Antigenicity of converting phages obtained from Clostridium botulinum types C and D

Integration of Complete Plasmids Containing Bont Genes into Chromosomes of Clostridium parabotulinum, Clostridium sporogenes, and Clostridium argentinense

At least 40 toxin subtypes of botulinum neurotoxins (BoNTs), a heterogenous group of bacterial proteins, are produced by seven different clostridial species. A key factor that drives the diversity of neurotoxigenic clostridia is the association of bont gene clusters with various genomic locations including plasmids, phages and the chromosome. Analysis of Clostridium sporogenes BoNT/B1 strain CDC 1632, C. argentinense BoNT/G strain CDC 2741, and Clostridium parabotulinum BoNT/B1 strain DFPST0006 genomes revealed bont gene clusters within plasmid-like sequences within the chromosome or nested in large contigs, with no evidence of extrachromosomal elements. A nucleotide sequence (255,474 bp) identified in CDC 1632 shared 99.5% identity (88% coverage) with bont/B1-containing plasmid pNPD7 of C. sporogenes CDC 67071; CDC 2741 contig AYSO01000020 (1.1 MB) contained a ~140 kb region which shared 99.99% identity (100% coverage) with plasmid pRSJ17_1 of C. argentinense BoNT/G strain 89G; and DFPST0006 contig JACBDK0100002 (573 kb) contained a region that shared 100% identity (99%) coverage with the bont/B1-containing plasmid pCLD of C. parabotulinum Okra. This is the first report of full-length plasmid DNA-carrying complete neurotoxin gene clusters integrated in three distinct neurotoxigenic species: C. parabotulinum, C. sporogenes and C. argentinense.

Pseudolysogeny

Pseudolysogeny can be defined as the stage of stalled development of a bacteriophage in a host cell without either multiplication of the phage genome (as in lytic development) or its replication synchronized with the cell cycle and stable maintenance in the cell line (as in lysogenization), which proceeds with no viral genome degradation, thus allowing the subsequent restart of virus development. This phenomenon is usually caused by unfavorable growth conditions for the host cell (such as starvation) and is terminated with initiation of either true lysogenization or lytic growth when growth conditions improve. Pseudolysogeny has been known for tens of years; however, its role has often been underestimated. Currently, it is being considered more often as an important aspect of phage-host interactions. The reason for this is mostly an increased interest in phage-host interactions in the natural environment. Pseudolysogeny seems to play an important role in phage survival, as bacteria in a natural environment are starved or their growth is very slow. This phenomenon can be an important aspect of phage-dependent bacterial mortality and may influence the virulence of some bacterial strains.

Unique ganglioside binding by botulinum neurotoxins C and D-SA

The botulinum neurotoxins (BoNTs) are the most potent protein toxins for humans. There are seven serotypes of BoNTs (A-G), based on a lack of cross-antiserum neutralization. The BoNT/C and BoNT/D serotypes include mosaic toxins that are organized as D-C and C-D toxins. One BoNT D-C mosaic toxin, BoNT/D-South Africa (BoNT/D-SA), was not fully neutralized by immunization with a vaccine composed of either prototype BoNT/C-Stockholm or BoNT/D-1873. Whereas several BoNT serotypes utilize dual receptors (gangliosides and proteins) to bind to and enter neurons, the basis for BoNT/C and BoNT/D entry into neurons is less well understood. Recent studies solved the crystal structures of the receptor-binding domains of BoNT/C, BoNT/D, and BoNT/D-SA. Comparative structural analysis showed that BoNT/C, BoNT/D and BoNT/D-SA lacked components of the ganglioside-binding pocket that exists within other BoNT serotypes. With the use of structure-based alignments, biochemical analyses, and cell-binding approaches, BoNT/C and BoNT/D-SA have been shown to possess a unique ganglioside-binding domain, the ganglioside-binding loop. Defining how BoNTs enter host cells provides insights towards understanding the evolution and extending the potential therapeutic and immunological values of the BoNT serotypes.

Identification of structural genes forClostridium botulinumtype C neurotoxin-converting phage particles

The structural genes for strain C-Stockholm (c-st) phage particles, a representative type C toxin-converting phage of Clostridium botulinum, have been determined. First, by determining the N-terminal amino acid sequences of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) bands of c-st phage particles, it became clear that four proteins, 14, 25, 32 and 42 kDa, are the products of the ORFs, cst166, cst165, cst160 and cst164, respectively, of the c-st phage genome. The Western blot analyses reacting these phage bands with an antiphage serum prepared previously indicated that the products of cst165 and cst160 are the main proteins of the phage particles. Then, six candidates for the phage structural proteins, including cst165 and cst160 gene products, were prepared as recombinant proteins. Also, the protein corresponding to the cst164 gene product was excised from SDS-PAGE gels. The antibodies against these seven proteins were prepared in rabbits, and finally, the reaction of these antibodies to the c-st phage particles was analyzed by electron microscopy. It was concluded that a sheath protein and a head protein of the c-st phage are the products of genes cst160 and cst165, respectively, and that these two proteins are conserved in the other three converting phages, but not in the nonconverting phage.

The genome sequence of Clostridium botulinum type C neurotoxin-converting phage and the molecular mechanisms of unstable lysogeny

Botulinum neurotoxins (BoNTXs) produced by Clostridium botulinum are among the most poisonous substances known. Of the seven types of BoNTXs, genes for type C1 and D toxins (BoNTX/C1 and D) are carried by bacteriophages. The gene for exoenzyme C3 also resides on these phages. Here, we present the complete genome sequence of c-st, a representative of BoNTX/C1-converting phages. The genome is a linear double-stranded DNA of 185,682 bp with 404-bp terminal direct repeats, the largest known temperate phage genome. We identified 198 potential protein-coding regions, including the genes for production of BoNTX/C1 and exoenzyme C3. Very exceptionally, as a viable bacteriophage, a number of insertion sequences were found on the c-st genome. By analyzing the molecular structure of the c-st genome in lysogens, we also found that it exists as a circular plasmid prophage. These features account for the unstable lysogeny of BoNTX phages, which has historically been called "pseudolysogeny." The PCR scanning analysis of other BoNTX/C1 and D phages based on the c-st sequence further revealed that BoNTX phages comprise a divergent phage family, probably generated by exchanging genomic segments among BoNTX phages and their relatives.

Molecular characterization of GroES and GroEL homologues from Clostridium botulinum

We report novel findings of significant amounts of 60- and 10-kDa proteins on SDS-PAGE in a culture supernatant of the Clostridium botulinum type D strain 4947 (D-4947). The N-terminal amino acid sequences of the purified proteins were closely related to those of other bacterial GroEL and GroES proteins, and both positively cross-reacted with Escherichia coli GroEL and GroES antibodies. Native GroEL homologue as an oligomeric complex is a weak ATPase whose activity is inhibited by the presence of GroES homologue. The 2634-bp groESL operon of D-4947 was isolated by PCR and sequenced. The sequence included two complete open reading frames (282 and 1629 bp), which were homologous to the groES and groEL gene family of bacterial proteins. Southern and Northern blot analyses indicate that the groESL operon is encoded on the genomic DNA of D-4947 as a single copy, and not on that of its specific toxin-converting phage.

Role of C-terminal region of HA-33 component of botulinum toxin in hemagglutination

Using SDS-PAGE, we found that one subcomponent, hemagglutinin (HA-33), from the Clostridium botulinum progenitor toxin of type D strain 1873 and type C strain Yoichi had slightly smaller molecular sizes than those of type C and D reference strains, but other components did not. Based on N- and C-terminal sequence analyses of HA-33, a deletion of 31 amino acid residues from the C-terminus at a specific site was observed in the HA-33 proteins of both strains. The progenitor toxins from both strains showed poor hemagglutination activities, titers of 2(1) or less, which were much lower than titers from the reference strains (2(6)), and did not bind to erythrocytes. These results suggest strongly that the short C-terminal region of the HA-33 plays an essential role in the hemagglutination activity of the botulinum progenitor toxin. Additionally, a sequence motif search predicted that the C-terminal region of HA-33 has a carbohydrate-recognition subdomain.

Molecular composition of the 16S toxin produced by a Clostridium botulinum type D strain, 1873

The 16S toxin was purified from a Clostridium botulinum type D strain 1873 (D-1873). Furthermore, the entire nucleotide sequences of the genes coding for the 16S toxin were determined. It became clear that the purified D-1873 16S toxin consists of neurotoxin, nontoxic nonhemagglutinin (NTNH), and hemagglutinin (HA), and that HA consists of four subcomponents, HA1, HA2, HA3a, and HA3b, the same as type D strain CB16 (D-CB16) 16S toxin. The nucleotide sequences of the nontoxic components of these two strains were also found to be identical except for several bases. However, the culture supernatant and the purified 16S toxin of D-1873 showed little HA activity, unlike D-CB16, though the fractions successively eluted after the D-1873 16S toxin peak from an SP-Toyopearl 650S column showed a low level of HA activity. The main difference between D-1873 and D-CB16 HA molecules was the mobility of the HA1 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Therefore it was presumed that the loss of HA activity of D-1873 16S toxin might be caused by the differences of processing HA after the translation.

Mosaic structures of neurotoxins produced from Clostridium botulinum types C and D organisms

We isolated the gene encoding a botulinum neurotoxin (BoNT) of 1285 amino acids with a molecular weight of 147,364 from the toxigenic bacteriophage d-sA of Clostridium botulinum type D strain South African (Dsa). The BoNT of Dsa (BoNT/Dsa) is composed of three regions on the basis of the homology to BoNT types C1 (BoNT/C1) and D (BoNT/D). The N-terminal (Met-1 to Val-522) and the C-terminal regions (Trp-945 to Glu-1285) have high identity to corresponding regions of BoNT/D (96% identity) and BoNT/C1 (74% identity), respectively. The core region (Pro-523 to Lys-944) is common to three toxins (83% to 92% identity). These results suggest that neurotoxins produced from Clostridium botulinum types C and D are composed in a mosaic-like fashion.

Characterization of nontoxic-nonhemagglutinin component of the two types of progenitor toxin (M and L) produced by Clostridium botulinum type D CB-16

A 9.8-kbp DNA fragment which contained a neurotoxin gene and its upstream region was cloned from Clostridium botulinum type D strain CB-16. Nucleotide sequencing of the fragment revealed that genes encoding for hemagglutinin (HA) subcomponents and one for a nontoxic-nonhemagglutinin (NTNH) component were located upstream of the neurotoxin gene. This strain produced two toxins of different molecular size (approximately 300 kDa and 500 kDa) which were designated as progenitor toxins (M and L toxins). The molecular size of the NTNH component of L toxin was approximately 130 kDa on SDS-PAGE and its N-terminal amino acid sequence was M-D-I-N-D-D-L-N-I-N-S-P-V-D-N-K-N-V-V-I which agreed with that deduced from the nucleotide sequence. In contrast, the M toxin had a 115-kDa NTNH component whose N-terminal sequence was S-T-I-P-F-P-F-G-G-Y-R-E-T-N-Y-I-E, corresponding to the sequence from Ser141 of the deduced sequence. A 15-kDa fragment, which was found to be associated with an M toxin preparation, possessed the same N-terminal amino acid sequence as that of the 130-kDa NTNH component. Furthermore, five major fragments generated by limited proteolysis with V8 protease were shown to have N-terminal amino acid sequences identical to those deduced from the nucleotide sequence of 130-kDa NTNH. These results indicate that the 130-kDa NTNH of the L toxin is cleaved at a unique site, between Thr and Ser, leading to the 115-kDa NTNH of the M toxin.

Two different types of ADP-ribosyltransferase C3 from Clostridium botulinum type D lysogenized organisms

We examined production of ADP-ribosyltransferase C3 in 11 strains of Clostridium botulinum type C and D and their nontoxigenic derivatives. Antisera to C3 proteins of type C organisms divided C3 proteins roughly into at least two groups, bearing no relation to their bacterial types. The C3 gene of type D strain South African was isolated from a toxigenic phage library, and the complete sequence of the C3 gene was determined. The C3 protein of type D strain South African had 98% homology to the C3 protein of type C strain 003-9 and 66% homology to that of type D strain 1873. These results indicate that there are two types of C3 protein in type D organisms, as there are in type C organisms.

Cloning of the structural gene for Clostridium botulinum type C1 toxin and whole nucleotide sequence of its light chain component

The toxigenicity of Clostridium botulinum type C1 is mediated by specific bacteriophages. DNA was extracted from one of these phages. Two DNA fragments, 3 and 7.8 kb, which produced the protein reacting with antitoxin serum were cloned by using bacteriophage lambda gt11 and Escherichia coli. Both DNA fragments were then subcloned into pUC118 plasmids and transferred into E. coli cells. The nucleotide sequences of the cloned DNA fragments were analyzed by the dideoxy chain termination method, and their gene products were analyzed by Western immunoblot. The 7.8-kb fragment coded for the entire light chain component and the N terminus of the heavy chain component of the toxin, whereas the 3-kb fragment coded for the remaining heavy chain component. The entire nucleotide sequence for the light chain component was determined, and the derived amino acid sequence was compared with that of tetanus toxin. It was found that the light chain component of C1 toxin possessed several amino acid regions, in addition to the N terminus, that were homologous to tetanus toxin.

Cloning and complete nucleotide sequence of the gene for the main component of hemagglutinin produced by Clostridium botulinum type C

In Clostridium botulinum types C and D, phage conversion to toxin and hemagglutinin (HA) production has been reported. DNA was extracted from a converting type C Stockholm phage, c-st, and a fragment (7.8 kilobase pairs) coding for the parts of both toxin and HA was cloned. The gene for HA was recloned, and the complete nucleotide sequence was determined. The molecular mass of this gene product was 33 kilodaltons, and it showed HA activity. The HA preparation partially purified from a type C Stockholm culture demonstrated two major bands (33 and 53 kilodaltons) on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with or without reducing agent. The amino acid sequence of the N terminus of the 33-kilodalton component of the native HA preparation, which was determined by a direct protein microsequencing procedure, was identical to that deduced from the nucleotide sequence of cloned HA gene. These data indicate that the cloned gene product (33 kilodaltons) is an important component of HA.

The complete nucleotide sequence of the gene coding for botulinum type C1 toxin in the C-ST phage genome

Two DNA fragments, 3 kbp and 7.8kbp, which encode the type C1 botulinum neurotoxin gene, were obtained from toxigenic bacteriophage DNA by treatment with a restriction enzyme. They were cloned into the plasmid vectors for nucleotide sequence determination. The nucleotide sequence contained a single open reading frame coding for 1,291 amino acids corresponding to a polypeptide with a molecular weight of 149,000. The amino acid sequence of the C1 toxin has a few regions highly homologous with tetanus toxin.

Characterization of bacteriophage nucleic acids obtained from Clostridium botulinum types C and D

Nontoxigenic strains of Clostridium botulinum types C and D are converted to toxigenic strains by infection with specific Tox+ bacteriophages. The nucleic acids were extracted from five converting phages, c-st, c-468, c-203, c-d6f, and d-1873, and one nonconverting phage, c-n71, and treated with nucleases. The nucleic acids isolated were not digested by RNase A, but were digested by DNase I and exonuclease III, indicating that they were double-stranded DNA. On the basis of the restriction endonuclease digestion patterns on 0.8% agarose gel electrophoresis, the length of c-st, c-n71, c-468, and c-d6f phage DNAs was estimated to be about 110 kilobase pairs and that of c-203 and d-1873 was about 150 kilobase pairs. The digestion patterns of c-st, c-468, and c-n71 phage DNAs by PstI and HindIII were very similar. High homology was observed in the dot hybridization test. For other phages and nucleases, a good similarity was not observed. Only a little similarity was observed between c-203 and c-d6f phages. The existence of the structural genes for the toxin in both c-st and c-n71 phages was confirmed by the hybridization test with these phage DNAs and the oligonucleotide probe which represented the DNA sequence predicted for the N-terminal amino acids (2 to 17) of C. botulinum type C toxin. The loss of the converting ability of c-n71 phage may be caused not by the deletion of the tox+ gene but rather by the base mutation in c-st phage DNA.

Analysis of antigenicity of Clostridium botulinum type C1 and D toxins by polyclonal and monoclonal antibodies

Clostridium botulinum type C1 toxin was purified from C-Stockholm (C-ST), and D toxin was purified from D-1873 and D-South African. Polyclonal antibodies against these toxins were prepared in rabbits. Twenty-eight monoclonal antibodies to these toxins were also prepared with BALB/c myeloma cells. The antibodies were analyzed by both enzyme-linked immunosorbent assay (ELISA) and a toxin neutralization test. ELISA was performed with the three purified toxins and heavy-chain (Hc) and light-chain (Lc) components derived from C-ST and D-1873 toxins. A neutralization test was carried out with 11 toxin preparations (7 from type C and 4 from type D cultures). ELISA results indicated that there exists at least one common antigenic determinant on each of the Hc and Lc components of the three purified toxins. The results of the neutralization test also indicated that type C1 and D toxin preparations contain several common antigenic sites in their molecules. Some are common to toxins from several specific cultures, whereas others are common to toxins from a large number of cultures. It was speculated that toxins from two type C strains are composed of Hc and Lc components which are somewhat similar to those of D-1873 and C-ST toxins, respectively.

Four different monoclonal antibodies against type C1 toxin of Clostridium botulinum

Monoclonal antibodies against type C1 toxin produced by Clostridium botulinum type C strain Stockholm (C-ST) were prepared by fusion of BALB/c myeloma cells P3X63-Ag8, with spleen cells from the mice immunized by C-ST toxoid. About 5% of single-cell colonies in wells were found to produce antibodies against the toxin as determined by an enzyme-linked immunosorbent assay (ELISA). Four different hybridoma cell lines, no. 9, 12, 14, and 17, were established, cloned by limiting dilution, and intraperitoneally injected into mice to obtain the ascites fluids containing high-titered antibodies. The reactions of these antibodies to type C1 and D toxins of strains C-ST, D-1873, and D-South African (D-SA) were observed by both neutralization and ELISA tests. Three monoclonal antibodies, no. 9, 14, and 17, reacted with C-ST toxin, but only no. 17 highly neutralized the toxin. These antibodies did not react with type D toxins. On the contrary, no. 12 reacted with toxins of both C-ST and D-SA (but not of D-1873) and commonly neutralized these two toxins. This indicates that there is a common antigenic part between C-ST and D-SA toxin molecules which participates in the toxin-neutralizing reaction. The neutralization profiles of C-ST toxin by no. 12 and 17 antibodies were different in a time-to-death test of mice. The mechanisms of neutralization by no. 12 and 17 may be different.

Homogeneity and heterogeneity of toxins produced by Clostridium botulinum type C and D strains

Five Clostridium botulinum strains were used in the present work, two of type C, C-Stockholm (C-ST) and C-CB19, and three of type D, D-South African (D-SA), D-1873, and D-CB16. The toxins, except for those of C-CB19 and D-CB16, were purified, and antisera were prepared in rabbits. To clarify the antigenicity of the toxins, neutralization and agar gel double-diffusion tests were performed. Anti-C-ST toxin serum neutralized two kinds of type C(1) toxin to a similar extent. Antisera against D-SA and D-1873 toxins, however, showed different neutralizing activity toward three type D toxins. Precipitin lines formed between D-SA and D-1873 toxins, and their antisera spurred to each other. From anti-D-SA toxin serum, two neutralizing fractions, one which neutralized D-SA, D-1873, and D-CB16 and one which neutralized only D-SA, were obtained. These results indicate that the antigenicities of D-SA and D-1873 toxins are not identical. Anti-C-ST toxin serum produced cross-neutralization on type D toxins, although the neutralization titer differed depending on the kind of toxin used. A precipitin line was formed with D-SA toxin, but not with D-1873; the developed line spurred to the C-ST toxin precipitin line. Two anti-D toxin sera also caused cross-neutralization on type C(1) toxins. However, the neutralizing activity of each serum to the same type C(1) toxin was different, and only anti-D-SA toxin serum developed a precipitin line with C-ST toxin which spurred to the D-SA toxin precipitin line. From anti-D-SA toxin serum, two different fractions capable of neutralizing C(1) and D toxins were obtained; one neutralized C-ST, C-CB19, and D-SA toxins, but not D-1873 and D-CB16, and the other neutralized all five toxins. There may be at least two common parts among C-ST, C-CB16, and D-SA toxin molecules.

Antigenic similarity of toxins produced by Clostridium botulinum type C and D strains

Antisera against purified type C1 toxin of Clostridium botulinum and its heavy-chain component cross-neutralized type D toxin. Antisera against partially purified type D toxin cross-neutralized type C1 toxin. From the latter serum, a component which neutralized only type D toxin and a component which equally neutralized both C1 and D toxins were obtained. We concluded that the cross-neutralization was not due to the fact that type C and D strains produce both C1 and D toxins but rather to the fact that the toxins have an antigen(s) common to their molecules. The results of the agar gel-double-diffusion test also supported this conclusion.

Use of ganglioside affinity filters to identify toxigenic strains of Clostridium botulinum types C and D

Clostridium botulinum neurotoxin is synthesized by toxic clones grown anaerobically on ganglioside affinity filters. The toxin binds to the filters and is detected by reaction with 125I-immunoglobulin G from type-specific antitoxin. Toxin spots from culture filtrates were similarly identified. The C. botulinum type C and D strains were selected for developing this affinity filter assay because synthesis of the C1 and D toxins is bacteriophage dependent. Toxigenic clones were distinguished from prophage-cured atoxigenic derivatives. These studies represent a first step toward the development of a general nonbiological screening procedure for identifying botulinal toxin and toxigenic cells. The affinity filter methodology should facilitate genetic analysis of the basis of C. botulinum toxicity.

Phage conversion to hemagglutinin production in Clostridium botulinum types C and D

Five toxigenic strains of Clostridium botulinum types C and D were incubated at 37 degrees C for 7 days in 15 ml of the following media: LYG medium, cooked-meat medium, egg meat medium, and N-Z-amine medium. The supernatants of these cultures were tested for hemagglutinin production with 1% erythrocytes obtained from mice, guinea pigs, chickens, sheep, monkeys, and humans. Four toxigenic strains produced hemagglutinin. The highest hemagglutinin titer was obtained with a combination of human erythrocytes and cultures incubated in LYG medium. When the same experiment was carried out with many nontoxigenic strains, hemagglutination was observed in only one strain, C-N71. Strains producing hemagglutinin also produced phages. The phages obtained from toxin- and hemagglutinin-producing strains converted nontoxigenic indicator strains to produce both toxin and hemagglutinin. The phage obtained from a toxin-positive hemagglutinin-negative strain could only induce cultures to produce toxin, and the phage from a toxin-negative hemagglutinin-positive strain could only induce production of hemagglutinin. These studies suggest that the production of hemagglutinin by C. botulinum types C and D is governed by bacteriophages and that hemagglutinin production can be transmitted separately or concomitantly with toxin production.