Mapping of a gene in Vibrio cholerae that determines the antigenic structure of cholera toxin

Interspecies recombination between enterococci: genetic and phenotypic diversity of vancomycin-resistant transconjugants

Handwerger and colleagues demonstrated that a particular clinical isolate of Enterococcus faecium, designated GUC, and here redesignated as GUCR, can conjugatively transfer vancomycin resistance. The vancomycin resistance is encoded by a chromosomally born linked set of genes in the donor, designated the vanA cluster, to the chromosome of an E. faecalis recipient, JH2-2. Here it is reported that an earlier isolate of E. faecium from the same patient who later harbored the vancomycin-resistant E. faecium GUCR lacks the vanA gene cluster but is otherwise similar (by SmaI chromosomal fingerprint and metabolic fingerprinting) to the vancomycin-resistant GUCR. Therefore, "GUCS" is a strong suspect as the base strain for the clinical acquisition of the vanA cluster present in GUCR. Thirteen laboratory-generated vanA transconjugants derived from conjugation between GUCR and JH2-2 were subjected to further analysis, allowing a comparison between transfer in the laboratory and transfer that occurred in the clinical setting. Surprisingly, each JH2-2 transconjugant had a unique constellation of abilities to oxidize various members of a panel of potential carbon sources. This pattern was stable for each transconjugant, and it was not changed by growing the strains with or without vancomycin. Transconjugants had pulsed-field gel electrophoretic (PFGE) patterns largely consistent with that of JH2-2, the recipient in conjugation experiments. However, PFGE analysis showed that a large but variable amount of DNA, between 145 kb and 277 kb, was transferred into different transconjugants. The mechanism appears to be conjugative transposition in which new DNA is added to the pre-existing genome rather than substituting for a segment in the recipient. Mapping and hybridization studies of several transconjugants showed that each received similar, but not exactly the same, DNA fragment of at least 30 kb from the donor. Sequencing of 16S ribosomal genes was used to confirm that the recipient and donor strains used in transconjugation experiments were different species. Sequence analysis was also used to consider the possibility that rRNA operons might be mobilized in conjugation, but no evidence for the transfer of rDNA operons was found. An apparent insertion sequence in E. faecium almost identical to IS 1485 and 57% sequence identity to IS 199 of Streptococcus mutans was found in the region of DNA transferred. The results imply new consequences of conjugative transfer and interspecies recombination.

Mapping of a gene that regulates hemolysin production in Vibrio cholerae

A gene that regulates the hemolysin structural gene (hly) was found to be tightly linked to the tox-1000 locus of Vibrio cholerae RJ1 and separated from hly by a large section of the V. cholerae genetic map. This hemolysin regulatory gene was designated hlyR.

Phosphate repression of phage protein synthesis during infection by choleraphage phi 149

A synthetic medium for choleraphage phi 149 growth, in which the concentration of phosphate ions plays a significant role, has been defined. Upon infection, choleraphage phi 149 DNA binds to the cell membrane at three to four sites. The host macromolecular syntheses are shut off by 10 min after infection and the synthesis of phage-specific DNA is detectable after 20 min of infection. The phage utilizes primarily the host DNA degradation products for its own DNA synthesis. When added during the first 20 min of infection both nalidixic acid and novobiocin inhibit phage growth. The effects of these antibiotics are not pronounced when added late during infection. Pulse labeling of ultraviolet-irradiated infected cells at different times during infection has allowed identification of about 50 phage-specific proteins of which 19 are structural proteins. These proteins appear during the infection cycle in two distinct phases, early and late. When infection is carried out in high-phosphate medium, none of the late proteins is synthesized. Eleven of the 26 early proteins detected are DNA-binding proteins.

Genetic mapping of Vibrio cholerae enterotoxin structural genes

The structural genes which constitute the cholera toxin operon, ctxAB, were genetically mapped in the Vibrio cholerae El Tor strain RV79. This strain of V. cholerae contains two copies of the ctx operon located on a 7-kilobase-pair tandemly duplicated region. We began by isolating a vibriophage VcA1 insertion mutation in one of the two ctxA genes located in this region. The mutant carrying this ctxA::VcA1 insertion, DC24, was converted to a VcA1-facilitated donor by introduction of the conjugal plasmid pSJ15, which carries an inserted copy of a defective VcA1-like prophage. The donor characteristics of DC24(pSJ15) indicated that the ctxA::VcA1 insertion mutation was near the trp region of the V. cholerae chromosome. Subsequent RV79 three-factor crosses were performed between VcA1-facilitated donors and recipient strains carrying one of two structural gene mutations in ctx, either delta ctxA23P Kmr or delta ctx-7922. The former was constructed by an in vivo marker exchange procedure and could be scored either by its kanamycin resistance phenotype or by its lack of DNA sequences homologous to the ctxA region. The delta ctx-7922 mutation is a total deletion of both ctx copies of strain RV79. The three-factor cross data strongly suggest that the two ctx loci of RV79 map between the nal and his genes of V. cholerae in the trp nal his linkage group. Physical analysis and heterologous crosses between an RV79 El Tor donor and a 569B classical recipient indicates that one of the two 569B ctx operon copies maps in the same region as the RV79 ctx loci (i.e., linked to nal). Together with previously published observations, these data show that the ctx structural genes are not closely linked to other genes known to affect toxin production in V. cholerae.

Mapping of chromosomal genes that determine the El Tor biotype in Vibrio cholerae

The El Tor biotype of Vibrio cholerae has several characteristics that differentiate it from the classical biotype of V. cholerae. Among these are production of soluble hemolysin(s), a cell-associated hemagglutinin for chicken erythrocytes, resistance to polymyxin B, and resistance to bacteriophages of Mukerjee group IV. In the present study, we located the determinants for hemolysin (hly), chicken erythrocyte hemagglutinin (cha), and polymyxin B resistance (pmx) on the genetic map of V. cholerae. Transposon-facilitated recombination was used to perform conjugal matings between El Tor donor strains and classical recipient strains of V. cholerae. Recombinants were selected that had inherited specific nutritional markers from the donor strains and streptomycin resistance from the recipient strains. The recombinants were tested for the presence or absence of the unselected donor markers hly, cha, and pmx. These three El Tor biotype markers were found to be closely linked to each other and were located between the pyrA-201 and his-2 loci on the genetic map of V. cholerae.

Production of cholera toxin-like toxin by Vibrio mimicus and non-O1 Vibrio cholerae: batch culture conditions for optimum yields and isolation of hypertoxigenic lincomycin-resistant mutants

Vibrio mimicus 61892, isolated in 1977 from a case of watery diarrhea in Bangladesh, produces an enterotoxin which possesses activity in Y-1 mouse adrenal cells and in rabbit ileal loops which is identical to the prototype cholera toxin (CT) produced by Vibrio cholerae 569B. The neutralization of the adrenal cell activity of 61892 toxin and 569B CT by homologous and heterologous antisera generates parallel titration curves which show complete neutralization in all cases. Paired titrations in the ganglioside GM1 enzyme-linked immunosorbent assay (using either CT or Escherichia coli heat-labile toxin antitoxin) of both toxins indicates that 61892 toxin is antigenically indistinguishable from 569B CT. The specific activity of the two toxins in the rabbit ileal loop is virtually identical. Batch culture production of CT-like toxin and CT by isolates of V. mimicus and different biotypes of V. cholerae was found to be highest in shake flask cultures of Casamino Acids-yeast extract broth grown at 27 degrees C with vigorous aeration. Incorporation of lincomycin into the growth medium at a concentration of 50 micrograms/ml increased yields from wild-type strains. Dramatically higher yields were obtained when a spontaneous resistance mutant of strain 61892 was grown in the presence of 200 to 300 micrograms of lincomycin per ml. Under these conditions, yields of CT-like toxin were increased by 300- to 500-fold, and the highest yields reached more than 100 micrograms/ml after 44 h of culture. This is substantially higher than that reported in the literature for CT production by any strain of V. cholerae, including hypertoxigenic strain 569B.

Evidence that a non-O1 Vibrio cholerae produces enterotoxin that is similar but not identical to cholera enterotoxin

Cholera-like enterotoxin produced by a non-O1 strain of Vibrio cholerae, S7 (S7 enterotoxin), isolated from human diarrheal stool, was purified, and its physicochemical, biological, and immunological properties were compared with those of cholera enterotoxin from V. cholerae O1 569B (CT) and an enterotoxin produced by another non-O1 V. cholerae (E8498 enterotoxin) reported previously (Yamamoto et al., Infect. Immun. 39:1128-1135, 1983). The purified S7 enterotoxin had physicochemical properties different from those of CT and E8498 enterotoxin. S7 enterotoxin had greater relative mobility in conventional polyacrylamide gel disc electrophoresis and a lower isoelectric point, and its B subunit was smaller than those of CT and E8498 enterotoxin. The results of sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis suggested that the size of the aggregate of the B subunits of S7 enterotoxin is larger than that of CT and E8498 enterotoxin. The biological and immunological properties of S7 enterotoxin were indistinguishable from those of CT and E8498 enterotoxin. These results indicate that non-O1 vibrios may produce more than one kind of cholera-like enterotoxin: one which is identical to CT (E8498 enterotoxin type) and another which is not identical to CT (S7 enterotoxin type).

Genetic mapping of the tox-1000 locus of Vibrio cholerae El Tor strain RJ1

The results of a genetic cross between a Vibrio cholerae RJ1 donor and a V. cholerae 3083-2 recipient suggest that the map position of tox-1000 is between his and trp.