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D M Livermore

Publications and source records attributed to D M Livermore.

At least 91 records · Page 5Linked to original sources
PubMed1999

Biochemical characteristics of a carbapenemase from an Acinetobacter baumannii isolate collected in Buenos Aires, Argentina.

Three carbapenem-resistant Acinetobacter baumannii isolates were collected at a hospital in Buenos Aires, Argentina. Isoelectric focusing revealed multiple beta-lactamases, with two of the isolates showing identical profiles. A pI 6.9 carbapenemase with a molecular weight of 30 kDa was purified from one of these two isolates. The enzyme was predominantly a penicillinase, with its highest Vmax for oxacillin but highest Vmax/Km for benzylpenicillin. First-generation cephalosporins and imipenem were weaker substrates than penicillins, and oxyimino-aminothiazolyl cephalosporins were essentially stable. Meropenem-hydrolysing activity was not detected, despite resistance. The carbapenemase was inhibited by clavulanic acid and tazobactam, but not by EDTA. These kinetics place the enzyme into functional group 2; as an oxacillinase it could be placed in sub-group 2d or, as a zinc-independent carbapenemase, in sub-group 2f.

Acinetobacter↗
PubMed1999

Laboratory mutants of OXA-10 beta-lactamase giving ceftazidime resistance in Pseudomonas aeruginosa.

Several extended-spectrum beta-lactamases (ESBLs) belonging to molecular Class D have been described from Pseudomonas aeruginosa isolates collected in Turkey. Four of these, OXA-11, -14, -16 and -17, are derivatives of OXA-10 beta-lactamase. We tried to select similar mutants in vitro from OXA-10-producing transconjugants of P. aeruginosa, using a multistep method on ceftazidime-containing agars. Forty-four such mutants were obtained; all had increased resistance to ceftriaxone, cefsulodin, cefepime, cefpirome, latamoxef, aztreonam and, especially, ceftazidime whereas MICs of piperacillin, carbenicillin, cefotaxime, cefoperazone and carbapenems were little altered. Genes related to blaOXA-10 were sequenced from five mutants. One mutant enzyme had aspartate instead of glycine at position 157, and corresponded exactly to natural OXA-14 beta-lactamase. Another mutant strain appeared to have both OXA-14 and a new pI 6.2 enzyme, designated OXA-M102, with serine instead of alanine at position 124 and aspartate instead of glycine at position 157. This latter variant resembled natural OXA-16 enzyme, which has threonine at position 124 and aspartate at position 157. The remaining three mutant enzymes differed from any so far found in wild-type isolates. Two had leucine replacing tryptophan at position 154 (this enzyme was named OXA-M101) while the third (OXA-M103) had a pI of 7.6, and had lysine instead of asparagine at position 143. A different mutation at this position was previously found in OXA-11, a wild-type OXA-10 mutant. Thus, some of the ESBL mutants selected (OXA-14 and OXA-M102) correspond exactly or almost exactly to ESBLs found in wild-types, whereas others (OXA-M101 and OXA-M103) were totally new.

Ceftazidime↗
PubMed1999

Quality of antimicrobial susceptibility testing in the UK: a Pseudomonas aeruginosa survey revisited.

As part of a programme to assess the usefulness of routine antimicrobial susceptibility data as a surveillance tool, we reviewed the results of a national survey of resistance in Pseudomonas aeruginosa, undertaken in 1993. Twenty-four UK laboratories contributed isolates for centralized MIC testing, indicating also their own susceptibility test data. As reported previously (Chen et al. (1995) Journal of Antimicrobial Chemotherapy 35, 521-34), the rate of false resistance (isolates reported susceptible, but found resistant on MIC testing/all isolates reported susceptible) was 0.6-8%, according to the antimicrobial and breakpoint. Review showed that this favourable position reflected the fact that >88% of isolates were susceptible to any given antimicrobial and--in most cases--were correctly reported as such. Reporting was more erratic for resistant isolates: for beta-lactams and amikacin, isolates resistant at the highest MIC breakpoints were equally likely to be reported as 'susceptible' or 'resistant'; such misreporting was less common with ciprofloxacin and gentamicin but still occurred in 9-20% of cases. Conversely, up to 73% of the isolates reported as resistant proved to be susceptible at high breakpoints, and up to 44% were susceptible at low breakpoints. Miscategorizations did not reflect failure to detect particular mechanisms but, rather, the fact that MIC and zone breakpoints for P. aeruginosa serve to cut 'tails' of resistant organisms from continuous distributions, not to distinguish discrete populations. In this situation, some disagreement between routine tests and MICs is inevitable, but the frequency at which highly resistant isolates were reported as sensitive is disturbing. For surveillance, we conclude that resistance rates based on routine tests are unreliable for P. aeruginosa. This situation may improve with greater standardization of routine testing, but the continuous susceptibility distributions without discrete resistant and susceptible populations militate against perfect agreement. Despite these deficiencies, routine data should allow trend analysis.

Anti-Bacterial Agents↗
PubMed1999

In vitro activities of antibiotics alone and in combination against Brucella melitensis at neutral and acidic pHs.

Brucellae survive acidic pHs in phagolysosomes. Azithromycin, streptomycin, and quinolones were active against Brucella melitensis at pH 7.0 but not at pH 5.0; rifampin and doxycycline retained activity at pH 5.0. Regardless of pH, azithromycin-rifampin and ofloxacin-rifampin showed less synergy than established streptomycin-doxycycline and rifampin-doxycycline combinations.

Azithromycin↗
PubMed1999

OXA-17, a further extended-spectrum variant of OXA-10 beta-lactamase, isolated from Pseudomonas aeruginosa.

Pseudomonas aeruginosa isolates 871 and 873 were isolated at Hacettepe University Hospital in Ankara and were highly resistant to ceftazidime (MIC, 128 microg/ml). Each produced three beta-lactamases, with pIs of 5.3, 6.1, and 7.9. The beta-lactamase with a pI of 5.3 was previously shown to be PER-1 enzyme. The antibiograms of the isolates were not entirely explained by production of PER-1 enzyme, insofar as ceftazidime resistance was incompletely reversed by clavulanate. The enzymes with pIs of 6.1 and 7.9 were therefore investigated. The enzyme with a pI of 6.1 proved to be a novel mutant of OXA-10, which we designated OXA-17, and had asparagine changed to serine at position 73 of the protein. When cloned into Escherichia coli XL1-blue, OXA-17 enzyme conferred greater resistance to cefotaxime, latamoxef, and cefepime than did OXA-10, but it had only a marginal (two- to fourfold) effect on the MIC of ceftazidime. This behavior contrasted with that of previous OXA-10 mutants, specifically OXA-11, -14, and -16, which predominately compromise ceftazidime. Extracted OXA-17 enzyme had relatively greater activity than OXA-10 against oxacillin, cloxacillin, and cefotaxime but, in terms of kcat/Km, it had lower catalytic efficiency against most beta-lactams. The enzyme with a pI of 7.9 was shown by gene sequencing to be OXA-2.

Anti-Bacterial Agents↗
PubMed1998

Unusual tazobactam-sensitive AmpC beta-lactamase from two Escherichia coli isolates.

Two Escherichia coli isolates were studied. MIC patterns and hydrolysis assays suggested that they hyperproduced AmpC beta-lactamase, but synergy between ceftazidime and tazobactam was greater than between ceftazidime and Ro 48-1256, whereas the converse pattern is typical of AmpC hyperproducers. Studies with purified beta-lactamase from one of the isolates confirmed that tazobactam was a 100-fold stronger inhibitor than for the classical E. coli AmpC enzyme. Moreover, in contrast to typical AmpC types, the new enzyme had greater affinity for cephaloridine than for benzylpenicillin.

Bacterial Proteins↗
PubMed1998

Can susceptibility to an antimicrobial be restored by halting its use? The case of streptomycin versus Enterobacteriaceae.

To test the widespread view that resistance disappears in the absence of antimicrobial use, we tested streptomycin against 477 Enterobacteriaceae from the Royal London Hospital. Twenty per cent proved resistant although streptomycin is little used at the hospital and streptomycin resistance in gram-negative bacteria is caused by mechanisms that do not compromise the drugs that are used. Up to 70% of the observed resistance was associated with cross-resistance to spectinomycin and the presence of ant(3")-Ia, an integron-associated gene carried in Tn21-type transposons. This genetic organization may have conserved streptomycin resistance in the absence of direct selection pressure.

Anti-Bacterial Agents↗
PubMed1998

Epidemiological typing of klebsiellae with extended-spectrum beta-lactamases from European intensive care units.

Extended-spectrum beta-lactamases (ESBLs) are an increasing cause of resistance to oxyimino-aminothiazolyl cephalosporins, especially in klebsiellae. In a recent survey we detected ESBLs in 220 (23%) of 966 consecutive klebsiellae from 35 intensive care units (ICUs) in southern and western Europe. The present study examined the extent to which this distribution reflected epidemic strain spread, as against the distribution of ESBL genes into unrelated strains. All 220 ESBL producers were subjected to capsular serotyping and pulsed-field gel DNA electrophoresis (PFGE). Beta-Lactamases were typed for strains isolated on three or more occasions, with the emphasis on SHV enzymes, as these were commoner than TEM variants. Serotyping and PFGE typing defined 85 distinct strains, from 23 of the 35 participating centres. Of 14 centres that contributed five or more ESBL producers, all sent representatives of more than one strain, and two centres sent members of ten or more different strains in contributions of 17-21 ESBL-producing isolates. Nevertheless, epidemic strains-defined as those represented by three or more isolates-accounted for a majority (61%) of the collection. Fifty-two isolates of the same serotype K25 (occasionally acapsular) strain with SHV-4 beta-lactamase were recovered at two French hospitals and one in Belgium. This strain has been found by others in France, and has become particularly widespread. Another single strain was found in two separate Portuguese centres, and many individual hospitals had one or more epidemic strain(s), as well as a scatter of diverse ESBL producers. Major variation in antibiogram and plasmid profile was apparent within strains, with some intra-strain variation in beta-lactamase type. These data imply a fluid situation, with resistance determinants being gained, modified or lost. The endemicity of ESBL producers is disturbing since it limits the potential for control by blocking strain spread, while the diversity within strains is disturbing because it complicates the design of antibiotic policies even during 'single strain' outbreaks.

Anti-Bacterial Agents↗
PubMed1998

Beta-lactamase-mediated resistance and opportunities for its control.

Clinical use of beta-lactams has selected for beta-lactamase-producing organisms. Numerous beta-lactamases are known, and sequencing allows them to be divided into four Classes, A to D, with Classes A and C being the most important. Pharmaceutical chemists have responded to the spread of beta-lactamase-producing organisms by developing stable agents and inhibitors. Stability in penicillins and cephalosporins is achieved by attaching a bulky substituent to the amino group of 6-aminopenicillanic acid or 7-aminocephalosporanic acid, or by replacing the hydrogen on carbon 6 (penicillins) or 7 (cephalosporins) with an alpha-methoxy group. In carbapenems, stability is achieved by incorporation of a simple trans-6-hydroxyethyl group. Beta-lactamase-inhibitory activity occurs in many beta-lactam classes but only clavams and penicillanic acid sulphones have been developed specifically as beta-lactamase inhibitors. These inhibit most Class A and some Class D enzymes but act poorly against Class B and C enzymes. Their success is affected by the amount of enzyme, the permeability of the bacterial cell wall, the partner beta-lactam and the pH. Piperacillin/tazobactam, which combines a good inhibitor of Class A enzymes with a broad-spectrum, easily-protected penicillin, has wide activity against common pathogens, the major exceptions being strains of Enterobacter, Serratia and Citrobacter freundii that produce large amounts of Class C enzymes, and Gram-positive cocci with modified penicillin-binding proteins. Beta-lactamase-stable beta-lactams and inhibitor combinations overcome many existing resistance mechanisms but are themselves selecting new resistances. Few new beta-lactams able to overcome these resistances are advanced in development and consequently the opportunities for control lie mostly in the more prudent use of compounds already available.

Anti-Bacterial Agents↗