Influence of inoculum size on activity of cefoperazone, cefotaxime, moxalactam, piperacillin, and N-formimidoyl thienamycin (MK0787) against Pseudomonas aeruginosa

Microscopic Analysis of Bacterial Inoculum Effect Using Micropatterned Biochip

Antimicrobial resistance has become a major problem in public health and clinical environments. Against this background, antibiotic susceptibility testing (AST) has become necessary to cure diseases in an appropriate and timely manner as it indicates the necessary concentration of antibiotics. Recently, microfluidic based rapid AST methods using microscopic analysis have been shown to reduce the time needed for the determination of the proper antibiotics. However, owing to the inoculum effect, the accurate measurement of the minimal inhibitory concentration (MIC) is difficult. We tested four standard bacteria: Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Enterococcus faecalis, against five different antibiotics: piperacillin, cefotaxime, amikacin, levofloxacin, and ampicillin. The results showed that overall, the microfluidic system has a similar inoculum effect compared to the conventional AST method. However, due to the different testing conditions and determination protocols of the growth of the microfluidic based rapid AST, a few results are not identical to the conventional methods using optical density. This result suggests that microfluidic based rapid AST methods require further research on the inoculum effect for practical use in hospitals and can then be used for effective antibiotic prescriptions.

Imipenem/Cilastatin: The First Carbapenem Antibiotic

Imipenem/cilastatin is the first of a new class of β-lactam antibiotics called carbapenems. The antibacterial spectrum of imipenem exceeds any antibiotic investigated to date and includes gram-positive, gram-negative, and anaerobic organisms. Only methicillin-resistant organisms, Strep. faecium, Pseudomonas cepacia, and Pseudomonas maltophilia have been shown to be resistant. Imipenem is administered in a 1:1 ratio with cilastatin, which inhibits a renal enzyme (dehydropeptidase) and improves urinary recovery of imipenem. The elimination half-life of both compounds is 1.0 hours and recommended doses are 0.25–0.5 g iv q6h. Adverse events are similar in nature and incidence to β-lactam antibiotics, with phlebitis/thrombophlebitis, diarrhea, nausea, skin rash, and elevations of hepatic enzymes most common.

Clinical studies in phase II and III trials have shown imipenem/cilastatin to be effective in soft tissue infections, endocarditis, obstetrics and gynecology, complicated urinary tract infections, mixed anaerobic-aerobic infections, osteomyelitis, bacteremias, and pneumonias. Several comparative clinical trials have shown imipenem/cilastatin to be equal in efficacy to combination therapy. Imipenem/cilastatin may prove to be an alternative to combination antibiotic therapy because of its extremely broad spectrum of activity.

Burn Wound Sepsis

Wound sepsis remains perhaps the most feared sequela in the patient who has suffered major burn injuries and leads to overwhelming mortality among patients with extensive burn wounds. The presence of large areas of devitalized, necrotic tissue, coupled with the profound immunosuppression that usually follows major injury, sets the stage for rapid microbial proliferation in the wound; when microbes invade subjacent, previously vi able tissues, invasive burn wound sepsis is defined. Top ical antimicrobial drugs probably have only a limited effect in preventing wound sepsis, and organisms now frequently emerge that are resistant to the commonly used topical agents. Salient factors in the diagnosis and treatment of invasive wound sepsis are discussed in this review. Prevention of wound sepsis, however, is one of the primary objectives of current burn care. Early burn wound excision and immediate wound closure with autograft or a suitable biologic dressing has emerged as the best means for prevention of generalized wound sepsis.

A review of the pharmacokinetics and pharmacodynamics of aztreonam

The monobactam aztreonam is currently being re-examined as a therapeutic agent in light of the global spread of carbapenem resistance in aerobic Gram-negative bacilli and aztreonam's stability to Ambler class B metallo-β-lactamases. Of particular interest are the pharmacokinetic and pharmacodynamic properties of aztreonam alone and in combination with β-lactamase inhibitors. The choice of inhibitor may vary depending on the spectrum of β-lactamases produced by Enterobacteriaceae. The monobactam ring is also being used to produce new developmental monobactams. Thus, a greater understanding of aztreonam pharmacokinetics and dynamics is of great relevance in drug development. This review summarizes the pharmacokinetic profile of aztreonam in man and its pharmacodynamics in human and pre-clinical studies when studied alone and with β-lactamase inhibitors.

Vancomycin and maltodextrin affect structure and activity of Staphylococcus aureus biofilms

Hyperosmotic agents such as maltodextrin negatively impact bacterial growth through osmotic stress without contributing to drug resistance. We hypothesized that a combination of maltodextrin (osmotic agent) and vancomycin (antibiotic) would be more effective against Staphylococcus aureus biofilms than either alone. To test our hypothesis, S. aureus was grown in a flat plate flow cell reactor. Confocal laser scanning microscopy images were analyzed to quantify changes in biofilm structure. We used dissolved oxygen microelectrodes to quantify how vancomycin and maltodextrin affected the respiration rate and oxygen penetration into the biofilm. We found that treatment with vancomycin or maltodextrin altered biofilm structure. The effect on the structure was significant when they were used simultaneously to treat S. aureus biofilms. In addition, vancomycin treatment increased the oxygen respiration rate, while maltodextrin treatment caused an increase and then a decrease. An increased maltodextrin concentration decreased the diffusivity of the antibiotic. Overall, we conclude that (1) an increased maltodextrin concentration decreases vancomycin diffusion but increases the osmotic effect, leading to the optimum treatment condition, and (2) the combination of vancomycin and maltodextrin is more effective against S. aureus biofilms than either alone. Vancomycin and maltodextrin act together to increase the effectiveness of treatment against S. aureus biofilm growth.

Development and qualification of a pharmacodynamic model for the pronounced inoculum effect of ceftazidime against Pseudomonas aeruginosa

Evidence is mounting in support of the inoculum effect (i.e., slow killing at large initial inocula [CFUo]) for numerous antimicrobials against a variety of pathogens. Our objectives were to (i) determine the impact of the CFUo of Pseudomonas aeruginosa on ceftazidime activity and (ii) to develop and validate a pharmacokinetic/pharmacodynamic (PKPD) mathematical model accommodating a range of CFUo. Time-kill experiments using ceftazidime at seven concentrations up to 128 mg/liter (MIC, 2 mg/liter) were performed in duplicate against P. aeruginosa PAO1 at five CFUo from 10(5) to 10(9) CFU/ml. Samples were collected over 24 h and fit by candidate models in NONMEM VI and S-ADAPT 1.55 (all data were comodeled). External model qualification integrated data from eight previously published studies. Ceftazidime displayed approximately 3 to 4 log(10) CFU/ml net killing at 10(6.2) CFUo and concentrations of 4 mg/liter (or higher), less than 1.6 log(10) CFU/ml killing at 10(7.3) CFUo, and no killing at 10(8.0) CFUo for concentrations up to 128 mg/liter. The proposed mechanism-based model successfully described the inoculum effect and the concentration-independent lag time of killing. The mean generation time was 28.3 min. The effect of an autolysin was assumed to inhibit successful replication. Ceftazidime concentrations of 0.294 mg/liter stimulated the autolysin effect by 50%. The model was predictive in the internal cross-validation and had excellent in silico predictive performance for published studies of P. aeruginosa ATCC 27853 for various CFUo. The proposed PKPD model successfully described and predicted the pronounced inoculum effect of ceftazidime in vitro and integrated data from eight literature studies to support translation from time-kill experiments to in vitro infection models.

Parameterization of inoculum effect via mathematical modeling: aminoglycosides against Staphylococcus aureus and Escherichia coli

Inoculum effect describes the inoculum size dependent changes in minimum inhibitory concentrations (MIC) exhibited by antibiotic-bacterium combinations demonstrating such effect. Traditionally, inoculum effect has been loosely defined based on the extent of increase in the MIC with respect to the increase in inoculum size. In most studies, assessment of MIC data has relied on the arbitrary selection of a point of reference for both baseline MIC and inoculum size. More importantly, this conventional method of assessment does not permit information conveyed in a complete MIC versus inoculum size profile to be fully explored. To undertake these issues, a mathematical model was developed for the description of the entire inoculum effect profile. With the employment of three key parameter estimates, i.e., the baseline MIC, the threshold inoculum size at which the increase in MIC commences, and the rate of increase in MIC with respect to inoculum size, both the shape and location of the profile could be adequately defined. To verify the application of this model, a series of four aminoglycosides were tested against standard strains of E. coli and S. aureus. Results showed a good degree of organism specificity and antibiotic-class dependency of the inoculum effect profiles. Analysis of the parameter estimates obtained provided further support for these observations. In conclusion, the mathematical model developed in the present study adequately described the inoculum effect exhibited by the various aminoglycoside-bacterium combinations tested. The parameter estimates generated by the modeling approach allowed comparison and quantitative analysis of the inoculum effect profiles with minimal difficulties.

Comparison of the inoculum effects of members of the family Enterobacteriaceae on cefoxitin and other cephalosporins, beta-lactamase inhibitor combinations, and the penicillin-derived components of these combinations

We compared the inoculum effects of 105 recent clinical isolates of the family Enterobacteriaceae on cefoxitin, other cephalosporins, aztreonam, and three beta-lactamase inhibitors (clavulanic acid, sulbactam, and tazobactam) and their penicillin-derived components. Piperacillin and aztreonam showed the largest inoculum effect, and cefoxitin showed the smallest. The other cephalosporins tested (cefotetan, ceftizoxime, and ceftriaxone) showed an intermediate inoculum effect. In general, the inoculum effect was of greater magnitude for the penicillin and beta-lactamase inhibitor combinations than for the cephalosporins tested. Bactericidal activity was assayed and morphologic changes were monitored for selected strains exhibiting a large inoculum effect. MICs correlated with bactericidal activity at an inoculum level of 10(5) CFU/ml, while activity at 10(8) CFU/ml was variable. Cefoxitin demonstrated the least filamentous transformation and the most rapid bactericidal activity. Aztreonam showed the most marked filamentous transformation and was no longer bactericidal at 10(8) CFU/ml. The beta-lactamase inhibitor combinations showed variable bactericidal activity, and regrowth occurred with a number of strains with all three agents tested.

Effects of beta-lactam antibiotics imipenem/cilastatin and cefodizime on cellular and humoral immune responses in BALB/c-mice

The effects of a 7-day chemotherapy with two broad-spectrum beta-lactam antibiotics (imipenem/cilastatin and cefodizime) on the humoral and cellular immune responses in BALB/c-mice were investigated. Antibiotic dosages were calculated on a body weight basis from therapeutical dosages in human medicine. Treatment of experimental mice with imipenem/cilastatin and cefodizime did not influence the production of immunoglobulines (IgM and IgG) nor the delayed type hypersensitivity to oxazolone. In vitro, exposure of human granulocytes to imipenem/cilastatin and cefodizime did not interfere with their phagocytic activity as determined by chemiluminescence assay. Subinhibitory concentrations of both antibiotics modified Staphylococcus aureus and made them more susceptible for granulocyte phagocytosis in chemiluminescence assays.

Postantibiotic and bactericidal effect of imipenem against Pseudomonas aeruginosa

The postantibiotic effect of imipenem on Pseudomonas aeruginosa was studied at different inocula using one ATCC strain and four clinical isolates. The postantibiotic effect was measured using two different methods: viable counts and bioluminescence assay of intracellular bacterial ATP. The postantibiotic effect could be demonstrated with both methods (viable counts 1-2 h, ATP assay 3-5 h) for all strains at an inoculum of 10(6) CFU/ml. When the inoculum was raised to 10(8) CFU/ml, no postantibiotic effect could be observed with either method using routine growth conditions. This disappearance of the postantibiotic effect coincided with a loss of bactericidal effect of imipenem when high inocula were used. Improved oxygenation of the cultures restored the bactericidal and postantibiotic effects of imipenem at high inocula.

Recent advances in the treatment of pneumonia in the intensive care unit

The treatment of pneumonia acquired in the intensive care unit (ICU) includes intravenous antibiotics and supportive care. In many cases, the aetiologic agent of infection is not clear and empirical broad-spectrum antibiotic regimens are commonly used. Combinations of beta-lactam and aminoglycoside agents are particularly popular due to the high incidence of Gram-negative bacillary and Staphylococcus aureus pneumonias in the hospital setting. Several new approaches to treatment of pneumonia in the ICU are currently being evaluated including single-agent empirical treatment with broad-spectrum beta-lactam agents; intrabronchial aminoglycoside instillation therapy; oral quinolone agents for treatment of Gram-negative bacillary pneumonia; and passive immune therapy. Conventional and experimental therapy are discussed in this report.

Agents for the treatment of Pseudomonas aeruginosa infections

Pseudomonas aeruginosa is the most common pathogen of Pseudomonas species. One of the most virulent organisms pathogenic to man, P aeruginosa can cause a variety of infections in humans. Despite the introduction of many new antimicrobial agents with enhanced activity against P aeruginosa, the high mortality rate associated with the organism over the past two decades continues.

Imipenem/cilastatin. A review of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy

Imipenem is the first available semisynthetic thienamycin and is administered intravenously in combination with cilastatin, a renal dipeptidase inhibitor that increases urinary excretion of active drug. In vitro studies have demonstrated that imipenem has an extremely wide spectrum of antibacterial activity against Gram-negative and Gram-positive aerobic and anaerobic bacteria, even against many multiresistant strains of bacteria. It is very potent against species which elaborate beta-lactamases. Imipenem in combination with equal doses of cilastatin has been shown to be generally well tolerated and an effective antimicrobial for the treatment of infections of various body systems. It is likely to be most valuable as empirical treatment of mixed aerobic and anaerobic infections, bacteraemia in non-neutropenic patients and serious hospital-acquired infections.

In vitro activity of piperacillin, ticarcillin, and mezlocillin alone and in combination with aminoglycosides against Pseudomonas aeruginosa

A total of 103 isolates of Pseudomonas aeruginosa were studied to compare the in vitro effectiveness of three beta-lactam antibiotics (piperacillin, ticarcillin, and mezlocillin) when used alone and in combination with four aminoglycosides (tobramycin, gentamicin, amikacin, and netilmicin). All drugs were tested as single agents against a standard inoculum (5 X 10(5) CFU/ml). The three antipseudomonal penicillins were also tested against the isolates at a higher inoculum concentration (10(7) CFU/ml). Synergy testing was performed by the two-dimensional checkerboard method and was defined by a fractional bactericidal index of less than or equal to 0.5 and bacterial killing accomplished at antibiotic concentrations no greater than those achievable in serum. All combinations were assessed for synergy. The degree of synergy was further analyzed by dividing the isolates into groups based on their susceptibility and resistance to the individual agents in the combination. The overall effectiveness of the various aminoglycoside-antipseudomonal penicillin combinations was assessed regarding their ability to kill the isolates either as single agents or through synergy. Piperacillin was the most active antipseudomonal penicillin, and tobramycin and amikacin were the most active aminoglycosides when used as single agents. When tested against isolates at a higher inoculum concentration, ticarcillin was significantly more active than the other beta-lactams. The highest degree of overall synergy was noted with gentamicin-ticarcillin (78.2% of strains) and amikacin-piperacillin (77% of strains). When assessed for overall effectiveness, all combinations containing amikacin were the most active. The combination of amikacin-piperacillin was the most effective, with activity against 96% of all isolates.

Inoculum effect of beta-lactam antibiotics on Enterobacteriaceae

Seven beta-lactam antibiotics were studied for both their antimicrobial activity and the degree to which they produced inoculum effect on Escherichia coli, Klebsiella pneumoniae, and Salmonella typhimurium. Aztreonam, cefoperazone, and ceftazidime were poorly bactericidal, caused marked bacterial filamentation, and exhibited a large inoculum effect on E. coli, K. pneumoniae, and S. typhimurium. Cefotaxime and ceftriaxone were more rapidly bactericidal, caused only a moderate amount of filamentous forms, and exhibited a modest inoculum effect, while cefoxitin and imipenem both were rapidly bactericidal and exhibited only a minimal-to-no-inoculum effect. The inoculum effect did not correlate with drug stability during incubation with the bacteria. Inoculum effect on these species of the family Enterobacteriaceae appears to be a manifestation of increase in optical density secondary to the development of filamentous bacterial forms with an increase in bacterial mass during exposure to antibiotics which are not rapidly bactericidal. These observations have a clear significance for the susceptibility testing of beta-lactam antibiotics when turbidity is used as a parameter to determine presence of bacterial growth.

Identification and antibiotic susceptibility of bacterial isolates from burned patients

We retrieved bacterial blood isolates from 397 adult burned patients admitted over a 7-year period. Sixty-two patients (15.6 per cent) developed true-positive bacterial blood cultures (judged non-contaminants), and of these 30 (48.4 per cent) expired. Pseudomonas aeruginosa (24 isolates), Staphylococcus aureus (19) and Klebsiella pneumoniae (19) were the most frequent isolates. In vitro susceptibilities of 149 isolates were determined to 12 antibiotics (gentamicin, amikacin, ticarcillin, piperacillin, mezlocillin, azlocillin, cefazolin, cefotaxime, ceftazidime, cefoperazone, thienamycin and ticarcillin-clavulinic acid) using agar diffusion assay. Thienamycin proved the most active agent (97 per cent of isolates susceptible). Cefoperazone was the most active cephalosporin (95 per cent susceptible). Twenty-eight organisms demonstrated multiple drug resistance; patients with such organisms had a 71 per cent mortality. Thienamycin was the most active agent against such isolates (27/28 susceptible). Susceptibilities of all 149 isolates to combinations of antibiotics were calculated, assuming no synergism or antagonism; some combinations of third-generation cephalosporins with the newer penicillins may prove to be as effective as combinations including aminoglycosides.

In vitro activity of BMY-28142 against pediatric pathogens, including isolates from cystic fibrosis sputum

The antibacterial activity of BMY-28142, a new aminothiazole cephalosporin, was measured by standardized broth microdilution and agar dilution methods against 450 gram-positive and gram-negative bacteria isolated from pediatric infections, including acute pulmonary exacerbations of cystic fibrosis. BMY-28142 activity was compared with that of aminoglycosides, beta-lactams, chloramphenicol, trimethoprim-sulfamethoxazole, vancomycin, and clindamycin. The activity of BMY-28142 in combination with other antimicrobial agents against Pseudomonas aeruginosa was also determined. Furthermore, the effects of inoculum and pH on BMY-28142 activity were evaluated. BMY-21842 was active against most of the gram-positive and gram-negative isolates, with the exception of methicillin-resistant Staphylococcus aureus and Pseudomonas cepacia. The combination of BMY-28142 with tobramycin was often synergistic, and combinations of BMY-28142 with either polymyxin B or imipenem were usually antagonistic. BMY-28142 antibacterial activity could be adversely affected at extremes of medium pH and by high inoculum densities.

Safety and efficacy of imipenem/cilastatin in treatment of complicated urinary tract infections

Forty-three patients were treated with imipenem/cilastatin for urinary tract infections. The patients were predominantly men (77 percent), 60 years of age or older (81 percent), and had infections caused by Pseudomonas aeruginosa (58 percent). Forty of 43 cases were complicated (including tumor, stone, obstruction, and renal insufficiency). Approximately 33 percent of patients were febrile (temperature greater than 100.4 degrees). All patients received 500 mg of imipenem/cilastatin intravenously every eight hours. Microbiologic eradication was defined as a sterile urine culture sample obtained from two consecutive urine cultures, one during therapy and one five to nine days after therapy. All patients experienced clinical improvement with microbiologic eradication. No patients experienced drug-related clinical adverse effects.

Gram-negative bacillary infections. Pathogenic and pathophysiologic correlates

Gram-negative bacillary infections continue to be extremely important. Escherichia coli is the single most frequently encountered pathogen, followed by organisms belonging to the tribe Klebsiella-Enterobacter-Serratia and Proteus-Providencia. Pseudomonas aeruginosa, although it receives considerable (perhaps excessive) attention, is found relatively less frequently, occurring principally in the hospitalized patient who is immunocompromised. Many factors, both host and microbial, are responsible for invasiveness, virulence, and pathogenicity of gram-negative bacilli, but their relative roles, importance, and the pathophysiologic reactions they trigger are yet to be precisely defined. Certain aspects of many (but certainly not all) of the pathogenic correlates considered important in gram-negative bacillary infections, such as microbial flora, local barriers, surface and serum antibodies, complement, cell-mediated immunity, slime production, capsules, pili, endotoxin, cell wall components, extracellular products, and inoculum size are discussed herein. Points at which preventive or therapeutic strategies might be developed are offered. The benefits of antibiotics in managing susceptible gram-negative bacillary infections appear to be plateauing. If further advances are to be made in the therapy of these infections, new approaches to rapidly identifying the responsible etiologic agent and a better understanding of the factors responsible for invasiveness, virulence, and pathogenicity are needed.

Review of the in vitro spectrum of activity of imipenem

Imipenem (N-formimidoyl thienamycin, MK0787), a new carbapenem was found to have the widest antimicrobial activity of currently available beta-lactam drugs. Enterobacteriaceae had minimal inhibitory concentrations of imipenem of 8.0 micrograms/ml or less for 99.8 percent of clinical isolates. Only rare strains of Enterobacter species and Proteus mirabilis have higher imipenem minimal inhibitory concentration results. Hemophilus and Neisseria species were inhibited, but minimal inhibitory concentrations of imipenem were higher than those reported for third-generation cephalosporins. Only Pseudomonas maltophilia and Pseudomonas cepacia strains were imipenem resistant (MIC50 greater than 32 micrograms/ml) among the commonly isolated non-enteric gram-negative bacilli. All anaerobes were found susceptible to imipenem with the exception of some strains of Clostridium difficile. Staphylococcus species and non-enterococcal streptococci were very susceptible to imipenem. Streptococcus faecalis had higher minimal inhibitory concentrations of imipenem (MIC90 3.1 micrograms/ml) and S. faecium strains were frankly resistant. Methicillin-resistant S. aureus isolates had a MIC90 of 27.2 micrograms imipenem/ml. Imipenem was generally bactericidal except for marked minimal inhibitory and minimal bactericidal concentration differences with enterococci, Listeria, methicillin-resistant staphylococci, and some P. aeruginosa strains. The minimal inhibitory and minimal bactericidal concentrations of imipenem were not significantly influenced by organism inoculum size, probably because of its beta-lactamase stability to nearly all commonly encountered bacterial enzymes. Imipenem was found to be an excellent inhibitor of beta-lactamases and a potent enzyme inducer. The induction characteristic seems responsible for the antagonistic interactions of imipenem with some enzyme-labile beta-lactams in combination. Imipenem had limited stability in some in vitro susceptibility test systems. The 10 micrograms disk test or dry-form broth micro-dilution systems were preferred, applying the interpretive criteria from the National Committee for Clinical Laboratory Standards (M2-A3). Imipenem-resistant strains were rarely found in clinical practice and bacteria resistant to newer beta-lactams and aminoglycosides were generally very susceptible to this new carbapenem.

In vitro activity of imipenem--a review

A review is given of the microbiological properties of imipenem, a new carbapenem antibiotic with an exceptionally broad spectrum of antibacterial activity. An evaluation of results of numerous in vitro studies reveals that imipenem effectively inhibited growth of 53 of 55 bacterial species, the mean MIC90 being less than 8 mg/l. The MIC90 for cocci, with the exception of Staphylococcus epidermidis, is in the range of 0.01-3.1 mg/l. The MIC90 for all Enterobacteriaceae is equal to or less than 8 mg/l. Pseudomonas aeruginosa and other non-fermentative gram-negative bacteria are generally susceptible to imipenem, only Pseudomonas maltophilia and Pseudomonas cepacia showing intrinsic resistance. Imipenem is currently the most active drug available against anaerobic bacteria, the MIC usually being below 1 mg/l even for Bacteroides fragilis. Rare bacteria such as Nocardia asteroides, Listeria monocytogenes or fast growing Mycobacterium spp. which cause difficult-to-treat infections are also susceptible to imipenem. Increases in inoculum size have only a minimal effect on activity of the drug. In most species the MBC only slightly exceeded the MIC; however in the case of Streptococcus faecalis the MBC value was many times the MIC value. Synergism has been observed in combinations of imipenem with aminoglycosides, and antagonism in combinations with other beta-lactam antibiotics against Pseudomonas aeruginosa and Serratia marcescens. Imipenem is stable in the presence of the common chromosomal and plasmid-mediated enzymes. Induction of inactivating enzymes was observed in staphylococci, Pseudomonas aeruginosa and Serratia marcescens.

Comparative activities of ciprofloxacin, ticarcillin, and tobramycin against experimental Pseudomonas aeruginosa pneumonia

The therapeutic efficacy of ciprofloxacin, an investigational quinoline derivative, was compared with those of ticarcillin and tobramycin in guinea pigs with experimental Pseudomonas aeruginosa pneumonia. Guinea pigs challenged with tracheal instillations of 10(8) CFU of P. aeruginosa developed acute pneumonia, for which survival rates were: controls, 0%; ticarcillin treatment, 37%; ciprofloxacin treatment, 57%; and tobramycin treatment, 69%. Intrapulmonary killing of P. aeruginosa was greater (P less than 0.05) in animals treated with ciprofloxacin or tobramycin than in groups treated with ticarcillin. A more chronic, nonfatal form of bronchopneumonia caused by P. aeruginosa was induced with agar beads impregnated with bacteria for pulmonary challenge. In this model, ciprofloxacin treatment resulted in significantly (P less than 0.001) greater intrapulmonary killing than did any other therapy. These data suggest that ciprofloxacin may be useful in the treatment of acute and more-chronic forms of pulmonary infection caused by P. aeruginosa.

Inoculum effect of new beta-lactam antibiotics on Pseudomonas aeruginosa

The degree of the inoculum effect shown by the new beta-lactam antibiotics with Pseudomonas aeruginosa was investigated, and the antibiotics were divided into three groups based upon the observations. The group 1 antibiotics (cefotaxime, moxalactam, cefoperazone, azlocillin, piperacillin, and aztreonam) demonstrated a large inoculum effect, were poorly bactericidal, produced aberrant, elongated bacilli, and did not inhibit the increase in turbidity of high inocula during an 18-h incubation. The group 2 antibiotics (ceftazidime and ticarcillin) were slowly bactericidal, caused minimal formation of aberrant, elongated bacilli, and slowly decreased the turbidity of high inocula. The group 3 antibiotics (imipenem and gentamicin) were bactericidal, did not cause the formation of elongated bacilli, and decreased the turbidity of high inocula rapidly. Data are presented which suggest that the inoculum effect seen with the group 1 beta-lactam antibiotics is related to (i) the poor intrinsic antibactericidal activity of these antibiotics for P. aeruginosa at the inocula tested and (ii) failure of these antibiotics to inhibit the formation of aberrant and filamentous bacilli, which can result in increased bacterial mass and turbidity.

Antagonism by chloramphenicol of broad-spectrum beta-lactam antibiotics against Klebsiella pneumoniae

Chloramphenicol combined with cefotaxime, moxalactam, cefoperazone, aztreonam, or imipenem was tested in vitro against clinical isolates of Klebsiella pneumoniae. By time-kill cultures (killing curves), chloramphenicol interfered with activity of all five beta-lactams. When chloramphenicol was added before the beta-lactams, the action of cefotaxime, moxalactam, or cefoperazone against all isolates was antagonized at all times tested. The action of aztreonam was antagonized against four of six isolates. With imipenem, antagonism occurred against half of the isolates at some time during 24 h when chloramphenicol was added simultaneously, provided that a sufficient inoculum of K. pneumoniae was employed. Generally, less antagonism resulted when chloramphenicol was added after the cephalosporins. Interference of bactericidal activity of three new cephalosporins by chloramphenicol has potential clinical relevance to the therapy of gram-negative bacillary meningitis. The lesser antagonism of aztreonam and imipenem by chloramphenicol is of uncertain clinical relevance but indicates that this in vitro phenomenon may apply to a wide range of beta-lactam antibiotics.

In vitro activity of U-63196E, a new cephalosporin, against clinical bacterial isolates

The in vitro activity of U-63196E, a new cephalosporin, was compared with those of other extended-spectrum cephalosporins and penicillins against clinical bacterial isolates. Against Pseudomonas aeruginosa, the activity of U-63196E was comparable to those of cefoperazone and piperacillin, each of which inhibited 90% of strains at concentrations of less than or equal to 16 micrograms/ml. The new drug also demonstrated activity against a variety of other bacterial species, but it was generally less active than cefotaxime, moxalactam, and cefoperazone against members of the family Enterobacteriaceae and staphylococci. The presence of any 1 of 10 plasmid-mediated beta-lactamases in a series of otherwise isogenic laboratory strains of P. aeruginosa resulted in a significant reduction in the activity of U-63196E in comparison with its activity against the parent strain, which lacks these enzymes. Combinations of U-63196E with tobramycin demonstrated bacteriostatic synergism against 11 of 20 clinical isolates of P. aeruginosa.

Comparative efficacies of piperacillin, azlocillin, ticarcillin, aztreonam, and tobramycin against experimental Pseudomonas aeruginosa pneumonia

The therapeutic efficacies of the newer beta-lactam antibiotics piperacillin, azlocillin and aztreonam were compared with the efficacies of ticarcillin and tobramycin in a guinea pig model of experimental Pseudomonas aeruginosa pneumonia. For animals challenged with 2 X 10(8) CFU, tobramycin treatment resulted in survival rates and intrapulmonary killing of pseudomonads which were significantly greater than those found with any of the beta-lactam agents. There were no significant differences noted among the individual beta-lactam agents. When animals were challenged with 200-fold-fewer organisms (10(6) CFU), there was no significant difference between the efficacy of tobramycin and those of the various beta-lactams. These data suggest that tobramycin is particularly valuable in treating more severe P. aeruginosa pneumonia, whereas a number of different beta-lactam agents are of equivalent value in less severe lung infections.

Imipenem versus moxalactam in the treatment of serious infections

Imipenem (formerly imipemide, N-formimidoyl thienamycin, or MK0787) was compared to moxalactam in a randomized therapeutic trial involving 39 evaluable patients with serious bacterial infections. Of those treated with imipenem, 89% were cured or improved versus 60% for moxalactam (P = 0.06). Although mucocutaneous fungal infections occurred in both groups (25 and 10%, respectively), Streptococcus faecalis superinfection was seen in two patients in the moxalactam group only. Adverse drug reactions occurred with both drugs, although bleeding occurred in three patients treated with moxalactam.

Moxalactam (latamoxef). A review of its antibacterial activity, pharmacokinetic properties and therapeutic use

Moxalactam (latamoxef) is a new synthetic oxa-beta-lactam antibiotic administered intravenously or intramuscularly. It has a broad spectrum of activity against Gram-positive and Gram-negative aerobic and anaerobic bacteria, is particularly active against Enterobacteriaceae and is resistant to hydrolysis by beta-lactamases. Moxalactam has moderate activity against Pseudomonas aeruginosa, but on the basis of present evidence can not be recommended as sole antibiotic treatment of known or suspected pseudomonal infections. Like the related compounds, the cephalosporins, moxalactam is effective in the treatment of complicated urinary tract infections and lower respiratory tract infections caused by Gram-negative bacilli. As moxalactam is also active against Bacteroides fragilis it has considerable potential in the treatment of intra-abdominal infections in patients with normal immunological mechanisms, as well as in immunocompromised patients, when used alone or in combination with other antibiotics. Likewise, its ready penetration into the diseased central nervous system, its high level of activity against Gram-negative bacilli, and the lack of necessity to monitor drug plasma concentrations, indicate its potential value in the treatment of neonatal Gram-negative bacillary meningitis. Further clinical experience is needed before it can be determined whether moxalactam alone can be used in the treatment of conditions for which the aminoglycosides are drugs of choice, but if established as equally effective, moxalactam has the advantage of being devoid of nephrotoxicity. Bleeding is a potentially serious problem, however, particularly in the elderly, malnourished and in the presence of renal impairment.

The in vitro activity of N-formimidoyl thienamycin compared with other broad-spectrum cephalosporins and with clindamycin and metronidazole

N-formimidoyl thienamycin is a new semisynthetic beta-lactam antibiotic still awaiting clinical trials. We have investigated the in vitro activity of N-formimidoyl thienamycin against 413 fresh clinical isolates and compared it to other new beta-lactam drugs and to clindamycin and metronidazole in the agar dilution test. All coliforms were inhibited by less than or equal to 1 mg/l and no member of the Proteus group was resistant to more than 8 mg/l of N-formimidoyl thienamycin. The MICs for Pseudomonas aeruginosa were less than or equal to 4 mg/l. Beta-haemolytic streptococci, pneumococci and staphylococci, including methicillin-resistant strains, were all inhibited by 0.125 mg/l or less. Streptococcus faecalis strains were all susceptible to less than or equal to 2 mg/l. N-formimidoyl thienamycin was highly active against the anaerobes tested. N-formimidoyl thienamycin exhibited bactericidal activity, and changes in inoculum size had little effect on the MICs. This data has shown that N-formimidoyl thienamycin has excellent antibacterial activity and an unusually broad spectrum of activity.

Cefotaxime. A review of its antibacterial activity, pharmacological properties and therapeutic use

SYNOPSIS

Cefotaxime is a new 'third generation' semisynthetic cephalosporin administered intravenously or intramuscularly. It has a broad spectrum of activity against Gram-positive and Gram-negative aerobic and anaerobic bacteria, and is generally more active against Gram-negative bacteria than the 'first' and 'second generation' cephalosporins. Although cefotaxime has some activity against Pseudomonas aeruginosa, on the basis of present evidence it cannot be recommended as sole antibiotic therapy for pseudomonal infections. However, cefotaxime has been effective in treating infections due to other 'difficult' organisms, such as multidrug-resistant Enterobacteriaceae. Like other cephalosporins, cefotaxime is effective in treating patients with complicated urinary tract and lower respiratory tract infections, particularly pneumonia caused by Gram-negative bacilli. High response rates have also been achieved in patients with Gram-negative bacteraemia. Although favourable clinical results have been obtained in patients with infections caused by mixed aerobic/anaerobic organisms (such as peritonitis or soft tissue infections), the relatively low in vitro activity of cefotaxime against Bacteroides fragilis may restrict its usage in situations where this organism is the suspected or proven pathogen. In preliminary studies, males and females treated with a single intramuscular dose of cefotaxime for uncomplicated gonorrhoea caused by penicillinase-producing strains of Neisseria gonorrhoeae responded very favourably. Encouraging results have also been reported in open studies in children including neonates, treated with cefotaxime for meningitis and various other serious infections. In some situations, cefotaxime has been given in combination with another antibiotic such as an aminoglycoside, but the merits of such a combination have not been clearly established. Whether cefotaxime alone is appropriate therapy for conditions previously treated with aminoglycosides (other than pseudomonal infections) also needs additional clarification, but if established as equally effective in such conditions cefotaxime offers potentially important clinical and practical advantages in its apparent lack of serious adverse effects and freedom from the need to undertake drug plasma concentration monitoring.

Cefoperazone (Cefobid, Pfizer)

Cefoperazone is a new beta-lactam antibiotic that possesses a broad spectrum of activity against gram-positive and gram-negative organisms. Cefoperazone differs from all previous cephalosporins in that it has exceptional activity against P. aeruginosa. The other distinguishing feature of cefoperazone is its high rate of biliary excretion, which will allow for treatment of biliary tract infections. Renal elimination accounts for only 20 percent of the agent's elimination; dosage modification is not necessary in decreased renal function. The clinical response rate of infections to cefoperazone is similar to that of moxalactam, cefotaxime, or the cephalosporins in general. The overall incidence of side effects was 14 percent in U.S. trials, with skin rash, fever, or urticaria occurring in 1 percent; phlebitis and injection-site pain in 2 percent; and diarrhea in 5 percent. As with the other third-generation cephalosporins, cefoperazone requires close scrutiny because of its expected high cost and the lack of comparative trials with standard antibiotic regimens.

Bactericidal and synergistic activity of moxalactam alone and in combination with gentamicin against Pseudomonas aeruginosa

The bactericidal activity of moxalactam, alone and in combination with gentamicin, was studied with macrobroth two-dimensional checkerboard and killing curve techniques against gentamicin-resistant and -susceptible strains of Pseudomonas aeruginosa. Moxalactam was bactericidal at concentrations equal to or at least two to four times its inhibitory concentrations. Synergy at clinically applicable concentrations of moxalactam and gentamicin occurred with 6 of 14 gentamicin-resistant strains and 4 of 4 gentamicin-susceptible strains by the checkerboard technique and with 7 of 14 gentamicin-resistant strains by the killing curve technique. Synergy between moxalactam and gentamicin against gentamicin-resistant strains of P. aeruginosa is unpredictable and strain- and method-dependent.

Pseudomonas aeruginosa

P. aeruginosa is widely distributed in nature and in the hospital environment with a predilection for moist areas. Its inherent resistance to many antimicrobials and its ability to produce many enzymes contribute to its pathogenic potential as both a primary and a secondary cause of infection. It is easily grown and identified in the microbiology laboratory. However, susceptibility testing remains a problem. Currently, the best approach to treatment is an aminoglycoside and an antipseudomonal beta-lactam antimicrobial. Typing can differentiate strains, but should be reserved for specific epidemiologic problems.

Piperacillin sodium: antibacterial spectrum, pharmacokinetics, clinical efficacy, and adverse reactions

Piperacillin sodium is a beta lactam antibiotic with a broad range of antibacterial activity that includes gram-negative bacilli, gram-positive cocci (except penicillinase-producing S. aureus) and anaerobic pathogens such as Clostridium difficile, and Bacteroides fragilis. Piperacillin inhibits many of the members of the Enterobacteriaceae, including Klebsiella sp and Pseudomonas, at lower concentrations than required for carbenicillin and ticarcillin. Piperacillin sodium is administered by intramuscular and intravenous injection and is widely distributed throughout body fluids and tissues. Like other newer penicillins, piperacillin is excreted by both renal and biliary mechanisms. The primary route of elimination is by glomerular filtration, which results in high urinary concentrations of the unchanged compound. Piperacillin has been approved for patients with serious infection caused by susceptible strains of specific organisms in intra-abdominal, urinary tract, gynecologic, lower respiratory tract, skin and skin structure, bone and joint, and gonococcal infections and septicemia. As with other penicillins, piperacillin has a low frequency of toxicity. The usual dose of piperacillin in adults with serious infections with normal renal function is 3-4 g every 4-6 hr as a 20-30 min infusion, with a maximum dose of 24 g per day. It is stable in most large volume parenteral solutions. Less serious infectins (requiring smaller dosages) may be treated by intramuscular injection; however, no more than 2 g should be given at any one injection site. Overall, piperacillin has a greater degree of activity than other penicillins. Evidence from prospective studies indicates that piperacillin is a highly effective agent for the treatment of patients with infections caused by susceptible organisms.

[N-formimidoyl-thienamycin: in vitro activity in bacteria with resistance to beta-lactam antibiotics or gentamicin]

The minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of N-formimidoyl-thienamycin were determined for 25 strains each of Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Serratia marcescens and Pseudomonas aeruginosa which were resistant to gentamicin and/or acylureido penicillins or cefotaxime, cefoperazone or moxalactam, and for 38 strains of the group D streptococci. The drug was very active against the most frequently encountered gram-negative resistant causative organisms of nosocomial infections. The MIC ranged from 0.25-1 mg/l for multiresistant Enterobacteriaceae, and from 0.5-4 mg/l for multiresistant P. aeruginosa. The group D streptococci (enterococci) showed a low MIC (median: 0.75 mg/l); the median of the MBC was greater than 64 mg/l, however.

Comparison of cefoperazone with penicillin, ampicillin, gentamicin, and chloramphenicol in the therapy of experimental meningitis

Cefoperazone was compared with penicillin against Streptococcus pneumoniae, gentamicin against Escherichia coli, and ampicillin and chloramphenicol against Haemophilus influenzae in the therapy of experimental meningitis in rabbits. Meningitis was produced by intracisternal inoculation into cerebrospinal fluid, and all antibiotics were administered intravenously over 8 h in dosages that would achieve serum levels comparable to those found in humans. The mean percent penetration into purulent cerebrospinal fluid, expressed as (cerebrospinal fluid concentration/serum concentration) x 100%, was 2.6% for penicillin, 22.0% for gentamicin, 12.1% for ampicillin, 23.8% for chloramphenicol, and 6.4% for cefoperazone. The mean cerebrospinal fluid antibiotic concentrations exceeded the minimum bactericidal concentration for the test strain in each experimental model, except for ampicillin in experimental meningitis due to the beta-lactamase-producing H. influenzae. Cefoperazone produced a significantly faster bactericidal effect after 4 h of treatment when compared with penicillin (P = 0.037) and ampicillin (P = 0.01) in meningitis caused by S. pneumoniae and H. influenzae (ampicillin susceptible), respectively. In meningitis caused by E. coli, cefoperazone was significantly (P = 0.006) more rapidly bactericidal after 8 h of treatment when compared to gentamicin. In addition, cefoperazone was significantly more rapidly bactericidal than either ampicillin or chloramphenicol in experimental meningitis due to beta-lactamase-producing H. influenzae. Cefoperazone deserves further evaluation in the therapy of bacterial meningitis in humans.

Comparative activities of N-formimidoyl thienamycin, ticarcillin, and tobramycin against experimental Pseudomonas aeruginosa pneumonia

The therapeutic efficacies of disodium ticarcillin, tobramycin sulfate, and N-formimidoyl thienamycin (MK0787) were compared in guinea pigs with experimentally induced Pseudomonas aeruginosa pneumonia. Survival rates were 35% for ticarcillin, 80% for tobramycin, and 75% for N-formimidoyl thienamycin. Numbers of viable Pseudomonas organisms in lungs approximately 3 h after the first dose of drug were nearly 10-fold fewer in tobramycin- or N-formimidoyl thienamycin-treated animals than in ticarcillin-treated animals. Our data suggest that N-formimidoyl thienamycin may have therapeutic efficacy against respiratory infections with P. aeruginosa equivalent to that of tobramycin.

The effect of N-formimidoyl thienamycin, ceftazidime, cefotiam, ceftriaxone and cefotaxime on non-fermentative Gram-negative rods, Aeromonas, Plesiomonas and Enterobacter agglomerans

Thirty-one species (185 strains) of non-fermentative gram-negative rods (excluding Pseudomonas aeruginosa) as well as 45 strains of Aeromonas spp., 15 strains of Plesiomonas shigelloides and 68 strains of Enterobacter agglomerans were tested in microdilution procedures against N-formimidoyl thienamycin, ceftazidime, cefotiam, ceftriaxone and cefotaxime. N-formimidoyl thienamycin was the most effective drug as far as the spectrum of these bacterial groups and potency is concerned; ceftazidime was the second most effective agent. Ceftriaxone and cefotaxime were similar in their activity (against a smaller spectrum), while cefotiam showed little effect. There were occasional differences between MBC and MIC values which were most notable with ceftazidime, cefotiam, ceftriaxone and cefotaxime against E. agglomerans.

Effects of azlocillin in combination with clavulanic acid, sulbactam, and N-formimidoyl thienamycin against beta-lactamase-producing, carbenicillin-resistant Pseudomonas aeruginosa

We investigated the effects of the combination of azlocillin with the beta-lactamase inhibitors clavulanic acid and sulbactam and with N-formimidoyl thienamycin against strains of Pseudomonas aeruginosa with R-factor-mediated carbenicillin resistance. The 10 strains tested (1 R-, 9 R+) were isogenic, except for the presence of individual plasmids determining each of nine plasmid-mediated beta-lactamases found in P. aeruginosa. We utilized a checkerboard technique for testing antibiotic combinations. Low concentrations of clavulanic acid produced synergy with azlocillin against the strains producing the TEM-1, TEM-2, PSE-1, PSE-3, and PSE-4 beta-lactamases; for the strains producing the OXA-1, OXA-2, OXA-3, and PSE-2 beta-lactamases, such synergy was not found. With sulbactam, synergy was demonstrated in all strains except that producing PSE-2 beta-lactamase; for several strains, however, the concentration of sulbactam required to produce synergy was substantially higher than that for clavulanic acid. N-Formimidoyl thienamycin was highly active as a single agent against all of the strains, regardless of beta-lactamase production. The combination of N-formimidoyl thienamycin and azlocillin produced synergy against only two of the strains tested.

Antimicrobial activity, pharmacokinetics, adverse reactions, and therapeutic indications of cefoperazone

Cefoperazone is a parenteral cephalosporin antibiotic that is pending approval by the U. S. Food and Drug Administration. Compared to most other cephalosporins cefoperazone has a greatly expanded spectrum of bactericidal activity that encompasses most aerobic gram-positive bacteria except enterococci, most aerobic gram-negative bacteria, including a majority of Pseudomonas aeruginosa strains, and a number of pathogenic anaerobic bacteria. Its long serum half-life, approximately two hours, permits a twelve hourly dosing schedule. No dosage modification is required in patients with renal insufficiency, and only minor modification is needed in patients with hepatic insufficiency or biliary obstruction. Clinical trials have established cefoperazone's efficacy in lower respiratory tract infections, urinary tract infections, and a variety of other bacterial infections. Adverse reactions have been infrequent, and few serious reactions have been identified. Cefoperazone is a promising new agent for the treatment of gram-negative bacillary and polymicrobial infections, especially in settings that require empiric therapy.

Comparative in vitro activities of N-formimidoyl thienamycin and moxalactam against nonfermentative aerobic gram-negative rods

N-Formimidoyl thienamycin was the most active drug against strains of Pseudomonas aeruginosa with a 90% minimum inhibitory concentration of 1.25 mug/ml. With the exception of P. maltophilia, thienamycin was as active or more active than moxalactam against other species of pseudomonads and against other genera of nonfermenters.

Factors affecting the in vitro activity of cefoperazone against the Bacteroides fragilis group

The in vitro activity of cefoperazone against 32 strains of bacteria of the Bacteroides fragilis group was determined on four media by using a variety of test parameters. Lower mean minimal inhibitory concentrations (MICs) were obtained on Mueller-Hinton blood agar and supplemented brain heart infusion agar than were obtained on brucella laked blood agar or Wilkins-Chalgren agar. Higher MICs were obtained with 6-h inocula than with 24-h inocula, and slightly higher MICs were obtained with tests read at 48 as compared with 24 h. Conducting tests in an anaerobic glove box had little effect. The greatest differences in mean MICs were seen with inoculum densities of 10(4) and 10(5) colony-forming units.