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Biomedical subjects

D M Livermore

Publications and source records attributed to D M Livermore.

At least 163 records · Page 9Linked to original sources
PubMed1991

Invalidity for Pseudomonas aeruginosa of an accepted model of bacterial permeability to beta-lactam antibiotics.

The accepted model for the penetration of beta-lactam antibiotics into gram-negative bacteria is that proposed by Zimmermann and Rosselet (Antimicrob. Agents Chemother. 12:368-372, 1977). The model assumes (i) that diffusion of the antibiotic molecules across the outer membrane obeys Fick's law and can be characterized by a permeability constant for any given combination of organism and drug, (ii) that drug hydrolysis within the periplasm obeys Michaelis-Menten kinetics, and (iii) that a steady state is rapidly attained between drug uptake and hydrolysis. The model has allowed accurate prediction of antibiotic MICs for Escherichia coli strains from a knowledge of their beta-lactamase production and permeability characteristics. It has been suggested that the model is inappropriate for Pseudomonas aeruginosa, but attempts to confirm this have been bedevilled by experimental difficulties in estimating permeability coefficients for this species. In the present study, we tested a prediction of the model that the overall resistance of P. aeruginosa transconjugants containing a plasmid-encoded beta-lactamase should continue to depend partly on permeability. Transconjugants with PSE-4 beta-lactamase were constructed in host strains with widely different levels of intrinsic, presumably impermeability-determined resistance. Contrary to the prediction of the model, all the transconjugants developed identical overall levels of resistance to substrate beta-lactams, such as azlocillin and cefoperazone, irrespective of the initial levels of intrinsic resistance of the recipient strains. We conclude that the model is inappropriate for P. aeruginosa, and possible explanations for the organism's behavior are discussed.

Anti-Bacterial Agents↗
PubMed1991

Mechanisms of resistance to beta-lactam antibiotics.

The use of new beta-lactam antibiotics has led to selection of novel forms of resistance. Newer-generation cephalosporins, ureidopenicillins and monobactams, but not carbapenems (e.g. imipenem), are labile to Class I chromosomal beta-lactamases and tend to select mutants of E. cloacae and P. aeruginosa that constitutively synthesize high levels of these enzymes. Selection occurs more rarely with Serratia, Citrobacter and indole positive protease. Third-generation cephalosporins and monobactams, but not carbapenems and cephamycins, are inactivated by a range of new variants of the well-known TEM and SHV-1 plasmid-mediated beta-lactamases. These variants have arisen by spontaneous mutation and differ only marginally in amino-acid sequence from their parent enzymes. Already prevalent in France, their potential for spread is a major concern. Outer membrane impermeability is another source of resistance to beta-lactams in Gram-negative bacteria. Rare in enterobacteria, it is more important in P. aeruginosa, where broad and narrow spectrum forms can arise via mutation. The narrow spectrum form involves loss of the D2 outer membrane protein and gives resistance to carbapenems but not other beta-lactams. The broad spectrum form, of unknown mechanism, simultaneously affects many beta-lactam and unrelated (e.g. quinolone) drugs, but not imipenem. Target (penicillin-binding protein) insensitivity also can mediate resistance, most notably in MRSA. Unlike the mechanisms described above, this affects all beta-lactams, including carbapenems as well as penicillins and cephalosporins.

Anti-Bacterial Agents↗
PubMed1990

Adaptive resistance to aminoglycoside antibiotics from first-exposure down-regulation.

Adaptive resistance to the bactericidal effect of an aminoglycoside antibiotic was induced in Pseudomonas aeruginosa and other aerobic gram-negative bacilli by initial exposure to the drug. Both subinhibitory and inhibitory concentrations produced resistance in bacterial cells surviving the effects of the initial ionic binding. Development of drug refractoriness required an adaptive period of growth, was enhanced by the continued presence of drug, and reversed after several hours of growth in drug-free medium. Unstable resistance was not explained by selection of mutants. The mechanism of adaptive resistance was down-regulation of aminoglycoside uptake during the period of accelerated energy dependent drug transport (EDP II). Down-regulation induced by gentamicin or tobramycin produced cross-resistance to other aminoglycosides. The kinetics of unstable first-exposure resistance suggests that a continuous drug level might not provide the most effective therapy with aminoglycosides and gives rationale to larger initial and longer interval bolus dosing.

Anti-Bacterial Agents↗
PubMed1990

Interactions of tazobactam and clavulanate with inducibly- and constitutively-expressed Class I beta-lactamases.

Clavulanate and tazobactam (YTR 830) were tested as inhibitors and inducers of the AmpC-type Class I beta-lactamases of Pseudomonas aeruginosa, Enterobacter cloacae, Citrobacter freundii, Serratia marcescens, Morganella morganii and the Ic beta-lactamase of Proteus vulgaris. Both clavulanate and tazobactam inhibited the Pr. vulgaris Class Ic beta-lactamase and potentiated ticarcillin and piperacillin against beta-lactamase derepressed variants of this species. Tazobactam, but not clavulanate, also had some ability to inhibit the AmpC Class I enzymes of M. morganii, C. freundii, Ps. aeruginosa, E. cloacae and S. marcescens. The piperacillin + tazobactam combination, unlike ticarcillin + clavulanate, showed some degree of synergy against most derepressed strains of these species. This behaviour partly depended upon the greater inhibitory activity of tazobactam for the enzymes, but also on piperacillin being easier to potentiate than ticarcillin. The synergy between piperacillin and tazobactam was greatest for M. morganii and C. freundii, least for Ps. aeruginosa and E. cloacae. Unfortunately, it is in the last two species that these enzymes pose the greatest resistance threat. Tazobactam caused little or no antagonism of piperacillin against beta-lactamase inducible species, whereas clavulanate antagonized ticarcillin against beta-lactamase inducible strains of E. cloacae and M. morganii (not other species). The antagonism of ticarcillin was attributable to beta-lactamase induction. The lack of antagonism with the tazobactam+piperacillin combination was related to tazobactam being a weaker inducer than clavulanate, not to piperacillin being less susceptible to antagonism than ticarcillin.

Bacteria↗
PubMed1990

Response of chemostat cultures of Pseudomonas aeruginosa to carbapenems and other beta-lactams.

Cultures of Pseudomonas aeruginosa were grown at different rates in a chemostat and challenged continuously or intermittently with ceftazidime, imipenem, meropenem or piperacillin. The killing rate was related to the bacterial growth rate; fast-growing cells being killed more rapidly than slow-growing ones. Mutants that were stably derepressed (i.e. constitutive) for chromosomal beta-lactamase expression were selected when a beta-lactamase inducible (i.e. typical) strain was challenged with ceftazidime or piperacillin. Addition of the beta-lactamase inhibitor tazobactam to piperacillin did not prevent selection. There was a lag of c. 24-48 h post-challenge before totally derepressed mutants were detectable. Once selected, the derepressed organisms were stable and were not outgrown by inducible cells if these were readded in the absence of selective antibiotics. Selection of resistant mutants was not observed with imipenem, despite the known tendency of this drug to select carbapenem-impermeable mutants of P. aeruginosa in vivo. Imipenem, but not ceftazidime or meropenem, caused a significant post-antibiotic effect after single or repeated dosage.

Anti-Bacterial Agents↗
PubMed1990

Resistance mechanisms of multiresistant serotype 012 Pseudomonas aeruginosa isolated in Europe.

Serotype 012 Pseudomonas aeruginosa resistant to gentamicin (MIC greater than 4 mg/l) and carbenicillin (MIC greater than 128 mg/l) occur widely in Europe and are homogeneous in their phenotypic and genetic properties. It has been suggested that a single multiresistant strain of this serotype has become widespread. This study examined the resistance mechanisms present in multiresistant serotype 012 P. aeruginosa isolates from Austria, Belgium, France, Greece, Italy, Holland, West Germany and the UK. Disseminated isolates produced a PSE-1 type beta-lactamase, correlating with their resistance to the known substrates of this enzyme (carbenicillin, azlocillin and cefsulodin). These isolates also reacted with gene probes for the aminoglycoside modifying enzymes AAC(6')I and ANT(3'). The probe for AAC(6')I is known to cross-react with the gene for AAC(6')II. The fact that the organisms were resistant to netilmicin, gentamicin, sisomicin and tobramycin, but less so to amikacin, suggested that the latter enzyme was produced rather than AAC(6')I. PSE-1 beta-lactamase and the gene for AAC(6')I/AAC(6')II were absent from the International Antigenic Typing Scheme 012 reference strain, which was sensitive to beta-lactams and aminoglycosides, and also from a multiresistant serotype 012 strain isolated at a London burns unit in 1987. These organisms have been shown previously to be distinct from the disseminated multiresistant strain in their phenotypic properties. Two 012 isolates appeared to have additional resistance determinants to amikacin and isepamicin.

Aminoglycosides↗
PubMed1990

Selection and transformation of non-beta-lactamase-mediated insusceptibility to beta-lactams in Haemophilus influenzae: lack of cross-resistance between carbapenems and other agents.

Non-beta-lactamase-mediated insusceptibility to beta-lactams was studied in Haemophilus influenzae. Mutants resistant to ampicillin were selected in a susceptible isolate, while phenotypically similar resistance was transformed from four wild-type isolates into a susceptible recipient. Ampicillin-selected mutants generally had reduced susceptibility to penicillins, cephalosporins and aztreonam. Reduced susceptibility to meropenem was less common, and susceptibility to imipenem was unaffected. MICs of three cephalosporins and aztreonam were much higher for the ampicillin-resistant transformants than for the recipient strain, and were generally equal to, or within one doubling dilution of, the concentrations that inhibited the donor parents. In contrast, reduced susceptibility to imipenem observed with one parent (MIC of 4 mg/l) was not transferred to the recipient (MIC of 0.5 mg/l). Meropenem inhibited 22 of 24 transformants tested at less than or equal to 0.12 mg/l, compared with MICs of greater than or equal to 0.5 mg/l for the donor parents and 0.03 mg/l for the recipient. These results indicated that the mechanisms that confer non-beta-lactamase-mediated insusceptibility to penicillins, cephalosporins and aztreonam in H. influenzae have little or no effect on susceptibility to carbapenems.

Ampicillin Resistance↗
PubMed1990

Biochemical characterization of a beta-lactamase that hydrolyzes penems and carbapenems from two Serratia marcescens isolates.

Reexamination of Serratia marcescens isolates obtained in 1982 revealed two organisms that were resistant to the penem FCE 22101 (MIC, 512 micrograms/ml) and imipenem (MIC, 16 micrograms/ml) and that had slightly reduced susceptibilities to meropenem (MIC, 0.12 micrograms/ml). MICs of these agents for typical S. marcescens isolates were 1 to 8, 0.25 to 0.5, and 0.03 micrograms/ml, respectively. The two isolates were fully susceptible to broad-spectrum cephalosporins, and only one was highly resistant to ampicillin and carbenicillin (MICs, greater than 1,024 micrograms/ml). Both isolates had beta-lactamases that focused at pIs 8.2 and 9.7. The penicillin-resistant isolate additionally produced the TEM-1 enzyme. The enzymes with pIs of 8.2 and 9.7 were separated by cation-exchange chromatography. The pI 8.2 beta-lactamase was a class I enzyme of the type found in most S. marcescens isolates. It was almost inactive against carbapenems and penems, as was the class I enzyme from another S. marcescens strain. The pI 9.7 enzyme hydrolyzed penems and carbapenems rapidly: kcat (turnover number) values for FCE 22101, imipenem, and meropenem were 3.4, 26, and 1% of the kcat value for cephaloridine, respectively; kcat/Km values were 140, 915, and 150% of the kcat/Km value for cephaloridine, respectively. Otherwise, the pI 9.7 enzyme had predominantly penicillinase activity. It was inhibited more readily by clavulanate than by tazobactam and was inactivated by the chelating agents EDTA and ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Expression of the pI 9.7 enzyme was not associated with any plasmid, and production was not transferred to Escherichia coli K-12 recipients, even after the mobilizing plasmid pUZ8 was inserted into the S. marcecens donor strains.

Carbapenems↗
PubMed1990

Antibiotic uptake and transport by bacteria.

A few antibiotics, e.g. polymyxins, disorganize the bacterial wall. For most, however, the wall is an obstacle to be crossed. The extent of this barrier varies with the target to be reached, the drug and the bacterial species. Staphylococci and streptococci have only capsular material and peptidoglycan external to the cytoplasmic membrane (CM). These components afford little shielding and agents with targets on the outer surface of the CM (e.g. vancomycin and beta-lactams) have unhindered target access. Antibiotics with cytoplasmic or ribosomal targets must however cross the CM, usually by active transport. Mycobacteria have outer barriers beyond the CM, as do Gram-negative species. The latter organisms have an outer membrane (OM) located externally to the peptidoglycan and CM. Being hydrophilic, most antibiotics cross the OM by passive diffusion through pores composed of 'porin' proteins. Uptake varies with the drug's charge, size and hydrophilicity, also with the number of pores. Antibiotics larger than 800 D are excluded. Once across the OM, the antibiotics have access to the CM, which must be crossed by those with ribosomal or cytoplasmic targets. Resistance can arise by porin loss, or by loss of an active uptake pathway in the CM. Often the barrier is not absolute: rather uptake is reduced relative to drug removal or detoxification. Occasionally, e.g. with tetracyclines, resistance entails drug excretion or 'coating' of the ribosome.

Anti-Bacterial Agents↗
PubMed1990

Mechanisms and clinical significance of resistance to new beta-lactam antibiotics.

The newer beta-lactam antibiotics, including various ureidopenicillins, second- and third-generation cephalosporins, carbapenems and monobactams, are active against many bacteria that are resistant to older penicillins, such as ampicillin and ticarcillin. Nonetheless, resistance to the newer agents can arise, and in this article the mechanisms whereby it does so are reviewed and their clinical significance is discussed.

Anti-Bacterial Agents↗
PubMed1989

Multiresistant serotype O 12 Pseudomonas aeruginosa: evidence for a common strain in Europe.

A survey was made of serotype association and multiple antibiotic resistance in strains of Pseudomonas aeruginosa in Europe. Of 208 epidemiologically distinct strains from 16 laboratories in 10 countries, 48 were resistant to carbenicillin (MIC greater than 128 micrograms/ml) and gentamicin (MIC greater than 4 micrograms/ml), and 12 of these strains were of serotype O 12. Representative O 12 strains from different countries were compared with two multiresistant O 12 strains isolated 5 years apart, from a British burns unit and the antibiotic sensitive serotype reference strain. All O 12 strains were similar by phage and pyocin typing but lysogenic phage profiles indicated that two strains (the later burns isolate and the serotype strain) were distinct from the others. Electrophoretic characterization of outer membrane proteins, lipopolysaccharides and esterase enzymes corroborated the relationship of the strains and restriction fragment length polymorphism of DNA fragments hybridized with a cDNA probe copy of rRNA from P. aeruginosa provided further proof of their relatedness. We propose that the uniformity of characters of multiresistant O 12 P. aeruginosa in Europe is suggestive of a common origin for the strains.

Anti-Bacterial Agents↗
PubMed1989

In-vitro activity of tigemonam, an oral monobactam, against gram-negative rods, including variants in beta-lactamase-production.

Tigemonam was compared in vitro with other orally active beta-lactams, aztreonam and ciprofloxacin against a selection of Gram-negative clinical isolates and laboratory-derived beta-lactamase-producing variants. Of the orally active beta-lactams, tigemonam was the most potent, with a spectrum of activity similar to that of aztreonam. This included stability to plasmid-mediated beta-lactamases and poor induction of chromosomal beta-lactamases. The susceptibility to Class I enzymes was only clinically significant for derepressed Enterobacter cloacae mutants. Tigemonam may have a valuable role in the management of infection caused by enterobacteria, particularly if bioavailability following oral administration is confirmed in human subjects.

Anti-Bacterial Agents↗
PubMed1989

In-vitro interactions of FCE 22101 with aminoglycosides against gram-negative rods.

Agar dilution MICs of FCE 22101 were measured for 894 consecutively-isolated Gram-negative rods from clinical specimens (413 Escherichia coli, 104 Klebsiella spp., 54 Enterobacter spp., 19 Citrobacter spp., 131 Proteus spp., 43 Acinetobacter spp., 9 Serratia spp., 3 Providencia spp., 4 Yersinia spp., 3 Hafnia spp. and 111 Pseudomonas spp.). Excluding Pseudomonas spp., 98% of these isolates were susceptible to 8 mg/l FCE 22101. Resistance (MIC greater than 8 mg/l) varied from 11% in Serratia spp. to 0% in Citrobacter spp. Interactions between FCE 22101 and gentamicin, tobramycin and amikacin were tested by the chequerboard method for 150 of the isolates. The geometric mean sigma FICs were 0.80 for FCE + gentamicin, 0.78 for FCE + tobramycin and 0.79 for FCE + amikacin. Synergy, defined at FIC index (sigma FIC) less than 0.5, for at least one combination, was found in only 16/150 (11%) of the isolates. Most other isolates showed an additive response (sigma FIC = greater than 0.5-1). No antagonism was detected. No significant difference was observed between the various species tested, except that sigma FIC values were lower (geometric mean sigma FIC = 0.61-0.68) for non-fermentative species than for fermentative species (geometric mean sigma FIC = 0.83-0.86). Likewise, geometric sigma FIC values were lower (0.67-0.69) for isolates resistant to FCE and/or aminoglycosides than for those susceptible to both components (sigma FIC 0.85-0.88).

Amikacin↗
PubMed1989

Interactions of FCE 22101 with class I beta-lactamases.

Hyperproduction of chromosomal Class I beta-lactamase causes resistance to newer cephalosporins in Enterobacter cloacae, Citrobacter freundii, Serratia spp. and indole-positive Proteeae. We examined the interactions of FCE 22101 with these enzymes, measuring (i) antibiotic lability to pure enzyme, (ii) inducing power and (iii) activity against beta-lactamase-inducible strains and their -depressed and -basal mutants. Turnover numbers of FCE 22101 were between 0.7 and 1.5 molecules hydrolysed/min/enzyme molecule, compared with ranges of 0.5-40 for cefotaxime (950 for the Proteus vulgaris cephalosporinase) and 4400-22,000 for cephaloridine. FCE 22101 induced enzyme synthesis strongly below the MIC and antagonized labile weak inducers such as cefotaxime and mezlocillin. Like imipenem, but unlike newer cephalosporins, FCE 22101 had equal activity against inducible strains and derepressed mutants, and did not significantly select derepressed mutants from inducible populations. beta-Lactamase basal mutants of E. cloacae and C. freundii were four- to eight-fold more sensitive than inducible and derepressed organisms whereas beta-lactamase-basal, -inducible and -derepressed Proteeae and Serratia spp. were equally sensitive. This difference reflected the larger amounts of enzyme produced by the E. cloacae and C. freundii strains, rather than kinetic differences in enzyme activity.

Anti-Bacterial Agents↗
PubMed1989

Reaction products of chloramphenicol acetyltransferases from enterobacteria and Haemophilus influenzae.

The reaction products of Types I, II and III chloramphenicol acetyltransferases (CATs) from reference strains of Escherichia coli and Type II CATs from clinical isolates of Haemophilus influenzae were examined by HPLC. Types I and III CATs produced 1, 3 diacetylchloramphenicol at pH 6.8 and 7.8 whereas Type II enzymes failed to produce this diester at either pH. All the CATs produced monoacetylchloramphenicol at both pH values. Neither mono- nor di-acetylchloramphenicol was stable. Breakdown of enzymically-produced mono- and di-esters proceeded more rapidly at pH 7.8 than at pH 6.8. Similar breakdown behaviour was observed with synthetically manufactured 1, 3 diacetylchloramphenicol.

Acetylation↗