Search PubMed⌕ Search

Biomedical subjects

S G Amyes

Publications and source records attributed to S G Amyes.

At least 37 records · Page 2Linked to original sources
PubMed1998

Genes and spectrum: the theoretical limits.

Antibiotic resistance can result either from mutations within a chromosomal gene or from mobile genes imported from outside. In the last 15 years, some of these mobile genes have shown a propensity to adapt to successive antibiotic challenges, the most versatile being the class A beta-lactamases. The TEM and SHV beta-lactamase nuclei, usually after one initial critical mutation, allow a series of successive mutations that increase the spectrum to hydrolyze most cephalosporins. The class C beta-lactamases also show some versatility; while it migrates from the chromosome, subtle changes can occur in the gene to broaden the spectrum. Trimethoprim resistance has shown less adaptability in gram-negative bacteria, but in gram-positive organisms the plasmid has captured the chromosomal dihydrofolate reductase of Staphylococcus epidermidis, and a minimal number of changes have occurred that decrease the binding of trimethroprim. Other resistance mechanisms appear less adaptable, relying rather on the importation of new genes to cope with new challenges.

Amino Acid Sequence↗
PubMed1998

New trimethoprim-resistant dihydrofolate reductase cassette, dfrXV, inserted in a class 1 integron.

The nucleotide sequence of a plasmid-borne trimethoprim resistance gene from a commensal fecal Escherichia coli isolate revealed a new dihydrofolate reductase gene, dfrXV, which occurred as a gene cassette integrated in a site-specific manner in a class 1 integron. The new gene shows 84% nucleotide identity and the predicted protein shows 90% amino acid identity with dfrI and DHFR type I, respectively. Genes for spectinomycin resistance, aadA1 [ant (3")-Ia], and sulfonamide resistance, sulI, were located downstream of dfrXV in a manner identical to that in pLMO229.

Amino Acid Sequence↗
PubMed1998

Molecular analysis of and identification of antibiotic resistance genes in clinical isolates of Salmonella typhi from India.

A representative sample of 21 Salmonella typhi strains isolated from cultures of blood from patients at the Christian Medical College and Hospital, Vellore, India, were tested for their susceptibilities to various antimicrobial agents. Eleven of the S. typhi strains possessed resistance to chloramphenicol (256 mg/liter), trimethoprim (64 mg/liter), and amoxicillin (>128 mg/liter), while four of the isolates were resistant to each of these agents except for amoxicillin. Six of the isolates were completely sensitive to all of the antimicrobial agents tested. All the S. typhi isolates were susceptible to cephalosporin agents, gentamicin, amoxicillin plus clavulanic acid, and imipenem. The antibiotic resistance determinants in each S. typhi isolate were encoded by one of four plasmid types. Plasmid-mediated antibiotic resistance genes were identified with specific probes in hybridization experiments; the genes responsible for chloramphenicol, trimethoprim, and ampicillin resistance were chloramphenicol acetyltransferase type I, dihydrofolate reductase type VII, and TEM-1 beta-lactamase, respectively. Pulsed-field gel electrophoresis analysis of XbaI-generated genomic restriction fragments identified a single distinct profile (18 DNA fragments) for all of the resistant isolates. In comparison, six profiles, different from each other and from the resistance profile, were recognized among the sensitive isolates. It appears that a single strain containing a plasmid conferring multidrug-resistance has emerged within the S. typhi bacterial population in Vellore and has been able to adapt to and survive the challenge of antibiotics as they are introduced into clinical medicine.

Chloramphenicol O-Acetyltransferase↗
PubMed1998

Transferable trimethoprim resistance in Shigella strains.

The susceptibility patterns of 35 Shigella isolates (16 S. flexneri, 14 S. dysenteriae and 5 S. sonnei) to trimethoprim (Tp) and various antibiotics including amoxycillin, amoxycillin-clavulanic acid, nalidixic acid, ciprofloxacin, ceftazidime and ceftriaxone, were investigated. Twenty-two (62.8%) strains were resistant to Tp with a minimal inhibitory concentration (MIC50) value of 512 mg/L. Only six isolates were amoxycillin resistant, to which clavulanic acid restored sensitivity in all of them. None of the isolates were resistant either to extended spectrum cephalosporins or to quinolones. Resistance to Tp was transferred from 7 of the 22 isolates (31.8%) to the recipient Escherichia coli K12. Tp MIC values of the transconjugants were 512 mg/L. In no strain could amoxycillin resistance be transferred. Our results indicate that as the prevalence of transferable Tp resistance in Shigella isolates in Izmir is substantially high, alternative antimicrobial agents should be considered for empirical antibiotic therapy.

Amoxicillin↗
PubMed1998

Strategies and options for minimizing resistance emergence in pulmonary infections.

The early-onset hospital pulmonary gram-negative infections may respond to ciprofloxacin and co-amoxiclav without significant resistance development. Penicillin-resistant Streptococcus pneumoniae may be treated with macrolides, fluoroquinolones, and glycopeptides. The late-onset hospital pathogens all seem to have developed resistance to cephalosporins, so greater reliance is now made on the fluoroquinolones and carbapenems when aminoglycoside therapy is considered undesirable.

Acinetobacter↗
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

TEM- and SHV-derived extended-spectrum beta-lactamases: relationship between selection, structure and function.

The later-generation cephalosporins were developed to overcome beta-lactamases which conferred resistance to earlier beta-lactam drugs. Within two years of their clinical introduction, the ubiquitous TEM and SHV plasmid-encoded beta-lactamase genes underwent simple point mutations that changed key amino acids around the active site of the protein and enabled the enzyme to bind and hydrolyse these new drugs. Successive mutations interacted in concert, radically increasing the enzymes' abilities to bind and confer resistance to later-generation cephalosporins. These modified enzymes have been classified in three groups, based on activity, and altered functions have been correlated with changes in the enzyme structure.

Amino Acid Sequence↗