Modeling and biochemical analysis of the activity of antibiofilm agent Dispersin B

Bacteria in a biofilm are enmeshed in a self-synthesized extracellular polysaccharide matrix (PGA), which is a linear polymer of beta(1,6)-linked N-acetylglucosamine (GlcNAc) residues. Dispersin B (DspB), a soluble glycoside hydrolase produced by the periodontal pathogen Actinobacillus actinomycetemcomitans degrades PGA. The enzyme DspB is an alpha/beta TIM-barrel protein and belongs to family 20 glycosyl hydrolases members. The enzyme activity of DspB with regard to its substrate specificity towards beta(1,6)-linked GlcNAc polymers and its endo/exo character was investigated through ligand docking and the hydrolysis of synthetic oligosaccharides. Ligand docking analysis suggested that beta(1,6)-linked GlcNAc oligosaccharide bound to the active site better that beta(1,4)-linked GlcNAc oligosaccharide. Our combined results indicate that DspB is an exo-acting enzyme that hydrolyzes beta(1,6)-linked N-acetylglucosamine oligomers.

Both leukotoxin and poly-N-acetylglucosamine surface polysaccharide protect Aggregatibacter actinomycetemcomitans cells from macrophage killing

Two virulence factors produced by the periodontopathogen Aggregatibacter actinomycetemcomitans are leukotoxin, a secreted lipoprotein that kills human polymorphonuclear leukocytes and macrophages, and poly-N-acetylglucosamine (PGA), a surface polysaccharide that mediates intercellular adhesion, biofilm formation and detergent resistance. In this study we examined the roles of leukotoxin and PGA in protecting A. actinomycetemcomitans cells from killing by the human macrophage cell line THP-1. Monolayers of THP-1 cells were infected with single-cell suspensions of a wild-type A. actinomycetemcomitans strain, or of isogenic leukotoxin or PGA mutant strains. After 48h, viable bacteria were enumerated by dilution plating, macrophage morphology was evaluated microscopically, and macrophage viability was measured by a Trypan blue dye exclusion assay. The number of A. actinomycetemcomitans CFUs increased approximately twofold in wells infected with the wild-type strain, but decreased by approximately 70-90% in wells infected with the leukotoxin and PGA mutant strains. Infection with the wild-type or leukotoxin mutant strain caused a significant decrease in THP-1 cell viability, whereas infection with the PGA mutant strain did not result in any detectable changes in THP-1 viability. Pre-treatment of wild-type A. actinomycetemcomitans cells with the PGA-hydrolyzing enzyme dispersin B rendered them sensitive to killing by THP-1 cells. We concluded that both leukotoxin and PGA are necessary for evasion of macrophage killing by A. actinomycetemcomitans.

Aggregatibacter actinomycetemcomitans serotype f O-polysaccharide mediates coaggregation with Fusobacterium nucleatum

BACKGROUND/AIMS

Intergeneric bacterial coaggregation may play an important role in plaque development.

METHODS

In this study we investigated the coaggregation reaction between two periodontal pathogens, Aggregatibacter actinomycetemcomitans and Fusobacterium nucleatum.

RESULTS

Previous studies showed that A. actinomycetemcomitans serotype b strains coaggregate with F. nucleatum strain PK1594, and that A. actinomycetemcomitans serotype b O-polysaccharide (O-PS) is the receptor responsible for coaggregation between A. actinomycetemcomitans and F. nucleatum. A. actinomycetemcomitans serotype f O-PS has been shown to be structurally and antigenically related to serotype b O-PS. In the present study we show that A. actinomycetemcomitans strain CU1060N, a serotype f strain, also coaggregated with F. nucleatum PK1594. Like coaggregation between serotype b strains and F. nucleatum, coaggregation between CU1060N and F. nucleatum was inhibited by galactose. An O-PS mutant of CU1060N failed to coaggregate with F. nucleatum.

CONCLUSION

We concluded that A. actinomycetemcomitans serotype f O-PS, like serotype b O-PS, mediates coaggregation between A. actinomycetemcomitans and fusobacteria.

A survey of rodent-borne pathogens carried by wild-caught Norway rats: A potential threat to laboratory rodent colonies

Summary

Unintentional infection of laboratory rodents can compromise scientific research as well as the health of the animals and animal handlers. The source of contamination often is unknown, but may be introduced by wild rats from surrounding environments. To determine whether rats in Baltimore, Maryland, USA carry infectious agents commonly found in laboratory rodent colonies, we live-trapped 162 rats during 2005 to 2006 and screened them for a panel of viruses, bacteria and parasites. Antibodies against rat coronavirus/sialodacryoadenitis virus (91.7%), Mycoplasma pulmonis (72.9%), cilia-associated respiratory bacillus (52.1%), rat parvovirus/rat minute virus (29.2%), Kilham rat virus (10.4%), Toolan's H-1 virus (10.4%), Sendai virus (4.2%) and Theiler's mouse encephalomyelitis virus (4.2%), were detected in wild-caught Norway rats. Antibodies against reovirus and pneumonia virus of mice were not detected in wild Norway rats. Endoparasites, including Nippostrongylus braziliensis (71.6%), Rodentolepis nana or Hymenolepis diminuta (34.4%), Hetarakis spumosa (24.1%) and Trichuris muris (14.8%), as well as ectoparasites (14.8%), were identified in wild-caught rats. The risk of pathogen transmission from wild-caught rats to laboratory colonies needs to be mitigated by minimizing exposures rather than assuming wild animals represent a minimal hazard.

Role of active-site residues of dispersin B, a biofilm-releasing beta-hexosaminidase from a periodontal pathogen, in substrate hydrolysis

Dispersin B (DspB), a family 20 beta-hexosaminidase from the oral pathogen Aggregatibacter actinomycetemcomitans, cleaves beta(1,6)-linked N-acetylglucosamine polymer. In order to understand the substrate specificity of DspB, we have undertaken to characterize several conserved and nonconserved residues in the vicinity of the active site. The active sites of DspB and other family 20 hexosaminidases possess three highly conserved acidic residues, several aromatic residues and an arginine at subsite -1. These residues were mutated using site-directed mutagenesis and characterized for their enzyme activity. Our results show that a highly conserved acid pair in beta-hexosaminidases D183 and E184, and E332 play a critical role in the hydrolysis of the substrates. pH activity profile analysis showed a shift to a higher pH (6.8) in the optimal activity for the E184Q mutant, suggesting that this residue might act as the acid/base catalyst. The reduction in k(cat) observed for Y187A and Y278A mutants suggests that the Y187 residue (unique to DspB) located on a loop might play a role in substrate specificity and be a part of subsite +1, whereas the hydrogen-bond interaction between Y278A and the N-acetyl group might help to stabilize the transition state. Mutation of W237 and W330 residues abolished hydrolytic activity completely suggesting that alteration at these positions might collapse the binding pocket for the N-acetyl group. Mutation of the conserved R27 residue (to R27A or R27K) also caused significant reduction in k(cat) suggesting that R27 might be involved in stabilization of the transition state. From these results, we conclude that in DspB, and possibly in other structurally similar family 20 hydrolases, some residues at the active site assist in orienting the N-acetyl group to participate in the substrate-assisted mechanism, whereas other residues such as R27 and E332 assist in holding the terminal N-acetylglucosamine during the hydrolysis.

Poly-N-acetylglucosamine mediates biofilm formation and detergent resistance in Aggregatibacter actinomycetemcomitans

Clinical isolates of the periodontopathogen Aggregatibacter actinomycetemcomitans form matrix-encased biofilms on abiotic surfaces in vitro. A major component of the A. actinomycetemcomitans biofilm matrix is poly-beta-1,6-N-acetyl-d-glucosamine (PGA), a hexosamine-containing polysaccharide that mediates intercellular adhesion. In this report, we describe studies on the purification, structure, genetics and function of A. actinomycetemcomitans PGA. We found that PGA was very tightly attached to A. actinomycetemcomitans biofilm cells and could be efficiently separated from the cells only by phenol extraction. A. actinomycetemcomitans PGA copurified with LPS on a gel filtration column. (1)H NMR spectra of purified A. actinomycetemcomitans PGA were consistent with a structure containing a linear chain of N-acetyl-d-glucosamine residues in beta(1,6) linkage. Genetic analyses indicated that all four genes of the pgaABCD locus were required for PGA production in A. actinomycetemcomitans. PGA mutant strains still formed biofilms in vitro. Unlike wild-type biofilms, however, PGA mutant biofilms were sensitive to detachment by DNase I and proteinase K. Treatment of A. actinomycetemcomitans biofilms with the PGA-hydrolyzing enzyme dispersin B made them 3 log units more sensitive to killing by the cationic detergent cetylpyridinium chloride. Our findings suggest that PGA, extracellular DNA and proteinaceous adhesins all contribute to the structural integrity of the A. actinomycetemcomitans biofilm matrix.

Detachment and killing of Aggregatibacter actinomycetemcomitans biofilms by dispersin B and SDS

The periodontopathogen Aggregatibacter actinomycetemcomitans forms tenacious biofilms on abiotic surfaces in vitro. The objective of the present study was to measure the susceptibility of A. actinomycetemcomitans biofilms to detachment and killing by the anionic surfactant sodium dodecyl sulfate (SDS). We found that biofilms formed by a wild-type strain were resistant to detachment by SDS. In contrast, biofilms formed by an isogenic mutant strain that was deficient in the production of PGA (poly-N-acetyl-glucosamine), a biofilm matrix polysaccharide, were sensitive to detachment by SDS. Pre-treatment of wild-type biofilms with dispersin B, a PGA-degrading enzyme, rendered them sensitive to detachment by SDS and resulted in a > 99% increase in SDS-mediated cell killing. We concluded that PGA protects A. actinomycetemcomitans cells from detachment and killing by SDS. Dispersin B and SDS may be useful agents for treating chronic infections caused by A. actinomycetemcomitans and other PGA-producing bacteria.

A second Aggregatibacter actinomycetemcomitans autotransporter adhesin exhibits specificity for buccal epithelial cells in humans and Old World primates

Previous work showed that the Aggregatibacter actinomycetemcomitans adhesin Aae demonstrated species specificity and tissue tropism to buccal epithelial cells (BECs) derived from humans and Old World primates, but a second, lower-affinity adhesin was noted. This study was designed to determine if Omp100 (also known as ApiA), a surface-expressed A. actinomycetemcomitans adhesin, is that second adhesin and if so to investigate its tissue tropism and species specificity. A targeted mutagenesis protocol was used to construct an isogenic apiA mutant and an aae apiA double mutant with wild-type A. actinomycetemcomitans. In addition, Escherichia coli strain DH5alpha was used to express apiA to further assess binding parameters. Results indicated that the apiA mutant strain showed significantly less binding to BECs than its parent strain (P < or = 0.05). Further, binding mediated by ApiA was specific to BECs from humans and Old World primates, as seen in both wild-type A. actinomycetemcomitans and E. coli expressing ApiA (P < or = 0.05). Pretreatment of wild-type A. actinomycetemcomitans cells with anti-ApiA antiserum reduced binding in a dose-dependent manner. The aae apiA double mutant completely abrogated A. actinomycetemcomitans binding to both human and Old World primate BECs. Taken together, these studies indicate that ApiA and Aae, in concert, modulate binding of A. actinomycetemcomitans to human BECs. Since the BEC is a prominent reservoir for A. actinomycetemcomitans, identification of this second adhesin could lead to important therapeutic strategies.

Poly-N-acetylglucosamine mediates biofilm formation and antibiotic resistance in Actinobacillus pleuropneumoniae

Most field isolates of the swine pathogen Actinobacillus pleuropneumoniae form tenacious biofilms on abiotic surfaces in vitro. We purified matrix polysaccharides from biofilms produced by A. pleuropneumoniae field isolates IA1 and IA5 (serotypes 1 and 5, respectively), and determined their chemical structures by using NMR spectroscopy. Both strains produced matrix polysaccharides consisting of linear chains of N-acetyl-D-glucosamine (GlcNAc) residues in beta(1,6) linkage (poly-beta-1,6-GlcNAc or PGA). A small percentage of the GlcNAc residues in each polysaccharide were N-deacetylated. These structures were nearly identical to those of biofilm matrix polysaccharides produced by Escherichia coli, Staphylococcus aureus and Staphylococcus epidermidis. PCR analyses indicated that a gene encoding the PGA-specific glycoside transferase enzyme PgaC was present on the chromosome of 15 out of 15 A. pleuropneumoniae reference strains (serotypes 1-12) and 76 out of 77 A. pleuropneumoniae field isolates (serotypes 1, 5 and 7). A pgaC mutant of strain IA5 failed to form biofilms in vitro, as did wild-type strains IA1 and IA5 when grown in broth supplemented with the PGA-hydrolyzing enzyme dispersin B. Treatment of IA5 biofilms with dispersin B rendered them more sensitive to killing by ampicillin. Our findings suggest that PGA functions as a major biofilm adhesin in A. pleuropneumoniae. Biofilm formation may have relevance to the colonization and pathogenesis of A. pleuropneumoniae in pigs.

Synergistic activity of dispersin B and cefamandole nafate in inhibition of staphylococcal biofilm growth on polyurethanes

Antibiotic therapies to eradicate medical device-associated infections often fail because of the ability of sessile bacteria, encased in their exopolysaccharide matrix, to be more drug resistant than planktonic organisms. In the last two decades, several strategies to prevent microbial adhesion and biofilm formation on the surfaces of medical devices, based mainly on the use of antiadhesive, antiseptic, and antibiotic coatings on polymer surfaces, have been developed. More recent alternative approaches are based on molecules able to interfere with quorum-sensing phenomena or to dissolve biofilms. Interestingly, a newly purified beta-N-acetylglucosaminidase, dispersin B, produced by the gram-negative periodontal pathogen Actinobacillus actinomycetemcomitans, is able to dissolve mature biofilms produced by Staphylococcus epidermidis as well as some other bacterial species. Therefore, in this study, we developed new polymeric matrices able to bind dispersin B either alone or in combination with an antibiotic molecule, cefamandole nafate (CEF). We showed that our functionalized polyurethanes could adsorb a significant amount of dispersin B, which was able to exert its hydrolytic activity against the exopolysaccharide matrix produced by staphylococcal strains. When microbial biofilms were exposed to both dispersin B and CEF, a synergistic action became evident, thus characterizing these polymer-dispersin B-antibiotic systems as promising, highly effective tools for preventing bacterial colonization of medical devices.

Susceptibility of staphylococcal biofilms to enzymatic treatments depends on their chemical composition

Bacterial infections are serious complications after orthopaedic implant surgery. Staphylococci, with Staphylococcus epidermidis as a leading species, are the prevalent and most important species involved in orthopaedic implant-related infections. The biofilm mode of growth of these bacteria on an implant surface protects the organisms from the host's immune system and from antibiotic therapy. Therapeutic agents that disintegrate the biofilm matrix would release planktonic cells into the environment and therefore allow antibiotics to eliminate the bacteria. An addition of a biofilm-degrading agent to a solution used for washing-draining procedures of infected orthopaedic implants would greatly improve the efficiency of the procedure and thus help to avoid the removal of the implant. We have previously shown that the extracellular staphylococcal matrix consists of a poly-N-acetylglucosamine (PNAG), extracellular teichoic acids (TAs) and protein components. In this study, we accessed the sensitivity of pre-formed biofilms of five clinical staphylococcal strains associated with orthopaedic prosthesis infections and with known compositions of the biofilm matrix to periodate, Pectinex Ultra SP, proteinase K, trypsin, pancreatin and dispersin B, an enzyme with a PNAG-hydrolysing activity. We also tested the effect of these agents on the purified carbohydrate components of staphylococcal biofilms, PNAG and TA. We found that the enzymatic detachment of staphylococcal biofilms depends on the nature of their constituents and varies between the clinical isolates. We suggest that a treatment with dispersin B followed by a protease (proteinase K or trypsin) could be capable to eradicate biofilms of a variety of staphylococcal strains on inert surfaces.

A survey of zoonotic pathogens carried by Norway rats in Baltimore, Maryland, USA

Norway rats (Rattus norvegicus) carry several zoonotic pathogens and because rats and humans live in close proximity in urban environments, there exists potential for transmission. To identify zoonotic agents carried by rats in Baltimore, Maryland, USA, we live-trapped 201 rats during 2005-2006 and screened them for a panel of viruses, bacteria, and parasites. Antibodies against Seoul virus (57.7%), hepatitis E virus (HEV, 73.5%), Leptospira interrogans (65.3%), Bartonella elizabethae (34.1%), and Rickettsia typhi (7.0%) were detected in Norway rats. Endoparasites, including Calodium hepatica (87.9%) and Hymenolepis sp. (34.4%), and ectoparasites (13.9%, primarily Laelaps echidninus) also were present. The risk of human exposure to these pathogens is a significant public health concern. Because these pathogens cause non-specific and often self-limiting symptoms in humans, infection in human populations is probably underdiagnosed.

Polysaccharide intercellular adhesin or protein factors in biofilm accumulation of Staphylococcus epidermidis and Staphylococcus aureus isolated from prosthetic hip and knee joint infections

Nosocomial staphylococcal foreign-body infections related to biofilm formation are a serious threat, demanding new therapeutic and preventive strategies. As the use of biofilm-associated factors as vaccines is critically restricted by their prevalence in natural staphylococcal populations we studied the distribution of genes involved in biofilm formation, the biofilm phenotype and production of polysaccharide intercellular adhesin (PIA) in clonally independent Staphylococcus aureus and Staphylococcus epidermidis strains isolated from prosthetic joint infections after total hip or total knee arthroplasty. Biofilm formation was detected in all S. aureus and 69.2% of S. epidermidis strains. Importantly, 27% of biofilm-positive S. epidermidis produced PIA-independent biofilms, in part mediated by the accumulation associated protein (Aap). Protein-dependent biofilms were exclusively found in S. epidermidis strains from total hip arthroplasty (THA). In S. aureus PIA and proteins act cooperatively in biofilm formation regardless of the infection site. PIA and protein factors like Aap are of differential importance for the pathogenesis of S. epidermidis in prosthetic joint infections (PJI) after THA and total knee arthroplasty (TKA), implicating that icaADBC cannot serve as a general virulence marker in this species. In S. aureus biofilm formation proteins are of overall importance and future work should focus on the identification of functionally active molecules.

Prenatal diagnosis of glycogen storage disease type IV

BACKGROUND

Glycogen storage disease type IV (GSD-IV) is a rare autosomal recessive disorder due to mutations in the GBE1 gene causing deficiency of the glycogen branching enzyme (GBE). Prenatal diagnosis has occasionally been performed by the measurement of the GBE activity in cultured chorionic villi (CV) cells.

METHODS

Two unrelated probands with severe hypotonia at birth and death during the neonatal period were diagnosed with GSD-IV on the basis of postmortem histological findings. DNA analysis revealed truncating GBE1 mutations in both families.

RESULTS

Prenatal diagnosis was performed in subsequent pregnancies by determination of branching enzyme activity and DNA analysis of CV or cultured amniocytes. Detailed autopsies of the affected fetuses at 14 and 24 weeks of gestation demonstrated intracellular inclusions of abnormal glycogen characteristic of GSD-IV.

CONCLUSION

Prenatal diagnosis of GSD-IV by DNA analysis is highly accurate in genetically confirmed cases.

Biofilm formation is prevalent among field isolates of Actinobacillus pleuropneumoniae

A total of 77 field isolates and 15 reference strains of the porcine respiratory pathogen Actinobacillus pleuropneumoniae were tested for their ability to form biofilms in a polystyrene microtiter plate assay. More than half of all field isolates, which included strains representing serotypes 1, 5 and 7, but only two reference strains (serotypes 5B and 11) exhibited biofilm formation. Strains that formed biofilms in microtiter plates also formed thick biofilms at the air-liquid interface when cultured in glass tubes with agitation. The biofilm formation phenotype was maintained indefinitely when cultures were passaged on agar but was lost after one or two passages in broth. Our findings indicate that biofilm formation is a prevalent phenotype among A. pleuropneumoniae field isolates, and that this phenotype may have been previously overlooked because of its tendency to be lost upon subculturing in broth. Biofilm formation may have relevance to the colonization, pathogenesis and transmission of this bacterium.

The Actinobacillus actinomycetemcomitans autotransporter adhesin Aae exhibits specificity for buccal epithelial cells from humans and old world primates

Cells of the gram-negative periodontopathogen Actinobacillus actinomycetemcomitans express a surface-exposed, outer membrane autotransporter protein, designated Aae, which has been implicated in epithelial cell binding. We constructed a mutant strain of A. actinomycetemcomitans that contained a transposon insertion in the Aae structural gene (aae) and tested the mutant to determine its ability to bind to buccal epithelial cells (BECs) isolated from healthy volunteers. Significantly fewer mutant cells than wild-type cells bound to BECs. A broad-host-range plasmid that contained an intact aae gene driven by a heterologous tac promoter restored the ability of the mutant strain to bind to BECs at wild-type levels. This plasmid also conferred upon Escherichia coli the ability to express the Aae protein on its surface and to bind to human BECs. Aae-expressing E. coli also bound to BECs isolated from six Old World primates but not to BECs isolated from four New World primates or nine other nonprimate mammals, as well as to human gingival epithelial cells but not to human pharyngeal, palatal, tongue, bronchial, or cervical epithelial cells. Our findings indicate that Aae mediates binding of A. actinomycetemcomitans to BECs from humans and Old World primates and that this process may contribute to the host range specificity and tissue tropism exhibited by this bacterium.

Structural analysis of dispersin B, a biofilm-releasing glycoside hydrolase from the periodontopathogen Actinobacillus actinomycetemcomitans

Bacteria in a biofilm are enmeshed in a self-synthesized extracellular polysaccharide matrix that holds the bacteria together in a mass and firmly attaches the bacterial mass to the underlying surface. A major component of the extracellular polysaccharide matrix in several phylogenetically diverse bacteria is PGA, a linear polymer of N-acetylglucosamine residues in beta(1,6)-linkage. PGA is produced by the Gram-negative periodontopathogen Actinobacillus actinomycetemcomitans as well as by the Gram-positive device-associated pathogen Staphylococcus epidermidis. We recently reported that A.actinomycetemcomitans produces a soluble glycoside hydrolase named dispersin B, which degrades PGA. Here, we present the crystal structure of dispersin B at 2.0A in complex with a glycerol and an acetate ion at the active site. The enzyme crystallizes in the orthorhombic space group C222(1) with cell dimensions a=41.02A, b=86.13A, c=185.77A. The core of the enzyme consists a (beta/alpha)(8) barrel topology similar to other beta-hexosaminidases but significant differences exist in the arrangement of loops hovering in the vicinity of the active site. The location and interactions of the glycerol and acetate moieties in conjunction with the sequence analysis suggest that dispersin B cleaves beta(1,6)-linked N-acetylglucosamine polymer using a catalytic machinery similar to other family 20 hexosaminidases which cleave beta(1,4)-linked N-acetylglucosamine residues.

Genes involved in the synthesis and degradation of matrix polysaccharide in Actinobacillus actinomycetemcomitans and Actinobacillus pleuropneumoniae biofilms

Biofilms are composed of bacterial cells embedded in an extracellular polysaccharide matrix. A major component of the Escherichia coli biofilm matrix is PGA, a linear polymer of N-acetyl-D-glucosamine residues in beta(1,6) linkage. PGA mediates intercellular adhesion and attachment of cells to abiotic surfaces. In this report, we present genetic and biochemical evidence that PGA is also a major matrix component of biofilms produced by the human periodontopathogen Actinobacillus actinomycetemcomitans and the porcine respiratory pathogen Actinobacillus pleuropneumoniae. We also show that PGA is a substrate for dispersin B, a biofilm-releasing glycosyl hydrolase produced by A. actinomycetemcomitans, and that an orthologous dispersin B enzyme is produced by A. pleuropneumoniae. We further show that A. actinomycetemcomitans PGA cross-reacts with antiserum raised against polysaccharide intercellular adhesin, a staphylococcal biofilm matrix polysaccharide that is genetically and structurally related to PGA. Our findings confirm that PGA functions as a biofilm matrix polysaccharide in phylogenetically diverse bacterial species and suggest that PGA may play a role in intercellular adhesion and cellular detachment and dispersal in A. actinomycetemcomitans and A. pleuropneumoniae biofilms.

Enzymatic detachment of Staphylococcus epidermidis biofilms

The gram-positive bacterium Staphylococcus epidermidis is the most common cause of infections associated with catheters and other indwelling medical devices. S. epidermidis produces an extracellular slime that enables it to form adherent biofilms on plastic surfaces. We found that a biofilm-releasing enzyme produced by the gram-negative periodontal pathogen Actinobacillus actinomycetemcomitans rapidly and efficiently removed S. epidermidis biofilms from plastic surfaces. The enzyme worked by releasing extracellular slime from S. epidermidis cells. Precoating surfaces with the enzyme prevented S. epidermidis biofilm formation. Our findings demonstrate that biofilm-releasing enzymes can exhibit broad-spectrum activity and that these enzymes may be useful as antibiofilm agents.

One of two human lactoferrin variants exhibits increased antibacterial and transcriptional activation activities and is associated with localized juvenile periodontitis

The iron-binding protein lactoferrin is a ubiquitous and abundant constituent of human exocrine secretions. Lactoferrin inhibits bacterial growth by sequestering essential iron and also exhibits non-iron-dependent antibacterial, antifungal, antiviral, antitumor, anti-inflammatory, and immunoregulatory activities. All of these non-iron-dependent activities are mediated by the highly charged N terminus of lactoferrin. In this study we characterized a Lys/Arg polymorphism at position 29 in the N-terminal region of human lactoferrin that results from a single nucleotide polymorphism in exon 1 of the human lactoferrin gene. We expressed cDNAs encoding both lactoferrin variants in insect cells and purified the two proteins by ion exchange chromatography. The two lactoferrin variants exhibited nearly identical iron-binding and iron-releasing activities and equivalent bactericidal activities against a strain of the gram-negative bacterium Actinobacillus actinomycetemcomitans. When tested against the gram-positive species Streptococcus mutans and Streptococcus mitis, however, lactoferrin containing Lys at position 29 exhibited significantly greater bactericidal activity than did lactoferrin containing Arg. In addition, the Lys-containing lactoferrin stimulated bovine tracheal epithelial cells to synthesize much higher levels of tracheal antimicrobial peptide mRNA than did the Arg-containing variant. A genotyping assay that distinguished between the two alleles based on a polymorphic EarI restriction site showed that the Lys and Arg alleles had frequencies of 24% and 76%, respectively, among 17 healthy human subjects, and 72% and 28%, respectively, among nine patients with localized juvenile periodontitis. Our findings suggest that these two lactoferrin variants are functionally different and that these differences may contribute to the pathogenesis of localized juvenile periodontitis.

Detachment of Actinobacillus actinomycetemcomitans biofilm cells by an endogenous beta-hexosaminidase activity

When cultured in broth, fresh clinical isolates of the gram-negative periodontal pathogen Actinobacillus actinomycetemcomitans form tenaciously adherent biofilm colonies on surfaces such as plastic and glass. These biofilm colonies release adherent cells into the medium, and the released cells can attach to the surface of the culture vessel and form new colonies, enabling the biofilm to spread. We mutagenized A. actinomycetemcomitans clinical strain CU1000 with transposon IS903phikan and isolated a transposon insertion mutant that formed biofilm colonies which were tightly adherent to surfaces but which lacked the ability to release cells into the medium and disperse. The transposon insertion in the mutant strain mapped to a gene, designated dspB, that was predicted to encode a secreted protein homologous to the catalytic domain of the family 20 glycosyl hydrolases. A plasmid carrying a wild-type dspB gene restored the ability of biofilm colonies of the mutant strain to disperse. We expressed A. actinomycetemcomitans DspB protein engineered to contain a hexahistidine metal-binding site at its C terminus in Escherichia coli and purified the protein by using Ni affinity chromatography. Substrate specificity studies performed with monosaccharides labeled with 4-nitrophenyl groups showed that DspB hydrolyzed the 1-->4 glycosidic bond of beta-substituted N-acetylglucosamine, which is consistent with the known functions of other family 20 glycosyl hydrolases. When added to culture medium, purified DspB protein, but not heat-inactivated DspB, restored the ability of the mutant strain to release cells and disperse. DspB protein also caused the detachment of cells from preformed biofilm colonies of strain CU1000 grown attached to plastic and the disaggregation of highly autoaggregated clumps of CU1000 cells in solution. We concluded that dspB encodes a soluble beta-N-acetylglucosaminidase that causes detachment and dispersion of A. actinomycetemcomitans biofilm cells.

Tight-adherence genes of Actinobacillus actinomycetemcomitans are required for virulence in a rat model

Actinobacillus actinomycetemcomitans is a Gram-negative coccobacillus that has been associated with localized aggressive periodontitis and infections of the heart, brain, and urinary tract. Wild-type clinical isolates have the remarkable ability to adhere tenaciously and nonspecifically to solid surfaces such as glass, plastic, and hydroxyapatite. Adherence by A. actinomycetemcomitans is mediated by the tight-adherence (tad) gene locus, which consists of 14 genes (flp-1-flp-2-tadV-rcpCAB-tadZABCDEFG). All but 2 of the genes have been shown to be required for the secretion and assembly of long, bundled Flp1 fibrils. To test whether the tad locus is required for colonization and disease, we developed a rat model for periodontal disease. To mimic the natural route of infection, Sprague-Dawley rats were inoculated orally by adding bacteria directly to their food for 8 days. After inoculation with wild-type or mutant strains defective in adherence (flp-1 and tadA), the rats were assessed for colonization of the oral cavity and pathogenesis. Wild-type A. actinomycetemcomitans was able to colonize and persist for at least 12 weeks in the oral cavity, elicit a humoral immune response, and cause significant bone loss in rats. In contrast, rats fed flp-1 or tadA mutant strains showed no bone loss and their immune responses were indistinguishable from those of the uninoculated controls. These results demonstrate the critical importance of the tad locus in the colonization and pathogenesis of A. actinomycetemcomitans.

Biofilm growth and detachment of Actinobacillus actinomycetemcomitans

The gram-negative, oral bacterium Actinobacillus actinomycetemcomitans has been implicated as the causative agent of several forms of periodontal disease in humans. When cultured in broth, fresh clinical isolates of A. actinomycetemcomitans form tenacious biofilms on surfaces such as glass, plastic, and saliva-coated hydroxyapatite, a property that probably plays an important role in the ability of this bacterium to colonize the oral cavity and cause disease. We examined the morphology of A. actinomycetemcomitans biofilm colonies grown on glass slides and in polystyrene petri dishes by using light microscopy and scanning and transmission electron microscopy. We found that A. actinomycetemcomitans developed asymmetric, lobed biofilm colonies that displayed complex architectural features, including a layer of densely packed cells on the outside of the colony and nonaggregated cells and large, transparent cavities on the inside of the colony. Mature biofilm colonies released single cells or small clusters of cells into the medium. These released cells adhered to the surface of the culture vessel and formed new colonies, enabling the biofilm to spread. We isolated three transposon insertion mutants which produced biofilm colonies that lacked internal, nonaggregated cells and were unable to release cells into the medium. All three transposon insertions mapped to genes required for the synthesis of the O polysaccharide (O-PS) component of lipopolysaccharide. Plasmids carrying the complementary wild-type genes restored the ability of mutant strains to synthesize O-PS and release cells into the medium. Our findings suggest that A. actinomycetemcomitans biofilm growth and detachment are discrete processes and that biofilm cell detachment evidently involves the formation of nonaggregated cells inside the biofilm colony that are destined for release from the colony.