Susceptibility of Enterobacter to cefamandole: evidence for a high mutation rate to resistance

AmpD, essential for both beta-lactamase regulation and cell wall recycling, is a novel cytosolic N-acetylmuramyl-L-alanine amidase

In enterobacteria, the ampD gene encodes a cytosolic protein which acts as a negative regulator of beta-lactamase expression. It is shown here that the AmpD protein is a novel N-acetylmuramyl-L-alanine amidase (E.C.3.5.1.28) participating in the intracellular recycling of peptidoglycan fragments. Surprisingly, AmpD exhibits an exclusive specificity for substrates containing anhydro muramic acid. This anhydro bond is mainly found in the peptidoglycan degradation products formed by the periplasmic lytic transglycosylases and thus might behave as a 'recycling tag' allowing the enzyme to distinguish these fragments from the newly synthesized peptidoglycan precursors. The AmpD substrate (or substrates) which accumulates in the absence of the corresponding enzymatic activity acts as an intracellular positive effector for beta-lactamase expression and might represent an element of a communication network between the chromosome and the cell wall peptidoglycan.

Comparison of the E test and microdilution for detection of beta-lactam-resistant mutants that are stably derepressed for type I beta-lactamase

The activities of cefotaxime, ceftazidime, piperacillin, and aztreonam were compared in the E test and broth microdilution test against 30 gram-negative bacterial mutants derepressed for type I beta-lactamases. The results demonstrated complete agreement between 24-h MICs of 80 to 83% and essential agreement between 24-h MICs of 90 to 97%. When sufficient growth was present for the E test to be read at 6 h, the essential agreement between 6- and 24-h E-test MICs was 100% for ceftazidime, piperacillin, and aztreonam and 85% for cefotaxime.

The changing ecology of hospital bacteria and the selective role of cephalosporins

More than 12,800 clinical isolates from 115,373 in-patient specimens obtained at the Sir Charles Gairdner Hospital, Perth, Western Australia, were identified and analysed statistically for relationships with usage of three generations of cephalosporins over the 5-year period from July 1984 to June 1989. A positive relationship between cephalosporin usage and significantly increasing isolation rates for those species capable of producing chromosomal beta-lactamases was observed. Simultaneously, a small increase in the isolation frequency of non-chromosomal beta-lactamase-producing strains was noted and no correlation with cephalosporin usage was demonstrated. The trend toward predomination in the hospital environment of strains possessing substantial cephalosporin resistance has implications for future antimicrobial policy, choice of empiric therapy and the predictive value of standard antimicrobial susceptibility tests.

Resistance to third generation cephalosporins: the current situation

Newer beta-lactam antibiotics, notably the third generation cephalosporins (3 GC) have been designed for providing high intrinsic potency against a large variety of microorganisms. Bacterial resistance can occur however, and nowadays, clinicians are concerned by novel situations where even most recently developed compounds can be ineffective. A first situation is generated by bacteria which produce great amounts of chromosomal cephalosporinase. The resistance emerges during therapy, in hospital isolates which are classified as susceptible with conventional susceptibility testing. The prevalence of 3 GC resistance among these gram-negative rods with inducible beta-lactamase seems to increase in some institutions but the significance of susceptibility testing in this regard is doubtful. It is probably more important to note that the prevalence of gram-negative rods with inducible beta-lactamases remains stable. A second problem arose with the abrupt development of plasmid mediated beta-lactamases markedly active against 3 GC. This resistance is underestimated because some strains fall into susceptibility range of 3 GC as determined by MICs or inhibition zone sizes. These extended spectrum enzymes are now distributed over four continents and represent a growing threat.

Multiply beta-lactam resistant Enterobacter cloacae infections linked to the environmental flora in a unit for cardiothoracic and vascular surgery

During the period March 1987-May 1988, postoperative infection or colonization with Enterobacter cloacae occurred in 9/379 (2.4%) patients who underwent cardiovascular surgery. Five of the patients were infected with multiply beta-lactam resistant E. cloacae, of whom 4 had been infected with an identical, resistant strain during intervals of months. This strain was also found in the environmental flora of the cardiovascular operating suite and in a sink reservoir in the surgery department. All 4 patients with the identical resistant strain had serious complications during the postoperative period with symptoms of septicaemia in 3, multiorgan failure and shock in 2, and mediastinitis in 3. The single resistant strain of a different serotype was also associated with severe postoperative complications. The 4 sensitive strains were all different serotypes. None caused septicaemia, one was associated with mediastinitis, another with an uncomplicated sternum infection, and 2 were from sputum. In the 3 latter patients with sensitive strains and few postoperative complications, cephalosporins had not been used during the pre- or postoperative period.

In vitro activity of U-76,252 (CS-807), a new oral cephalosporin

U-76,252 is the prodrug of U-76,253. MICs of U-76,253 were 0.015 to 0.06 microgram/ml for greater than or equal to 90% of the strains of Streptococcus spp., Haemophilus influenzae, and Proteus mirabilis; 0.25 to 1 microgram/ml for Branhamella catarrhalis, Escherichia coli, Klebsiella spp., and Citrobacter diversus; 1 to 8 micrograms/ml for Staphylococcus spp.; and 2 to greater than 16 micrograms/ml for other members of the family Enterobacteriaceae and Aeromonas hydrophila; for 72% of the latter group, MICs were less than or equal to 4 micrograms/ml. MICs for Pseudomonas aeruginosa and Enterococcus faecalis were greater than 16 micrograms/ml.

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.

Emergence of resistance in gram-negative bacteria during therapy with expanded-spectrum cephalosporins

To assess the clinical importance of emergence of beta-lactam resistance caused by stable derepression of chromosomal beta-lactamases, sequential cultures from patients treated with expanded-spectrum cephalosporins were monitored for the persistence of bacteria possessing these enzymes. Antibiotic susceptibilities and beta-lactamase production before and after cefoxitin induction were determined in sequential isolates of individual bacterial strains. Of 49 strains isolated from 44 patients, 25 strains (51%) were eradicated by cephalosporin therapy, 17 strains (35%) persisted with unchanged susceptibility in sequential cultures, and 7 strains (14%) from 7 patients developed multiple beta-lactam resistance during cephalosporin therapy. In 6 of the 7 strains, resistance was associated with stable derepression of beta-lactamases. In the patient group whose strains developed resistance, subsequent use of non-beta-lactam antibiotics was more frequent and mortality was higher.

Common mechanism of ampC beta-lactamase induction in enterobacteria: regulation of the cloned Enterobacter cloacae P99 beta-lactamase gene

Expression of the chromosomal beta-lactamase from the ampC gene in inducible in both Enterobacter cloacae and Citrobacter freundii. Cloning of ampC as well as its regulatory gene, ampR, from E. cloacae P99 revealed a gene organization indentical to that of C. freundii in the corresponding region. Although almost no similarities could be found between the restriction maps of ampC and ampR in the two species, the genes cross-hybridize. Also, both ampR gene products have a size of about 31,000. The regulatory features of E. cloacae beta-lactamase induction are very similar to those in C. freundii, i.e., beta-lactamase synthesis is repressed by AmpR in the absence, and stimulated in the presence, of inducer. The AmpR function can be transcomplemented between the two species, but there are quantitative regulatory aberrations in such hybrids, in contrast to the total complementation obtained within each system. These results suggest that the mechanism of beta-lactamase induction is the same in E. cloacae, C. freundii, and other gram-negative bacteria with inducible chromosomal beta-lactamase expression.

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.

Microbiological investigation of cephalosporins

The most important role of the clinical microbiology laboratory is to advise clinicians in their choice of antimicrobial therapy. While the application of modern laboratory techniques is enabling sensitivity testing to cephalosporins to be performed with increasing precision, the ability to predict accurately clinical efficacy has not improved in parallel. For the cephem group in particular, the present confusion as to the numerical value of breakpoints and their interpretation, and the overuse of 'class testing' are making the task of the clinical microbiologist more difficult. For most purposes, simple disc sensitivity testing of cephems gives sufficient information, and it is simple to carry out, as no special media or growth conditions are required. Further studies are required to answer an outstanding question of great importance, namely, what the clinical prognostic significance is of results of sensitivity testing of 'methicillin-resistant' Staphylococcus aureus and coagulase-negative staphylococci, as these organisms often appear sensitive to cephems in vitro. For the research worker, the cephems provide tools of almost unrivalled power in the investigation of such microbiologically important topics as cell wall synthesis, bacteriolysis, membrane function and various aspects of enzyme regulation and inhibition. Relatively minor changes in the structure of cephem molecules can markedly affect their binding to bacteria, thus allowing probing of the functions of the individual penicillin-binding proteins. In Gram-negative bacteria, membrane function can be selectively changed by the action of subinhibitory concentrations of cephems, as it is intimately connected to the integrity of the peptidoglycan moiety. Induction and derepression of beta-lactamases may be responsible for a new type of bacterial resistance.

Emergence of resistance in gram-negative bacteria: a risk of broad-spectrum beta-lactam use

A number of new beta-lactam antibiotics have been developed to overcome bacterial resistance to older agents. Such resistance usually is caused by plasmid-mediated, constituently produced beta-lactamases. Second- and third-generation cephalosporins, ureidopenicillins, acylamino penicillins, and monobactams generally are resistant to hydrolysis by these enzymes. However, inducible beta-lactamases may confer resistance to these antibiotics. This induction may occur spontaneously or in response to cefoxitin or other beta-lactam agents. The mechanisms by which inducible enzymes produce this resistance are reviewed and implications for the prophylactic and therapeutic use of newer beta-lactams are considered.

Endemic emergence of cephalosporin-resistant Enterobacter: relation to prior therapy

The “second” and “third” generation cephalosporins offer striking antimicrobial activity against a wide spectrum of Enterobacteriaceae. Nevertheless, mutants resistant to these drugs have emerged in both laboratory and clinical settings. For example, before the commercial availability of the third-generation agents, we treated three cardiac surgery patients for Enterobacter mediastinitis with aminoglycosides and high doses of cefamandole. In two, initial treatment failed due to emergence of strains that were not only resistant to cefamandole, but also to then experimental third-generation drugs. Despite such reports and in vitro studies of the mechanisms of resistance, the frequency with which broad-spectrum cephalosporin resistance develops in clinical practice is not clear. To help delineate this problem, we have reviewed our hospital's experience with Enterobacter strains resistant to newer cephalosporins (using cefamandole and cefotaxime as prototypes) and the relation of resistant strains to cephalosporin use, with special attention to our cardiac surgery patients.

Cefotiam susceptibility testing criteria

Interpretive zone size breakpoints for diffusion tests with 30-micrograms cefotiam disks are diameters of greater than or equal to 18 mm for susceptible and less than or equal to 14 mm for resistant strains. The standard control strains Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923 should both give zones 27 to 33 mm in diameter. Tests with 579 clinical isolates yielded an unacceptably high rate of very major discrepancies between disk tests and microdilution tests; such discrepancies were especially common among Enterobacter spp. Additional studies support the concept that standard microdilution tests and standard disk diffusion tests may fail to detect a potential for resistance among some microorganisms.

Population analysis of susceptibility to cefotaxime and desacetyl-cefotaxime in Staphylococcus and Enterobacteriaceae

Population analyses of susceptibility to cefotaxime (CTX) and desacetyl-cefotaxime (DCTX) of strains of Staphylococcus and some genera of Enterobacteriaceae were carried out. DCTX, which is the main metabolite of CTX, has antimicrobial activity. The penicillinase-producing strains of S. aureus and S. epidermidis were homogeneous as regards susceptibility to both agents. CTX was about 4-8 times more active than DCTX. The methicillin-resistant strains contained a sub-population of resistant bacteria with both CTX and DCTX. The frequency of resistant bacteria was 10(-6) - 10(-5). The E. coli strain was homogeneous to both agents. The strains of Enterobacter cloacae and Citrobacter freundii had a sub-population of resistant bacteria with both agents. The frequency of resistant bacteria was 10(-7) - 10(-4.5). In Klebsiella pneumoniae no resistant sub-population was found. CTX was about four times more active than DCTX with the strains of the Enterobacteriaceae. DCTX had no advantage over CTX as regards homogeneity of susceptibility of the populations examined. CTX seems applicable for treatment of infections with E. coli, Klebsiella pneumoniae, and penicillinase-producing, methicillin-susceptible Staphylococcus, but should not be used alone in treatments of infections with Enterobacter cloacae or Citrobacter freundii.

Selection of resistant mutants of Citrobacter freundii by second and third-generation cephalosporins and imipenem

Using a single-step selection procedure, resistant mutants could be obtained from three clinical isolates of Citrobacter freundii with two second-generation and four third-generation cephalosporins but not with imipenem. All mutants showed a drastically increased beta-lactamase activity and were cross-resistant to all the cephalosporins examined. Combinations of cloxacillin with the cephalosporins were markedly synergistic, suggesting the principal role of the cephalosporinase in the resistance of these mutants.

Development of resistance to cephalosporins in clinical strains of Citrobacter spp

The predominant beta-lactam antibiogram of Citrobacter freundii resembles that of Enterobacter cloacae in demonstrating resistance to cephalothin and cefoxitin with susceptibility to the newer cephalosporins. Four representative strains of C. freundii were reversibly induced to high-level beta-lactamase production by cefoxitin, and mutants with stable, high-level production were selected with cefamandole. The mutants were resistant to several second- and third-generation cephalosporins. Comparisons of isoelectric points and substrate profiles of beta-lactamases from wild-type, induced wild-type, and mutant organisms suggested a close relationship to those from E. cloacae and indicated that C. freundii mutants, like those of E. cloacae, were derepressed for production of beta-lactamase. One primary isolate of C. freundii resembled the mutants in all characteristics. In contrast, most strains of Citrobacter diversus were susceptible to all cephalosporins, and two representative strains showed neither inducible nor mutational resistance. Cefoxitin induction to enhanced beta-lactamase production was demonstrated in a cephalothin-resistant isolate, and a derepressed mutant was selected with cefotaxime. The beta-lactamase from this C. diversus strain differed substantially in substrate profile from that of E. cloacae and C. freundii.

Transfer of the chromosomal bla gene from Enterobacter cloacae to Escherichia coli by RP4::mini-Mu

The resistance gene for beta-lactamase-stable cephalosporins from Enterobacter cloacae was transferred to Escherichia coli by the aid of RP4::mini-Mu. The R-prime plasmids generated carried 60 to 80 kilobases (kb) of E. cloacae DNA and coded for the chromosomal E. cloacae beta-lactamase. The gene was fully expressed in the recipient. Restriction endonuclease EcoRI fragments of the R-prime plasmid pBP100 were cloned into the vector pBP328, yielding the plasmid pBP102 with a size of 14 kb. A restriction map of this plasmid was constructed. By digesting pBP102 into seven PstI fragments, ligating the fragments, and looking for the smallest plasmid generated, pBP103 was isolated. It consisted of three PstI fragments, two of them (together 4.2 kb) necessary for resistance. During the experiment (performed in a recA+ background) the largest PstI fragment had undergone a substitution of a 0.3-kb segment of pBP102 by a 0.7-kb segment in pBP103 (as deduced by heteroduplex analysis). The bla gene of resistant E. cloacae strains was dominant over the gene of susceptible organisms.

Antibacterial activity of cefoperazone alone and in combination against cephalosporinase-producing Enterobacter cloacae

The activity of cefoperazone against a strain with an inducible cephalosporinase and a mutant that produces the enzyme constitutively indicates that the low inducer activity of this antibiotic plays an important role in its activity against Enterobacter cloacae.

Antimicrobial susceptibility testing (AST): a review of changing trends, quality control guidelines, test accuracy, and recommendation for the testing of beta-lactam drugs

The review that follows presents the changing trends in antimicrobial susceptibility testing observed from the author's clinical laboratory experience and the proficiency testing surveys of the College of American Pathologists (CAP). The CAP Microbiology Surveys show a clear trend toward standardized test methods of the National Committee for Clinical Laboratory Standard (NCCLS) and greater compliance with specified methods' technical steps. This has favorably influenced the laboratory performance on proficiency challenges where a 3-5% improvement has been noted over the last 5 years for the disk tests (overall acceptable rate of 95.2% in 1981). A concurrent increase in dilution test use, mainly broth microdilution methods, has resulted in greater than 25% of larger hospital laboratories reporting results as MICs (overall acceptable or good performance = 98%). Automated systems use also continues to increase, with user performance being monitored at an acceptable level. Quality control frequency may be reduced to once weekly without compromising test accuracy or patient care, but only after adequate daily or concurrent QC performance has been documented. Most methods continue to have problems in testing enterococci, methicillin-resistant staphylococci, and the class-disk concept appears to be less applicable. Recommendations are made for the testing of the newer semisynthetic penicillins and cephalosporins based on their spectrum comparability and cross-resistance studies with bacteria possessing known susceptibility or resistance mechanisms. The concept of "spectrum-class" is introduced with peer drugs within classes. The general trends and quality of antimicrobial susceptibility tests seem outstanding and point toward continued excellent intra- and interlaboratory reproducibility at the national level, primarily due to the efforts of the inspection and accreditation agencies, CAP, CDC, and other concerned professional groups.

What do beta-lactamases mean for clinical efficacy?

beta-Lactamases have proved to be extremely important in influencing therapy with penicillins and cephalosporins against gram-positive and gram-negative aerobic and anaerobic species. Both plasmid mediated beta-lactamases which are primarily of a constitutive penicillinase type and the inducible chromosomal enzymes which are primarily cephalosporinases are important. The use of penicillins to treat Haemophilus, Neisseria gonorrhoeae, Escherichia coli, Klebsiella, Salmonella, Shigella and Pseudomonas infections must be based upon the relative incidence of beta-lactamase producing strains. In the same manner cephalosporins can be used to treat infections due to Enterobacter, Serratia and Bacteroides only if the compounds are beta-lactamase stable and not good inducers of beta-lactamase activity. Although altered permeability is important in the resistance of some Pseudomonas and Enterobacter to beta-lactams, the resistance really is due to a combination of reduced entry of molecules and strategically placed beta-lactamases. It is only in some Streptococcus pneumoniae, methicillin-resistant Staphylococcus aureus and Streptococcus faecalis strains that altered penicillin-binding proteins make a significant contribution to the resistance to beta-lactams. beta-lactamases will continue to be the most important factor in clinically significant resistance of bacteria to both penicillins and cephalosporins.

Activity of cefoperazone against ampicillin-resistant bacteria in agar and broth dilution tests

Examination of the activity of cefoperazone against ampicillin-resistant, gramnegative bacteria in agar dilution and simultaneously in broth dilution revealed that strains could be divided into three classes: class I strains were susceptible in agar (mean minimal inhibitory concentration [MIC], 0.5 mg/liter) as well as in broth dilution (mean MIC, 1.5 mg/liter), class II strains were susceptible in agar (MIC, 0.9 mg/liter), but resistant in broth dilution (MIC, 182 mg/liter); and class III strains were highly resistant in both test systems. Among 100 randomly selected ampicillin-resistant Escherichia coli cultures, 51 belonged to class I and 49 belonged to class II. Class III E. coli strains were much rarer. Similar results were obtained with cefamandole and cephalothin, but not with six other second-and third-generation cephalosporins. MICs of cefoperazone against cultures of all three classes were influenced by initial inoculum size. The inoculum effect was greatest with class II strains. Examination of bactericidal activity by cefoperazone showed killing of class I and class II E. coli strains and of class III strains of other genera during the first hours of incubation and regrowth after the drug was destroyed by the action of TEM beta-lactamase (penicillinase). Representative class I bacteria produced 10 to 100 times less TEM beta-lactamase than did class II strains. It appeared that the quantitative difference in TEM production was the reason for the different resistance phenotypes in class I and class II strains. Salmonella and Klebsiella strains of class III produced the same amounts of TEM beta-lactamase as did class II E. coli strains. Probably, some factors other than beta-lactamase contributed to the class III phenotype in these species.

Cefoperazone disk diffusion susceptibility test: confirmation of the tentative interpretive criteria, Pseudomonas aeruginosa cross-resistance, and determination of quality control performance limits

Cefoperazone disk diffusion test and minimum inhibitory concentration comparison studies were performed on 421 recent bacterial isolates, using 30- and 75-micrograms commercially prepared disks. Acceptable correlation coefficients (-0.82 to -0.86) and very major (false-susceptible) interpretive error rates (less than 1%) were obtained with both disk concentrations. The interpretive criteria for both disks were identical. Using the preferred 75-micrograms disk, the Thornsberry et al. criteria (J. Clin. Microbiol. 15:769-776, 1982) of greater than or equal to 18 mm = susceptible (less than or equal to 32 micrograms/ml) and less than or equal to 14 mm = resistant (greater than 64 micrograms/ml) resulted in only 5.5% of strains having indeterminate-range zone diameters; the 30-micrograms disk had 6.9% of strains with indeterminate zone diameters. The 75-micrograms disk, excluding the testing of enterococci, minimized the very major and other interpretive errors to less than 5%. Larger zone diameters will contribute few technical problems with either disk concentration. Data from 1,320 zone diameters submitted for each quality control strain indicated no significant (P greater than 0.05) difference between disks made by the three major manufacturers, and consistent results were obtained within each laboratory with numerous lots of Mueller-Hinton agar (except for one manufacturer). Individual daily test and accuracy quality control ranges were calculated from clinical investigator laboratory data at 16 hospitals based on mean zone sizes and from an additional 8 laboratories with both mean and median calculations. The quality control data were nearly identical, and ranges calculated by the two methods were very similar. Susceptibility tests of Pseudomonas aeruginosa indicate that the cefoperazone disk or minimum inhibitory concentration test would accurately predict P. aeruginosa susceptibility test results for other pseudomonas-active cephalosporins (cefsulodin and ceftazidime), thus producing no very major interpretive errors.

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.

Cefotaxime therapy of serious bacterial infection in adults

We evaluated the efficacy, safety, and tolerance of cefotaxime in 35 adults (25 with pleuropulmonary infections, 7 with genitourinary tract infections, and 3 with soft tissue infections). Of these 35 patients, 18 (51.4%) were seriously or critically ill. In vitro susceptibility testing revealed that 90.4% of the pathogens isolated were susceptible to cefotaxime (minimal inhibitory concentration, less than 8 micrograms/ml), 4.8% were intermediately susceptible (minimal inhibitory concentration, 8 to 32 micrograms/ml), and 4.8% were resistant (minimal inhibitory concentration, greater than 32 micrograms/ml). A total of 34 of the 35 patients (97%) were clinically and bacteriologically cured of their infections. Adverse reactions occurred in two patients, but these reactions did not require interruption of therapy.

Comparative activity of N-formimidoyl thienamycin with third generation cephalosporins and ureido penicillins against multiple resistant Serratia marcescens

Twenty multiple resistant clinical isolates were tested with N-formimidoyl thienamycin, moxalactam, cefotaxime, cefoperazone, and the three ureidopenicillins: azlocillin, mezlocillin, and piperacillin. A concentration of less than 0.97 microgram/ml inhibited 100% of organisms for N-f-thienamycin and cefotaxime, 90% for moxalactam, and 60% for cefoperazone. An increase in inoculum from 10(3) to 10(6) cells reduced activity fourfold for 95% of isolates with cefoperazone, 70% with N-formimidoyl thienamycin, 65% for cefotaxime, but only 15% for moxalactam. For ureidopenicillins, 85% of strains tested had MIC's less than or equal to 15.6 micrograms/ml. An inoculum effect was observed in only 35-50%. At 10(3), the cidal concentration was the same or twofold greater than the inhibitory level for N-f-thienamycin and cephalosporins in 70% of strains tested and 65% for penicillins. With 10(6), the 70% value remained for N-f-thienamycin but was reduced to 45% for cefotaxime and 25% for moxalactam; 85% demonstrated greater than eightfold differences with cefoperazone. Single step high-level resistance was observed to moxalactam (20%). Carbenicillin resistant strains were cross-resistant to the ureidopenicillins. N-f-thienamycin and cefotaxime appeared comparable, although important differences between morphological alteration and metabolism may influence their therapeutic effectiveness.

Therapy of lower respiratory tract infections with moxalactam

Moxalactam was evaluated in the therapy of lower respiratory tract infections in 40 patients. The most common organisms isolated were Streptococcus pneumoniae (37.2%) and Haemophilus influenzae (21.5%). Gram-negative enteric organisms were isolated from six patients. No patient was evaluated as a treatment failure; however, two patients died of unrelated causes either during therapy or in the immediate posttherapy period. We determined the comparative minimal inhibitory concentrations of moxalactam, cefamandole, and cephalothin for our aerobic clinical isolates. Susceptibilities of the anaerobic isolates were measured by the Kirby-Bauer method. All isolates were susceptible to moxalactam. Moxalactam was found to be highly effective in the therapy of lower respiratory tract infections.

Comparative in vitro studies of Ro 13-9904, a new cephalosporin derivative

The in vitro activity of Ro 13-9904, a new cephalosporin derivative, was compared with the activities of cephalothin, cefamandole, cefoxitin, cefotaxime, and moxalactam against 591 clinical isolates of gram-negative and gram-positive organisms. The spectra of activity and potency of Ro 13-9904 and cefotaxime were quite similar; they were the most active agents against Enterobacteriaceae, Streptococcus pyogenes, Haemophilus influenzae, Neisseria gonorrhoeae, and Neisseria meningitidis. Moxalactam was only slightly less active against these organisms. Ro 13-9904, cefotaxime, and moxalactam were approximately equal in activity against Pseudomonas aeruginosa; concentrations of 50 to 100 microgram/ml inhibited over 90% of the strains tested. Cefamandole and cephalothin were the most active drugs tested against staphylococci. Moxalactam demonstrated the highest intrinsic activity against Bacteroides fragilis; a concentration of 1.6 microgram/ml inhibited over 50% of the strains. All six of the antibiotics were essentially inactive against group D streptococci. The action of all of the antibiotics was bactericidal, with minimal bactericidal concentrations generally being no more than twofold greater than minimal inhibitory concentrations. The only exception to this was found when large inocula of Staphylococcus aureus were tested. Increased inoculum size generally sharply reduced the activity of Ro 13-9904, cefotaxime, and moxalactam against Enterobacteriaceae and P. aeruginosa.

Role of a cefoxitin-inducible beta-lactamase in a case of breakthrough bacteremia

Development of resistance during therapy with cefamandole contributes to treatment failure. A simple cefoxitin disk test was recently described which detects a cefamandole-active inducible beta-lactamase not otherwise detectable with cefamandole as the inducer. A case of breakthrough Enterobacter bacteremia due to selection of a resistant subpopulation is reported in an immunocompromised patient. The use of this simple disk test in selected clinical cases is advocated.

Evaluation of cefamandole susceptibility testing of Enterobacteriacea by the autobac 1 system

A total of 509 clinical isolates of Enterobacteriaceae were tested for susceptibility to cefamandole by Autobac 1 and Bauer-Kirby disk diffusion methods, using commercially available 30-micrograms cefamandole disks. Minimal inhibitory concentrations were determined for all organisms showing major or very major discrepancies. Overall agreement between Autobac 1 and disk diffusion was 89.8%, with 5.1% major or very major and 5.1% minor discrepancies. When considering only the genera for which 20 or more isolates were tested, overall agreement was 90.8%. Discrepancies for Escherichia coli showed a trend toward resistance by Autobac 1, with minimal inhibitory concentrations generally in agreement with disk diffusion results. No trends were detected for other genera. The rate of agreement was lower for Enterobacter species (75.4%), but minimal inhibitory concentrations, determined for all discrepancies in this genus, agreed with Autobafc 1 as often a with disk diffusion results.

In vitro evaluation of Ro 13-9904

The in vitro activity of a new investigational cephalosporin, Ro 13-9904, was compared with those of four cephalosporins (cephalothin, cefamandole, cefoxitin, and moxalactam), five semisynthetic penicillins (mezlocillin, piperacillin, carbenicillin, ticarcillin, and azlocillin), and the aminoglycoside tobramycin. Ro 13-9904 inhibited 75% of all isolates of Enterobacteriaceae at a concentration of 6.25 micrograms/ml, including Enterobacter spp., Serratia marcescens, and indole-positive Proteus spp. Ro 13-9904 was only minimally active against Pseudomonas aeruginosa and enterococci. There was no significant change (greater than or equal to 2 dilutions) in drug activity when tested in various pH and media. A significant inoculum effect occurred when the size of the inoculum was increased from a concentration of 10(5) to 10(7) organisms per ml.

Moxalactam (LY127935), a new semisynthetic 1-oxa-beta-lactam antibiotic with remarkable antimicrobial activity: in vitro comparison with cefamandole and tobramycin

Moxalactam (LY127935) exhibited greater in vitro activity than cefamandole and tobramycin against clinical isolates of Enterobacteriaceae, Aeromonas hydrophila, and Pseudomonas maltophilia. The activities of the three drugs against other microorganisms were as follows: for staphylococci, cefamandole = tobramycin greater than moxalactam; for streptococci, cefamandole greater than moxalactam greater than tobramycin; and for Pseudomonas aeruginosa, tobramycin greater than moxalactam greater than cefamandole. Moxalactam also demonstrated significant activity against the Bacteroides fragilis group and other anaerobes. Moxalactam was comparable to cefotaxime (HR756) in its inhibition of cephalothin-resistant and aminoglycoside-resistant clinical isolates.

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.

In vitro activity of cefamandole, cefoxitin, cefuroxime, and carbenicillin, alone and in combination with aminoglycosides against Serratia marcescens

Synergistic antibiotic studies were undertaken to compare the effectiveness of two new beta-lactamase resistant cephalosporins, cefamandole, and carbenicillin, with four aminoglycosides against clinical strains of Serratia marcescens. The strains demonstrated various combinations of resistance and/or susceptibility to the antibiotics tested. Tobramycin was the most effective aminoglycoside when used in combination with beta-lactam antibiotics. Carbenicillin and cefamandole demonstrated similar activity with aminoglycosides in synergy experiments. Tobramycin-carbenicillin was found to be the superior pairs as indicated by the total number of strains inhibited. This combination was the only one effective against certain high drug resistant strains and the strain resistant to all four aminoglycosides. Carbenicillin or cefamandole with tobramycin exhibited comparable activity against multiple drug resistant organisms. However, mutants significantly more resistant to cefamandole developed during susceptibility testing. The findings of this study have clinical relevance for treating infections by this formidable pathogen.

In vitro activity of LY127935

LY127935 is a unique new beta-lactam antibiotic. Its activity against 536 clinical isolates was studied by using microdilution methods of susceptibility testing and compared with the activities of cefamandole, cefoxitin, and cephalothin. The lowest concentrations required to inhibit at least 90% of strains tested (MIC(90)s) of LY127935 for Staphylococcus aureus, Streptococcus pyogenes, Streptococcus agalactiae, and Streptococcus pneumoniae ranged from 2 to 8 mug/ml. The MIC(90)s for other staphylococci and streptococci were higher. The MIC(90)s for Enterobacteriaceae and Pseudomonas species ranged from 0.12 to 8 mug/ml and 8 to >32 mug/ml, respectively. The MIC(90)s for anaerobes ranged from 2 to >32 mug/ml. As determined by MIC(90)s, LY127935 was consistently the least active antibiotic against facultatively anaerobic gram-positive cocci and the most active against aerobic and facultatively anaerobic gram-negative bacilli. Its position with respect to activity against anaerobes varied from being the most active against Bacteroides fragilis and Clostridium perfringens to the least active against anaerobic cocci. In a population of multidrug-resistant isolates, concentrations of 8 mug or less of LY127935 per ml inhibited 82% of Enterobacteriaceae; concentrations of 32 mug or less per ml inhibited 100% of Enterobacteriaceae and 40% of P. aeruginosa. Increasing the inoculum size by 100-fold did not increase the minimal inhibitory concentrations of LY127935 or cefoxitin but did increase minimal inhibitory concentrations of cefamandole and cephalothin for some Enterobacteriaceae. All four drugs were bactericidal; minimal bactericidal concentrations were the same or one concentration higher than minimal inhibitory concentrations for 91 to 96% of strains tested. The broad spectrum and marked in vitro activity of LY127935 make it a promising new antibiotic.

Antibacterial activity of a new parenteral cephalosporin--HR 756: comparison with cefamandole and ceforanide

HR 756, a new parenteral cephalosporin that is beta-lactamase resistant, was tested against 271 bacterial isolates. Both agar and broth dilution testing were employed, using two media and two inoculum sizes of bacteria. Antibacterial activity of the drug was compared to that of cefamandole (CFM) and ceforanide (CFN). In agar, HR 756 was more active than CFM and CFN against all bacteria tested except isolates of Staphylococcus aureus, which were better inhibited by CFM. HR 756 exhibited some antipseudomonas activity in agar, although a marked inoculum effect was apparent. A comparison of median minimum inhibitory and bactericidal concentrations in broth showed again that HR 756 was the most active of these three drugs. HR 756 demonstrated enhanced antibacterial activity compared to CFM and CFN against bacteria sensitive to all three drugs as well as against more resistant isolates of Serratia marcescens, Enterobacter species, and indole-positive Proteus. As with other cephalosporins, results for most bacteria were affected by inoculum size, medium, and type of dilution test employed in in vitro studies.

Emergence of resistance to cefamandole: possible role of cefoxitin-inducible beta-lactamases

Selection of resistance to cefamandole has been observed, and the drug has failed to protect animals lethally infected with certain Enterobacteriaceae that appeared to be highly susceptible in vitro. Using spectrophotometric assays, some of these organisms were found to produce beta-lactamases highly active against cefamandole. Cefoxitin, a poor enzyme substrate, was found to be superior to both cephalothin and cefamandole in induction of these enzymes. A simple disk induction test was developed and used to examine 147 Enterobacteriaceae for production of these beta-lactamases. The enzymes were found in 69% of cephalothin-resistant, cefamandole-susceptible strains and in only 3% of strains susceptible to both cephalothin and cefamandole. They were most prevalent among isolates of Enterobacter, indole-positive Proteus, and Serratia. Since selection of resistance and therapeutic failures have occurred most often among these genera, the relationship between presence of inducible enzymes and outcome of therapy should be examined further in humans.

Cefuroxime: a review of its antibacterial activity, pharmacological properties and therapeutic use

Cefuroxime is a new semisynthetic cephalosporin for parenteral administration. It is resistant to destruction by beta-lactamases produced by staphylococci and most Gram-negative aerobic bacteria and is active against many bacteria resistant to cephalothin. Cefuroxime is the most active of the cephalosporins against gonococci and Haemophilus influenzae particularly against beta-lactamase producing strains. Given by intramuscular or intravenous injection cefuroxime is effective against a wide variety of infections caused by Gram-positive or Gram-negative aerobes, but has no effect against infections caused by Pseudomonas aeruginosa or B. fragilis. Cefuroxime is of value in the treatment of respiratory infections due to Haemophilus influenzae and Streptocococcus pneumoniae and is useful against cephalosporin-resistant Klebsiella and Enterobacter infections. Cefuroxime is an alternative to spectinomycin for the treatment of beta-lactamase producing Neisseria gonorrhoeae infections. It is generally well tolerated and appears not to be nephrotoxic when given alone at usual dosages.