Bacteriophage-mediated transduction of antibiotic resistance in enterococci.

AIMS: Temperate bacteriophages are bacterial viruses that transfer genetic information between bacteria. This phenomenon is known as transduction, and it is important in acquisition of bacterial virulence genes and antimicrobial resistance determinants. The aim of this study was to demonstrate the role of bacteriophages in gene transfer (antibiotic resistance) in enterococci. METHODS AND RESULTS: Three bacteriophages from environmental samples isolated on pig host strains of Enterococcus gallinarum and Enterococcus faecalis were evaluated in transduction experiments. Antibiotic resistance was transferred from Ent. gallinarum to Ent. faecalis (tetracycline resistance) and from Ent. faecalis to Enterococcus faecium, Enterococcus hirae/durans and Enterococcus casseliflavus (gentamicin resistance). CONCLUSIONS: Bacteriophages play a role in transfer of antibiotic resistance determinants in enterococci. SIGNIFICANCE AND IMPACT OF THE STUDY: This study confirms previous suggestions on transduction in enterococci, in particular on interspecies transduction. Interspecies transduction is significant because it widens the range of recipients involved in antimicrobial resistance transfer.

Cloning of the structural gene (hly) for the haemolysin of Vibrio cholerae El Tor strain 017.

We have cloned the DNA encoding the haemolysin of Vibrio cholerae El Tor strain 017 into the plasmid vector pBR322. The resultant plasmid, pPM431, has a 6.2-kb PstI DNA insert which leads to the production of the haemolysin in Escherichia coli K-12. Deletion analysis and transposon mutagenesis have allowed us to localize several regions affecting haemolysin production. A number of these mutants have been analysed in E. coli K-12 minicells. Three proteins have been identified: A, 80 kDal; B, 71 kDal; and C, 22 kDal. A is the haemolysin which appears to be cell-associated in E. coli K-12, and B and C are required for its efficient production. We suggest that the genes for proteins A, B and C be designated hlyA, hlyB and hlyC, respectively.

Molecular cloning using immune sera of a 22-kDal minor outer membrane protein of Vibrio cholerae.

Using antisera prepared against live Vibrio cholerae we have selected several recombinant DNA clones, plasmids pPM440, pPM450 and pPM460, encoding the gene for a 22-kDal V. cholerae peptidoglycan-associated-outer-membrane protein. This is a minor protein in V. cholerae but is expressed in large amounts when the cloned gene is present in Escherichia coli K-12, where it is exposed on the cell surface as judged by ELISA. We have localized the gene within the cloned DNA by transposon mutagenesis and deletion analysis followed by analysis of whole cells and minicells to identify the plasmid-encoded proteins. The DNA region encoding the protein seems to be conserved between El Tor and Classical strains as judged by Southern DNA hybridization.

Sequence of a variant Shiga-like toxin type-I operon of Escherichia coli O111:H-.

PCR amplification was used to screen faecal isolates of Escherichia coli from a 12-month-old boy with haemolytic uraemic syndrome for the presence of Shiga-like toxin (SLT)-encoding genes. One isolate, belonging to serotype O111:H-, was positive for SLT-I by this method. UV induction indicated that the strain was lysogenic for a lambdoid bacteriophage, but this did not encode the toxin. Southern hybridization analysis of chromosomal DNA revealed that the SLT-I gene was located on an 8.5-kb EcoRI fragment. SLT-I was further localized to within a 3.0-kb SphI-EcoRI fragment. A separate subclone contained a 3.75-kb HindIII fragment, 1.18 kb of which was common to both. Nucleotide sequence analysis of derivatives of these clones revealed that the SLT-I A subunit gene from E. coli O111:H- differed from the previously published sequences for SLT-I by 5 bp [resulting in two amino acid (aa) changes]. It was more closely related to the gene encoding the A subunit of the Shiga toxin from Shigella dysenteriae type 1, from which it differed by 3 bp (resulting in one aa change). The DNA sequence of the B subunit-encoding gene was identical to that of the other two toxins. The region of DNA upstream from the SLT-I of E. coli O111:H- contained an IS element, as well as a region with strong homology to a portion of the genome of bacteriophage lambda.

Demonstration of clonal variation amongst O-antigen serotype variants of Escherichia coli O2 and O18 using DNA probes to the rfb region of the E. coli strain B41 (O101:K99/F41).

The confirmation at the DNA level of the existence of clonal variants within Escherichia coli O2 and O18 serotypes has been shown by Southern hybridization analysis of restriction endonuclease digested genomic DNA and subsequent probing with contiguous subclones of the E. coli O101 rfb region. The O101 rfb subclones are believed to represent a conserved region of DNA (Heuzenroeder et al. Molec. Microbiol, in press) and identify serotype variants by means of restriction fragment length polymorphisms (RFLP) within homologous DNA of O2 and O18 E. coli. A number of different restriction enzymes have been used singly and in combination to digest the genomic DNA, thereby allowing construction of restriction maps of the region displaying homology to the O101 rfb region subclones. This analysis further substantiates previously defined evolutionary relationships between O2 and O18 E. coli. These simple probes appear to be able to provide the same clonal information as a battery of isoenzyme, outer membrane protein (OMP) and lipopolysaccharide (LPS) analyses.

Lysogenic phage in Salmonella enterica serovar heidelberg (Salmonella Heidelberg): implications for organism tracing.

A phage typing system using a group of 11 closely related phage (as judged by Southern analysis and restriction fragment length polymorphism analysis) was able to distinguish at least six phage types in Salmonella heidelberg of human and animal origin. Restriction fragment length polymorphism analysis using cosmid probes from S. heidelberg confirmed that most S. heidelberg isolates belong to a single 'clonal' group. Southern analysis using DNA isolated from each of the testing phage group showed that phage types 4, 5 and 6 carry closely related endogenous or lysogenic phage. Induction of a lysogenic phage Hlp-4 (Heidelberg lysogenic phage) from type 4 could become lysogenic and convert phage types 1 and 3 to phage type 4 and phage type 5 to a non-typable phenotype.

Distribution of two hemolytic toxin genes in clinical and environmental isolates of Aeromonas spp.: correlation with virulence in a suckling mouse model.

Previous studies have shown that two hemolytic toxins, HlyA and AerA, contribute to the virulence of Aeromonas hydrophila. A survey was performed to gauge the distribution of hlyA and aerA genes in clinical and environmental Aeromonas isolates. For A. hydrophila, A. veronii biotype sobria and A caviae, 96%, 12% and 35% of strains, respectively, were hlyA positive, whereas, 78%, 97%, 41%, respectively, were aerA positive. All virulent A. hydrophila isolates were hlyA+ aerA+. This genotype was most common in A. hydrophila (75.4%) followed by A. caviae (29.4%) and A. veronii biotype sobria (9.6%). For A. hydrophila, a two-hemolytic toxin model of virulence provides the best prediction of virulence in an animal model.

A new fimbrial type (PCFO9) on enterotoxigenic Escherichia coli 09:H- LT+ isolated from a case of infant diarrhea in central Australia.

The genes determining the biosynthesis of a new putative colonization factor, designated PCF09 have been cloned from an LT+ enterotoxigenic Escherichia coli 09:H- isolated during an outbreak of infant diarrhea in Central Australia. Electron microscopy has shown it to be of the fibrillar type. Purification of the major pilin subunit showed it to have a size of approximately 27 kDa. NH2-terminal analysis of the major subunit has shown the PCFO9 determinant to be distinct from other fimbriae although there is some conservation of certain residues. A synthetic oligodeoxynucleotide probe based on the NH2-terminal amino acid sequence of the purified protein has been used in Southern hybridization analyses to define the region on pPM1320 encoding the structural gene for the major pilin subunit.

Effect of lipopolysaccharide core synthesis mutations on the production of Vibrio cholerae O-antigen in Escherichia coli K-12.

The rfb genes of Vibrio cholerae O1 (Ogawa serotype) were subcloned into a derivative of pBR322. This plasmid was transformed into several Escherichia coli K-12 mutant strains which produce an incomplete lipopolysaccharide (LPS)-core-oligosaccharide region. The data indicate that the V. cholerae O-antigen is assembled onto the E. coli LPS and that at least two glucoses are needed in the core in order to achieve a high level of production. These data are consistent with the reported presence of glucose in the V. cholerae LPS-core-oligosaccharide region.

Molecular cloning and expression in Escherichia coli K-12 of chromosomal genes determining the O antigen of an E. coli O2: K1 strain.

The rfb gene cluster which determines the biosynthesis of the O2 O antigen has been cloned from an Escherichia coli O2: K1 strain isolated from a case of septicaemia in chickens. The region required for expression of the O antigen in E. coli K-12 was localised to a 10.7 to 14.15-kb segment which was shown to be chromosomal in origin with a close linkage to the gnd and his genetic loci.

Measurement of virulence of aeromonads using a suckling mouse model of infection.

Virulent and avirulent strains of Aeromonas spp. were identified and virulence quantified using an animal model. Virulence was measured by determining a 50% lethal dose (LD50) 43 h after oral administration of live bacteria. The LD50 of virulent Aeromonas isolates ranged from log10 7.53 (mean) organisms to log10 8.88 (mean). Some isolates were avirulent in this model. Detection of cytotoxic activity in culture supernatants correlated with virulence (Fisher exact test, P = 0.0029). There was no correlation between LD50 and the source of the isolate, beta-haemolysis or lipopolysaccharide (LPS) banding profile on SDS-PAGE. In this animal model, virulence was multifactorial in that: (i) bacterial multiplication in the gut was associated with fatal infection; (ii) the increase in bacterial numbers in the gut of mice administered a lethal dose of bacteria was accompanied by accumulation of fluid; and (iii) there was evidence of extraintestinal spread of infection. Protection of suckling mice by rabbit antiserum to Aeromonas cell envelopes was observed.

A study of the transfer of tetracycline resistance genes between Escherichia coli in the intestinal tract of a mouse and a chicken model.

Experiments to demonstrate the transfer of genes within a natural environment are technically difficult because of the unknown numbers and strains of bacteria present, as well as difficulties designing adequate control experiments. The results of such studies should be viewed within the limits of the experimental design. Most experiments to date have been based on artificial models, which only give approximations of the real-life situation. The current study uses more natural models and provides information about tetracycline resistance as it occurs in wild-type bacteria within the environment of the normal intestinal tract of an animal. Tetracycline sensitive, nalidixic acid resistant Escherichia coli isolates of human origin were administered to mice and chicken animal models. They were monitored for acquisition of tetracycline resistance from indigenous or administered donor E. coli. Five sets of in vivo experiments demonstrated unequivocal transfer of tetracycline resistance to tetracycline sensitive recipients. The addition of tetracycline in the drinking water of the animals increased the probability of transfer between E. coli strains originating from the same animal species. The co-transfer of unselected antibiotic resistance in animal models was also demonstrated.

Clinical evaluation of a peptide-ELISA based upon N-terminal B-cell epitope of the VapA protein for diagnosis of Rhodococcus equi pneumonia in foals.

A total of 227 field samples from naturally exposed foals aged between 3 weeks and 6 months were used in an evaluation of a peptide-based enzyme-linked immunosorbent assay (ELISA) for diagnosis of Rhodococcus equi infection. A biotinylated peptide derived from the virulence-associated protein A (VapA) of R. equi, a horse pathogen, was synthesized and designated as PN11-14. The peptide corresponds to the N-terminal B-cell epitope TSLNLQKDEPNGRASDTAGQ of the VapA protein. Based upon a serum immunoglobulin (Ig)G titre of 512 as a positive cut-off value for the R. equi infection, the ELISA provided the overall sensitivity of 47.62%, specificity of 69.67% and an accuracy of 59.47% with a positive predictive value of 57.47% for true R. equi pneumonia. The assay was improved by detecting VapA-specific IgGb antibodies against N-terminal B-cell epitope of the VapA protein rather than IgG antibodies. The VapA-IgGb ELISA showed the overall sensitivity of 70.47%, specificity of 72.13% and accuracy of 71.36% with a positive predictive value of 68.52%. Diagnosis of R. equi disease in 6-week-old foals showed that the VapA-IgGb ELISA provided an increasing trend (P=0.0572) in sensitivity of 82.4% in comparison with the VapA-IgG ELISA which showed the sensitivity of 58.8%. However, differences in specificity of both tests were statistically insignificant (P=0.357) as analysed by the McNemar test. These results indicated that detection of VapA-specific IgGb antibodies may be a better predictor of R. equi disease in foals.

Use of AFLP and PFGE to discriminate between Salmonella enterica serovar Typhimurium DT126 isolates from separate food-related outbreaks in Australia.

In 2001 Salmonella enterica serovar Typhimurium definitive phage-type (DT) 126 was isolated at higher frequency in Australia compared to other S. Typhimurium phage types and in comparison to previous years. Associated with this increase was the implication of this phage type in a number of food-related outbreaks. We compared fluorescent amplified fragment length polymorphism (FAFLP) to pulsed-field gel electrophoresis (PFGE), the current 'gold standard' for molecular typing of Salmonella for the discrimination between outbreak-associated isolates and epidemiologically unrelated DT126 strains. FAFLP showed a greater ability to discriminate between isolates than PFGE, with 16 groups of clusters or individual isolates with < 90% similarity to each other compared to three groups as determined by PFGE. Both methods were able to discriminate between isolates from two separate outbreaks in South Australia and isolates associated with an outbreak at a restaurant in New South Wales. The resolving power of both methods was not sufficient to separate all epidemiologically unrelated DT126 isolates from the outbreak isolates. We conclude that amplified fragment length polymorphism is a useful tool to assist in the discrimination of S. Typhimurium DT126 isolates.

Recognition of a B-cell epitope of the VapA protein of Rhodococcus equi in newborn and experimentally infected foals.

The aim of this study was to evaluate the usefulness of the previously identified B-cell epitope TSLNLQKDEPNGRASDTAGQ of the VapA protein of Rhodococcus equi and its association with R. equi pneumonia. A modified peptide designated PN11-14 corresponding to the epitope was recognized by all sera from experimentally infected foals with virulent R. equi ATCC103+ containing the virulence plasmid but not by its plasmid-cured derivative ATCC103- strain. Marked levels of VapA-specific immunoglobulin (Ig)G were detected in all sera from the ATCC103+ infected foals at 2 weeks after the infection. One control animal had high titres as determined by the peptide enzyme-linked immunosorbent assay (ELISA), indicating the ELISA may not absolutely differentiate between foals with R. equi pneumonia and healthy exposed foals in farms where the prevalence of disease is high. However, numbers of animals used were small. Further evaluation of the peptide ELISA with field samples is necessary to determine whether the assay is diagnostically useful. This study showed that levels of passive transfer of maternal IgG antibodies to the epitope in newborn foals could be measured. Interestingly, the maternally derived antibodies were found to significantly (P<0.05 by Student's t-test) decline 2 weeks after birth. Seroconversion against naturally occurring VapA expressing R. equi could be detected in some foals at 4 weeks of age. Antibodies to the epitope peaked and were significantly (P<0.05) greater in foals aged between 6 and 8 weeks. These results indicated that the peptide ELISA could be used to monitor anti-VapA antibodies in foals, particularly those at the age of 4-6 weeks. It is possible that the ELISA may be of some use as a diagnostic test on farms where R. equi is non-endemic. Further studies using large number of field samples are needed to verify this assumption.

Periplasmic maltose-binding protein confers specificity on the outer membrane maltose pore of Escherichia coli.

ompB mutants of Escherichia coli K-12 are markedly deficient in porin in their outer membrane. This results in a decreased rate of uptake for many substrates: the maltose pore (lambda receptor) can in some circumstances, in the absence of the periplasmic maltose-binding protein, compensate for the consequent defects in permeability to lactose, mannitol, glycylglycyl-L-valine, and tri-L-ornithine. It is postulated that the maltose-binding protein associates with the maltose pore and confers on it the specificity for maltose, and that the absence of the maltose-binding protein leaves the pore open and results in enhanced transmembrane diffusion of molecules other than maltose. This paper presents evidence to support this hypothesis.

The tsx protein of Escherichia coli can act as a pore for amino acids.

The tsx protein is known to be a specific diffusion pathway for nucleosides. The ability of this protein to facilitate the transport of molecules other than nucleosides was examined in strains lacking detectable amounts of porin (ompB mutants). The tsx protein was shown to promote serine, glycine, and phenylalanine transport and to have no effect on either glucose or arginine transport.

Colonization factor antigen CFA/IV (PCF8775) of human enterotoxigenic Escherichia coli: nucleotide sequence of the CS5 determinant.

Human enterotoxigenic Escherichia coli isolates expressing the colonization factor antigen CFA/IV (previously designated PCF8775) produce plasmid-encoded CS5 fimbriae. The nucleotide sequence of the region encoding the major CS5 fimbrial subunit was determined. The subunit is synthesized as a precursor of 203 amino acids (20.85 kDa) with a mature protein of 181 amino acids corresponding to a size of 18.6 kDa. The CS5 subunit shows homology to the corresponding component of porcine enterotoxigenic E. coli F41, particularly within the signal sequence and at the carboxy terminus.

B-Cell epitope mapping of the VapA protein of Rhodococcus equi: implications for early detection of R. equi disease in foals.

Linear B-cell epitopes of the Rhodococcus equi virulence-associated protein (VapA) were mapped using a synthetic peptide bank in this study. The peptides were screened in an enzyme-linked immunosorbent assay (ELISA) with a total of 70 sera from foals with current R. equi disease (51 sera), as well as from foals that had either recovered from R. equi infection 10 months previously (3 sera) or that had no known history of R. equi disease (16 sera). An epitope with the sequence NLQKDEPNGRA was identified and was universally recognized by all 51 sera from foals with R. equi disease and was not recognized by any of the other sera. There was poor reactivity between all sera and peptides relating to other areas of the VapA protein. It is proposed that an ELISA based upon a defined peptide epitope may be used in an improved serological diagnostic test for R. equi infection in foals.

Characterization of the small cryptic plasmid, pIMVS1, of Salmonella enterica ser. Typhimurium.

The nucleotide sequence has been determined of a small cryptic plasmid of Salmonella Typhimurium, designated pIMVS1, which correlated with an outbreak of gastroenteritis. This plasmid has a size of 3357 bp with no known functions. It does not encode any protein products but shows homology to several well-characterized plasmids. The replication system with RNAI and RNAII and the replication origin (oriV) is highly similar to that of plasmid p15A. It also has a putative mobilisation origin similar to that of ColE1.