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PubMed1979

In vitro response of Enterobacter to ampicillin.

Three strains of Enterobacter were studied for their response to ampicillin. They exhibited a basic level of resistance that depended on the medium used and high-level mutational resistance at a frequency of 10(-5) to 10(-7). Two classes of mutants were selected, one of which showed markedly enhanced antibiotic inactivation as indicated by a biological assay and the other of which resembled the wild type in this regard. Both mutants showed cross-resistance to other beta-lactam antibiotics. The results explained discrepancies between traditional broth dilution minimum inhibitory concentration tests and early read automated procedures.

Ampicillin↗
PubMed1980

Induction of beta-lactamase by various beta-lactam antibiotics in Enterobacter cloacae.

The induction of beta-lactamase in Enterobacter cloacae GN5797 was studied by using 23 beta-lactam antiobiotics, including newly introduced drugs, as inducers. the beta-lactam antibiotics can be classified into three groups on the basis of their inducer activity. Among the tested cephalosporins, cephamycin derivatives such as cefoxitin, cefmetazole, and YM09330 had high inducer activity even at low drug concentrations. On the other hand, cefoperazone, cefsulodin, piperacillin, and apalcillin showed low inducer activity when compared with the other cephalosporins.

Anti-Bacterial Agents↗
PubMed1980

Purification and properties of a cephalosporinase from Enterobacter cloacae.

A cephalosporin beta-lactamase (cephalosporinase) was extracted from Enterobacter cloacae GN7471 and purified by means of column chromatography. The resulting preparation gave a single protein band upon polyacrylamide gel electrophoresis. The enzyme's isoelectric point was 8.4, and its molecular weight was 44,000. The optimal pH was 8.5, and the optimal temperature was 40 degrees C. The enzyme hydrolyzed cephalosporins much more readily than penicillins. The enzyme activity was inhibited by iodine, semisynthetic penicillins, cefuroxime-type cephalosporins, and cephamycin derivatives. The enzymological properties of the purified enzyme were compared with those of beta-lactamases derived from other gram-negative enteric bacteria.

Cephalosporinase↗
PubMed1982

Mutational enzymatic resistance of Enterobacter species to beta-lactam antibiotics.

Mutants with enhanced beta-lactam resistance were selected from strains of Enterobacter cloacae and E. aerogenes by using three antibiotics. High-level beta-lactamase-producing mutants had similar degrees of increased resistance, enzyme substrate profiles, and isoelectric (pI) values irrespective of the selective agent. Reverse mutants from a resistant E. cloacae mutant regained the susceptibility pattern originally exhibited by the wild type, or were of enhanced susceptibility, and no longer expressed increased beta-lactamase production. beta-Lactamases of the mutants were similar in pI values to the wild-type enzyme. The increased resistance of the mutants therefore appeared to be accounted for by increased beta-lactamase production.

Anti-Bacterial Agents↗
PubMed1982

Trapping of nonhydrolyzable cephalosporins by cephalosporinases in Enterobacter cloacae and Pseudomonas aeruginosa as a possible resistance mechanism.

Resistance to cefotaxime (CTA) and ceftriaxone (CTR) in Enterobacter cloacae and Pseudomonas aeruginosa was investigated in several strains which are susceptible or resistant to these agents. All strains produced a chromosomally mediated cephalosporinase of the Richmond type 1. beta-Lactamases in susceptible strains were inducible, whereas resistant strains produced the enzymes constitutively. CTA and CTR were very poor substrates but potent inhibitors of all enzymes. Binding to, rather than hydrolysis by, beta-lactamases was assumed to be a major reason for resistance, and combination experiments supported this assumption. Dicloxacillin, which did not inhibit the growth and which was a poor inducer but a strong inhibitor of these beta-lactamases, exerted strong synergistic activity when combined with CTA or CTR in strains which produced large amounts of beta-lactamase constitutively. Cefoxitin, on the other hand, poorly active alone, but a good inducer, strongly antagonized CTA or CTR in susceptible strains producing inducible enzymes. In marked contrast to CTA and CTR were the findings with cefsulodin. Cefsulodin was active against CTA- and CTR-resistant Pseudomonas, and its activity was hardly influenced by dicloxacillin or cefoxitin. Since cefsulodin was found to have a very low affinity for all cephalosporinases, these findings corroborate the assumption that binding of nonhydrolyzable cephalosporins, rather than hydrolysis by cephalosporinases, may play an important role in resistance to these agents and other newer cephalosporins in Enterobacteriaceae, as well as in other gram-negative bacteria.

Cephalosporinase↗
PubMed1982

Outer membrane permeation of beta-lactam antibiotics in Escherichia coli, Proteus mirabilis, and Enterobacter cloacae.

Mutant strains lacking outer membrane protein(s) were isolated from Escherichia coli, Proteus mirabilis, and Enterobacter cloacae. The outer membrane protein(s) of P. mirabilis and E. cloacae corresponding to E. coli porin were identified on the basis of their function, namely, their ability to allow the permeation of glucose as demonstrated by [14C]glucose uptake by intact cells. P. mirabilis has only one outer membrane pore protein (molecular weight, 40,000), but E. cloacae has at least two such proteins (molecular weights, 37,000 and 39,000 to 40,000). When the bacteria lost these proteins or porin, the outer membrane permeation of cefazolin was found to be greatly reduced in these three species. Such a change in the outer membrane permeation closely correlated with a significant decrease in the bacterial susceptibility to cephalosporins, including cefoxitin. These results suggested that the main pathway for cephalosporin permeation is the pore made up of these proteins. The 39,000- to 40,000-molecular-weight protein in E. cloacae was also assumed to play an important role in the outer membrane permeation of tetracycline and chloramphenicol. On the other hand, the permeation route of penicillins was obscure. The susceptibility to penicillins, except in some cases, was little influenced by the absence of the proteins. Ampicillin was found to pass through the outer membrane via the same route as the cephalosporins, but the possibility that ampicillin and other penicillins possess another unknown route for outer membrane permeation could not be ruled out.

Anti-Bacterial Agents↗
PubMed1983

Characterization of beta-lactamase induction in Enterobacter cloacae.

The induction of beta-lactamase was studied in a strain of Enterobacter cloacae. A wide variety of beta-lactam compounds were found to induce beta-lactamase in this organism, and the degree of induction was directly related to the stability of the inducer to degradation by the enzyme. The kinetics of the induction process were consistent with a system normally under repressor control, suggesting a direct interaction of the beta-lactam compound with a repressor protein in the E. cloacae cells. Although these characteristics are common to many inducible systems in gram-negative organisms, the induction of beta-lactamase in this strain was not subject to catabolite repression with glucose and remained unaffected by exogenous cyclic AMP in the culture medium. This suggests that the organization and function of the beta-lactamase regulatory genes in E. cloacae are unlike those of other inducible gene systems, such as those composing the well-characterized lactose operon in Escherichia coli.

Cefoxitin↗
PubMed1983

Aztreonam, cefoperazone, and gentamicin in the treatment of experimental Enterobacter aerogenes endocarditis in rabbits.

The effectiveness of aztreonam, cefoperazone, and gentamicin alone and in combination was evaluated in Enterobacter aerogenes endocarditis in rabbits. The minimal inhibitory concentration/minimal bactericidal concentration ratios for E. aerogenes were as follows: aztreonam, 0.4/0.4 microgram/ml; cefoperazone, 0.8/0.8 microgram/ml; and gentamicin, 3.1/3.1 micrograms/ml. With an inoculum of 10(9) organisms per ml, aztreonam and cefoperazone were equivalent in reducing titers of E. aerogenes in broth, and both drugs demonstrated an increased rate of reduction when gentamicin was added; gentamicin alone was least effective. E. aerogenes endocarditis in rabbits was treated intramuscularly with aztreonam (60 mg/kg) every 6 h, with cefoperazone (60 mg/kg) every 6 h, with gentamicin (1.7 mg/kg) every 8 h, and with aztreonam plus gentamicin or cefoperazone plus gentamicin for 5 and 10 days, respectively. All of the therapeutic regimens were effective in reducing vegetation titers as compared with untreated controls. Aztreonam plus gentamicin was more effective than either aztreonam or gentamicin alone. Cefoperazone plus gentamicin was more effective than cefoperazone alone but was not more effective than gentamicin alone. Neither aztreonam and cefoperazone nor aztreonam and gentamicin differed significantly, but gentamicin was significantly more effective than cefoperazone. Aztreonam plus gentamicin did not differ significantly in effectiveness from cefoperazone plus gentamicin. Aztreonam gave a peak level of about 135 micrograms/ml and a half-life of 0.7 h. Cefoperazone gave a peak level of about 155 micrograms/ml and a half-life of 1.1 h. Gentamicin gave a peak level of 7.4 micrograms/ml and a half-life of 1.3 h.

Animals↗
PubMed1983

Efficacy of BRL 25000 against Serratia marcescens, Enterobacter cloacae, and Citrobacter freundii in urinary tract infections.

Synergism between amoxicillin and clavulanic acid was not expected against cephalosporinase-producing bacterial strains because clavulanic acid has little inhibitory action on cephalosporinases. However, in a clinical trial of BRL 25000 (amoxicillin-clavulanic acid), excellent results were obtained in complicated urinary tract infections caused by Serratia marcescens, Enterobacter cloacae, and Citrobacter freundii strains which produced cephalosporinase and were highly resistant to amoxicillin alone. The good clinical efficacy of BRL 25000 in such urinary tract infections was probably due to the fact that the urinary concentration of clavulanic acid was higher than its minimal inhibitory concentrations for these strains.

Amoxicillin↗
PubMed1984

Mezlocillin and ticarcillin alone and combined with gentamicin in the treatment of experimental Enterobacter aerogenes endocarditis.

The efficacies of mezlocillin and ticarcillin, each alone and in combination with gentamicin, in the therapy of experimental left-sided Enterobacter aerogenes endocarditis in rabbits were compared. Each beta-lactam was administered intramuscularly at a dose of 180 mg/kg every 6 h either alone or with gentamicin (1.7 mg/kg intramuscularly every 8 h). Bacterial populations at the start of therapy (7 days after initiation of infection) were 9 to 10 log10 CFU/g of vegetation. Ticarcillin produced concentrations in serum that were twice those produced by mezlocillin, but the therapeutic ratios of mezlocillin and ticarcillin (ratio of peak level in serum to MBC) were the same. All of the therapeutic regimens given for either 5 or 10 days were effective in reducing vegetation counts when compared with the untreated controls (P less than 0.01 for all comparisons), except mezlocillin alone and ticarcillin alone, which caused insignificant reductions in counts after 5 days of therapy (P greater than 0.05). After 10 days of therapy, the only regimen that was significantly different from another was that of mezlocillin plus gentamicin, which was significantly better than that of ticarcillin alone (P less than 0.01). These studies document that mezlocillin and ticarcillin were both effective in reducing the numbers of E. aerogenes CFU in vegetations in rabbits with experimental endocarditis when the drugs were given over a prolonged course. More rapid and extensive reduction in vegetation counts was achieved with combinations of an aminoglycoside plus mezlocillin or ticarcillin. Mortality was significantly less among rabbits treated with mezlocillin plus gentamicin.

Animals↗
PubMed1985

Role of beta-lactam hydrolysis in the mechanism of resistance of a beta-lactamase-constitutive Enterobacter cloacae strain to expanded-spectrum beta-lactams.

Enterobacter cloacae strains producing chromosomally mediated beta-lactamase constitutively show high degrees of resistance to most of the third-generation beta-lactams. It has been proposed that this resistance is due to the nonhydrolytic binding or trapping of beta-lactams by the enzyme. We found that the outer membrane of E. cloacae strain 55M indeed had permeability to cefazolin about 14-fold lower than that of Escherichia coli, and that the number of beta-lactamase molecules produced by this constitutive mutant was exceptionally large (2 X 10(5) per cell). These conditions are expected to produce a low degree of resistance, but could not explain the high resistance level of the mutant. We showed that the beta-lactamase of this strain hydrolyzed third-generation beta-lactams at measurable rates. Although the V max for these compounds was less than 0.01% of that for cefazolin, the enzyme could hydrolyze them at rates comparable to the rate for cefazolin when the substrate concentration was near 0.1 microM, a concentration thought to be physiologically relevant for the inhibition of cell growth, because of the exceptionally high affinity of the enzyme to many third-generation compounds. Calculations based on kinetic parameters of the enzyme, outer membrane permeability, and affinity toward penicillin-binding proteins succeeded in predicting the MICs for several third-generation beta-lactams. The data suggest that hydrolysis may be more important than nonhydrolytic binding for the expression of the resistant phenotype, and that studies on the susceptibility of beta-lactams to beta-lactamases should be carried out at physiologically relevant, very low concentrations of the drug, rather than the customary very high concentrations, such as 100 microM.

Anti-Bacterial Agents↗
PubMed1985

Role of beta-lactamases and outer membrane proteins in multiple beta-lactam resistance of Enterobacter cloacae.

The chromosomal beta-lactamase and outer membrane proteins of Enterobacter cloacae were examined to determine their relative contributions to multiple antibiotic resistance in this organism. Mutants altered in beta-lactamase expression, whether derived in the laboratory or recovered from patients treated with one of the new beta-lactam antibiotics, were found to have no detectable alterations in outer membrane proteins. Derepression of beta-lactamase in these mutants was associated with high-level resistance to multiple beta-lactam antibiotics, while loss of inducible beta-lactamase (i.e., production of basal enzyme levels only) was associated with acquisition of susceptibility to many beta-lactam antibiotics, including cephalothin. In contrast, alteration in outer membrane proteins was associated with only moderate-level resistance to beta-lactam antibiotics. However, this included resistance to such drugs as amdinocillin and Sch 34343, which were unaffected by derepression of beta-lactamase. Resistance to chloramphenicol and tetracycline also accompanied changes in outer membrane proteins. Although the outer membrane proteins of various strains of E. cloacae were similar, there did appear to be some major strain-to-strain variations. Thus, it appears that alterations in both beta-lactamase and outer membrane proteins can affect the susceptibility of E. cloacae to many antibiotics. However, alterations in beta-lactamase alone are sufficient to produce high-level multiple beta-lactam resistance in this organism.

Anti-Bacterial Agents↗
PubMed1985

Resistance caused by decreased penetration of beta-lactam antibiotics into Enterobacter cloacae.

Strains of Enterobacter cloacae were selected on the basis of resistance to aztreonam, ceftazidime, moxalactam, or imipenem. All strains produced the same E2 beta-lactamase, with an isoelectric point greater than 9.5 and with high hydrolytic activity in the presence of cephaloridine. Resistance to beta-lactams could not be correlated with the amount of beta-lactamase present in the various strains. beta-Lactamase activity was induced strongly by moxalactam and imipenem in the wild-type and moxalactam-resistant strains, with beta-lactamase representing as much as 4% of the total cellular protein after induction (2 X 10(5) molecules per cell). Ceftazidime and aztreonam were poor inducers. None of the antibiotics studied was readily hydrolyzed by the E2 beta-lactamase; aztreonam and moxalactam inhibited the enzyme with apparent Ki values of 1.2 and 100 nM, respectively. Aztreonam, which bound covalently to the E2 beta-lactamase with a half-life of 2.3 h at 25 degrees C, was used to measure penetrability of beta-lactam into the periplasmic space of the resistant E. cloacae strains. In all of the E2-producing organisms studied, a significant permeability barrier existed. A maximum concentration of 0.02 microgram of aztreonam per ml should have saturated the periplasmic beta-lactamase in the highest enzyme producers studied. However, fully active beta-lactamase was observed in the periplasm of cells exposed to aztreonam at concentrations at least 1,000-fold higher than that theoretically necessary to inhibit the total enzyme within the cell. Thus, the major cause for resistance to beta-lactam antibiotics in these E. cloacae strains was lack of penetration across the outer membrane.

Anti-Bacterial Agents↗
PubMed1985

Enoxacin compared with cefoperazone for the treatment of experimental Enterobacter aerogenes endocarditis.

This study compared enoxacin administered orally with cefoperazone administered intramuscularly for the treatment of Enterobacter aerogenes endocarditis in rabbits. The MICs and MBCs of both enoxacin and cefoperazone for an inoculum of 10(5) CFU/ml of the E. aerogenes strain used were 0.8 micrograms/ml, respectively. With an inoculum of 10(8) organisms per ml, enoxacin at 2 and 5 micrograms/ml and cefoperazone at 60 and 155 micrograms/ml were effective in reducing titers of E. aerogenes in broth. E. aerogenes endocarditis in rabbits was treated with enoxacin (100 or 25 mg/kg orally every 6 h) or cefoperazone (60 mg/kg intramuscularly every 6 h) for 5 or 10 days. Enoxacin at 100 and 25 mg/kg significantly reduced bacterial titers of vegetations compared with those of untreated controls. Enoxacin at 100 mg/kg was significantly more effective than enoxacin at 25 mg/kg and cefoperazone. Enoxacin at 25 mg/kg and cefoperazone did not differ significantly. Cefoperazone and controls did not differ significantly. In uninfected rabbits single doses of cefoperazone achieved much higher concentrations in serum than single doses of enoxacin (25 and 100 mg/kg). The half-lives of enoxacin at 25 and 100 mg/kg were approximately three times longer than that of cefoperazone.

Animals↗
PubMed1986

Multiply resistant mutants of Enterobacter cloacae selected by beta-lactam antibiotics.

Mutants of Enterobacter cloacae, selected in vitro with ceftriaxone, ceftazidime, carumonam, or aztreonam, fell into several distinct classes. Three mutants highly resistant to nearly all beta-lactam antibiotics were stably derepressed for beta-lactamase production. Although no other changes could be detected, virulence in a mouse septicemia model was decreased in two of these mutants. One mutant, 908-Ssi, showed selectively decreased susceptibility to ampicillin and cefotetan. A change in beta-lactamase expression was thought to be responsible for this. Alterations in the production of two outer membrane proteins with molecular sizes of 36.5 and 39 kilodaltons were responsible for multiple antibiotic resistance in two mutants, both of which acquired a low level of resistance to beta-lactam antibiotics. Whereas one of the mutants, AMA-R, simultaneously acquired resistance to chloramphenicol and trimethoprim, the other, AZT-R, became hypersusceptible to these and other hydrophobic agents. Both strains had drastically reduced virulence in mice.

Anti-Bacterial Agents↗
PubMed1987

Contribution of beta-lactamase hydrolysis and outer membrane permeability to ceftriaxone resistance in Enterobacter cloacae.

Mechanisms of ceftriaxone resistance were examined in Enterobacter cloacae. Clones were selected from four strains: susceptible (S), resistant (R1), selected by plating on ceftriaxone-containing agar, and highly resistant (R2), selected in ceftriaxone-treated mice infected with S clones. According to 14C-labeled beta-lactam binding assays, ceftriaxone resistance was not associated with altered target proteins. R1 and R2 clones stably produced 50 to 1,500 times more beta-lactamase than S clones; this production increased after cefoxitin induction in all S and some R1 clones. Experiments conducted with strain 218 suggested that ceftriaxone resistance involved beta-lactamase hydrolysis. Half-lives for the beta-lactamase-beta-lactam complexes at 37 degrees C were 0.4 and 2.2 min for ceftriaxone and Sch 34343, a drug not affected by the resistance, respectively; in chromatography experiments, 218 intact R1 cells (2 x 10(9) to 3 x 10(9) CFU) suspended in ceftriaxone-containing buffer (2 micrograms/ml) hydrolyzed 80% of the antibiotic in 30 min. Three observations also suggested decreased permeability in some clones, (i) Most of the R1 and R2 clones showed decreased expression of outer membrane proteins of 37,000 to 38,000 molecular weight (37K to 38K proteins) by electrophoresis, often associated with increased amounts of 42K protein. (ii) [14C]Sch 34343 labeling of intact cells proceeded more slowly in 218 R2 (with altered 37K to 38K proteins) than in 218 R1 (without this alteration), a difference persisting after competition with unlabeled cloxacillin. Delays in binding were not caused by different enzymatic activities, since 218 R1 and 218 R2 produce, in similar amounts, beta-lactamases undistinguishable in isoelectric point and Km of cephaloridine. (iii) Intact cells from 218 R2 hydrolyzed ceftriaxone more slowly (20% in 30 min) than did those from 218 R1. In 218 R1, beta-lactamase overproduction was responsible for a 15- to 200-fold increase in the MIC's of ceftriaxone, ceftazidime, carbenicillin, piperacillin, moxalactam, aztreonam, carumonam, and BMY 28142. Imipenem and Sch 34343 were not affected; an additional three- to fivefold increase in the MIC's of these antibiotics (with the exception of piperacillin, imipenem, Sch 34343), seen with 218 R2, was associated with decreased permeability.

Anti-Bacterial Agents↗
PubMed1987

Molecular genetic analysis of cephalosporinase production and its role in beta-lactam resistance in clinical isolates of Enterobacter cloacae.

Two strains of Enterobacter cloacae were isolated from a patient before (strain MHN1) and during (strain MHN2) treatment with moxalactam and gentamicin. Strain MHN1 exhibited inducible ampC cephalosporinase production. In contrast, strain MHN2 expressed the enzyme constitutively at a 3,000-fold higher level. With the Escherichia coli ampC gene as a hybridization probe it was shown that the genomic arrangement of the ampC region was the same in both strains. To gain more insight into regulatory phenomena, the ampC genes were cloned, and their expression was studied in E. coli K-12. The ampC gene from MHN1 behaved normally and conferred inducible beta-lactam resistance. A regulatory region of at least 800 base pairs involved in controlling repression-induction was located immediately upstream of ampC. Surprisingly, when present in E. coli the ampC gene from MHN2 no longer overproduced the cephalosporinase, and inducible expression was observed. This indicates that in MHN2 stable cephalosporinase overproduction is controlled by another factor which is not linked to the ampC gene.

Anti-Bacterial Agents↗