able to form a biofilm similar to that of the wild type. MR/P ON formed a three-dimensional biofilm structure as early as 18 h after the initiation of the biofilm, while MR/P OFF and the wild type did not. After 7 days, however, P. mirabilis HI4320 formed a 65-mum-thick biofilm, while the thickest MR/P ON and MR/P OFF biofilms were only 12 mum thick. We concluded that MR/P fimbriae are expressed by most P. mirabilis cells infecting the urinary tract, dictate the localization of bacteria in the bladder, and contribute to biofilm formation.

In vivo expression of the mannose-resistant fimbriae of Photorhabdus temperata K122 during insect infection

Photorhabdus temperata K122 is an entomopathogenic bacterium symbiotically associated with nematodes of the family Heterorhabditidae: Surface fimbriae are important for the colonization of many pathogenic bacteria, and here we report the nucleotide sequence and analysis of the expression of a 12-kbp fragment encoding the mannose-resistant fimbriae of P. temperata (mrf). The mrf gene cluster contains 11 genes with an organization similar to that of the mrp locus from Proteus mirabilis. mrfI (encoding a putative recombinase) and mrfA (encoding pilin), the first gene in an apparent operon of nine other genes, are expressed from divergent promoters. The mrfI-mrfA intergenic region contains inverted repeats flanking the mrfA promoter. This region was shown to be capable of inversion, consistent with an ON/OFF regulation of the operon. In in vitro liquid cultures, both orientations were detected. Nevertheless, when we analyzed the expression of all of the genes in the mrf locus by semiquantitative reverse transcription-PCR during infection of Galleria mellonella (greater wax moth) larvae, expression of mrfA was not detected until 25 h postinfection, preceding the death of the larvae at 32 h. In contrast, mrfJ (a putative inhibitor of flagellar synthesis) was expressed throughout infection. Expression of mrfI was also detected only late in infection (25 to 30 h), indicating a possible increase in inversion frequency at this stage. In both in vitro liquid cultures and in vivo larval infections, the distal genes of the operon were expressed at substantially lower levels than mrfA. These results indicate the complex regulation of the mrf cluster during infection.

Development of an intranasal vaccine to prevent urinary tract infection by Proteus mirabilis

Proteus mirabilis commonly infects the complicated urinary tract and is associated with urolithiasis. Stone formation is caused by bacterial urease, which hydrolyzes urea to ammonia, causing local pH to rise, and leads to the subsequent precipitation of magnesium ammonium phosphate (struvite) and calcium phosphate (apatite) crystals. To prevent these infections, we vaccinated CBA mice with formalin-killed bacteria or purified mannose-resistant, Proteus-like (MR/P) fimbriae, a surface antigen expressed by P. mirabilis during experimental urinary tract infection, via four routes of immunization: subcutaneous, intranasal, transurethral, and oral. We assessed the efficacy of vaccination using the CBA mouse model of ascending urinary tract infection. Subcutaneous or intranasal immunization with formalin-killed bacteria and intranasal or transurethral immunization with purified MR/P fimbriae significantly protected CBA mice from ascending urinary tract infection by P. mirabilis (P < 0.05). To investigate the potential of MrpH, the MR/P fimbrial tip adhesin, as a vaccine, the mature MrpH peptide (residues 23 to 275, excluding the signal peptide), and the N-terminal receptor-binding domain of MrpH (residues 23 to 157) were overexpressed as C-terminal fusions to maltose-binding protein (MBP) and purified on amylose resins. Intranasal immunization of CBA mice with MBP-MrpH (residues 23 to 157) conferred effective protection against urinary tract infection by P. mirabilis (P < 0.002).

Identification of MrpI as the sole recombinase that regulates the phase variation of MR/P fimbria, a bladder colonization factor of uropathogenic Proteus mirabilis

Proteus mirabilis is a common cause of urinary tract infection (UTI) in individuals with structural abnormalities or long-term catheterization. The expression of mannose-resistant/Proteus-like (MR/P) fimbria is phase variable because of the inversion of a 251 bp DNA fragment that carries the promoter for the mrp operon. Previous studies have shown that mrpI, which is transcribed divergently from the mrp operon, encodes a recombinase capable of switching the orientation of this invertible element. In this study, we constructed isogenic mrpI null mutants from a clinical isolate of P. mirabilis, HI4320. A polymerase chain reaction (PCR)-based invertible element assay revealed that the isogenic mrpI null mutants were locked in one phase, either expressing (locked on) MR/P fimbriae or not (locked off), which indicated that MrpI was the sole recombinase that regulated the phase variation of MR/P fimbria. The locked-on and locked-off mutants were evaluated for virulence in the CBA mouse model of ascending UTI by co-challenges with each other and with the wild-type strain. Results from these experiments demonstrated conclusively that the MR/P fimbria was a critical bladder colonization factor of uropathogenic P. mirabilis and also suggested that the ability to switch off the expression of MR/P fimbria might be important for kidney colonization.

Bacterial adhesion to sulcular epithelium in periodontitis

The purpose of this study was to investigate, by electron microscopy, the type of bacterial attachment to the sulcular epithelium in periodontitis. Gingiva biopsies were observed in a transmission electron microscope using cytochemical staining with ruthenium red for glycocalyx visualisation. In addition, subgingival plaque samples and biopsies from the sulcular epithelium in periodontitis from the patients were estimated microbiologically. Aerobic bacteria only were estimated in the subgingival plaque and both aerobic and anaerobic bacteria in the gingival biopsies. No bacterial internalisation could be observed. Fimbria-mediated adhesion as the only type of bacterial attachment and a large diversity of bacterial glycocalyces were detected. As the fimbrial adhesins of putative periodontal pathogens are able in vitro to induce inflammation and bone resorption via stimulation of the proinflammatory cytokine production, the demonstrated fimbrial adhesins suggest the significant role of bacterial adhesion to sulcular epithelium in periodontitis.

New aspects of the role of MR/P fimbriae in Proteus mirabilis urinary tract infection

Proteus mirabilis, a common cause of urinary tract infection (UTI), produces a number of different fimbriae including mannose-resistant Proteus-like fimbriae (MR/P). The precise role of different P. mirabilis fimbriae in ascending UTI has not yet been elucidated. In this study, a clinical isolate of P. mirabilis and an isogenic mutant unable to express MR/P were tested using different experimental approaches. They were tested for their ability to cause infection in an ascending co-infection model of UTI and in a haematogenous model in the mouse. In both models, the mutant was less able than the wild-type strain to colonise the lower and upper urinary tracts although infectivity was not abolished. In vitro adherence to uroepithelial cells was also assessed. Significant differences in adherence between both strains were observed at 1 h but not at 15 min post infection. We have also shown that a wild-type strain carries two copies of the mrpA gene. These data reinforce the importance of MR/P fimbriae in P. mirabilis UTI although other virulence factors may be necessary for efficient colonisation and development of infection.

Requirement of MrpH for mannose-resistant Proteus-like fimbria-mediated hemagglutination by Proteus mirabilis

Two new genes, mrpH and mrpJ, were identified downstream of mrpG in the mrp gene cluster encoding mannose-resistant Proteus-like (MR/P) fimbriae of uropathogenic Proteus mirabilis. Since the predicted MrpH has 30% amino acid sequence identity to PapG, the Galalpha(1-4)Gal-binding adhesin of Escherichia coli P fimbriae, we hypothesized that mrpH encodes the functional MR/P hemagglutinin. MR/P fimbriae, expressed in E. coli DH5alpha, conferred on bacteria both the ability to cause mannose-resistant hemagglutination and the ability to aggregate to form pellicles on the broth surface. Both a DeltamrpH mutant expressed in E. coli DH5alpha and an isogenic mrpH::aphA mutant of P. mirabilis were unable to produce normal MR/P fimbriae efficiently, suggesting that MrpH was involved in fimbrial assembly. Amino acid residue substitution of the N-terminal cysteine residues (C66S and C128S) of MrpH abolished the receptor-binding activity (hemagglutinating ability) of MrpH but allowed normal fimbrial assembly, supporting the notion that MrpH was the functional MR/P hemagglutinin. Immunogold electron microscopy of P. mirabilis HI4320 revealed that MrpH was located at the tip of MR/P fimbriae, also consistent with its role in receptor binding. The isogenic mrpH::aphA mutant of HI4320 was less able to colonize the urine, bladder, and kidneys in a mouse model of ascending urinary tract infection (P < 0.01), and therefore MR/P fimbriae contribute significantly to bacterial colonization in mice. While there are similarities between P. mirabilis MR/P and E. coli P fimbriae, there are more notable differences: (i) synthesis of the MrpH adhesin is required to initiate fimbrial assembly, (ii) MR/P fimbriae confer an aggregation phenotype, (iii) site-directed mutation of specific residues can abolish receptor binding but allows fimbrial assembly, and (iv) mutation of the adhesin gene abolishes virulence in a mouse model of ascending urinary tract infection.

MrpB functions as the terminator for assembly of Proteus mirabilis mannose-resistant Proteus-like fimbriae

Insertional mutagenesis studies of mrpB, a putative pilin-encoding open reading frame of the mrp gene cluster, which encodes mannose-resistant Proteus-like (MR/P) fimbriae of Proteus mirabilis, indicate that MrpB functions as the terminator for fimbrial assembly.