In vitro and in vivo antibacterial effects of combinations of beta-lactam antibiotics

Dual beta-lactam therapy for serious Gram-negative infections: is it time to revisit?

We are rapidly approaching a crisis in antibiotic resistance, particularly among Gram-negative pathogens. This, coupled with the slow development of novel antimicrobial agents, underscores the exigency of redeploying existing antimicrobial agents in innovative ways. One therapeutic approach that was heavily studied in the 1980s but abandoned over time is dual beta-lactam therapy. This article reviews the evidence for combination beta-lactam therapy. Overall, in vitro, animal and clinical data are positive and suggest that beta-lactam combinations produce a synergistic effect against Gram-negative pathogens that rivals that of beta-lactam-aminoglycoside or beta-lactam-fluoroquinolone combination therapy. Although the precise mechanism of improved activity is not completely understood, it is likely attributable to an enhanced affinity to the diverse penicillin-binding proteins found among Gram negatives. The collective data indicate that dual beta-lactam therapy should be revisited for serious Gram-negative infections, especially in light of the near availability of potent beta-lactamase inhibitors, which neutralize the effect of problematic beta-lactamases.

In vitro activities of antibiotic combinations against clincal isolates of Pseudomonas aeruginosa

Combination therapy has been recommended to treat Pseudomonas aeruginosa infections worldwide. The purpose of the present study was to determine the in vitro activities of piperacillin, cefepime, aztreonam, amikacin, and ciprofloxacin alone and in combination against 100 clinical isolates of P. aeruginosa from one medical center in southern Taiwan. The combination susceptibility assay was performed using the checkerboard technique. The percentage of resistance of P. aeruginosa to single agents in our study was relatively high for the Asia-Pacific area, except to aztreonam. Piperacillin plus amikacin exhibited the highest potential for synergy (59/100) in this study. Moreover, a high percentage of synergism was also noted with amikacin combined with cefepime (7/100) or aztreonam (16/100). The combination of two beta-lactams, such as cefepime with piperacillin, and aztreonam with cefepime or piperacillin, showed synergistic effects against some P. aeruginosa isolates. Although ciprofloxacin is a good anti-pseudomonal agent, a very low potential for synergy with other antibiotics was demonstrated in this study. No antagonism was exhibited by any combination in our study. Among piperacillin-resistant strains, there was synergy with a beta-lactam plus amikacin, including the combination of piperacillin and amikacin. However, the combination of two beta-lactams, such as piperacillin and cefepime or aztreonam, did not have any synergistic activity against these strains. In summary, the combinations of amikacin with the tested beta-lactams (piperacillin, aztreonam, cefepime) had a greater synergistic effect against P. aeruginosa, even piperacillin-resistant strains, than other combinations. Understanding the synergistic effect on clinical strains may help clinicians choose better empirical therapy in an area with high prevalence of multidrug-resistant P. aeruginosa.

Determination of intracellular and extracellular beta-lactamase activities of Pseudomonas aeruginosa after exposure to beta-lactams in vitro and in vivo

Beta-lactamase production in Pseudomonas aeruginosa was determined in in-vitro models and in rat pouch infection models after exposure to ceftazidime, imipenem, and piperacillin. Exposure of 28 P. aeruginosa strains to 1/4 minimum inhibitory concentration (MIC) of ceftazidime, imipenem, and piperacillin for 24 h enhanced intracellular beta-lactamase activities in 14, 22, and 6 strains, respectively, of the 28 clinical strains tested, and enhanced extracellular beta-lactamase activities which were not detected without exposure to antibiotics, in 7, 23, and 1 of the 28 strains, respectively. Extracellular beta-lactamase activity from P. aeruginosa S-1278, producing an inducible beta-lactamase, scarcely increased after exposure to ceftazidime and piperacillin 24 h after incubation, while the activity increased after exposure to imipenem over the range of 1/8 to 8 MIC. In the rat granuloma pouch models infected with P. aeruginosa S-1278, ceftazidime and piperacillin, after single administration (20 mg/kg) and serial administration (20 mg/kg per day x 3 days), did not enhance extracellular beta-lactamase activities. However, the activities were enhanced with single and serial administrations of imipenem, and levels over 10 mU/ml were detected until the third day. The beta-lactamase activity, similar to the activity found in rat pouches after serial administration of imipenem, inactivated various cephalosporins. In conclusion, extracellular beta-lactamase activity was detected both in vitro and in vivo after exposure to a good inducer, and extracellular beta-lactamase remained at infection site at levels that could inactivate cephalosporins.

Ureidopenicillins and beta-lactam/beta-lactamase inhibitor combinations

Although research and development of new penicillins have declined, penicillins continue to be essential antibiotics for the treatment and prophylaxis of infectious diseases. The most recent additions are the ureidopenicillins and beta-lactam/beta-lactamase inhibitor combinations. This article reviews the spectrum of activity, toxicity, pharmacokinetics, and clinical uses of the ureidopenicillins, and the beta-lactam/beta-lactamase inhibitor combination agents.

Antipseudomonal penicillins

Antipseudomonal penicillins retain most of the antibacterial activity of penicillin and aminopenicillins. This group of penicillins has added activities against many gram-negative rods, including P. aeruginosa. Similar to the earlier penicillins, this group continues to be susceptible to hydrolysis by many beta-lactamases and are, therefore, not consistently active against Staphylococcus, some gram-negative rods, and certain beta-lactamase-producing gram-negative anaerobes. The ureidopenicillins, especially piperacillin, appear to have better activity against Enterococcus, Klebsiella, and P. aeruginosa than ticarcillin. The advantages over the newer cephalosporins are (1) better activity against Enterococcus, (2) more consistent activity against Clostridium, and (3) more consistent synergy with aminoglycosides. The ureidopenicillins have certain advantages over carboxypenicillins, including lower sodium load, less frequent hypokalemia, reduced platelet dysfunction, minimal dosage adjustment in patients with renal failure, and a wider spectrum of antibacterial activity, especially against Enterococcus, Pseudomonas, and Klebsiella. The utility of the antipseudomonal penicillins by themselves is limited as agents for monotherapy when the infecting organism is not known. In addition, monotherapy is not recommended in certain infections to avoid the development of resistance. When combined with a beta-lactamase inhibitor or with an aminoglycoside, however, some of the weaknesses can be overcome.

Overview of synergy with reference to double beta-lactam combinations

Combination antimicrobial therapy is used to expand the bacterial coverage over a single agent, to prevent the emergence of resistant organisms, to decrease toxicity by allowing lower doses of both agents, or for synergy. Synergy is one of the most common of these reasons, especially in serious infections. The introduction of new broad-spectrum beta-lactam antimicrobials has led to their combination in the treatment of seriously ill patients. Whereas a combination of an aminoglycoside and a beta-lactam antimicrobial is frequently synergistic, much less is known about synergy between combinations of beta-lactams. In vitro testing shows most combinations of two beta-lactams to be indifferent or additive in their effects; rarely does synergy occur. Antagonism can sometimes be seen, particularly with combinations involving cefoxitin or imipenem, especially if the treated organism is Enterobacter or Pseudomonas. Results of clinical trials comparing double beta-lactam (DBL) therapy with aminoglycoside/beta-lactam combinations show no difference in clinical response rates. Highly active DBL combinations may substitute for standard aminoglycoside-containing regimens in certain situations, even though they are not reliably synergistic. However, in the treatment of seriously ill patients such combinations may be less desirable.

Significance of inducible cephalosporinase remaining in the experimentally infected rat granuloma pouch after beta-lactam therapy

We studied the influence of inducible cephalosporinase on levels of secondarily administered beta-lactam antibiotics in exudates using experimentally infected rat granuloma pouches. Cefoperazone or cefmetazole was administered intramuscularly at a dose of 100 mg/kg of body weight to rats at 2 and 8 h after infection of rat pouches with Serratia marcescens W-24, which possesses an inducible type I beta-lactamase (cephalosporinase). Subsequently, cefotaxime or cefbuperazone was administered at an intravenous dose of 100 mg/kg to rats at 24 h postinfection. Levels of cefotaxime in the pouch exudates of the cefmetazole-pretreated group were lower than those in the control group, which was infected but not pretreated with antibiotics. This was due to the inactivation of cefotaxime by extracellular cephalosporinase which was induced by cefmetazole and which remained in the rat pouches. However, cefotaxime concentrations were not reduced in the cefoperazone-pretreated group because of the low inducibility of cefoperazone against cephalosporinase production. On the other hand, cefbuperazone concentrations were similar in all groups (control, cefoperazone pretreated, and cefmetazole pretreated), because cefbuperazone is more stable against this enzyme than cefotaxime is. In conclusion, concentrations of secondarily administered beta-lactam antibiotics are affected by inducibly produced cephalosporinase at the infection site when a good inducer like cefmetazole is administered beforehand.

Activity of cefotiam in combination with beta-lactam antibiotics on enterobacterial hospital strains

By using checkerboard titrations the effect of cefotiam combined with different beta-lactam antibiotics on fifty strains of Enterobacteriaceae moderately susceptible (minimal inhibiting concentration greater than or equal to 8 mg/l) or resistant (minimal inhibiting concentration greater than or equal to 64 mg/l) to cefotiam was evaluated. The following compounds were tested: cefamandole, cefazolin, cefmenoxime, cefotaxime, cefotiam, ceftazidime, cefuroxime, mecillinam and piperacillin. The synergistic effect varied markedly. The combination cefotiam-mecillinam showed the highest rate of synergistic activity. Antagonism was found in 1% of the combinations.

Diverse potential of beta-lactamase inhibitors to induce class I enzymes

The ability of various beta-lactamase inhibitors to induce class I beta-lactamases was assessed. Clavulanate was the most active compound, inducing Morganella morganii, Aeromonas caviae, and Enterobacter aerogenes over a broad concentration range and Citrobacter freundii, Pseudomonas aeruginosa, and Serratia marcescens at high concentrations. Disk approximation tests paralleled these results, with clavulanate, but not sulbactam or tazobactam, antagonizing the activity of several beta-lactams against these organisms.

Antibiotic combinations: should they be tested?

When antibiotic combinations are used to provide a broader spectrum of antimicrobial activity or in an attempt to prevent the emergence of resistant organisms, it is rarely necessary or practical to perform tests of drug interactions in vitro. In vitro testing of combinations may be useful when combinations are used in an attempt to attain synergistic interactions. In some cases, screening methods can be used as substitutes for formal synergy testing. This paper examines the mechanisms of antibiotic interaction leading to synergism or antagonism, surveys attempts to correlate in vitro observations with efficacy in animal models, and reviews clinical data providing evidence for or against a useful role of synergistic antibiotic interactions in the treatment of human infections.

Beta-lactamase alteration of beta-lactam inhibitory zones

A disk approximation test, in which the inhibitory zone of one beta-lactam antibiotic is truncated under influence of another beta-lactam (e.g. cefoxitin), was used as a screen for the presence of inducible beta-lactamase. By using beta-lactamase extracts and a specific inhibitor, it was shown that chromosomal beta-lactamase can indeed be the sole cause of truncated beta-lactam inhibitory zones.

Ability of newer beta-lactam antibiotics to induce beta-lactamase production in Enterobacter cloacae

The beta-lactamase inducing properties of various new beta-lactam antibiotics in two isogenic strains of Enterobacter cloacae were investigated. Beta-lactamase activity was measured two hours after addition of inducer to cells in the late logarithmic growth-phase. Beta-lactamase expression was highly dependent on the growth medium used, highest levels being obtained after induction with cefoxitin in Tryptic Soy broth, Mueller-Hinton broth and Nutrient broth. Upon induction the mutant 908 Ssi produced tenfold higher beta-lactamase levels than its parent wild type 908 Swi. Among the new antibiotics investigated, sulfoxides of several oxyimino-cephalosporins, HR 810, cefetamet, cefteram, carumonam and BRL 36650 were moderate or poor inducers. The penem FCE 22101 resembled imipenem in its strong inducing properties.

Cephalosporins in gram-positive infections

Infections caused by Gram-positive bacteria are an important and common cause of morbidity and mortality. Staphylococci and streptococci are the most frequent infecting organisms in skin and soft tissue infections, pneumonia, bone and joint infections, and endocarditis. Anaerobic Gram-positive bacteria such as Clostridia spp. cause infections that can rapidly produce tissue necrosis and death. The cephalosporins are indicated for the treatment of infections caused by Gram-positive bacteria in certain circumstances. These include selected patients with endocarditis, osteomyelitis, septic arthritis and cellulitis. They are also used as alternatives to the penicillins in penicillin-allergic patients and for 'mixed' infections caused by Gram-positive and Gram-negative organisms. This article discusses the indications for the cephalosporin group of antibiotics in patients suffering from infections caused by Gram-positive bacteria.

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.

Aztreonam

Aztreonam is a new, totally synthetic beta-lactamase agent--the first monobactam. It is highly resistant to hydrolytic inactivation caused by plasmid-mediated (except PSE-2 enzyme found in some Pseudomonas species) or chromosomally mediated beta-lactamases (except for K1 produced by rare strains of Klebsiella oxytoca). Accordingly, aztreonam remains active against many pathogens that are resistant to other beta-lactam antibiotics. The drug exhibits directed antibacterial activity against gram-negative organisms and is effective as monotherapy against most Enterobacteriaceae and Hemophilus and Neisseria species, including beta-lactamase-producing strains; it is not active against anaerobes or gram-positive organisms. Before culture results are known, it may be necessary to administer the agent empirically in combination with other antibiotics. Aztreonam is rapidly distributed to most body tissues and fluids when administered parenterally. Its serum half-life is 1.7 hours, suggesting a dosing interval of 6-8 hours for severe or life-threatening infections and 8-12 hours for moderately severe infections and urinary tract infections. It is primarily eliminated unchanged in the urine and in much lesser amounts as a microbiologically inactive metabolite; slight biliary excretion may occur. Aztreonam is well-tolerated, lacking any serious adverse hematologic, otic, or renal system effects. Its lack of effect on anaerobes helps to maintain resistance against colonization. Particularly in light of its safety and unique properties, aztreonam promises to be a useful alternative to aminoglycoside therapy.

Synergy of ciprofloxacin and azlocillin in vitro and in a neutropenic mouse model of infection

Combinations of ciprofloxacin with azlocillin, piperacillin and ticarcillin were tested in vitro against clinical isolates. Azlocillin plus ciprofloxacin showed synergy against 30% of Pseudomonas aeruginosa isolates; it was either synergistic or additive against 78% of all isolates tested even those resistant to the beta-lactam. Synergism was rarely noted for Klebsiella pneumoniae, Escherichia coli, Enterobacter spp. or Branhamella spp. isolates. Minimum inhibitory concentrations of ciprofloxacin plus azlocillin, plus piperacillin and plus ticarcillin against Pseudomonas spp. were reduced 4 or 2 fold, respectively. However, the combination azlocillin plus ciprofloxacin showed primarily indifference against gram-positive strains. Neutropenic mice infected with a lethal challenge of Pseudomonas spp. were protected by a combination of azlocillin and ciprofloxacin. Its additive and/or synergistic effects and expanded spectrum of activity against streptococci, methicillin-resistant staphylococci and JK corynebacteria may provide an alternative to traditional therapy.

Update on antibiotics in ocular infections

Each year, new antimicrobials are found or synthesized in an effort to improve the chance of overcoming infections. In the early 1950s, the only antibiotic available for ocular use was penicillin. Today, ophthalmologists can make a choice from a large selection of antibiotics for ocular infections. The majority of antibiotics have been literally unearthed, since worldwide soil surveys may have been the means of their discovery. In addition, synthetic derivatives of penicillin, cephalosporins, aminoglycosides, and tetracyclines, as well as drugs against tuberculosis and fungi, have become available, and new names have been added to the already bewildering list of less frequently used sulfonamides. However, it takes several years to appreciate the impact of new agents and the continued contribution of older ones. Constant reevaluation is mandatory. The real benefits as well as the untoward effects of a new antimicrobial agent may not be known until several years after the clinical introduction. In addition to approaching infection from the viewpoint of the offending organism and a specific antibiotic to address this organism, one may also approach this problem from the host's immunity. Until now, we have relied largely on the corticosteroids, but one must also consider various nonsteroidal anti-inflammatory agents and, even more importantly, the development of drugs to enhance the host's natural immunity.

Imipenem-induced resistance to antipseudomonal beta-lactams in Pseudomonas aeruginosa

Using clinical isolates of Pseudomonas aeruginosa, we studied the ability of imipenem to antagonize the activity of nine other antipseudomonal beta-lactam antimicrobial agents. Imipenem caused truncation of the zones of inhibition in a disk diffusion test for 91 to 100% of the strains, depending on the beta-lactam tested. Addition of subinhibitory concentrations of imipenem caused a fourfold or greater increase in MICs for 72 of 74 isolates and in 20 to 87% of the tests, again depending on the antibiotic tested. beta-Lactamase assays with both whole-cell suspensions and cell sonicates showed that exposure to subinhibitory concentrations of imipenem resulted in a beta-lactamase production supported the hypothesis that induction of beta-lactamase was responsible for antagonism. In hydrolysis studies with a beta-lactamase extract, most of the antagonized drugs were either not hydrolyzed or only poorly hydrolyzed. We conclude that imipenem induces significantly elevated levels of beta-lactamase in P. aeruginosa. This increase in beta-lactamase is associated with increased resistance of the organism to many other beta-lactam agents.

Management in the febrile, neutropenic patient with cancer: therapeutic considerations

Infection is the principal cause of death in neutropenic children and adults with neoplastic diseases. Various antibiotic regimens have been studied in clinical trials in an attempt to reduce this mortality. Recent trials have compared newer combinations of antibacterial agents (double beta-lactams) or monotherapy with the standard aminoglycoside-beta-lactam combinations. No significant differences are demonstrated in these trials with regard to efficacy. Emphasis has been on the reduction of toxicity. Although nephrotoxicity, ototoxicity, and hypokalemia have been of concern in the past and continue to be important, newer problems have emerged that involve disorders of coagulation, for example, prolongation of prothrombin time, disorders of platelet function, and clinical bleeding. Superinfection and the emergence of resistance during therapy have also been problematic. Cost must be considered an important factor in determining the selection of an antibacterial regimen. The combination of an aminoglycoside and a beta-lactam antibiotic remains the standard against which future combinations or monotherapy must be judged in clinical trials involving the febrile, neutropenic patient with cancer.

Antimicrobial combinations in the therapy of infections due to gram-negative bacilli

Antimicrobial combinations are used most frequently to provide broad-spectrum coverage; however, they are also frequently employed to enhance antimicrobial activity (synergism). Although there is extensive in vitro documentation of synergism for many antibiotic combinations, a clear advantage for these combinations has been difficult to demonstrate in clinical studies. Several types of combinations have been useful in clinical medicine and frequently result in synergism. These include combinations of a cell wall-active agent with an aminoglycosidic aminocyclitol, combinations of a beta-lactamase inhibitor with a beta-lactam, and combinations of agents that inhibit sequential steps in a metabolic pathway. Given its spectrum of activity, aztreonam will often be used with clindamycin or a beta-lactam antibiotic. Combinations of beta-lactams may be synergistic via several mechanisms. However, these combinations also exhibit significant potential for antagonism when used against gram-negative bacilli and, therefore, require careful evaluation prior to clinical use.

Moxalactam and piperacillin: a study of in vitro characteristics and pharmacokinetics in cancer patients

We evaluated the microbiologic characteristics including MIC determinations, synergy plate assays and serum bactericidal activity for two regimens being examined as empiric antibiotic therapy for febrile granulocytopenic cancer patients. The regimens consisted of moxalactam (4 g.i.v. q12h) plus piperacillin (75 mg/kg i.v. q6h) or moxalactam (as above) plus amikacin (levels adjusted to one hour post-infusion levels of 25 mg/l and troughs of 6-8 mg/l). Detailed pharmacokinetics were ascertained for the beta lactams. All drugs were active against a panel of 11 strains each of Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Staphylococcus aureus. The pharmacokinetic profile showed serum levels sufficient to provide good antimicrobial activity throughout the dosing interval. Both regimens displayed synergistic or partially synergistic activity in the main for the test organisms; moxalactam plus piperacillin produced good results against S. aureus and P. aeruginosa. In the serum bactericidal assays, the moxalactam-piperacillin combination produced significantly higher mean titers at both peak and trough when compared to the moxalactam-amikacin regimen. This may be because moxalactam acts as a beta lactamase inhibitor for both staphylococcal beta lactamase, as well as the Sabath-Abraham Id type beta lactamase carried by P. aeruginosa (among others). Moxalactam-piperacillin deserves extensive evaluation as empiric therapy for the febrile neutropenic cancer patients.

Temocillin. In vitro antibacterial activity

Temocillin, a 6-alpha-methoxy penicillin derivative, was tested in vitro against 516 recent clinical isolates of Enterobacteriaceae. The compound exhibited good antibacterial activity, with 95% of isolates inhibited by a range 2 to 16 mg/L. Further studies, against selected isolates resistant to ticarcillin, piperacillin and cefuroxime (Klebsiella oxytoca, 25; Enterobacter species, 34; and Citrobacter species, 5), showed about half of the isolates of K. oxytoca (11/25) to be resistant to aztreonam (MIC range 16-greater than or equal to 128 mg/L), but susceptible to temocillin, cefotaxime and latamoxef. In general, the resistant strains of Enterobacter species tested were not susceptible to cefotaxime (MIC range 16-128 mg/L), or aztreonam (MIC range 1.0-64 mg/L), and many exhibited reduced susceptibility to latamoxef (MIC range 2-128 mg/L). In contrast, all the strains were susceptible to temocillin (MIC range 4-16 mg/L). The bactericidal activity of temocillin was confirmed against selected aztreonam-resistant strains of K. oxytoca and Enterobacter cloacae by conventional time-kill studies, and against a strain of E. cloacae in an in vitro model system designed to simulate the temocillin concentration profiles attained in extravascular fluid such as peripheral lymph. In the time-kill studies, temocillin concentrations of 16 and 32 mg/L were shown to effectively reduce the numbers of viable bacteria by 99 and 99.9%, respectively, within 12 hours. In the in vitro model system the numbers of bacteria were reduced 99.9% over the initial 4-hour period. In combination with aminoglycoside antibiotics, temocillin exerted a synergistic or partially synergistic effect (sigma FIC less than or equal to 0.75) against the majority of strains of Pseudomonas aeruginosa tested. When combined with piperacillin, cefotaxime or latamoxef, temocillin, unlike cefoxitin, exhibited no antagonism against strains of Enterobacteriaceae producing inducible cephalosporinases.

Piperacillin. A review of its antibacterial activity, pharmacokinetic properties and therapeutic use

Piperacillin is one of the new generation of semisynthetic penicillins which can be administered intravenously or intramuscularly. It has a broad spectrum of activity against Gram-positive and Gram-negative aerobic and anaerobic bacteria. Although piperacillin has shown greater activity against beta-lactamase-producing organisms than the other penicillins, it is hydrolysed by the plasmid-mediated beta-lactamases (TEM-1). Activity against Pseudomonas aeruginosa is better than that of ticarcillin, carbenicillin and mezlocillin. Although only limited controlled studies have been reported, in those which have been conducted and in a larger number of open studies piperacillin was effective in the treatment of complicated urinary tract infections and lower respiratory tract infections, particularly pneumonia, caused by Gram-negative bacilli. Favourable clinical results have been obtained in patients with infections caused by mixed aerobic/anaerobic organisms (such as intra-abdominal infections) but the relatively average in vitro activity of piperacillin against Bacteroides fragilis may not indicate its usage in situations where this organism is the suspected or proven pathogen. Piperacillin in combination with an aminoglycoside or a 'third generation' cephalosporin gave encouraging results in the treatment of infections in immunocompromised patients, whilst its penetration into the diseased central nervous system and lack of toxicity indicate a potential value in the treatment of neonatal Gram-negative bacillary meningitis, particularly where the causative organism is Pseudomonas aeruginosa. Whether piperacillin alone is appropriate therapy for conditions usually treated with aminoglycosides (other than pseudomonal infections) needs additional clarification, but if established as equally effective in such conditions it has the advantages of its apparent lack of serious adverse effects and freedom from the need to undertake plasma concentration monitoring. These advantages would not, however, apply when considering one of the new (third generation) cephalosporins as alternative therapy in non-pseudomonal infections. Generally, however, it is still considered necessary to treat serious and complicated infections with combination therapy, either a cephalosporin, or in cases of resistance to P. aeruginosa an aminoglycoside.

Synergism at clinically attainable concentrations of aminoglycoside and beta-lactam antibiotics

We evaluated the in vitro synergistic activity at clinically attainable concentrations of combinations of aminoglycoside and beta-lactam antibiotics against 30 gentamicin-resistant clinical isolates of gram-negative bacilli. All 56 pairs of 4 aminoglycosides and 14 beta-lactams were evaluated. Combinations with amikacin demonstrated inhibitory synergistic activity in 29% of the assays, as compared with 22% for netilmicin (P = 0.018), 17% for gentamicin (P less than 0.001), and 13% for tobramycin (P less than 0.001). Among the beta-lactams, combinations with cefoperazone, ceftriaxone, or cefpiramide (SM-1652) demonstrated inhibitory synergistic activity most often (39, 38, and 35% of the assays, respectively) and with ceforanide, cefsulodin, and imipenem least often (less than or equal to 8% each). The most active combination was amikacin and ceftriaxone, with which 67% of the assays demonstrated inhibitory synergism. Isolates with high-level resistance to either antibiotic in a combination were unlikely to be inhibited synergistically by the combination. Further, combinations generally demonstrated little synergistic activity against isolates highly susceptible to beta-lactams.

Effect of aztreonam in combination with azlocillin or piperacillin on Pseudomonas aeruginosa

Aztreonam, a synthetic monobactam antimicrobial agent specifically active against aerobic, gram-negative microorganisms, was studied in combination with the extended-spectrum penicillins azlocillin and piperacillin against 46 strains of Pseudomonas aeruginosa. Of these strains, 4.3% were synergistically inhibited, and 19.7% showed evidence for an additive effect of the antibiotics. All other strains showed indifference. The addition of cefoxitin to these combinations increased the MICs of azlocillin and piperacillin by two to three tubes, whereas zero- to one-tube increases were noted for aztreonam MICs. Attempts to block cefoxitin-induced beta-lactamase production by using clindamycin were unsuccessful even at high clindamycin concentrations.

Unpredictable interactions between cefotetan and penicillins or aminoglycosides

The bacteriostatic and bactericidal activity of gentamicin, amikacin, piperacillin, azlocillin, ampicillin, benzyl penicillin, and oxacillin against 85 selected isolates was measured alone and in the presence of subinhibitory concentrations of cefotetan. A potential for antagonistic interactions between cefotetan and acylamino penicillins (piperacillin greater than azlocillin greater than ampicillin) were observed with some strains; synergy was suggested with other isolates, however, Bacteriostatic and bactericidal activity of the two aminoglycosides was enhanced by cefotetan with most gram-negative bacilli, but bactericidal activity was significantly diminished with 4 of 65 strains. Piperacillin and cefotetan were clearly antagonistic with 11 of 44 strains and synergistic with 4 of 44 strains of gram-negative bacilli: the remaining 21 strains gave off scale end points and could not be analyzed in that way.

In vitro activities of ureidopenicillins alone and in combination with amikacin and three cephalosporin antibiotics

The MIC and MBC activity of mezlocillin alone and in combination with two concentrations of ceftizoxime, moxalactam, and amikacin and a single concentration of cefoxitin was studied in a broth microdilution partial checkerboard against 472 strains of aerobic gram-negative and gram-positive bacteria. Azlocillin was tested alone and in the same combinations against Pseudomonas aeruginosa. Of the gram-negative bacilli tested, 38% were gentamicin resistant. Antagonism (less than or equal to a fourfold ureidopenicillin MIC increase) was observed frequently with combinations of ureidopenicillins plus cefoxitin and sporadically with ureidopenicillins plus ceftizoxime or moxalactam. Partial synergism (less than or equal to a fourfold ureidopenicillin MIC decrease) was evident with both combinations of ureidopenicillins plus amikacin and ureidopenicillins plus ceftizoxime or moxalactam, the percentage being dependent upon the individual species and combinations.

Comparative synergistic activity of ceftriaxone-piperacillin versus ceftriaxone-netilmicin

The effect of combination of ceftriaxone with piperacillin or netilmicin was studied in a total of 119 clinical isolates using the checkerboard titration technique. The isolates included Pseudomonas aeruginosa, Staphylococcus aureus, enterococci and various Enterobacteriaceae. Synergy was observed in all Streptococcus faecalis strains with both combinations. Whereas the ceftriaxone/netilmicin combination showed a higher rate of synergy against Pseudomonas aeruginosa, the rate of synergy against Enterobacteriaceae was the same for the two combinations. In no instance was antagonism encountered.

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.

Antimicrobial effect of beta-lactam antibiotic combinations against Bacteroides fragilis in vitro

Carbenicillin, piperacillin, and mezlocillin, when combined with cefoxitin, exhibited synergism against 20, 13, and 10 isolates of Bacteroides fragilis, respectively, and no antagonism was observed in 19 of 20 strains. These results suggest that if beta-lactamase was induced from B. fragilis by cefoxitin, it was ineffective against the antibiotics tested.

Characterization of beta-lactamase induction in Enterobacter cloacae

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

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.

Interaction of mezlocillin and cefoxitin against Proteus morganii in the granuloma pouch model

The interaction of cefoxitin and mezlocillin was studied in rats using the granuloma pouch technique. Proteus morganii strains against which cefoxitin and penicillins exhibited antagonistic antibacterial activity were selected for infection of the pouches. Animals were treated with cefoxitin and/or mezlocillin, and bacterial counts, drug levels and beta-lactamase production in the pouch exudate monitored. A significant decrease in bacterial counts was seen after administration of mezlocillin and cefoxitin in combination but not after the drugs administered alone. Drug concentrations of mezlocillin and/or cefoxitin corresponded directly to the rate of induction of beta-lactamase activity. Although both beta-lactam antibiotics induce beta-lactamase activity, administered in combination in vivo they appear to protect each other from being inactivated thus resulting in synergistic antibacterial activity.

Antagonism of carbenicillin and cefamandole by cefoxitin in treatment of experimental infections in mice

The ability of cefoxitin to antagonize the in vivo efficacy of cefamandole and carbenicillin as predicted by in vitro assays was analyzed in experimental infections in mice. Cefoxitin was administered in a nonprotective dose either at the time of challenge or simultaneously with the protective drug, 1 and 3.5 h postchallenge. In mice infected with Enterobacter cloacae, median 50% protective doses of cefamandole and carbenicillin were markedly increased by cefoxitin, especially when the latter was given at the time of challenge. The antagonistic effect was also associated with increased numbers of challenge bacteria present in animal heart blood within a 6.5-h period after infection. In infections with Pseudomonas aeruginosa, cefoxitin antagonized carbenicillin; however, the effect was less dramatic than that seen with E. cloacae. Antagonism in this model was pronounced with simultaneous administration of antagonizing and protective drugs. The antagonistic effects observed in all in vivo tests were not due to the selection of stable resistance to the protective drugs, but appeared to be due to a reversible induction of beta-lactamases by cefoxitin.

In vitro antagonism of beta-lactam antibiotics by cefoxitin

We assessed the extent and mechanisms of antagonism of beta-lactam antibiotics by cefoxitin. In tests with 41 gram-negative isolates, cefoxitin antagonized cephalothin, cefamandole, cefsulodin, cefotaxime, moxalactam, ampicillin, carbenicillin, piperacillin, mezlocillin, and azlocillin, but not cephalexin, mecillinam, or N-formimidoyl thienamycin. The extent of antagonism varied with the beta-lactam and genus studied. However, antagonism occurred most often with strains possessing inducible cephalosporinases. Antagonism of cephalothin and cefamandole correlated closely with the induction of beta-lactamases capable of inactivating these drugs. Although antagonism of the remaining drugs occurred more often with strains possessing inducible beta-lactamases, these enzymes did not inactivate the drugs. Morphological studies revealed that cefoxitin inhibited filamentation and lysis produced by various beta-lactam drugs. Results of this investigation suggest that cefoxitin antagonizes beta-lactams via (i) induction of drug-inactivating beta-lactamases, and (ii) the induction of beta-lactamases that cannot inactivate the drug but serve as barriers against access to target proteins. This barrier appears most efficient for drugs that bind to penicillin-binding proteins 1 and 3.

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

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

In vitro activity of moxalactam and mecillinam, singly and in combination, against multi-drug-resistant Enterobacteriaceae and Pseudomonas species

The in vitro interaction of moxalactam and mecillinam against multi-drug-resistant gram-negative enteric bacilli was studied by checkerboard microdilution susceptibility tests and by killing curve kinetics. Against Enterobacteriaceae, the combination was unpredictable; the frequencies of synergy, indifference, and antagonism were 11, 76, and 13%, respectively. Against Pseudomonas sp., the two drugs were consistently indifferent. Overall, the combination of moxalactam and mecillinam was no more active than moxalactam alone.