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C J Thomson

Publications and source records attributed to C J Thomson.

At least 37 records · Page 2Linked to original sources
PubMed1996

The 'hidden' carbapenemase of Aeromonas hydrophila.

It has been presumed that there are just two beta-lactamases in the motile Aeromonas species, a carbapenemase and a cephalosporinase, based on the premise that all beta-lactamases can be detected by hydrolysis of the chromogenic cephalosporin, nitrocefin. However, when it was recently found that a non-motile species of Aeromonas that causes furunculosis in salmon, contained three beta-lactamases, one of which was a carbapenemase which could not be detected with nitrocefin, it was hypothesised that genetic exchange could occur between fish pathogens and human pathogens resulting in the transfer of the carbapenemase-encoding gene. This could have a potentially serious impact on intensive therapy units where carbapenems are employed. The purpose of this study was to determine whether the human pathogen Aeromonas hydrophila demonstrated the same beta-lactamase profile. After anion and cation exchange chromatography had been employed to separate the beta-lactamases of a clinical strain of A. hydrophila, three different beta-lactamases were found, one of which is a carbapenemase which does not hydrolyse nitrocefin. It is, therefore, probable that many strains of Aeromonas spp. contain a similar array of beta-lactamases which include a carbapenemase that cannot be detected with nitrocefin. Similar carbapenemases may well remain hidden in other species of bacteria unless appropriate techniques to detect the enzymes are employed.

Aeromonas hydrophila↗
PubMed1996

Mutations of the gyrA gene of clinical isolates of Salmonella typhimurium and three other Salmonella species leading to decreased susceptibilities to 4-quinolone drugs.

The region of the gyrA gene encoding nucleotides 72 to 557 in ten Salmonella typhimurium clinical isolates received from Vellore, India and six British clinical isolates of a number of Salmonella species has been sequenced. Those exhibiting a decreased susceptibility to 4-quinolone drugs (MICs of ciprofloxacin ranging from 0.128 mg/L to 1.0 mg/L) have been shown to possess a number of different mutations in this region of the gene (Asp-87 to Gly, Asp-87 to Asn and Ser-83 to Phe). The mutations Asp-87 to Gly and Ser-83 to Phe/Asp-87 to Asn have yet to be reported occurring in S. typhimurium.

Amino Acid Sequence↗
PubMed1996

Mutations responsible for reduced susceptibility to 4-quinolones in clinical isolates of multi-resistant Salmonella typhi in India.

Twelve isolates of Salmonella typhi isolated in Vellore, India had reduced susceptibility to 4-quinolones (MIC of ciprofloxacin 0.256 mg/L). One isolate was isolated in 1992 but the remaining 11 were isolated in 1994. The section of the gyrA gene from codons 24 to 185, which includes the "Quinolone Resistance Determining Region", was amplified by the polymerase chain reaction and, after separating the amplified strands, the DNA was sequenced directly. In the one isolate from 1992, no alterations were seen in this region of gyrA, compared with the ciprofloxacin-sensitive isolates and presumably decreased 4-quinolone susceptibility resulted from reduced permeability of the cell outer membrane or another mechanism. In nine isolates from 1994, a substitution of phenylalanine for serine at position 83 of GyrA correlated with the decrease in 4-quinolone susceptibility. In the remaining two isolates, the novel substitution of aspartate to tyrosine at position 87 was found; in one isolate this substitution was coupled with another substitution at position 83 but in the other it was not.

Amino Acid Sequence↗
PubMed1995

Beta-lactam resistance in normal faecal flora from South Africa.

The genetic and biochemical basis of ampicillin resistance amongst the aerobic Gram-negative commensal faecal flora of healthy volunteers in South Africa has been determined. Amongst 608 ampicillin resistant strains isolated from 320 of the participants, 158 were able to transfer their ampicillin resistant determinants into Escherichia coli K-12 J62-2. Iso-electric focusing of the beta-lactamases, extracted from the transconjugants, demonstrated that ampicillin resistance resulted from the presence of the TEM-1, TEM-2 and SHV-1 beta-lactamases in 94.3%, 2.5% and 3.2% of isolates respectively. Endonuclease restriction digests of the plasmids isolated from the transconjugants showed that the beta-lactamase genes were present on a wide variety of plasmid types; 101 distinct plasmid endonuclease restriction patterns were identified. Transferable ampicillin resistance was associated with resistance to other antibiotics at the following frequencies: trimethoprim (48.7%), streptomycin (35.4%), tetracycline (27.2%), spectinomycin (9.5%), chloramphenicol (3.2%) and gentamicin (1.3%). One antibiotic resistance pattern, ampicillin and trimethoprim, predominated (28%). In total, 77.9% of the plasmids conferred resistance to other antibiotics raising the possibility that use of any of these agents, not simply ampicillin, may contribute to the maintenance of resistance genes.

Adolescent↗
PubMed1995

Prevalence and genetic location of non-transferable trimethoprim resistant dihydrofolate reductase genes in South African commensal faecal isolates.

In a recent survey of trimethoprim resistance, 357 Gram-negative aerobic organisms were isolated from healthy volunteers from rural and urban populations in South Africa. Trimethoprim resistance did not transfer to an Escherichia coli J62-2 recipient strain by conjugation in a liquid mating in 161 (45.1%) of the isolates. These isolates which did not transfer their resistance were probed with intragenic oligonucleotide probes for the types Ia, Ib, IIIa, V, VI, VII, VIII, IX, X and XII dihydrofolate reductase genes. Contrary to all previous data, the most prevalent dihydrofolate reductase gene in this group of non-transferable isolates which hybridized, was the type VII (38%) followed by the type Ia (25%), Ib (12%), V (1.7%) and VIII (1.2%). None of the strains hybridized to the types IIIa, VI, XI, X and the XII dihydrofolate reductase probes. Southern blots of plasmid and chromosomal DNA from selective isolates revealed that the type VII dihydrofolate reductase genes were located on the chromosome and were associated with the integrase gene of Tn21. However, the type Ib and V dihydrofolate reductase genes were all found on plasmids which could not be mobilized. The type Ia dihydrofolate reductase genes were found on both non-transferable plasmids and on the chromosome. The nature of the genetic structures associated with a dihydrofolate reductase gene strongly affects the means of spread of the gene in a population.

Base Sequence↗
PubMed1995

Back mutations to the TEM-1 beta-lactamase from TRC-1 lead to restored sensitivity to clavulanic acid.

Back mutations from the TRC-1 beta-lactamase to the TEM-1 enzyme were selected in vitro. The revertant beta-lactamase was obtained from Escherichia coli strain J62.2 carrying plasmid pUK901 which encodes the TRC-1 beta-lactamase. The revertant was obtained after repeated subculture of E. coli J62.2 (pUK901) in amoxycillin 512 mg/L for 5 days. The revertant beta-lactamase had the same pI as TEM-1 (5.4) and had restored inhibition by clavulanic acid (ID50 reduced from 4.2 microM to 0.15 microM). The prevalence of these beta-lactamases in the clinical population may be the result of a two-way flux, with mutations in both forward and backward directions.

Amoxicillin↗
PubMed1995

A piperacillin-tazobactam resistant Escherichia coli strain isolated from a faecal sample of a healthy volunteer.

As part of a surveillance programme of the prevalence of antibiotic resistance, the faecal bacteria of healthy people (n = 1348) were examined, and the antibiotic resistance of the Escherichia coli strains determined. One strain out of 142 amoxycillin-resistant isolates, E. coli strain 1662, was also resistant to piperacillin-tazobactam but susceptible to amoxycillin-clavulanic acid. The piperacillin-tazobactam resistance determinant was transferable to standard E. coli strains by conjugation. However, the strain produced a beta-lactamase with several characteristics very similar to those of the TEM-1 beta-lactamase, i.e. pI of 5.4, an M(r) value of 22,000 and a comparable substrate profile. The enzyme was as efficiently inhibited by clavulanic acid and tazobactam as the TEM-1 and TEM-2 beta-lactamases but more than the amoxycillin-clavulanic acid-resistant TRC-1 enzyme. The transferable resistance to piperacillin-tazobactam appears to be mediated by a novel resistance mechanism that has previously not been described.

Anti-Bacterial Agents↗
PubMed1994

Three beta-lactamases isolated from Aeromonas salmonicida, including a carbapenemase not detectable by conventional methods.

The beta-lactamases of seven strains of Aeromonas salmonicida subsp. achromogenes resistant to amoxicillin (MIC > 1024 mg/l) and responsible for furunculosis in farmed Atlantic salmon in Scotland were examined to establish the mechanisms of beta-lactam resistance. Separation of a cell-free extract on an isoelectric focusing gel stained with the chromogenic cephalosporin nitrocefin showed the presence of two beta-lactamases, one with a pI of 7.9 and the other with a pI of 6.0. Hydrolysis assays of cell-free extracts of these strains demonstrated carbapenemase, penicillinase and cephalosporinase activity. However, when the beta-lactamases were separated by anion exchange chromatography, the carbapenemase activity could not be retrieved in either of the peak fractions containing the separated enzymes that had been visualised by nitrocefin. Consequently, a novel carbapenemase was discovered which cannot be detected with nitrocefin.

Aeromonas↗
PubMed1994

Beta-lactam resistance in aerobic faecal flora from general practice patients in the UK.

One hundred faecal specimens submitted to a diagnostic laboratory in Edinburgh and found to be negative for gastrointestinal pathogens were examined for the presence of antibiotic-resistant bacteria. The results were compared with findings in the healthy population in the same area. The highest incidence of resistance was observed to cefuroxime (65%) and ampicillin (60%). Of the ampicillin-resistant isolates, 62% could transfer their resistance determinants to a standard Escherichia coli host strain. In 100% of these transconjugants ampicillin resistance was shown to result from the presence of the TEM-1 beta-lactamase which was identified in a heterogeneity of plasmid profiles. These plasmids commonly mediated resistance to streptomycin and tetracycline in addition to ampicillin.

Ampicillin Resistance↗
PubMed1993

Dissemination of the TEM-I beta-lactamase gene into disparate plasmids in faecal strains of Escherichia coli isolated in Vellore, south India.

The genetic basis for ampicillin resistance in commensal strains of Escherichia coli isolated in Vellore, south India has been examined. Of the 58 strains tested, 41% could transfer ampicillin resistance to a standard E. coli host strain. With the exception of one isolate, transferable ampicillin resistance was shown to result from the presence of the TEM-I beta-lactamase which was found on a wide variety of plasmid types.

Ampicillin Resistance↗
PubMed1993

Molecular epidemiology of the plasmid-encoded TEM-1 beta-lactamase in Scotland.

A survey of the beta-lactamases responsible for ampicillin resistance in urinary Escherichia coli isolated in central Scotland has been performed. The TEM-1 beta-lactamase was found to be most prevalent occurring in 88.2% of ampicillin-resistant isolates tested. Forty-six percent of the TEM-1 producing strains were able to transfer this resistance to E. coli J62-2 at 37 degrees C. Analysis of the resulting transconjugants revealed that the degree of resistance to amoxycillin and amoxycillin in combination with clavulanic acid was related to the specific activity of the TEM-1 beta-lactamase. The variation in specific activity was shown to be related to plasmid type as determined by restriction analysis. No obvious relationship between beta-lactamase specific activity and resistance to amoxycillin and amoxycillin plus clavulanic acid could be demonstrated in the original plasmid donor strains.

Amoxicillin↗
PubMed1993

The prevalence of antimicrobial resistance in human faecal flora in South Africa.

Between January and March 1992, 361 faecal specimens were collected from the healthy black population in the Transvaal Province of South Africa. Each specimen was examined for the prevalence of antimicrobial resistance in commensal bacteria. Volunteers, from both rural and urban dwellings, were divided into four age groups. The overall carriage rate of resistance varied from 88.6% for ampicillin, 74.2% for trimethoprim, 52.6% for chloramphenicol, 10.2% for nalidixic acid to 7.5% for gentamicin. The carriage of resistance found to each individual antimicrobial agent was slightly higher in the rural population rather than the urban population but there was no correlation between the prevalence of antimicrobial resistance and the age group.

Adolescent↗
PubMed1993

Selection of variants of the TEM-1 beta-lactamase, encoded by a plasmid of clinical origin, with increased resistance to beta-lactamase inhibitors.

A TEM-1 derived variant beta-lactamase with increased resistance to beta-lactamase inhibitors was selected in vitro. The variant beta-lactamase was obtained from the parent strain Escherichia coli J62-2 containing resistance plasmid R1 which encodes the TEM-1 beta-lactamase. The variants were obtained by repeated subculture of E. coli J62-2 (R1) for five days in the presence of sub-inhibitory concentrations of amoxycillin plus clavulanic acid. Comparison of this variant beta-lactamase with a clinically isolated TEM derived beta-lactamase, with increased resistance to beta-lactamase inhibitors, suggests that they are the same enzyme. Both beta-lactamases have the same pI at 5.25 and have similar ID50 values for the beta-lactamase inhibitors clavulanic acid, sulbactam and tazobactam.

Amoxicillin↗
PubMed1993

The role of thymine starvation in the expression of type IV plasmid-encoded trimethoprim-resistant dihydrofolate reductase.

Hyperproduction of the type IV plasmid-encoded dihydrofolate reductase was studied in Escherichia coli J62-2 (pUK1123). Hyperproduction of the enzyme was shown to occur not simply as a response to a given concentration of trimethoprim but also to the presence of thymidine in the medium. Before hyperproduction occurred the bacteria began to elongate and die, thus showing the symptoms of thymine starvation. Hyperproduction also required the presence of L-methionine, adenine and glycine, suggesting that the elevated production of the enzyme was a response to the ability of trimethoprim to starve the cell of thymine metabolites.

Adenine↗