Identification of genes essential for pellicle formation in Acinetobacter baumannii.

BACKGROUND: Acinetobacter baumannii is an opportunistic pathogen, which has the ability to persist in the clinical environment, causing acute and chronic infections. A possible mechanism contributing to survival of A. baumannii is its ability to form a biofilm-like structure at the air/liquid interface, known as a pellicle. This study aimed to identify and characterise the molecular mechanisms required for pellicle formation in A. baumannii and to assess a broad range of clinical A. baumannii strains for their ability to form these multicellular structures. RESULTS: Random transposon mutagenesis was undertaken on a previously identified hyper-motile variant of A. baumannii ATCC 17978 designated 17978hm. In total three genes critical for pellicle formation were identified; cpdA, a phosphodiesterase required for degradation of cyclic adenosine monophosphate (cAMP), and A1S_0112 and A1S_0115 which are involved in the production of a secondary metabolite. While motility of the A1S_0112::Tn and A1S_0115::Tn mutant strains was abolished, the cpdA::Tn mutant strain displayed a minor alteration in its motility pattern. Determination of cAMP levels in the cpdA::Tn strain revealed a ~24-fold increase in cellular cAMP, confirming the role CpdA plays in catabolising this secondary messenger molecule. Interestingly, transcriptional analysis of the cpdA::Tn strain showed significant down-regulation of the operon harboring the A1S_0112 and A1S_0115 genes, revealing a link between these three genes and pellicle formation. Examination of our collection of 54 clinical A. baumannii strains revealed that eight formed a measurable pellicle; all of these strains were motile. CONCLUSIONS: This study shows that pellicle formation is a rare trait in A. baumannii and that a limited number of genes are essential for the expression of this phenotype. Additionally, an association between pellicle formation and motility was identified. The level of the signalling molecule cAMP was found to be controlled, in part, by the cpdA gene product, in addition to playing a critical role in pellicle formation, cellular hydrophobicity and motility. Furthermore, cAMP was identified as a novel regulator of the operon A1S_0112-0118.

Comparative analysis of surface-exposed virulence factors of Acinetobacter baumannii.

BACKGROUND: Acinetobacter baumannii is a significant hospital pathogen, particularly due to the dissemination of highly multidrug resistant isolates. Genome data have revealed that A. baumannii is highly genetically diverse, which correlates with major variations seen at the phenotypic level. Thus far, comparative genomic studies have been aimed at identifying resistance determinants in A. baumannii. In this study, we extend and expand on these analyses to gain greater insight into the virulence factors across eight A. baumannii strains which are clonally, temporally and geographically distinct, and includes an isolate considered non-pathogenic and a community-acquired A. baumannii. RESULTS: We have identified a large number of genes in the A. baumannii genomes that are known to play a role in virulence in other pathogens, such as the recently studied proline-alanine-alanine-arginine (PAAR)-repeat domains of the type VI secretion systems. Not surprising, many virulence candidates appear to be part of the A. baumannii core genome of virulent isolates but were often found to be insertionally disrupted in the avirulent A. baumannii strain SDF. Our study also reveals that many known or putative virulence determinants are restricted to specific clonal lineages, which suggests that these virulence determinants may be crucial for the success of these widespread common clones. It has previously been suggested that the high level of intrinsic and adaptive resistance has enabled the widespread presence of A. baumannii in the hospital environment. This appears to have facilitated the expansion of its repertoire of virulence traits, as in general, the nosocomial strains in this study possess more virulence genes compared to the community-acquired isolate. CONCLUSIONS: Major genetic variation in known or putative virulence factors was seen across the eight strains included in this study, suggesting that virulence mechanisms are complex and multifaceted in A. baumannii. Overall, these analyses increase our understanding of A. baumannii pathogenicity and will assist in future studies determining the significance of virulence factors within clonal lineages and/or across the species.

H-NS plays a role in expression of Acinetobacter baumannii virulence features.

Acinetobacter baumannii has become a major problem in the clinical setting with the prevalence of infections caused by multidrug-resistant strains on the increase. Nevertheless, only a limited number of molecular mechanisms involved in the success of A. baumannii as a human pathogen have been described. In this study, we examined the virulence features of a hypermotile derivative of A. baumannii strain ATCC 17978, which was found to display enhanced adherence to human pneumocytes and elevated levels of lethality toward Caenorhabditis elegans nematodes. Analysis of cellular lipids revealed modifications to the fatty acid composition, providing a possible explanation for the observed changes in hydrophobicity and subsequent alteration in adherence and motility. Comparison of the genome sequences of the hypermotile variant and parental strain revealed that an insertion sequence had disrupted an hns-like gene in the variant. This gene encodes a homologue of the histone-like nucleoid structuring (H-NS) protein, a known global transcriptional repressor. Transcriptome analysis identified the global effects of this mutation on gene expression, with major changes seen in the autotransporter Ata, a type VI secretion system, and a type I pilus cluster. Interestingly, isolation and analysis of a second independent hypermotile ATCC 17978 variant revealed a mutation to a residue within the DNA binding region of H-NS. Taken together, these mutants indicate that the phenotypic and transcriptomic differences seen are due to loss of regulatory control effected by H-NS.

The StkSR Two-Component System Influences Colistin Resistance in Acinetobacter baumannii.

Acinetobacter baumannii is an opportunistic human pathogen responsible for numerous severe nosocomial infections. Genome analysis on the A. baumannii clinical isolate 04117201 revealed the presence of 13 two-component signal transduction systems (TCS). Of these, we examined the putative TCS named here as StkSR. The stkR response regulator was deleted via homologous recombination and its progeny, ΔstkR, was phenotypically characterized. Antibiogram analyses of ΔstkR cells revealed a two-fold increase in resistance to the clinically relevant polymyxins, colistin and polymyxin B, compared to wildtype. PAGE-separation of silver stained purified lipooligosaccharide isolated from ΔstkR and wildtype cells ruled out the complete loss of lipooligosaccharide as the mechanism of colistin resistance identified for ΔstkR. Hydrophobicity analysis identified a phenotypical change of the bacterial cells when exposed to colistin. Transcriptional profiling revealed a significant up-regulation of the pmrCAB operon in ΔstkR compared to the parent, associating these two TCS and colistin resistance. These results reveal that there are multiple levels of regulation affecting colistin resistance; the suggested 'cross-talk' between the StkSR and PmrAB two-component systems highlights the complexity of these systems.

Contribution of serotype and genetic background to virulence of serotype 3 and serogroup 11 pneumococcal isolates.

The capsular serotype has long been associated with the virulence of Streptococcus pneumoniae. Here we present an in-depth study of phenotypic and genetic differences between serotype 3 and serogroup 11 S. pneumoniae clinical isolates from both the general and indigenous populations of Australia. Both serotypes/groups included clonally unrelated strains with differences in well-known polymorphic virulence genes, such as nanA and pspA, as demonstrated by multilocus sequence typing and Western blot analysis. Nonetheless, the serotype 3 strains were consistently and significantly more virulent in mice than the serogroup 11 strains. Despite extensive genomic analysis, noncapsular genes common to one serotype/group but not the other were not identified. Nevertheless, following the conversion of a serotype 11A isolate to serotype 3 and subsequent analysis in an intranasal infection model, it was evident that both capsular and noncapsular factors determine the virulence phenotype in mice. However, it appears that these noncapsular factors vary from strain to strain.

Resistance to pentamidine is mediated by AdeAB, regulated by AdeRS, and influenced by growth conditions in Acinetobacter baumannii ATCC 17978.

In recent years, effective treatment of infections caused by Acinetobacter baumannii has become challenging due to the ability of the bacterium to acquire or up-regulate antimicrobial resistance determinants. Two component signal transduction systems are known to regulate expression of virulence factors including multidrug efflux pumps. Here, we investigated the role of the AdeRS two component signal transduction system in regulating the AdeAB efflux system, determined whether AdeA and/or AdeB can individually confer antimicrobial resistance, and explored the interplay between pentamidine resistance and growth conditions in A. baumannii ATCC 17978. Results identified that deletion of adeRS affected resistance towards chlorhexidine and 4',6-diamidino-2-phenylindole dihydrochloride, two previously defined AdeABC substrates, and also identified an 8-fold decrease in resistance to pentamidine. Examination of ΔadeA, ΔadeB and ΔadeAB cells augmented results seen for ΔadeRS and identified a set of dicationic AdeAB substrates. RNA-sequencing of ΔadeRS revealed transcription of 290 genes were ≥2-fold altered compared to the wildtype. Pentamidine shock significantly increased adeA expression in the wildtype, but decreased it in ΔadeRS, implying that AdeRS activates adeAB transcription in ATCC 17978. Investigation under multiple growth conditions, including the use of Biolog phenotypic microarrays, revealed resistance to pentamidine in ATCC 17978 and mutants could be altered by bioavailability of iron or utilization of different carbon sources. In conclusion, the results of this study provide evidence that AdeAB in ATCC 17978 can confer intrinsic resistance to a subset of dicationic compounds and in particular, resistance to pentamidine can be significantly altered depending on the growth conditions.

Aqueous based synthesis of antimicrobial-decorated graphene.

Ramizol® (1,3,5-tris[(1E)-2'-(4'-benzoic acid)vinyl]benzene) is a potent amphiphilic anti-microbial agent. It is essentially a planar molecule and can interact with the surface of graphene via extended π-π interactions. Herein we demonstrate the utility of Ramizol® in potentially acting as a molecular 'wedge' to exfoliate graphene and stabilise it in water. The non-covalent attachment of Ramizol® on the graphene surface enables release of Ramizol® by altering the pH of the solution. Furthermore, the stabilised composite material demonstrates antibacterial activity against Staphylococcus aureus which leads to potential in biomedical applications with graphene acting as a drug carrier as well as enhancing the structural strength of the composite material.

A new antibiotic with potent activity targets MscL.

The growing problem of antibiotic-resistant bacteria is a major threat to human health. Paradoxically, new antibiotic discovery is declining, with most of the recently approved antibiotics corresponding to new uses for old antibiotics or structurally similar derivatives of known antibiotics. We used an in silico approach to design a new class of nontoxic antimicrobials for the bacteria-specific mechanosensitive ion channel of large conductance, MscL. One antimicrobial of this class, compound 10, is effective against methicillin-resistant Staphylococcus aureus with no cytotoxicity in human cell lines at the therapeutic concentrations. As predicted from in silico modeling, we show that the mechanism of action of compound 10 is at least partly dependent on interactions with MscL. Moreover we show that compound 10 cured a methicillin-resistant S. aureus infection in the model nematode Caenorhabditis elegans. Our work shows that compound 10, and other drugs that target MscL, are potentially important therapeutics against antibiotic-resistant bacterial infections.

Contribution of a genomic accessory region encoding a putative cellobiose phosphotransferase system to virulence of Streptococcus pneumoniae.

Streptococcus pneumoniae (the pneumococcus) is a formidable human pathogen, responsible for massive global morbidity and mortality. The ability to utilize carbohydrates in a variety of host niches appears to be integral to pneumococcal pathogenesis. In this study we investigated a genomic island, which includes a ROK family protein, a putative cellobiose phosphotransferase system (PTS) and a putative sulfatase. This accessory region is widespread in the pneumococcus in strains of various serotypes and levels of virulence. We have performed simple bioinformatic analysis of the region and investigated its role in vivo in 2 strains with markedly different virulence profiles (WCH206 of serotype 3, ST180; Menzies5 of serotype 11A, ST662). Deleting and replacing the entire island with an antibiotic resistance cassette caused the virulent serotype 3 strain to become attenuated in a murine pneumonia/sepsis model. Further mutants were constructed and used to show that various components of the island contribute significantly to the fitness of WCH206 in a variety of niches of this model, including the nasopharynx, ears and blood, but especially in the lungs. In addition, the island conferred a competitive advantage in nasopharyngeal colonization for the serotype 11A strain, which was essentially avirulent in the pneumonia/sepsis model. The contribution of this island to both pathogenesis and colonization may explain why this accessory region is widespread in the pneumococcus.

Adherence and motility characteristics of clinical Acinetobacter baumannii isolates.

Acinetobacter baumannii continues to be a major health problem especially in hospital settings. Herein, features that may play a role in persistence and disease potential were investigated in a collection of clinical A. baumannii strains from Australia. Twitching motility was found to be a common trait in A. baumannii international clone I strains and in abundant biofilm formers, whereas swarming motility was only observed in isolates not classified within the international clone lineages. Bioinformatic analysis of the type IV fimbriae revealed a correlation between PilA sequence homology and motility. A high level of variability in adherence to both abiotic surfaces and epithelial cells was found. We report for the first time the motility characteristics of a large number of A. baumannii isolates and present a direct comparison of A. baumannii binding to nasopharyngeal and lung epithelial cells.

A variable region within the genome of Streptococcus pneumoniae contributes to strain-strain variation in virulence.

The bacterial factors responsible for the variation in invasive potential between different clones and serotypes of Streptococcus pneumoniae are largely unknown. Therefore, the isolation of rare serotype 1 carriage strains in Indigenous Australian communities provided a unique opportunity to compare the genomes of non-invasive and invasive isolates of the same serotype in order to identify such factors. The human virulence status of non-invasive, intermediately virulent and highly virulent serotype 1 isolates was reflected in mice and showed that whilst both human non-invasive and highly virulent isolates were able to colonize the murine nasopharynx equally, only the human highly virulent isolates were able to invade and survive in the murine lungs and blood. Genomic sequencing comparisons between these isolates identified 8 regions >1 kb in size that were specific to only the highly virulent isolates, and included a version of the pneumococcal pathogenicity island 1 variable region (PPI-1v), phage-associated adherence factors, transporters and metabolic enzymes. In particular, a phage-associated endolysin, a putative iron/lead permease and an operon within PPI-1v exhibited niche-specific changes in expression that suggest important roles for these genes in the lungs and blood. Moreover, in vivo competition between pneumococci carrying PPI-1v derivatives representing the two identified versions of the region showed that the version of PPI-1v in the highly virulent isolates was more competitive than the version from the less virulent isolates in the nasopharyngeal tissue, blood and lungs. This study is the first to perform genomic comparisons between serotype 1 isolates with distinct virulence profiles that correlate between mice and humans, and has highlighted the important role that hypervariable genomic loci, such as PPI-1v, play in pneumococcal disease. The findings of this study have important implications for understanding the processes that drive progression from colonization to invasive disease and will help direct the development of novel therapeutic strategies.

The conformation and function of a multimodular glycogen-degrading pneumococcal virulence factor.

SpuA is a large multimodular cell wall-attached enzyme involved in the degradation of glycogen by the pathogenic bacterium Streptococcus pneumoniae. The deletion of the gene encoding SpuA from the bacterium resulted in a strain with reduced competitiveness in a mouse model of virulence relative to the parent strain, linking the degradation of host-glycogen to the virulence of the bacterium. Through the combined use of X-ray crystallography, small-angle X-ray scattering, and inhibitor binding, the molecular features involved in substrate recognition by this complex protein are revealed. This uniquely illustrates the complexity of the active site, the conformational changes incurred during carbohydrate binding by this protein, and the interaction and cooperation of its composite modules during this process. New insight into the function of this particular pneumococcal virulence factor is provided along with substantial contributions to the nascent framework for understanding the structural and functional interplay between modules in multimodular carbohydrate-active enzymes.

The pneumococcal two-component signal transduction system RR/HK06 regulates CbpA and PspA by two distinct mechanisms.

We have previously shown that CbpA, a major pneumococcal virulence factor, is regulated by the two-component signal transduction system RR/HK06 (A. J. Standish, U. H. Stroeher, and J. C. Paton, Proc. Natl. Acad. Sci. USA 102:7701-7706, 2005). However, additional unidentified regulated factors appeared to be responsible for differences in adherence and the ability of Streptococcus pneumoniae to cause disease in a mouse model. Here, we identified a number of other regulated genes by overexpressing the system. cbpA, along with a cotranscribed upstream gene, showed substantial increases in expression when RR06 was overexpressed in S. pneumoniae strains D39 and TIGR4. However, there were no other similarities between these strains. In D39, rr06 overexpression decreased expression of numerous factors, including the major virulence factor gene pspA. Further investigation of cbpA regulation by RR/HK06, using mutants with mutations in both HK06 and RR06, suggested that rather than the norm, cbpA transcription was activated when RR06 was in the nonphosphorylated form. Although other factors, such as pspA and gls24, are regulated by this system, these genes appear to be repressed when RR06 is in its phosphorylated form.

A pneumococcal MerR-like regulator and S-nitrosoglutathione reductase are required for systemic virulence.

A transcriptional regulator, NmlR(sp), has been identified in Streptococcus pneumoniae that is required for defense against nitric oxide (NO) stress. The nmlR(sp) gene is cotranscribed with adhC, which encodes an alcohol dehydrogenase that is able to reduce S-nitrosoglutathione (GSNO) with NADH as reductant. nmlR(sp) and adhC mutants exhibited a reduced level of NADH-GSNO oxidoreductase activity and were more susceptible to killing by NO than were wild-type cells. Comparison of the virulence of wild-type and mutant strains by use of a mouse model system showed that NmlR(sp) and AdhC do not play a key role in the adherence of pneumococci to the nasopharynx in vivo. An intraperitoneal challenge experiment revealed that both NmlR(sp) and AdhC were required for survival in blood. These data identify novel components of a NO defense system in pneumococci that are required for systemic infection.

Albomycin is an effective antibiotic, as exemplified with Yersinia enterocolitica and Streptococcus pneumoniae.

Albomycin belongs to the class of sideromycins, compounds composed of iron carriers linked to antibiotic moieties. Albomycin was found to be active against bacteria that have a functional ferric hydroxamate transport system meaning that bacteria will actively transport albomycin until they die. We examined the activity spectrum of albomycin for bacterial pathogens and found that Enterobacteriaceae except species of Proteus and Morganella were sensitive. Resistance in the two genera was due to the lack of the ferric hydroxamate transport system. Among Gram-positive bacteria, Staphylococcus aureus and Streptococcus pneumoniae were highly sensitive, whereas Streptococcus agalactiae, Streptococcus pyogenes, and Staphylococcus epidermidis were resistant. The in vivo efficacy of albomycin was examined in mice infected with S. pneumoniae or Yersinia enterocolitica. A single dose of 10mg albomycin/kg body weight reduced the colony-forming units of Y. enterocolitica by three to four orders of magnitude. A single dose of 1mg albomycin/kg body weight was sufficient to clear S. pneumoniae infections in mice. In direct competition experiments with wild-type S. pneumoniae and its albomycin-resistant mutant, the recovery rate of the mutant was lower than for the wild-type indicating that the mutant had reduced fitness in the mouse model. We conclude that albomycin is effective in clearing infections caused by both Gram-positive and Gram-negative bacteria in a mouse model. Albomycin treatment reduces the bacterial load allowing the immune system to remove residual albomycin-resistant bacteria, and as such would make albomycin-based antibiotics an adjunct to treatment. The ferrichrome transport system serves as a Trojan horse to get albomycin into bacteria.

Contributions of pneumolysin, pneumococcal surface protein A (PspA), and PspC to pathogenicity of Streptococcus pneumoniae D39 in a mouse model.

Successful colonization of the upper respiratory tract by Streptococcus pneumoniae is an essential first step in the pathogenesis of pneumococcal disease. However, the bacterial and host factors that provoke the progression from asymptomatic colonization to invasive disease are yet to be fully defined. In this study, we investigated the effects of single and combined mutations in genes encoding pneumolysin (Ply), pneumococcal surface protein A (PspA), and pneumococcal surface protein C (PspC, also known as choline-binding protein A) on the pathogenicity of Streptococcus pneumoniae serotype 2 (D39) in mice. Following intranasal challenge with D39, stable colonization of the nasopharynx was maintained over a 7-day period at a level of approximately 10(5) bacteria per mouse. The abilities of the mutant deficient in PspA to colonize the nasopharynx and to cause lung infection and bacteremia were significantly reduced. Likewise, the PspC mutant and, to a lesser extent, the Ply mutant also had reduced abilities to colonize the nasopharynx. As expected, the double mutants colonized less well than the parent to various degrees and had difficulty translocating to the lungs and blood. A significant additive attenuation was observed for the double and triple mutants in pneumonia and systemic disease models. Surprisingly, the colonization profile of the derivative lacking all three proteins was similar to that of the wild type, indicating virulence gene compensation. These findings further demonstrate that the mechanism of pneumococcal pathogenesis is highly complex and multifactorial but ascribes a role for each of these virulence proteins, alone or in combination, in the process.

The two-component signal transduction system RR06/HK06 regulates expression of cbpA in Streptococcus pneumoniae.

Streptococcus pneumoniae encounters a number of environmental niches in the body, including the nasopharynx, lungs, blood, middle ear, and brain. Recent studies have identified 13 putative two-component signal-transduction systems in S. pneumoniae, which are likely to be important for gene regulation in response to external stimuli. Here, we present conclusive evidence for the regulation of choline binding protein A (CbpA), a major pneumococcal virulence factor and protective antigen, by one of these two-component signal-transduction systems. We have demonstrated divergent expression of cbpA in unmarked hk06 and rr06 deletion mutants relative to wild-type S. pneumoniae D39 by using Western immunoblotting and real-time RT-PCR. Electrophoretic mobility-shift and solid-phase binding assays have demonstrated the binding of RR06 to the promoter region of cbpA, suggesting that RR06/HK06 directly regulates cbpA transcription. We have also shown that this system is important for the ability of the pneumococcus to adhere to epithelial cells in vitro and to survive and proliferate in an in vivo mouse model. Thus, the RR06/HK06 system has a significant role in pathogenesis, both in colonization and invasive disease.

Distribution of IS1358 and linkage to rfb-related genes in Vibrio anguillarum.

The insertion sequence IS1358 is linked to the rfb regions of both Vibrio cholerae O1 and O139, and its location was suggestive of a role in generating new combinations of rfb genes. This provoked an examination of the distribution and localization of IS1358 in Vibrio anguillarum. S11358 was widely distributed in a number of V. anguillarum serogroups. In particular, when cosmid clones of V. anguillarum O1 were screened with IS1358 and subsequently subcloned and sequenced, it was found that rfb-like genes were linked to this region. Furthermore, when the previously identified genes virA and virB from V. anguillarum O1, now known to be involved in LPS biosynthesis, were used as probes, it was discovered that they too are present on the same large EcoRI fragment as IS1358. This clearly indicated that IS1358 was linked to the rfb region of V. anguillarum O1. Further analysis of the location of IS1358 in other serotypes indicated that V. anguillarum O2 also has IS1358 associated with rfb-like genes. In V. anguillarum O2 there is more than one copy of IS1358, suggesting that this element is a site for recombination, gene duplication or that it may be capable of transposition. Following this latter premise, IS1358 elements from a variety of V. anguillarum strains have been cloned and sequenced. Only those strains with multiple copies of IS1358 produce a full-length putative transposase, as shown by protein overexpression, further strengthening the argument that the element is transposing within these strains.

Mutation of luxS of Streptococcus pneumoniae affects virulence in a mouse model.

The LuxS protein is required for the biosynthesis of the type 2 autoinducer (AI-2), which is involved in quorum sensing in a wide range of bacterial species. We have determined the effects of a defined luxS mutation on the virulence of Streptococcus pneumoniae. Although the luxS mutant displayed reduced virulence relative to its wild-type parent, the type 2 strain D39, it was by no means avirulent in a mouse model. After intranasal administration, the luxS mutant was able to colonize the nasopharynx of the mouse as efficiently as the wild type. However, it was less able to spread from the nasopharynx to the lungs or the blood. Intraperitoneal coadministration studies indicated that the luxS mutant was less fit and was readily outcompeted by wild-type D39. However, when administered on its own by this route, the mutant was able to proliferate and cause fatal systemic disease, albeit at a lower rate than the wild type. Western blot analysis of whole-cell lysates of the mutant and its parent did not reveal any differences in the levels of several well-characterized virulence proteins. However, analysis of Coomassie blue-stained protein profiles after separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that mutation of luxS had pleiotropic effects on protein expression in all cellular compartments. This is consistent with the product of luxS having a regulatory role in S. pneumoniae. This is the first report of a direct role for luxS (and by inference, AI-2) in the virulence of a gram-positive pathogen. However, the fact that mutagenesis of luxS does not completely attenuate S. pneumoniae has implications for the possible use of AI-2 antagonists for treatment of pneumococcal infections.

The human complement regulator factor H binds pneumococcal surface protein PspC via short consensus repeats 13 to 15.

The innate ability of Streptococcus pneumoniae to resist complement activation and complement-mediated phagocytosis may be a direct consequence of the ability of the bacteria to bind components of the complement regulatory system. One such component, factor H (fH), is a crucial fluid-phase negative regulator of the alternative pathway of complement and is utilized by a number of pathogenic organisms to resist complement attack. The pneumococcal surface protein C (PspC [also known as CbpA] and SpsA) has been shown to bind fH, although the exact binding site within one or more of the 20 short consensus repeats (SCRs) of the molecule is not known. The purpose of the current study was to map specific SCRs on fH responsible for this binding. Initial experiments utilizing type 2 pneumococcal strain D39 and its isogenic PspC-negative derivative (D39/pspC mutant) showed that fH binding was PspC dependent. A purified recombinant protein derivative of PspC that lacked the proline-rich region (PspCDeltaPro) had a reduced binding efficiency for fH, thereby directly showing the importance of this region for the fH interaction. We have specifically shown by inhibition experiments that SCRs responsible for heparin and C3b binding of fH are not involved in binding PspC and the interaction between fH and PspC is largely hydrophobic, since no inhibition was observed in the presence of high concentrations of NaCl. Construction of SCR proteins encompassing the whole fH molecule showed that SCRs 8 to 15 (SCR 8-15) mediated binding to PspC. Further localization experiments revealed that SCR 13 and SCR 15 were required for full binding, although partial binding was retained when either SCR was removed.