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

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.

Influence of beta-lactamase inhibitors on the potency of their companion drug with organisms possessing class I enzymes

The ability of beta-lactamase inhibitors to induce class I beta-lactamases in certain organisms in vitro suggests a potential for antagonism in vivo. Therefore, a study was designed to assess the ability of sulbactam and clavulanate to induce beta-lactamases in two strains each of Enterobacter cloacae, Citrobacter freundii, Serratia marcescens, and Pseudomonas aeruginosa both in vitro and in vivo. Induction in vitro was observed only with S. marcescens and P. aeruginosa and generally only when inhibitor concentrations greater than 2 micrograms/ml were examined. A mouse model of lethal infection, designed to detect in vivo antagonism arising from beta-lactamase induction, was used to determine what effect sulbactam and clavulanate would have on the 50% protective doses (PD50s) of cefoperazone and ticarcillin. Antagonism (a significant increase in the PD50) was observed in only 4 of 32 tests. Three of these involved antagonism of cefoperazone by clavulanate, and one involved antagonism of cefoperazone by sulbactam. In 6 of 32 tests, enhancement of efficacy (a significant decrease in PD50) was observed. In four of these, sulbactam enhanced cefoperazone; in one, sulbactam enhanced ticarcillin; and in one, clavulanate enhanced ticarcillin. Four of the six cases of enhancement occurred when the beta-lactamase inhibitor was given at the time of challenge. None of these positive or negative in vivo effects were predicted by in vitro tests. These data suggest that beta-lactamase inhibitors can influence the in vivo potency of their companion drug in both a beneficial and detrimental fashion against organisms with class I beta-lactamases and that these effects cannot be predicted from in vitro assays.

Remarks on the screening of antibiotics for antibacterial activity

The general principles of screening antibiotics for antimicrobial activity are similar to those for screening for other pharmacological effects. The system should be adapted to the specific character of the test substance and the objectives of the program. In the screening of beta-lactams, standard tests, such as determination of the MICs, effects of inoculum size or activity against systemic infection in mice, should be supplemented by less conventional studies on for instance activity against dormant bacteria or, in the case of penems or carbapenems, stability in the presence of kidney and lung dehydropeptidases.

Comparative in vitro activity of SM7338, a new carbapenem antimicrobial agent

The comparative in vitro activity of SM7338 was tested against 670 routine clinical isolates and 130 cefoperazone-resistant isolates of bacteria by agar dilution methods. SM7338 was at least as active as imipenem against gram-negative organisms but was slightly less active against gram-positive organisms. SM7338 was particularly active against members of the family Enterobacteriaceae, with MICs for 90% of strains of less than or equal to 0.125 micrograms/ml for all species tested. Differences in activity between SM7338 and imipenem were particularly striking against Proteus vulgaris, Proteus mirabilis, and Morganella morganii, against which MICs of SM7338 and imipenem for 90% of strains were 0.125 and 4 micrograms/ml, respectively. The presence of unique plasmid-mediated beta-lactamases in Pseudomonas aeruginosa PU21 transconjugants did not affect activity substantially, except in the case of OXA-2 (eightfold-increased MIC) and OXA-3 (fourfold-increased MIC). SM7338 was also active against a laboratory-derived strain of P. aeruginosa which hyperproduced chromosomal beta-lactamase, inhibiting both the wild type and the mutant at a concentration of 1.0 micrograms/ml.

Comparative in vitro susceptibilities of 504 bacteremic isolates to ticarcillin plus clavulanic acid and other antimicrobial agents

A total of 504 clinical bacteremic isolates were tested for susceptibility to ticarcillin-clavulanic acid and 12 other antibiotics. Ticarcillin-clavulanic acid showed superior antibacterial activity compared to penicillin, mezlocillin, piperacillin, ticarcillin, gentamicin, and amikacin against bacteremic isolates of methicillin-susceptible Staphylococcus aureus and Staphylococcus epidermidis. However, ticarcillin-clavulanic acid's activity was inferior to that of vancomycin against methicillin-resistant isolates of S. aureus and S. epidermidis. For Escherichia coli, Klebsiella oxytoca, Proteus mirabilis, Providencia stuartii, and lactose nonfermenting aerobic gram-negative bacilli, the activity of ticarcillin-clavulanic acid surpassed that of mezlocillin, piperacillin, and ticarcillin. Of the antimicrobial agents tested, ticarcillin, piperacillin, ceftazidime, and amikacin were the most active antibiotics against Pseudomonas aeruginosa.

Resistance to ticarcillin-potassium clavulanate among clinical isolates of the family Enterobacteriaceae: role of PSE-1 beta-lactamase and high levels of TEM-1 and SHV-1 and problems with false susceptibility in disk diffusion tests

Thirty-four clinical isolates of the family Enterobacteriaceae from the University of Texas M. D. Anderson Cancer Center appeared resistant to ticarcillin-potassium clavulanate in agar dilution and broth macrodilution tests. Among those isolates producing a single non-class I beta-lactamase, resistance was due to production of high levels of TEM-1, SHV-1, or class IV enzymes. In five Escherichia coli isolates, production of low levels of PSE-1 was responsible for resistance which seemed due to rapid hydrolysis of ticarcillin rather than diminished susceptibility of PSE-1 to inhibition by potassium clavulanate. Comparisons of dilution and disk diffusion tests revealed major discrepancies, with 65% false susceptibility in the disk test. Revision of the interpretive criteria used for disk diffusion tests from less than or equal to 11 to less than or equal to 18 mm for resistance is proposed to resolve these discrepancies until clinical data are obtained which can be used to determine which in vitro test is most predictive of therapeutic outcome. These new criteria would diminish false susceptibility without introducing false resistance.

Sulbactam/ampicillin, a new beta-lactamase inhibitor/beta-lactam antibiotic combination

Sulbactam/ampicillin is a combination of a beta-lactamase inhibitor with minimal intrinsic antibacterial activity (sulbactam sodium), and an aminopenicillin (ampicillin sodium). The addition of sulbactam to ampicillin has no effect on the chemical stability of ampicillin in aqueous solution, and the administration guidelines of the combination are the same as for ampicillin alone. Sulbactam acts primarily by irreversible inactivation of beta-lactamases from most beta-lactamase-producing organisms. The pharmacokinetics of sulbactam are similar to those of ampicillin with an elimination half-life of about one hour in most patients. One difference is that serum and tissue concentrations of sulbactam are usually twice those of ampicillin, at equivalent doses. The sulbactam/ampicillin combination has been approved for the treatment of adults with intraabdominal, skin and skin structure, and gynecological infections due to beta-lactamase-producing bacteria such as Staphylococcus aureus, Escherichia coli, and species of Klebsiella and Bacteroides. Clinical studies to date have also shown the combination to be effective for the treatment of meningitis, pneumonia, gonorrhea, epiglottis, urinary tract infections, cervical adenitis, and as prophylaxis for abdominal and gynecological surgeries. Many of these studies, however, have included small numbers of patients and/or had design flaws. Adverse effects have been minor with most being attributed to the ampicillin component. Sulbactam/ampicillin compares favorably with other antibiotic regimens in terms of acquisition costs and ease of administration.

In vitro activities of ICI 194008 and ICI 193428, two new cephem antimicrobial agents

The in vitro activities of two new cephem antibiotics, ICI 193428 and ICI 194008, were compared with those of cefpirome, cefotaxime, ceftazidime, and piperacillin. Essentially all strains of the family Enterobacteriaceae were inhibited by both study drugs at concentrations of less than or equal to 4 micrograms/ml. Both new cephems were comparable to ceftazidime against Pseudomonas aeruginosa (MIC for 90% of strains, 8 micrograms/ml) and were the most active agents tested against Pseudomonas maltophilia (MIC for 90% of strains, 16 micrograms/ml).

Comparative in vitro activity of CGP 31608, a new penem antibiotic

The in vitro activity of a new penem antimicrobial agent, CGP 31608, was compared with those of imipenem, SCH 34343, and several other antimicrobial agents against approximately 600 bacterial isolates. CGP 31608 was active against gram-positive organisms, including methicillin-susceptible Staphylococcus aureus (MIC for 90% of the isolates [MIC90], 0.25 microgram/ml) and penicillin-susceptible streptococci (MIC90s, less than or equal to 2 micrograms/ml). Penicillin-resistant streptococci (including enterococci) and methicillin-resistant S. aureus were more resistant to the penem. Activities of CGP 31608 against members of the family Enterobacteriaceae were remarkably uniform, with MIC90s of 8 to 16 micrograms/ml. CGP 31608 was at least as active as imipenem and ceftazidime and more active than piperacillin against Pseudomonas aeruginosa. Drug activity was not influenced by the presence of any of 10 plasmid-mediated beta-lactamases. Against strains of Serratia marcescens, Enterobacter cloacae, and P. aeruginosa with derepressible chromosomally mediated beta-lactamases, the presence of cefoxitin did not induce increased resistance to CGP 31608. The new drug was also active against anaerobes (MIC90s, 0.25 to 8 micrograms/ml), Haemophilus influenzae (MIC90s, 0.5 to 1.0 micrograms/ml), and Legionella spp. (MIC90, 2 micrograms/ml). CGP 31608 showed an antibacterial spectrum similar to those of imipenem and SCH 34343 (except that the latter is not active against P. aeruginosa) but was generally less potent than these drugs. However, CGP 31608 demonstrated more activity (MIC90) than imipenem against P. aeruginosa, Pseudomonas cepacia, and methicillin-resistant Staphylococcus epidermidis and S. aureus.

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.

Mechanisms of resistance to cephalosporin antibiotics

Cephalosporins, like other beta-lactams, bind to the bacterial penicillin-binding proteins (PBPs). These correspond to the D-ala-D-ala trans-, carboxy- and endo-peptidases responsible for catalysing the cross-linking of newly formed peptidoglycan. Resistance arises when the PBPs-and particularly the transpeptidases-are modified, or when they are protected by beta-lactamases or 'permeability barriers'. Target-mediated cephalosporin resistance can involve either reduced affinity of an existing PBP component, or the acquisition of a supplementary beta-lactam-insensitive PBP. beta-lactamases are produced widely by bacteria and may be determined by chromosomal or plasmid DNA. The chromosomal beta-lactamases are species-specific, but can be classified into a few broad groups. The plasmid-mediated enzymes cross interspecific and intergeneric boundaries. The level of beta-lactamase-mediated resistance relates to the amount of enzyme produced with or without induction; to the location of the enzyme (extracellular for Gram-positive organisms and periplasmic in Gram-negative ones); and to the kinetics of the enzyme's activity. In Gram-positive organisms the PBPs are located on the outer aspect of the cytoplasmic membrane and so shielding by permeability barriers is minimal. In Gram-negative cells, however, the PBPs are protected by the outer membrane, which most beta-lactams cross by diffusion through aqueous pores composed of 'porin' proteins. In enterobacteria, a clear correlation exists between porin quantity and cephalosporin resistance, suggesting that the outer membrane is the sole barrier to drug entry. Such relationships are less clear for Pseudomonas aeruginosa, where the cell may contain additional barriers between the outer membrane and the PBPs. Although elevated cephalosporin resistance often is attributed to a single factor (PBP-modification, beta-lactamase action or impermeability) an organism's response to a drug often reflects the interplay of several factors. Mathematical models can be proposed to describe this interplay.

Effect of clavulanic acid on the activity of ticarcillin against Pseudomonas aeruginosa

We studied the ability of clavulanic acid (CA) to induce beta-lactamase in Pseudomonas aeruginosa isolates and what effect this might have on the susceptibilities to beta-lactam agents. We first used a disk approximation method to test 4 laboratory and 16 clinical P. aeruginosa isolates against antipseudomonal beta-lactam agents for truncation by CA and found this to be very common. All antimicrobial compounds except imipenem demonstrated truncation in the vicinity of CA. We also evaluated the extent to which chromosomal beta-lactamase is induced by CA and found this to occur to some degree in most isolates and to be dependent on the concentration of CA. Finally, we performed time kill curves on these isolates to compare bacterial growth in ticarcillin alone with growth in ticarcillin-CA (the CA at 2 or 4 micrograms/ml). We found that CA at this concentration has neither an antagonistic nor a synergistic antibacterial effect in combination with ticarcillin.

Antibiotic resistance of Pseudomonas species

Pseudomonas species are highly versatile organisms with genetic and physiologic capabilities that allow them to flourish in environments hostile to most pathogenic bacteria. Within the lung of the patient with cystic fibrosis, exposed to a number of antimicrobial agents, highly resistant clones of Pseudomonas are selected. These may have acquired plasmid-mediated genes encoding a variety of beta-lactamases or aminoglycoside modifying enzymes. Frequently these resistance determinants are on transposable elements, facilitating their dissemination among the population of bacteria. Mutations in chromosomal genes can also occur, resulting in constitutive expression of normally repressed enzymes, such as the chromosomal cephalosporinase of Pseudomonas aeruginosa or Pseudomonas cepacia. These enzymes may confer resistance to the expanded-spectrum beta-lactam drugs. Decreased cellular permeability to the beta-lactams and the aminoglycosides also results in clinically significant antibiotic resistance. The development of new drugs with anti-Pseudomonas activity, beta-lactam agents and the quinolones, has improved the potential for effective chemotherapy but has not surpassed the potential of the organisms to develop resistance.

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.

Comparative in vitro activities of cefpiramide and apalcillin individually and in combination

The in vitro activities of cefpiramide and apalcillin were compared with those of other third-generation cephalosporins and extended-spectrum penicillins against over 1,000 clinical bacterial isolates. The activity of cefpiramide against Pseudomonas aeruginosa was comparable to those of piperacillin and cefoperazone, inhibiting 90% of strains at concentrations less than or equal to 16.0 micrograms/ml. This drug was also active against a broad range of gram-negative organisms but was generally less active than many of the other cephalosporins tested against members of the family Enterobacteriaceae. The activity of cefpiramide against gram-positive organisms was comparable to that of cefoperazone. Apalcillin, along with ceftazidime, was the most active agent tested against P. aeruginosa and Acinetobacter calcoaceticus subsp. anitratus, inhibiting 90% of these strains at concentrations less than or equal to 8 micrograms/ml. Against other gram-negative and gram-positive organisms, its activity was similar to that of piperacillin. The activities of both cefpiramide and apalcillin were significantly reduced by the presence of several plasmid-mediated beta-lactamases in a series of otherwise isogenic strains of P. aeruginosa in comparison with their activities against a parent strain which lacks these enzymes. Many strains of Enterobacter cloacae were synergistically inhibited by the combination of gentamicin with either cefpiramide (5 of 10 strains) or apalcillin (6 of 10 strains). Most strains of P. aeruginosa were synergistically inhibited by the combination of gentamicin with either cefpiramide (8 of 10 strains) or apalcillin (10 of 10 strains). However, cefoxitin antagonized the activity of both cefpiramide and apalcillin against most of these same strains.

Imipenem (N-formimidoyl thienamycin): in vitro antimicrobial activity and beta-lactamase stability

In vitro studies with imipenem (N-formimidoyl thienamycin or MK0787) were performed with 8481 clinical isolates in three separate medical centers. More extensive comparative studies were also performed with 605 representative isolates, comparing imipenem to six other beta-lactams. Although the newer beta-lactams were often more active against susceptible species, imipenem demonstrated the broadest spectrum of antibacterial activity, with MIC 90s less than or equal to 4.0 micrograms/ml for all species tested except Pseudomonas maltophilia and P. cepacia. Imipenem was very active against all streptococci and staphylococci, in contrast to the third-generation cephalosporins. There was no evidence of cross-resistance between imipenem and the cephalosporins or penicillins. Resistance to hydrolysis by seven beta-lactamase preparations was documented for imipenem, cefotaxime, and moxalactam. Like many other beta-lactams, imipenem inhibited the Type I beta-lactamase produced by Enterobacter cloacae. Other beta-lactamases from gram-negative bacilli were also inhibited by high concentrations of imipenem.

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.

In vitro antagonism between N-formimidoyl thienamycin and aztreonam, ticarcillin and ticarcillin/clavulanic acid

One-hundred eighty-seven bacterial strains were tested by the two-disk-agar diffusion method for the interaction between N-formimidoyl thienamycin and aztreonam, ticarcillin and ticarcillin-clavulanic acid. An antagonism between N-formimidoyl thienamycin and the other 3 beta-lactams was noted in half of the evaluable tests, especially against Pseudomonas, Escherichia coli and Morganella.

Amoxicillin and potassium clavulanate: an antibiotic combination. Mechanism of action, pharmacokinetics, antimicrobial spectrum, clinical efficacy and adverse effects

The combination of amoxicillin and potassium clavulanate will soon be marketed in 2:1 and 4:1 fixed ratio dosage forms. In vitro and in vivo evidence suggests that clavulanic acid, a potent inhibitor of many bacterial beta-lactamase enzymes, will increase the spectrum of amoxicillin to include, at achievable serum concentrations, Haemophilus influenzae, H. ducreyi, Neisseria gonorrhoeae, Staphylococcus aureus and Branhamella catarralis and, at achievable urine levels, many beta-lactamase-producing strains of E. coli, Klebsiella, Proteus and Citrobacter. Both amoxicillin and clavulanic are well absorbed after oral administration, reach peak serum levels in 40-120 min and have similar half-lives of 45 to 90 min. This combination will be suitable for the treatment of complicated urinary tract infections, otitis media, sinusitis and respiratory tract infections. However, precise recommendations for its use will need to await further clinical trials that compare amoxicillin/clavulanate to alternative therapies.

Properties of PSE-2 beta-lactamase and genetic basis for its production in Pseudomonas aeruginosa

The properties of PSE-2 beta-lactamase have been examined by using two new PSE-2-producing plasmids, pMG33 and pMG74, as well as plasmid R151, found in Pseudomonas aeruginosa. PSE-2 beta-lactamase resembled other PSE enzymes in activity against carbenicillin, but it also resembled OXA enzymes, such as OXA-1, in rapid hydrolysis of oxacillin, cloxacillin, and methicillin and in inhibition by sodium chloride but not by cloxacillin. Antisera that inactivated TEM-1, TEM-2, OXA-1, or PSE-1 and PSE-4 beta-lactamase failed to cross-react with PSE-2, which thus appears to be immunologically distinct. The plasmids determining PSE-2 varied in geographical origin, size, transfer proficiency, and incompatibility specificity, but all determined resistance to carbenicillin, gentamicin, kanamycin, streptomycin, spectinomycin, sulfonamide, and tobramycin. From a pUZ8-R151 recombinant plasmid in Escherichia coli, the PSE-2 beta-lactamase gene could be transposed to a second plasmid in a 6.4-megadalton unit together with resistance to gentamicin, kanamycin, streptomycin, spectinomycin, sulfonamide, and tobramycin. Transposition was recA independent. We propose the designation Tn1404 for this unit, which, like transposons carrying OXA-1, PSE-1, PSE-4, and some transposons determining TEM-1, includes genes for beta-lactam, aminoglycoside, and sulfonamide resistance.

Rationale for use of antimicrobial combinations

In most cases antimicrobial combinations are employed to broaden the spectrum of coverage. This clinical application is likely to be successful as long as the combinations are not antagonistic. Most examples of antibiotic antagonism are those in which a bacteriostatic agent renders a bactericidal agent "static." Another type of antagonism occurs when cefoxitin (which has a propensity to induce beta-lactamase production) is combined with another beta-lactam antibiotic. Combination drug therapy prevents emergence of resistant strains in mycobacterial infections and in infections due to methicillin-resistant staphylococci and certain other resistant organisms. Synergistic combinations should allow the use of lower concentrations of drugs in combination and thus diminish the incidence of dose-related antibiotic toxicity, but the concept has met with only limited success so far. Three types of antimicrobial combination or interaction result in enhanced (synergistic) antimicrobial activity. These types include combinations of agents that inhibit bacterial cell wall synthesis with aminoglycosidic aminocyclitols, the use of beta-lactamase inhibitors in combination with beta-lactam antibiotics, and the administration of agents that act on sequential steps in one of the bacterial metabolic or synthetic pathways. Combinations of two beta-lactam antibiotics that bind to complementary penicillin-binding proteins may represent an analogous situation. Amdinocillin binds specifically to penicillin-binding protein 2, and in vitro studies have clearly demonstrated synergism when amdinocillin is combined with other penicillins and cephalosporins that have higher affinity for other penicillin-binding proteins.