Emergence of resistance to beta-lactam and aminoglycoside antibiotics during moxalactam therapy of Pseudomonas aeruginosa infections

Selected Fungal Natural Products with Antimicrobial Properties

Fungal natural products and their effects have been known to humankind for hundreds of years. For example, toxic ergot alkaloids produced by filamentous fungi growing on rye poisoned thousands of people and livestock throughout the Middle Ages. However, their later medicinal applications, followed by the discovery of the first class of antibiotics, penicillins and other drugs of fungal origin, such as peptidic natural products, terpenoids or polyketides, have altered the historically negative reputation of fungal "toxins". The development of new antimicrobial drugs is currently a major global challenge, mainly due to antimicrobial resistance phenomena. Therefore, the structures, biosynthesis and antimicrobial activity of selected fungal natural products are described here.

Emergence of Resistance inPseudomonas aeruginosaandAcinetobacterSpecies After the Use of Antimicrobials for Burned Patients

Objectives:

To evaluate the emergence of resistance ofPseudomonas aeruginosaandAcinetobacterspecies to imipenem, ciprofloxacin, or both after the use of these drugs and to compare resistant with susceptible isolates by molecular typing.

Design:

Cohort study.

Setting:

Burn intensive care unit (ICU) with 4 beds in a tertiary-care university hospital.

Methods:

During 16 months, surveillance cultures were performed for all patients admitted to the ICU. Demographic information was obtained for each patient. Molecular typing was done by pulsed-field gel electrophoresis using restriction enzymes for 71 isolates ofP. aeruginosaandAcinetobacterspecies.

Results:

Thirty-four patients were admitted and 22 were colonized by susceptibleP. aeruginosaorAcinetobacterspecies before they used the antimicrobials. Nine (41%) of these patients had a resistant isolate after antimicrobial use: 5 had used imipenem alone, 1 had used ciprofloxacin, and 3 had used both drugs. The interval between isolation of the susceptible and resistant isolates ranged from 4 to 25 days, but was 10 or more days for 6 patients. Molecular typing revealed that susceptible and resistant isolates from each patient were different and that although there were no predominant clones among susceptible isolates, there was a predominant clone among resistant isolatesof P. aeruginosaandof Acinetobacter.

Conclusions:

Resistance was not due to the acquisition of resistance mechanisms by a previously susceptible strain, but rather to cross-transmission. Although various measures involving antimicrobial use have received great attention, it would seem that practices to prevent cross-transmission are more important in controlling resistance.

Widespread dissemination of aminoglycoside resistance genes armA and rmtB in Klebsiella pneumoniae isolates in Taiwan producing CTX-M-type extended-spectrum beta-lactamases

Among 235 extended-spectrum beta-lactamase-producing Klebsiella pneumoniae (ESBL) isolates collected from a nationwide surveillance performed in Taiwan, 102 (43.4%) were resistant to amikacin. Ninety-two of these 102 (90.2%) isolates were carrying CTX-M-type beta-lactamases individually or concomitantly with SHV-type or CMY-2 beta-lactamases. The armA and rmtB alleles were individually detected in 44 and 37 of these 92 isolates, respectively. One isolate contained both armA and rmtB. The coexistence of the aac(6')-Il and rmtB genes was detected in three isolates. CTX-M-type beta-lactamase genes belonging to either group 1 (CTX-M-3 and CTX-M-15) or group 9 (CTX-M-14) were found in all armA- or rmtB-bearing ESBL-producing K. pneumoniae isolates, and all were conjugatively transferable. All except one of the isolates bearing armA produced CTX-M enzymes of group 1, and the remaining isolate bearing armA produced a group 9 CTX-M-type beta-lactamase. On the contrary, in the majority of rmtB carriers, the CTX-M-type beta-lactamase belonged to group 9 (62.2%). Molecular typing revealed that the amikacin-resistant ESBL-producing K. pneumoniae isolates were epidemiologically unrelated, indicating that the acquisition of resistance was not through the spread of a resistant clone or a resistance plasmid. A tandem repeat or multiple copies of bla(CTX-M-3) were found in some armA-bearing isolates. An ISEcp1 insert was found in all CTX-M ESBL-producing K. pneumoniae isolates carrying armA or rmtB. In conclusion, the concomitant presence of a 16S rRNA methylase gene (armA or rmtB) and bla(CTX-M) among amikacin-resistant ESBL-producing K. pneumoniae isolates is widespread in Taiwan.

Model system to evaluate the effect of ampD mutations on AmpC-mediated beta-lactam resistance

Mutations within the structural gene of ampD can lead to AmpC overproduction and increases in beta-lactam MICs in organisms with an inducible ampC. However, identification of mutations alone cannot predict the impact that those mutations have on AmpD function. Therefore, a model system was designed to determine the effect of ampD mutations on ceftazidime MICs using an AmpD(-) mutant Escherichia coli strain which produced an inducible plasmid-encoded AmpC. ampD genes were amplified by PCR from strains of E. coli, Citrobacter freundii, and Pseudomonas aeruginosa. Also, carboxy-terminal truncations of C. freundii ampD genes were constructed representing deletions of 10, 21, or 25 codons. Amplified ampD products were cloned into pACYC184 containing inducible bla(ACT-1)-ampR. Plasmids were transformed into E. coli strains JRG582 (AmpD(-)) and K-12 259 (AmpD(+)). The strains were evaluated for a derepressed phenotype using ceftazidime MICs. Some mutated ampD genes, including the ampD gene of a derepressed C. freundii isolate, resulted in substantial decreases in ceftazidime MICs (from >256 microg/ml to 12 to 24 microg/ml) for the AmpD(-) strain, indicating no role for these mutations in derepressed phenotypes. However, ampD truncation products and ampD from a partially derepressed P. aeruginosa strain resulted in ceftazidime MICs of >256 microg/ml, indicating a role for these gene modifications in derepressed phenotypes. The use of this model system indicated that alternative mechanisms were involved in the derepressed phenotype observed in strains of C. freundii and P. aeruginosa. The alternative mechanism involved in the derepressed phenotype of the C. freundii isolate was downregulation of ampD transcription.

AmpC and OprD are not involved in the mechanism of imipenem hypersusceptibility among Pseudomonas aeruginosa isolates overexpressing the mexCD-oprJ efflux pump

Pseudomonas aeruginosa strains that overexpress mexCD-oprJ become hypersusceptible to imipenem. Disruption of AmpC induction has been suggested to cause this phenotype. However, data from this study demonstrate that hypersusceptibility to imipenem can develop without changes in ampC expression or AmpC activity. Furthermore, hypersusceptibility is not caused by changes in expression of the outer membrane porin, OprD.

In vivo emergence of multidrug-resistant mutants of Pseudomonas aeruginosa overexpressing the active efflux system MexA-MexB-OprM

During a 6-month period, 21 pairs of Pseudomonas aeruginosa isolates susceptible (pretherapy) and resistant (posttherapy) to antipseudomonal beta-lactam antibiotics were isolated from hospitalized patients. In vivo emergence of beta-lactam resistance was associated with the overexpression of AmpC beta-lactamase in 10 patients. In the other 11 patients, the posttherapy isolates produced only low, basal levels of beta-lactamase and had increased levels of resistance to a variety of non-beta-lactam antibiotics (e.g., quinolones, tetracyclines, and trimethoprim) compared with the levels of beta-lactamase production and resistance of their pretherapy counterparts. These data suggested the involvement of the MexA-MexB-OprM active efflux system in the multidrug resistance phenotype of the posttherapy strains. Immunoblotting of the outer membrane proteins of these 11 bacterial pairs with a specific polyclonal antibody raised against OprM demonstrated the overexpression of OprM in all the posttherapy isolates. To determine whether mutations in mexR, the regulator gene of the mexA-mexB-oprM efflux operon, could account for the overproduction of the efflux system, sequencing experiments were carried out with the 11 bacterial pairs. Eight posttherapy isolates were found to contain insertions or deletions that led to frameshifts in the coding sequences of mexR. Two resistant strains had point mutations in mexR that yielded single amino acid changes in the protein MexR, while another strain did not show any mutation in mexR or in the promoter region upstream of mexR. Introduction of a plasmid-encoded wild-type mexR gene into five posttherapy isolates partially restored the susceptibility of the bacteria to selected antibiotics. These results indicate that in the course of antimicrobial therapy multidrug-resistant active efflux mutants overexpressing the MexA-MexB-OprM system may emerge as a result of mutations in the mexR gene.

Multiresistant Pseudomonas aeruginosa serogroup O:11 outbreak in an intensive care unit

OBJECTIVE: To determine whether 15 multiresistant Pseudomonas aeruginosa isolates from an intensive care unit (ICU) outbreak were related, were endemic, and belonged to the O:12 European clone. METHODS: Forty-six P. aeruginosa isolates from a large hospital were investigated with respect to their antibiotic resistance profiles, serogroups, bacteriocin types and DNA fingerprints obtained by pulsed-field gel electrophoresis (PFGE) of genomic DNA digested with Xbal. RESULTS: Fourteen of the ICU outbreak isolates were indeed identical with respect to their serogroup, O:11, pyocin type, 10/a, and PFGE type, A. Clone A was endemic and dominant throughout the hospital, even though, within the ICU, it underwent phenotypic alterations, such as loss of cell wall lipopolysaccharide side-chains, or acquisition of ceftazidime and imipenem resistance. Bacteriocin typing was more discriminatory than serotyping, but PFGE could differentiate further among phenotypically identical strains. It also allowed the tracking of an O:6 strain, as it was becoming gradually more resistant and undergoing a bacteriocin-type conversion while remaining genotypically unaltered. CONCLUSIONS: Using three typing methods, a nosocomial multiresistant strain distinct from the previously described dominant European O:12 clone was characterized, and the ability of PFGE to identify clonal isolates even when these appear phenotypically distinct was demonstrated.

Detection of resistance due to inducible beta-lactamase in Enterobacter aerogenes and Enterobacter cloacae

Thirty-six of 36 strains of Enterobacter cloacae and E. aerogenes with inducible beta-lactamase developed resistance when cefoxitin (inducer) was added to cefuroxime disks. Constitutive beta-lactamase producers (n = 23) were all resistant to cefuroxime. Cefuroxime resistance correlated with the amount of induced or constitutive beta-lactamase. Cefuroxime was a better indicator of induced resistance than cefamandole, cefazolin, cephalothin, ceftriaxone, cefotaxime, ticarcillin with or without clavulanic acid, or cefotetan. Induction by addition of cefoxitin to disks occasionally reduced zone sizes but not enough to change interpretations for ceftazidime, ceftizoxime, aztreonam, cefoperazone with or without sulbactam, and piperacillin with or without tazobactam. Most enterobacters were resistant to cefmetazole. The cefoxitin inducer-cefuroxime indicator method can be used in routine clinical laboratories to detect latent resistance due to chromosomally mediated inducible beta-lactamase in enterobacters.

In-vitro activity of ciprofloxacin and sixteen other antimicrobial agents against blood culture isolates

The in-vitro antibacterial activities of seventeen antimicrobial agents including ampicillin, amikacin, Augmentin, aztreonam, cefazolin, cefuroxime, cefotaxime, ceftizoxime, ceftriaxone, ciprofloxacin, cloxacillin, erythromycin, gentamicin, penicillin G, piperacillin and vancomycin were compared against 100 Gram-negative and Gram-positive strains isolated from blood culture specimens received at the King Abdulaziz University Hospital (KAUH), in Jeddah, Saudi Arabia. The antibacterial susceptibility was determined by the minimal inhibitory concentration (MIC), using an agar dilution method. Ciprofloxacin exhibited the greatest activity, inhibiting 90% of the tested strains (MIC90) at a concentration ranging from < 0.015-0.5 mg/L. Against cloxacillin resistant or susceptible strains of Staphylococcus aureus and coagulase-negative staphylococci, ciprofloxacin had similar activity with MIC90 of 0.2 mg/L. Salmonella typhi and salmonella species which were resistant to ampicillin and augmentin remained sensitive to ciprofloxacin (MIC90 < 0.015-0.125) mg/L.). Against gentamicin sensitive and resistant Pseudomonas aeruginosa and Pseudomonas species, ciprofloxacin MIC90 was 0.5 and 1 mg/L respectively. Aminoglycosides, third generation cephalosporins, aztreonam and antipseudomonal penicillins, on the other hand, showed high MIC90 well above the obtainable serum concentrations against Enterobacteriaceae and Pseudomonas species.

A 56-month prospective surveillance study on the epidemiology of aminoglycoside resistance in a Belgian general hospital

In this survey, we studied the effect of extensive amikacin usage on the epidemiology of aminoglycoside resistance in a general hospital. The baseline resistance in the 12 months before amikacin was 5.8% for amikacin, 15.2% for gentamicin, 16.4% for tobramycin and 14.0% for netilmicin. During the following 44 months, amikacin was the aminoglycoside of first choice. In the first 2 years of this phase, resistance to amikacin did not change significantly. Later, amikacin resistance rose significantly, mainly due to the introduction of amikacin-resistant Enterobacter aerogenes strains. In general there was a significant decrease in resistance to gentamicin and tobramycin. Resistance mechanisms were examined in 380 strains. AAC(3)V, and AAC(6')I alone or coupled with ANT(2") or AAC(3) were the most prevalent enzymes. In the amikacin phase, we noticed a significant increase of strains harbouring the AAC(6')I enzyme, while strains with the AAC(3)V were less frequently isolated. Strains with permeability resistance did not become more prevalent during the period of extensive amikacin use.

The influence of clinical use of antibiotics and the sensitivity of strains isolated from postoperative infections--a comparison of nosocomial pathogens with strains isolated from the bacterial flora of patients

In the present study, we investigated how the recent clinical use of antibiotics have altered the antibiotic susceptibility of strains isolated from postoperative infections, especially Gram-negative rods. For Pseudomonas aeruginosa, serogroup E strains accounted for about 20 per cent of postoperative infections, but were unable to be isolated from either the feces of patients on admission or from the appendix contents of patients with appendicitis. It therefore appeared that serogroup E strains were responsible for the nosocomial infections in our department. The strains of methicillin-resistant Staphylococcus aureus and Pseudomonas aeruginosa serogroup E, which we assumed to be nosocomial pathogens, acquired a high level of resistance to antibiotics soon after third-generation cephems became widely used. On the other hand, the antibiotic susceptibility of Enterobacter cloacae, Citrobacter freundii, and the serogroups of Pseudomonas aeruginosa other than E, which were considered to originate from the bacterial flora of patients, did not vary throughout the several years of the study period.

In-vitro activity of ciprofloxacin and other beta-lactam antibiotics against gentamicin- and carbenicillin-resistant isolates of Pseudomonas aeruginosa

The susceptibilities of gentamicin- and carbenicillin-resistant clinical isolates of Pseudomonas aeruginosa to newer beta-lactams, netilmicin and ciprofloxacin were studied by a broth microdilution technique. Imipenem, aztreonam and ceftazidime were active against most of the P. aeruginosa strains with minimum inhibitory concentration (MIC) for 90% of the isolates at clinically achievable levels. Piperacillin, azlocillin, cefotaxime, ceftriaxone, cefsulodin, cefoperazone and netilmicin showed poor activity against these organisms, ciprofloxacin exhibited poor activity, inhibiting only 30% of these strains.

In vitro antibacterial activity of KP-736, a new cephem antibiotic

KP-736, a new cephen antibiotic with a broad antibacterial spectrum and potent antipseudomonal activity, was evaluated for in vitro antibacterial activity in comparison with ceftazidime, cefotaxime, and cefpirome. KP-736 was significantly more active than the test compounds against gram-negative bacteria, including the Pseudomonas group and ceftazidime-, cefotaxime-, or imipenem-resistant strains, but less active against gram-positive bacteria. KP-736 had very high affinities for penicillin-binding protein 3 (PBP 3) of Escherichia coli K-12 and PBP 1A and PBP 3 of Pseudomonas aerugiosa NCTC 10490 and showed potent bactericidal activities against these two strains. It was stable to hydrolysis by penicillinases and cephalosporinases but was slightly hydrolyzed by oxyiminocephalosporinases and type II penicillinase.

Clinical isolate of a porinless Salmonella typhi resistant to high levels of chloramphenicol

We studied a clinical isolate of Salmonella typhi (strain 1895) characterized by resistance to 200 micrograms of chloramphenicol per ml despite the absence of chloramphenicol-inactivating activity. The outer membrane protein profile analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated a deficiency of one of the major protein species which may serve as a porin for entry of chloramphenicol. When the strain was transformed with a plasmid encoding chloramphenicol acetyltransferase, chloramphenicol added to the culture was not inactivated, suggesting a drastic reduction of permeability towards the drug. Moreover, transformants bearing a plasmid coding for the Escherichia coli OmpF porin became considerably more susceptible to chloramphenicol (40 micrograms/ml). On the other hand, transformants carrying a plasmid encoding the Salmonella typhi ompC gene remained as resistant to the drug as the parental strain, even though they overexpressed OmpC. These findings indicate that the lack of OmpF plays a major role in the resistance to chloramphenicol in strain 1895.

In-vitro susceptibility of clinical isolates of Pseudomonas aeruginosa to beta-lactam and aminoglycoside antibiotics

The in-vitro susceptibilities of 198 isolates of Pseudomonas aeruginosa from clinical human specimens were determined by an agar dilution technique against beta-lactams and aminoglycosides. These isolates were susceptible to imipenem, aztreonam and ceftazidime with the minimum inhibitory concentration (MIC) for 90% of the strains tested being 8, 16 and 8 micrograms/ml, respectively. Aminoglycosides, except amikacin, had low activity (MIC90 greater than 128 micrograms/ml).

In vitro and in vivo antibacterial activities of BO-1341, a new antipseudomonal cephalosporin

BO-1341, a new antipseudomonal semisynthetic cephalosporin, was evaluated for in vitro and in vivo antibacterial activities in comparison with ceftazidime, cefotaxime, and cefoperazone. The in vitro activity of BO-1341 was generally superior or comparable to the activities of the reference antibiotics against clinical isolates of the family Enterobacteriaceae. BO-1341 was highly active against Pseudomonas aeruginosa (MIC for 90% of the strains tested, 1.56 micrograms/ml), Pseudomonas maltophilia (MIC for 50% of the strains tested, 1.56 micrograms/ml), and Acinetobacter calcoaceticus (MIC for 90% of the strains tested, 3.13 micrograms/ml). Furthermore, BO-1341 was highly active against P. aeruginosa isolates resistant to the other antibiotics. Of 199 P. aeruginosa isolates tested, only 2 were resistant to BO-1341. These two strains were also resistant to ceftazidime, cefotaxime, and cefoperazone. Haemophilus influenzae, Branhamella catarrhalis, and nonenteric streptococci were also susceptible to BO-1341, but Staphylococcus aureus, Streptococcus faecalis, and Bacteroides fragilis were not susceptible to the compound. The protective efficacy against experimental infections in mice caused by nine strains of gram-negative bacteria, including P. aeruginosa, reflected the potent in vitro activity.

Relationship of antibiotic resistance phenotype to the R-pyocin susceptibility pattern in clinical isolates of Pseudomonas aeruginosa

One hundred sixteen clinical isolates of Pseudomonas aeruginosa were collected from 7 hospitals in Athens. All strains were studied for their susceptibility to cefotaxime, ceftazidime, carbenicillin, aztreonam, imipenem, nalidixic acid, ciprofloxacin, gentamicin, and chloramphenicol. In addition, the R-pyocin susceptibility pattern was determined and the strains were O-serotyped and tested for their agglutination in acriflavine. The isolates included 53 strains resistant to both gentamicin and carbenicillin, 13 to carbenicillin only, 20 to gentamicin only, and 30 sensitive to gentamicin and carbenicillin. The multiresistant isolates displayed relatively higher resistance to all other antibiotics except aztreonam and cefotaxime. Remarkably 30 out of 53 multiresistant isolates reacted with one pyocin only, namely pyocin R2. This R-pyocin response was not encountered in any other strains of the other antibiotic resistance phenotypes. These isolates belonged to the 0-12 serogroup. The 0-12 serogroup was represented only in a minority of strains giving other R-pyocin reactions. It is interesting that strains reacting with pyocin R5 only were mostly susceptible to antibiotics. The results clearly indicate lipopolysaccharide-core mutations in multiresistant clinical isolates of P. aeruginosa. Despite the fact that the R-pyocin resistance pattern can not define the precise possible defect, the multiple and high level resistance associated with R2-pyocin reaction seems to be an interesting trait.

Outer membrane alterations in multiresistant mutants of Pseudomonas aeruginosa selected by ciprofloxacin

Spontaneous mutants of Pseudomonas aeruginosa selected by ciprofloxacin were studied for outer membrane alterations. Acquisition of ciprofloxacin resistance was at least partially related to defects in lipopolysaccharide synthesis. When ciprofloxacin resistance was combined with resistance to beta-lactams and aminoglycosides, several alterations in outer membrane proteins were noted.

An international study on the occurrence of multiresistant bacteria and aminoglycoside consumption patterns

The correlation between aminoglycoside consumption patterns and the occurrence of aminoglycoside-resistant bacteria from 12 different countries was analyzed. Regional and national data were collected retrospectively and compared. There was evidence of a wide variation of the national aminoglycoside consumption patterns in the different countries. There was a striking correlation of gentamicin resistance and the total national aminoglycoside and national gentamicin consumption. In addition, there was a clear correlation between bacterial resistance inside and outside the hospital to the total amount of aminoglycoside, particularly gentamicin, consumption in hospitals. In a number of countries, an increase in the frequency of gentamicin-resistant bacterial strains could be noticed. For amikacin, only a correlation of staphylococcal resistance in hospitalized patients to the total amount of national aminoglycoside and amikacin consumption could be found.

Mutation of Salmonella paratyphi A conferring cross-resistance to several groups of antibiotics by decreased permeability and loss of invasiveness

A spontaneous one-step mutant of Salmonella paratyphi A selected on ampicillin showed cross-resistance to all beta-lactam antibiotics except imipenem and to aminoglycosides, chloramphenicol, tetracycline, trimethoprim, and quinolones. It also grew as small colonies. Examination of the cell envelope of the mutant showed a quantitative decrease in three major outer membrane proteins of 40.6, 39.6 (presumably porins), and 24 kilodaltons and quantitative as well as qualitative modifications in the ladder pattern of lipopolysaccharide. Direct evidence for decreased permeability in the mutant included reduced uptake of [3H]glucose and norfloxacin, reduced accessibility of aztreonam and benzylpenicillin to penicillin-binding proteins in whole cells, and decreased diffusion of lactose and cephaloridine into proteoliposomes that were reconstituted with outer membrane proteins from the mutant. There was also loss of invasiveness of the mutant into HeLa cells. We assume that a pleiotropic mutation was responsible for multiple alterations in the outer membrane components of the resistant mutant of S. paratyphi A.

Prospects for the prevention and control of gram-negative nosocomial infections

Nosocomial infections of bacterial origin continue to be a leading cause of morbidity and mortality among virtually all hospitalized patient populations. It is estimated that nearly 5% of all hospitalized individuals will acquire an infection during their stay. Approximately 3% of nosocomial infections will contribute to an eventual fatal outcome. Preeminent among nosocomial pathogens are the aerobic Gram-negative bacilli, with Escherichia coli, Pseudomonas aeruginosa and Klebsiella spp. being the causative agents for the majority of life-threatening infections. This review describes the current situation and future prospects for combatting these infections.

Bactericidal interactions of a beta-lactam and beta-lactamase inhibitors in experimental Pseudomonas aeruginosa endocarditis caused by a constitutive overproducer of type Id beta-lactamase

We investigated the in vitro and in vivo effects of a combination of a beta-lactam (ceftazidime) and a beta-lactamase inhibitor (dicloxacillin) to synergistically kill a ceftazidime-resistant variant, Pseudomonas aeruginosa PA-48, which overproduces type Id cephalosporinase constitutively. In vitro, dicloxacillin plus ceftazidime exerted bactericidal synergy at approximately 10(5) CFU/ml of inoculum (but not at approximately 10(7)-CFU inoculum), whereas other beta-lactamase inhibitors (sulbactam, clavulanic acid) showed no enhanced killing of PA-48 when combined with ceftazidime at clinically achievable levels for each agent. Dicloxacillin was a potent competitive inhibitor of the extracted Id cephalosporinase from strain PA-48 in short-term comixture studies (less than 10 min [Ki = 2 nM]); in contrast, longer-term comixture studies (90 min) indicated that dicloxacillin functions as a competitive substrate for the enzyme. Growth of PA-48 cells in the presence of dicloxacillin (12.5 to 100 micrograms/ml) had no significant effect on the production rates or functional activity of the Id enzyme. In experimental aortic valve endocarditis due to the ceftazidime-resistant variant (PA-48), rabbits received either no therapy, ceftazidime (25 mg/kg intramuscularly, every 4 h), or ceftazidime plus dicloxacillin (200 mg/kg intramuscularly, every 4 h). The combination regimen reduced mean bacterial densities of PA-48 within cardiac vegetations significantly below those in the other groups at both days 3 and 6 of treatment (P less than 0.005). However, mean vegetation bacterial densities remained greater than 6 log10 CFU/g in the combined treatment group. This modest in vivo synergistic effect (as compared to striking in vitro synergy at approximately 10(5)-CFU inoculum) most likely reflects the high densities of PA-48 achieved in vivo within cardiac vegetations (greater than 8 log10 CFU/g).

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

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

Clinical consequences of development of resistance to third generation cephalosporins

Eighteen patients are described in whom initially sensitive microorganisms were replaced by resistant isolates during administration of ceftriaxone (n = 8), cefoperazone (n = 5), moxalactam (n = 4), cefotaxime (n = 2) or ceftazidime (n = 1), despite combination with aminoglycosides. All patients had documented gram-negative infections; in 12 patients underlying haematological diseases were present. Resistant strains of Enterobacter cloacae (14), Serratia marcescens (4), Klebsiella oxytoca (3), Pseudomonas aeruginosa (2) and Citrobacter freundii (2) emerged within 2 to 19 (mean 9) days after the beginning of treatment. In 12 patients relapse or secondary infections occurred. Seven of the patients with haematological disorders died. Resistance development was seen in 8 of 29 patients on ceftriaxone and 4 of 10 patients on moxalactam during prospective evaluations; the other drugs were used sporadically. Thus, selection of resistant bacteria is relatively frequent and may have serious clinical consequences in patients with impaired host-defense mechanisms.

Resistance of Pseudomonas aeruginosa to beta-lactam antibiotics

A series of experiments were performed with P. aeruginosa to demonstrate which of the biochemical mechanisms are responsible for the resistance to the beta-lactam antibiotics. The constitutive beta-lactamase was isolated and characterized for the strain used as type OXA whose pI was 7.1, with a molar mass of 49 kg/mol. The strain was also submitted to a series of treatments with Tris and Tris-EDTA to disrupt the outer membrane. The treated cells demonstrated a ten-fold reduction in the MIC with cloxacillin, six-fold with penicillin, and five-fold with oxacillin. At least two different biochemical mechanisms were responsible for the resistance to the beta-lactams studied which could be important in the prevalence of P. aeruginosa in nosocomial infections.

Biological activity of BO-1236, a new antipseudomonal cephalosporin

BO-1236, a new cephalosporin having an N-methyl-5,6-dihydroxyisoindolinium moiety on the 3-methylene of the cephem, showed potent activity against gram-negative organisms, including Pseudomonas aeruginosa. The in vitro activity of BO-1236 was superior or comparable to that of ceftazidime, cefotaxime, and cefoperazone in susceptibility tests with clinical isolates. BO-1236 was significantly more active than ceftazidime against P. aeruginosa strains susceptible or resistant to ceftazidime or gentamicin or both. MBCs were usually close to MICs, both of which were influenced by inoculum size to about the same degree as those of the other beta-lactams. BO-1236 was stable to all types of beta-lactamases except type I oxyiminocephalosporin-hydrolyzing enzyme, by which BO-1236 was slightly hydrolyzed. BO-1236 showed protective activity superior to that of ceftazidime and cefotaxime in experimental infections in mice caused by two strains of P. aeruginosa and showed activity comparable to that of ceftazidime and cefotaxime against other gram-negative bacterial infections.

Resistant strains of Pseudomonas aeruginosa isolated after exposure to several beta-lactam antibiotics

Successive transfer of three clinical isolates of Pseudomonas aeruginosa in liquid medium containing serial dilutions of several beta-lactam antibiotics was used to isolate resistant variants. The alpha-carboxy-penicillins (carbenicillin and ticarcillin), the acylureidopenicillins (piperacillin and azlocillin) and cephalosporins (moxalactam, cefoperazone, cefsulodin and ceftazidime) were used as the selective antibiotics. Resistant variants were isolated from two of the three strains (strains 27 and 45), using an inoculum size of 10(4)-10(5) CFU/ml, which showed a mean 5 to 8-fold increase in MIC for most of the selected antibiotics. The 27-carbenicillin and 27-cefsulodin resistant variants showed beta-lactamase production similar to that of the parent. However, alterations were found in outer-membrane proteins and lipopolysaccharides. With azlocillin, moxalactam and ceftazidime as the selective antibiotics, resistant variants were isolated from strains 27 and 45 which showed a stable increased constitutive beta-lactamase production. From the third strain, 9150, the only variants isolated showed a two dilution-step increase in MIC to the antibiotics tested. The beta-lactamase production, outer-membrane proteins and lipopolysaccharides of these variants were similar to those of the parent.

Role of beta-lactamase in in vivo development of ceftazidime resistance in experimental Pseudomonas aeruginosa endocarditis

Two ceftazidime-resistant variants of Pseudomonas aeruginosa (PA-48, PA-60), obtained from cardiac vegetations of rabbits with endocarditis receiving ceftazidime therapy, were studied for mechanisms of resistance. Both resistant variants were stably derepressed for the type Id beta-lactamase, which was ceftazidime inducible in the parental strain (PA-96) used to initially infect the rabbits. There was no evidence of ceftazidime bioinactivation by the resistant strains, and their outer membrane permeabilities were comparable to those of the parental strain. No alterations were observed in patterns of outer membrane proteins or membrane lipopolysaccharides in the resistant variants as compared with the parental strain. Penicillin-binding protein patterns of the resistant variants revealed the absence of penicillin-binding protein 4 in both, with acquisition of a new protein of higher apparent molecular weight in PA-60. Calculation of the rate of appearance of ceftazidime in the periplasm at sub-MICs suggested that slow enzymatic hydrolysis of the beta-lactam, rather than nonhydrolytic trapping, was the major explanation for the induced resistance in vivo in strains PA-48 and PA-60.

Efficacy of ciprofloxacin in experimental aortic valve endocarditis caused by a multiply beta-lactam-resistant variant of Pseudomonas aeruginosa stably derepressed for beta-lactamase production

The emergence of multi-beta-lactam resistance is a limiting factor in treating invasive Pseudomonas infections with newer cephalosporins. The in vivo efficacy of ciprofloxacin, a new carboxy-quinolone, was evaluated in experimental aortic valve endocarditis caused by a strain of Pseudomonas aeruginosa which is stably derepressed for beta-lactamase production and is resistant to ceftazidime and multiple other beta-lactam agents. A total of 51 catheterized rabbits with aortic catheters in place were infected with this strain and then received no therapy (controls), ceftazidime (75 mg/kg per day), or ciprofloxacin (80 mg/kg per day). Ciprofloxacin sterilized all blood cultures and significantly lowered vegetation densities of P. aeruginosa by day 2 of treatment versus controls (P less than 0.0005) and animals receiving ceftazidime (P less than 0.0005). This beneficial effect of ciprofloxacin was also noted on therapy days 6 and 11. Ciprofloxacin rendered most vegetations (85%) culture negative over the 11-day treatment period and achieved bacteriologic cure in 73% of animals (P less than 0.0005 versus other therapy groups). Ciprofloxacin prevented bacteriologic relapse at 6 days posttherapy. No ciprofloxacin resistance was detected among Pseudomonas isolates from cardiac vegetations. Ciprofloxacin warrants further evaluation in vivo versus multi-drug-resistant gram-negative bacillary infections.

Evidence for multiple forms of type I chromosomal beta-lactamase in Pseudomonas aeruginosa

The multiple stages of derepression of the type I chromosomal beta-lactamase in Pseudomonas aeruginosa were examined. Mutants partially and fully derepressed for beta-lactamase were selected from a wild-type clinical isolate. An analysis of the beta-lactamase produced by these mutants and the induced wild type revealed significant differences in the products of derepression at each stage. Beta-lactamase produced by the fully derepressed mutant showed a lower affinity (Km, 0.113 mM) for cephalothin than that produced by the partially derepressed mutant (Km, 0.049 mM). However, due to a very large Vmax, the former possessed a much greater hydrolytic efficiency. Differences in substrate profile were also noted. Only beta-lactamase from the fully derepressed mutant hydrolyzed cefamandole, cefoperazone, and cefonicid. The partially derepressed mutant possessed a single beta-lactamase band with a pI of 8.4. The fully derepressed mutant possessed this band and an additional major band with a pI of 7.5. Induction of the wild type with cefoxitin produced both bands. The changes in physiologic parameters of the enzymes produced in the different stages of derepression suggest a complex system for beta-lactamase expression in P. aeruginosa. This may involve at least two distinct structural regions, each of which is under control of the same repressor.

Discrepancies between disk diffusion and broth susceptibility studies of the activity of ticarcillin plus clavulanic acid against ticarcillin-resistant Pseudomonas aeruginosa

Ticarcillin and clavulanic acid in combination were tested against 40 Pseudomonas aeruginosa isolates resistant to ticarcillin by disk diffusion. A total of 21 isolates (53%) were susceptible to ticarcillin-clavulanate by disk diffusion, under currently recommended criteria for ticarcillin susceptibility. Macro-broth dilution tests (ticarcillin plus clavulanic acid, 2 micrograms/ml) confirmed susceptibility (MIC less than or equal to 64 micrograms/ml) of only 8 (38%) of 21 isolates. Time-kill studies of disk diffusion susceptible isolates indicated 2 log10 or greater killing of most isolates at 6 h in broth containing ticarcillin (64 micrograms/ml) combined with clavulanic acid (1, 2, 5, or 10 micrograms/ml). After 6 h, regrowth was common in all concentrations of clavulanic acid except 10 micrograms/ml. Regrowth populations were resistant to ticarcillin-clavulanate by MIC determination. Poor bactericidal activity of ticarcillin-clavulanate against ticarcillin-resistant P. aeruginosa was confirmed, as most isolates did not undergo 99.9% or greater killing at 24 h in all concentrations of clavulanic acid. Serotype O-11 was our most common serotype and was associated with disk diffusion "pseudosusceptibility." Concomitant disk diffusion testing of ticarcillin-clavulanate and ticarcillin is recommended for testing the susceptibility of P. aeruginosa to ticarcillin-clavulanate by disk diffusion. P. aeruginosa isolates resistant to ticarcillin should as a rule be considered also resistant to ticarcillin-clavulanate, despite apparent susceptibility by disk diffusion.

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.

Cost-effective aminoglycoside therapy in surgical patients

There is a risk that the current pressures for hospital cost containment may result in inappropriately restrictive administrative measures. A failure to take into account all factors in estimating the cost of antibiotic therapy yields a greatly distorted view of the importance of acquisition costs. Higher-priced drugs may actually be more cost-effective if they have greater efficacy, wider therapeutic range, and/or are less costly to prepare and administer. Simple microcomputer modeling techniques may be easily employed to examine the economic consequences of therapeutic decisions.

In vitro activity of BRL 36650, a new semisynthetic penicillin

BRL 36650 [sodium 6 beta-(D-2-[(4-ethyl-2, 3-dioxopiperazin-1-yl)carbonylamino]-2-(3,4-dihydroxyphenyl) acetamido)-6 alpha-formamido-penicillinate] is a new semisynthetic penicillin. It was tested in vitro for activity against 884 organisms cultured from blood specimens of cancer patients. BRL 36650 had broad-spectrum activity against the gram-negative bacilli tested but had no gram-positive activity. The MIC against 90% of the Pseudomonas aeruginosa isolates was 3.12 micrograms/ml. The activity of BRL 36650 was superior to that of piperacillin, comparable or slightly inferior to that of aztreonam and ceftazidime, and lower than that of imipenem and amifloxacin. BRL 36650 should prove useful for the management of gram-negative bacillary infections, including those caused by P. aeruginosa.

Ticarcillin plus clavulanic acid versus moxalactam therapy of osteomyelitis, septic arthritis, and skin and soft tissue infections

A controlled, randomized study to compare the efficacy and safety of ticarcillin plus clavulanic acid with moxalactam was carried out in 25 evaluable patients with bone, joint, and skin or skin structure infections. Of the 13 patients in the ticarcillin plus clavulanic acid-treated group, nine had osteomyelitis, two had septic arthritis, one had cellulitis, and one had a wound infection. Four of the 12 moxalactam-treated patients had osteomyelitis, one had septic arthritis, and the other seven had cellulitis and/or infected ulcers. A total of 21 causative organisms were isolated in the group treated with ticarcillin plus clavulanic acid: Enterobacteriaceae (10), Pseudomonas aeruginosa (five), obligate anaerobes (three), Staphylococcus aureus (two), and Acinetobacter species (one). Cultures in the moxalactam-treated group yielded 23 pathogens: Enterobacteriaceae (seven), S. aureus (six), group B streptococci (four), P. aeruginosa (two), obligate anaerobes (two), Streptococcus pyogenes (one), and Aeromonas species (one). A cure or satisfactory response was achieved in 12 of the 13 (92 percent) patients who received ticarcillin plus clavulanic acid and in 10 of the 12 (83 percent) patients who received moxalactam. One patient with septic arthritis who received ticarcillin plus clavulanic acid had a relapse during therapy, as did one moxalactam-treated patient with a post-surgical wound infection. The other patient in whom moxalactam treatment failed had a wound infection that became reinfected. Some abnormalities in laboratory parameters occurred in each group, but none was severe enough to warrant discontinuation of treatment.

Complicated urinary tract infections

The distinction between complicated and uncomplicated urinary tract infections is usually based on the presence or absence of some abnormality in the path of urine flow. Other differences include characteristic patient profiles, usual setting, likely pathogens and their antimicrobial susceptibility, and treatment options. Complicated infections are most common in older patients, usually men with prostatic enlargement. Gram-negative bacilli are the most frequent pathogens, but Escherichia coli plays a less dominant role. Isolates from patients with complicated infections are more likely to be resistant to older antibiotics, and therapy is usually parenteral. Many complicated urinary tract infections are hospital-acquired, and are often due to indwelling urinary catheters. Resistance to antimicrobials among isolates is common in this setting. Nosocomial urinary tract infections contribute substantially to morbidity, mortality, and health care costs. Numerous effective treatment options are available for uncomplicated infections. The types of patients, predisposing factors, and organisms limit antimicrobial choices for complicated infections. Although new antibiotics offer certain advantages, final therapeutic success usually depends on resolving the disorder that predisposes the patient to infection.

Randomized, double-blind comparison of ceftazidime and moxalactam in complicated urinary tract infections

Sixty-seven patients with complicated urinary tract infections were randomized in double-blind fashion to ceftazidime or moxalactam (MOX). A total of 54 patients were evaluable, 27 in each group. Patients received 500 mg of antibiotic intravenously every 12 h, except for those with Pseudomonas aeruginosa randomized to MOX who received 2 g intravenously every 12 h. Toxic effects with ceftazidime were experienced by the following number of patients: pain with infusion, one; posttherapy diarrhea, one; liver function test elevations, two; and neutropenia, one. Toxic effects with MOX were experienced by the following number of patients: liver function test elevations, two; and prolonged prothrombin time, one. All resolved. At 1 week posttherapy, bacteriologic results were 74% cured, 11% relapsed, 15% reinfection with ceftazidime and 52% cured, 33% relapsed, and 19% reinfection with MOX. Ceftazidime was effective for infections caused by MOX-resistant P. aeruginosa. P. aeruginosa resistant to MOX and other beta-lactams was isolated from one patient after MOX therapy. Enterococcal reinfection was common in both groups.

Role of amikacin in the management of intra-abdominal sepsis

Controversy has developed regarding the antibiotic management of intra-abdominal sepsis because of the recent availability of the third-generation cephalosporins and ureidopenicillins as alternatives to traditional combination therapy (aminoglycosides plus clindamycin). Most observers now acknowledge the need to provide anti-anaerobic as well as anti-aerobic gram-negative drug coverage. Although most of the newer agents do provide such broad-spectrum coverage, doubt remains regarding their efficacy because of flaws in comparative study design and the observation that resistance to the newer agents, which may even extend to the aminoglycosides, can emerge in individual patients during single courses of antibiotic therapy. Indeed, such resistance is most likely to occur during the treatment of seriously ill, immunodepressed patients who have undergone multiple reoperation for persistent or recurrent intra-abdominal sepsis--the precise group for which the new drugs were most desired as less toxic alternatives to the aminoglycosides. On the basis of such observations, combination therapy with the aminoglycosides, appears to remain the most logical choice. In the setting of nosocomial sepsis and pathogen resistance to other aminoglycosides, amikacin may be especially effective. Recent surveillance data indicate that the use of amikacin under such circumstances not only may provide effective antibiotic therapy, but also may actually reduce the level of microbial resistance to the other aminoglycosides. Past concern regarding the development of resistance to amikacin has probably been excessive and should not deter the use of this agent under appropriate clinical circumstances.

Moxalactam in nosocomial infections with Serratia marcescens

Ten critically ill patients presenting with nosocomial infection caused by Serratia marcescens (SM) not responding to prior chemotherapy were treated in an open study with Moxalactam (MOX) alone [6] or in combination with an aminoglycoside [4]. In initial disc diffusion tests, all isolates of SM were highly susceptible to MOX. Clinically, three patients were cured and four improved. Three patients died: one from SM pneumonia, one from gangrenous cholecystitis and another from ARDS. Bacteriologically, SM were eliminated from blood cultures in all seven patients with septicemia but were recovered post mortem from the lung of one patient. In three cases with localized infection, SM were eliminated once and persisted twice. Selection of resistant SM was observed in three patients but became clinically relevant in one case only. Resistant SM strains also showed reduced susceptibility to other cephalosporins and aminoglycosides. Emergence of enterococci occurred four times, in two cases with clinical consequences. MOX is a useful drug for the treatment of SM infections, but a definite risk of selecting multiresistant SM strains and of enterococcal overgrowth must be kept in mind.

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.

Treatment with aztreonam or tobramycin in critical care patients with nosocomial gram-negative pneumonia

During the course of one year, 47 critical care patients with gram-negative bacillary pneumonia at Millard Fillmore Hospital were randomly assigned to aztreonam or tobramycin therapy (two to one). Of these, 40 were fully evaluable for microbiologic and clinical response. All evaluable patients had gram-negative organisms in tracheal aspirate culture specimens and confirmed susceptibility of the organism to both study drugs. There was no difference between the two groups with respect to the percentage of patients who received concurrent antibiotics for gram-positive organisms. More than 60 percent of the patients received mechanical ventilation. Essentially, all had new lung infiltrates as shown by chest radiography, leukocytosis, recent onset of fever, and increased volume of purulent secretions. Half had multilobar pulmonary infiltrates. Their mean age was 73 years, with none under age 50. Most had chronic obstructive pulmonary disease, congestive heart failure, or both. By the prognostic nutritional index criteria, over 70 percent were nutritionally deficient at entry. The majority of infections were caused by Pseudomonas, Enterobacter, Klebsiella, and Escherichia coli. Aztreonam eradicated 92 percent of the causative gram-negative organisms, compared with 57 percent for tobramycin (p less than 0.05). Aztreonam produced a favorable clinical response (cure or improvement) in 93 percent of patients, compared with 50 percent for tobramycin (p less than 0.05). There were no differences in the minor adverse effects observed in the two treatment groups. Overall, aztreonam was superior to tobramycin for treatment of pneumonia due to susceptible gram-negative bacteria in these critical care patients.

Use of aztreonam in the treatment of serious infections due to multiresistant gram-negative organisms, including Pseudomonas aeruginosa

Aztreonam is a novel antimicrobial agent belonging to the monobactam class of antibiotics. It inhibits both beta-lactamase-producing and non-beta-lactamase-producing aerobic gram-negative bacilli, but it has no activity against gram-positive species or against anaerobic species. The efficacy of aztreonam in the treatment of infection in 76 patients and its safety in 87 patients was evaluated. The majority (91 percent) of patients had significant underlying disease, and 47 percent were critically ill. Aztreonam produced an overall clinical response of 86 percent, with 10 of 11 cases of bacteremia cured, including four due to Pseudomonas aeruginosa, seven of eight cases of pneumonia, and seven of nine episodes of osteomyelitis. Infections due to bacteria resistant to ampicillin, carbenicillin, cefazolin, cefamandole, cefoxitin, and gentamicin were cured. Although 15 of 18 patients with exacerbations of pulmonary infection due to P. aeruginosa showed clinical improvement, bacteriologic cure was not achieved, as has been noted with other drugs. Similarly, patients with major underlying structural abnormalities of the urinary tract showed early relapses of bacteriuria. Aztreonam combined with antistaphylococcal, antistreptococcal, or antianaerobic agents provided an alternative to aminoglycoside use in these non-neutropenic patients. Administration of 1 or 2 g every eight hours yield serum bactericidal levels well in excess of 1:8 against all Enterobacteriaceae and some P. aeruginosa strains. There was a low incidence of adverse side effects, none serious. Overall, aztreonam is a useful alternative to the drugs available for use in hospital-acquired gram-negative infections and provides a chance for more directed therapy.

Selection of multiple antibiotic resistance by quinolones, beta-lactams, and aminoglycosides with special reference to cross-resistance between unrelated drug classes

The ability of three quinolones, two beta-lactams, and one aminoglycoside to select resistant mutants was examined in tests with 30 isolates of commonly encountered nosocomial pathogens. Ciprofloxacin and norfloxacin, two new quinolone derivatives, were no more likely to select resistant mutants than amikacin, whereas nalidixic acid, an older quinolone derivative, was the most likely of the six drugs examined to select resistant mutants. Mutational frequencies of 10(-7) to 10(-8) were observed in most instances. In general, the mutants were 8 to 16 times less susceptible to the drug used for selection. Although most quinolone-selected mutants were cross-resistant only to other drugs within this class, certain mutants of Klebsiella pneumoniae selected by nalidixic acid, ciprofloxacin, or norfloxacin were also less susceptible to beta-lactam antibiotics. This unusual pattern of multiple drug resistance was associated with changes in outer membrane proteins of the organism. Multiple drug resistance was also observed in beta-lactam-selected mutants of Enterobacter cloacae and Pseudomonas aeruginosa (beta-lactams), amikacin-selected mutants of Providencia stuartii and P. aeruginosa (aminoglycosides), and beta-lactam- or amikacin-selected mutants of Serratia marcescens (beta-lactams plus aminoglycosides). These results underscore the need to examine carefully the frequency with which resistance to any new antibiotic develops, as well as the patterns of multiple drug resistance which may occur simultaneously.