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

Combating bacteria and drug resistance by inhibiting mechanisms of persistence and adaptation

Antibiotics have revolutionized the treatment of infectious disease but have also rapidly selected for the emergence of resistant pathogens. Traditional methods of antibiotic discovery have failed to keep pace with the evolution of this resistance, which suggests that new strategies to combat bacterial infections may be required. An improved understanding of bacterial stress responses and evolution suggests that in some circumstances, the ability of bacteria to survive antibiotic therapy either by transiently tolerating antibiotics or by evolving resistance requires specific biochemical processes that may themselves be subject to intervention. Inhibiting these processes may prolong the efficacy of current antibiotics and provide an alternative to escalating the current arms race between antibiotics and bacterial resistance. Though these approaches are not clinically validated and will certainly face their own set of challenges, their potential to protect our ever-shrinking arsenal of antibiotics merits their investigation. This Review summarizes the early efforts toward this goal.

Epidemiological risk factors for isolation of ceftriaxone-resistant versus -susceptible citrobacter freundii in hospitalized patients

Antimicrobial resistance is an emerging problem among nosocomial bacteria. Risk factors for the recovery of ceftriaxone-resistant (CRCF) or -susceptible (CSCF) Citrobacter freundii in clinical cultures from hospitalized patients were determined by using a case-case-control study design. CRCF was isolated from 43 patients (case group 1) and CSCF was isolated from 87 patients (case group 2) over a 3-year period. Risk factors for CRCF were exposure to imipenem (odds ratio [OR], 7.5; 95% confidence interval [CI], 1.2 to 45.4), broad-spectrum cephalosporins (OR, 6.9; 95% CI, 1.8 to 26.7), vancomycin (OR, 3.0; 95% CI, 1.2 to 7.4), or piperacillin-tazobactam (OR, 2.6; 95% CI, 1.1 to 6.2), as well as hospital length of stay >or=1 week (OR, 3.6; 95% CI, 1.3 to 10.2) and intensive care unit (ICU) stay (OR, 2.6; 95% CI, 1.1 to 6.2). Risk factors for CSCF were peripheral vascular disease (OR, 23.2; 95% CI, 4.3 to 124.6), AIDS (OR, 9.5; 95% CI, 1.6 to 55.5), cerebrovascular disease (OR, 4.2; 95% CI, 1.6 to 10.8), and ICU stay (OR, 3.1; 95% CI, 1.8 to 5.4).

Serratia bacteremia in a large university hospital: trends in antibiotic resistance during 10 years and implications for antibiotic use

OBJECTIVE

To identify antibiotic resistance trends and risk factors for resistance of Serratia species to third-generation cephalosporins.

DESIGN

Retrospective survey of medical records.

SETTING

A 2,200-bed, tertiary-care hospital.

PATIENTS

One hundred twenty-two patients with Serratia bacteremia between January 1991 and June 2001.

METHODS

Infectious disease physicians collected data from medical records regarding patient demographics, underlying disease or condition, portal of entry, microorganism, antibiogram, complications, antibiotics received, and outcome.

RESULTS

Among 122 Serratia isolates, 117 (95.9%) were Serratia marcescens and 110 (90.2%) were of nosocomial origin. During the study period, the 122 isolates showed a high rate of resistance to third-generation cephalosporins (45.9%) and extended-spectrum penicillins (56.6%). The resistance rate to ciprofloxacin was 32.0%. The resistance rate to third-generation cephalosporins increased from 31.7% for 1991 to 1995 to 54.9% for 1996 to 1998 and 50.0% for 1999 to 2001. In the multivariate analysis, prior use of a second-generation cephalosporin (adjusted odds ratio [OR], 5.90; 95% confidence interval [CI90], 1.41 to 24.6; P = .015) or a third-generation cephalosporin (OR, 3.26; CI95, 1.20 to 8.87; P = .020) was a strong independent risk factor for resistance to third-generation cephalosporins. The overall case-fatality rate was 25.4% (Serratia bacteremia-related case-fatality rate, 13.1%).

CONCLUSION

Prior use of a second- or third-generation cephalosporin was the most important risk factor for bacteremia with Serratia resistant to third-generation cephalosporins, suggesting the need for antibiotic control. The potential role of patient-to-patient spread could not be fully evaluated in this retrospective study.

[Effectiveness of contact isolation in the control of multiresistant bacteria in an intensive care service]

AIM

To describe the frequency, characteristics and progression of critically ill patients admitted to the ICU, for whom isolation is indicated due to detection of multiresistant pathogenic bacteria, and to study the effectiveness of precautionary measures to avoid dissemination of these microorganisms.

PATIENTS AND METHODS

Prospective, observational, cohort study performed by a specially created working group of four nurses and an ICU specialist. The study included 55 patients in whom contact isolation was indicated (isolation rate, 15.2 per 100 patients), collected over a 16-month period.

RESULTS

The multiresistant bacteria responsible for isolation of the patients were: Pseudomonas aeruginosa (17 cases), Staphylococcus aureus (17 cases), Stenotrophomonas maltophilia (15 cases), Acinetobacter baumannii (4 cases) and extended-spectrum beta-lactamase (ESBL)- producing Enterobacteria (2 cases). Vancomycin-resistant Enterococcus spp. was not identified in any case. The mean duration of ICU isolation was 17.6 6 5.1 days (range 1-75). Multiresistant bacteria were classified as intra-ICU nosocomial in 39 cases (70.9%), extra-ICU nosocomial in 10 cases (18.2%) and community-acquired in 6 (10.9%). During the study period, no epidemic outbreak due to any of the controlled bacteria was detected. The multiresistant bacteria presented in the form of colonization in 41 cases (74.5%). The reasons for discontinuing isolation were death of the patient in 18 cases, transferal to a hospital ward (discharge from the ICU) in 19 cases, and eradication of the bacteria in 18 cases. Of the 55 patients with multiresistant bacteria, 35 (63.6%) died during hospitalization, and 23 of these (41.8%) during their stay in the ICU.

CONCLUSIONS

The implementation of a working team for early detection of multiresistant pathogenic bacteria resulted in application of contact isolation in 15.2% of patients admitted. Surveillance to fulfill isolation precautions in a medical-surgical ICU achieved an absence of epidemic outbreaks due to these bacteria during the study period.

Management of antimicrobial use in the intensive care unit

Indications for the use of antimicrobials in critically ill patients are similar to those for other hospitalised patients. However, the selection of agents depends on the particular characteristics of patients in the intensive care unit (ICU), the form of presentation of infection, the type of infection and the bacteriological features of the causative pathogens. The use of antimicrobials in patients admitted to medical-surgical ICUs varies between 33 and 53%. The selection of empirical antimicrobials to be included in treatment protocols of the most common infections depends on the strong interrelationship between patient characteristics, predominant pathogens in each focus. and antimicrobials used for treatment. Epidemiological studies carried out in the past have identified the microorganisms most frequently responsible for community-acquired and nosocomial infections in patients admitted to ICUs. Susceptibility to antimicrobial agents may be different between each geographical area, between each hospital and even within the same hospital service. In addition, susceptibility patterns may change temporarily in relation to the use of particular antimicrobials or in association with other unknown factors so that assessment of endemic antimicrobial resistance patterns is very useful in order to tailor the antimicrobial regimens of therapeutic protocols. Antimicrobial use should not be a routine procedure. The clinical course of the patient (an indicator of effectiveness) should be closely monitored as well as the possible appearance of adverse effects and/or multiresistant pathogens. Controls are based on the assessment of plasma drug concentrations and microbiological surveillance to detect the presence of multiresistant strains or new antibacterial-resistant pathogens. Prevention of the development of multiresistant pathogens is the main goal of the ICU antimicrobial policy. Although a series of general strategies to reduce the presence of multiresistant pathogens have been proposed, the implementation of these recommendations in ICUs requires the cooperation of a member of the intensive care team.

Clinical and molecular epidemiology of chromosome-mediated resistance to third-generation cephalosporins in Enterobacter isolates in eastern France

OBJECTIVE

To determine both the prevalence of group I beta-lactamase-related resistance and the clinical setting in which resistance to expanded-spectrum cephalosporins occurs.

METHODS

Isolates of Enterobacter spp. were sensitivity tested to a range of antibiotics, and selected isolates were DNA fingerprinted by pulsed-field gel electrophoresis. The medical records of all patients with positive cultures for Enterobacter spp. were reviewed to determine the effect of previous antibiotic treatment on the susceptibility profile of these organisms.

RESULTS

The crude incidence of colonization/infection (n = 315) was 0.51 per 100 patients and 0.73 per 1000 days of hospitalization. The 4-day and 7-day Kaplan-Meier rates of colonization/infection with Enterobacter were estimated to be 7.57% (standard deviation (SD = 3.26%) and 4.16% (SD = 2.88%)), respectively. The time lag to colonization/infection with isolates producing large amounts of Bush group 1 beta-lactamase (HLBL) (27.35 +/- 27.30 days) was significantly different from that to colonization/infection with wild-type isolates (13.59 +/- 17.93 days) (P = 0.036). Ninety-six isolates (30.5%) demonstrated acquired resistance to expanded-spectrum cephalosporins: 34 isolates (10.8%) produced extended-spectrum beta-lactamase, and 62 isolates (19.7%) produced HLBL. The 89 Enterobacter isolates susceptible to third-generation cephalosporins yielded 84 major DNA patterns, and the 45 HLBL isolates yielded 38 major DNA patterns. The risk of colonization/infection with HLBL-producing Enterobacter was higher in cases of antimicrobial treatment with third-generation cephalosporins or a fluoroquinolone, and in cases of urinary tract colonization/infection.

CONCLUSIONS

The judicious use in hospitals of both expanded-spectrum cephalosporins and other antibiotics such as fluoroquinolones is necessary to curtail the emergence of resistance in Enterobacter spp.

beta-Lactamase epidemiology and the utility of established and novel beta-lactamase inhibitors

beta-Lactamase inhibitor:beta-lactam combinations remain one of the most successful strategies for the treatment of bacterial infections. Over the last 20 years the number and diversity of serine and metallo active site beta-lactamases has increased dramatically. This review highlights some of the new additions to the beta-lactamase arena and discusses how the commercially available beta-lactamase inhibitors are keeping pace with the changing epidemiology of beta-lactamases. In addition, we survey the progress with the design of novel inhibitors of serine and metallo-beta-lactamases. Focus is given to the recent advances in the design of metallo-beta-lactamase inhibitors as these enzymes pose a serious emerging threat to the use of all beta-lactam based therapies.

High antibiotic consumption in Danish intensive care units?

Decreased antibiotic susceptibility among microorganisms isolated from intensive care unit (ICU) patients is found to be associated with high total antibiotic consumption or inappropriate use of antibiotics in the ICUs. The aims of this study were: 1) to characterize the antibiotic consumption in Danish ICUs, and in four ICUs with expectedly large differences in levels of antibiotic consumption, 2) to estimate the association between antibiotic susceptibility among isolated microorganisms and antibiotic consumption. This was done by: 1) a retrospective questionnaire study of the annual supply of antibiotics in 1995 to 30 ICUs in Denmark, and 2) a 2-month prospective study of patients and microbiological samples in four Danish ICUs in 1996. We found that the supply of antibiotics to Danish ICUs was substantial, with a median value of 124 DDD/100 patient days. No association was found between high consumption of antibiotics and decreased antibiotic susceptibility in the four ICUs.

Decreased antimicrobial resistance after changes in antibiotic use

Vancomycin-resistant enterococci (VRE) and methicillin-oxacillin-resistant Staphylococcus aureus (MRSA) originally predominated in large medical centers; however, these isolates are now common in community hospitals and community clinics. No simple answer is available regarding control of antimicrobial-resistant bacteria, especially VRE and MRSA, as their numbers increase and pose a more serious threat to patient care. The source of colonization is often difficult to identify because of transport of patients among different locations on the continuum of care (e.g., hospital to extended care facility to home and back). At one hospital, control strategies greatly reduced the occurrence of gram-negative bacteria such as VRE. Since 1994, VRE declined from 16% to 5%. Similarly, the number of MRSA isolates declined from 35% to 23%. These declines are attributed to a cohesive working relationship among pharmacists, microbiologists, and infectious disease physicians and personnel, and to a decision to decrease administration of cephalosporins in favor of piperacillin-tazobactam.

Differences in the resistant variants of Enterobacter cloacae selected by extended-spectrum cephalosporins

The rates of development of resistance to ceftriaxone, ceftazidime, cefepime, and cefpirome in 10 strains of Enterobacter cloacae were determined by daily transfer for 7 days to fresh medium containing twofold serial dilutions of the antibiotics. Development of resistance to ceftriaxone was the most rapid; this was followed by ceftazidime, cefpirome, and cefepime. Resistant variants selected by ceftriaxone and ceftazidime were cross-resistant and produced very high levels of beta-lactamase. On the other hand, resistant variants selected by cefepime and cefpirome often had moderately high levels of beta-lactamase and diminished levels of the 39- to 40-kDa porin protein.

A hospital outbreak of high-level beta-lactam-resistant Enterobacter spp.: association more with ampicillin and cephalosporin therapy than with nosocomial transmission

We studied an 8 month outbreak of a 7-fold increased isolation rate of high-level beta-lactam-resistant Enterobacter spp. from clinical infections (20 patients, 22 isolates: 20 E. cloacae, 2 E. aerogenes). In a case-control analysis the occurrence of resistant Enterobacter spp. was found to be associated with treatment with multiple antibiotics (p = 0.03), broad-spectrum beta-lactam agents (p = 0.0001) including ampicillin (p = 0.04), and cephalosporins (cefuroxime and cefotaxime, p = 0.004). Biochemical fingerprinting and pulsed-field gel electrophoresis (PFGE) typing showed no identity between the resistant isolates, indicating that neither cross-infection nor nosocomial transmission from a common source was the immediate cause of the problem. The outbreak was not paralleled by the overall Enterobacter spp. isolation rate or the antibiotic usage pattern in the hospital. Thus, the underlying cause of the outbreak remained obscure.

Beta-lactamases: current situation and clinical importance

Production of beta-lactamases is the most important means of bacterial resistance to beta-lactam antibiotics. The traditional, plasmid-mediated beta-lactamases were initially confined to Enterobacteriaceae but have now spread to other genera and species including Pseudomonas aeruginosa, Haemophilus influenzae, and Neisseria gonorrhoeae. Extended-spectrum plasmid-mediated beta-lactamases have been identified in Enterobacteriaceae, particularly Klebsiella spp. and Escherichia coli. They are capable of hydrolyzing second- and third-generation cephalosporins and older beta-lactams. Chromosomally mediated beta-lactamases may be produced constitutively or induced by the presence of a beta-lactam. Mutation within the bacterial genome may lead to constitutive enzyme hyperproduction. Strains that produce these enzymes are resistant to practically all beta-lactams, with the exception of carbapenems. The wide-spread occurrence of antibiotic resistance dictates that therapies be judiciously chosen with attention to the resistance patterns of the causative organisms.

Resistance characteristics of blood culture isolates of Enterobacter cloacae with special reference to beta-lactamases and relation to preceding antimicrobial therapy

Resistance characteristics of 53 blood culture isolates of E. cloacae were examined and correlated with antimicrobial treatment preceding bacteraemia. Resistance patterns of 22 antimicrobial agents, presence of resistant mutants, and inducibility of beta-lactamase were investigated; furthermore, population analysis and investigation of beta-lactamase production of selected isolates were performed. Thirty-two isolates (60%) were resistant to cephalothin and/or cefoxitin and/or ampicillin, and 14 isolates (26%) had further resistance characteristics, 7 of the 14 being resistant to non-beta-lactam antibiotics. All ampicillin-susceptible and 76% of cefotaxime-susceptible isolates had resistant mutants in the zone of inhibition when high inoculum was used. All isolates investigated had inducible chromosomal beta-lactamases, and, in addition, two isolates had an enzyme corresponding with TEM-1. Correlation of resistance patterns and antimicrobial treatment preceding bacteraemia showed that treatment with a third-generation cephalosporin was associated with beta-lactam multiresistance. In conclusion, susceptibility testing of beta-lactam antibiotics of Enterobacter must be interpreted with caution and monotherapy with an extended-spectrum cephalosporin should be avoided unless presence of resistant mutants and inducibility of beta-lactamase can be excluded.

In vitro activities of cefepime alone and with amikacin against aminoglycoside-resistant gram-negative bacteria

The in vitro activity of cefepime was compared with those of ceftazidime, cefotaxime, and cefpirome against aminoglycoside-resistant gram-negative bacteria. Cefepime was the most active cephalosporin, with a MIC for 90% of strains tested for all non-Pseudomonas aeruginosa species of less than or equal to 4 micrograms/ml. No cefepime resistance was encountered among members of the family Enterobacteriaceae. Of the 40 aminoglycoside-resistant P. aeruginosa isolates, 15% were resistant to cefepime, compared with 18% for ceftazidime, 30% for cefpirome, and 35% for cefotaxime. Synergism between cefepime and amikacin was observed and occurred most frequently in P. aeruginosa strains resistant to cefepime but susceptible to amikacin. In no case did cefepime and amikacin exhibit antagonism against P. aeruginosa.

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).

Nosocomial gram-negative bloodstream isolates: a comparison of in vitro antibiotic potency

Nosocomial bloodstream infections add to the morbidity, mortality and length of hospitalization that is attributed to the underlying diseases alone. We have compared the in vitro potency of fifteen antibiotics against 136 isolates from clinically significant nosocomial gram-negative bacteraemias. Ciprofloxacin was the most potent antibiotic and had the broadest spectrum of activity (98% of isolates susceptible, MIC90 range: 0.06-0.5 micrograms ml-1). We subjected all isolates to beta-lactamase induction but antibiotic susceptibility was unaffected by this procedure.

In vitro evaluation of BRL 42715, a novel beta-lactamase inhibitor

The penem BRL 42715, C6-(N1-methyl-1,2,3-triazolylmethylene)penem, is a potent inhibitor of a broad range of bacterial beta-lactamases, including the plasmid-mediated TEM, SHV, OXA, and staphylococcal enzymes, as well as the chromosomally mediated enzymes of Bacteroides, Enterobacter, Citrobacter, Serratia, Morganella, Escherichia, Klebsiella, and Proteus species. The concentration of BRL 42715 needed to reduce the initial rate of hydrolysis of most beta-lactamase enzymes by 50% was less than 0.01 micrograms/ml, which was 10- to 100-fold lower than for other beta-lactamase inhibitors. These potent inhibitory activities were reflected in the low concentrations of BRL 42715 needed to potentiate the antibacterial activity of beta-lactamase-susceptible beta-lactams. Concentrations of 0.25 micrograms/ml or less considerably enhanced the activity of amoxicillin against many beta-lactamase-producing strains. The MIC50 (MIC for 50% of strains tested) of amoxicillin for 412 beta-lactamase-producing members of the family Enterobacteriaceae fell from greater than 128 to 2 micrograms/ml in the presence of 1 microgram of BRL 42715 per ml, whereas 5 micrograms of clavulanic acid per ml brought the MIC50 down to 8 micrograms/ml. Among these 412 strains were 73 Citrobacter and Enterobacter strains, and 1 microgram of BRL 42715 per ml reduced the MIC50 of amoxicillin from greater than 128 to 2 micrograms/ml for the 48 cefotaxime-susceptible strains and from greater than 128 to 8 micrograms/ml for the 25 cefotaxime-resistant strains.

Resistance to third generation cephalosporins: the current situation

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

Septicaemia caused by an Enterobacter cloacae strain varying in resistance against cephalosporins

Enterobacter cloacae, sensitive to third-generation cephalosporins (cefotaxime and ceftazidime), was isolated from the stoma of a patient with leukaemia. One month later, he developed a fatal septicaemia, caused by an identical strain isolated from blood cultures. He had been treated with several antibacterial agents, including cefotaxime. The blood culture strain seemed to be a mixture of four variants with different resistance patterns to cefotaxime and ceftazidime. One variant was extremely sensitive to third-generation cephalosporins, one was completely resistant, and two showed variations in zone diameter within sensitivity group 2, both for cefotaxime and ceftazidime. Minimal inhibitory concentration (MIC) studies also showed different resistance patterns between the four variants. Similar variants were found when the stoma isolate was further investigated.

Activity of cefepime against ceftazidime- and cefotaxime-resistant gram-negative bacteria and its relationship to beta-lactamase levels

One hundred clinical isolates resistant to ceftazidime and/or cefotaxime were examined for susceptibility to cefepime. The most frequently encountered ceftazidime-cefotaxime-resistant strains belonged to the genera Enterobacter, Pseudomonas, and Citrobacter. Among these strains, 92% were resistant to cefoperazone, 91% were resistant to cefotaxime, 84% were resistant to ceftazidime, and 6% were resistant to cefepime. Of the members of the family Enterobacteriaceae, 57% were resistant to ceftriaxone. The six strains resistant to cefepime were all Pseudomonas aeruginosa and were resistant to both cefotaxime and ceftazidime. Cefepime-resistant P. aeruginosa strains had exceptionally high levels of beta-lactamase activity, higher than the levels found in strains resistant to ceftazidime but susceptible to cefepime. The beta-lactamases from the cefepime-resistant strains were type I (Richmond-Sykes), were constitutively produced, and did not have increased affinity or hydrolytic activity for cefepime. Thus, cefepime was active against most gram-negative bacteria which have developed resistance to the broad-spectrum cephalosporins, and resistance to cefepime in P. aeruginosa appears to be associated with higher beta-lactamase levels than in cefepime-susceptible strains.

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

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

Chromosomal beta-lactamase expression and resistance to beta-lactam antibiotics in Proteus vulgaris and Morganella morganii

Indole-positive members of the Proteeae usually have inducible expression of chromosomal beta-lactamases. Mutants with stably derepressed beta-lactamase expression occur in inducible populations at frequencies in the range of 10(-6) to 10(-8). The contribution of these beta-lactamases to drug resistance was examined in Morganella morganii and Proteus vulgaris. The M. morganii enzyme was a high-molecular-weight (49,000) class I cephalosporinase with low Vmax rates for ampicillin, carbenicillin, and and broad-spectrum cephalosporins. The P. vulgaris enzyme had a lower molecular weight (32,000) and high Vmax rates for ampicillin, cephaloridine, cefotaxime, and ceftriaxone. Imipenem and cefoxitin inactivated the P. vulgaris enzyme but were low-Vmax, low-Km substrates for that of M. morganii. Despite these differences, the two beta-lactamases caused similar resistance profiles. Ampicillin and cephaloridine were strong inducers for both species, and beta-lactamase-inducible strains and their stably derepressed mutants were resistant, whereas basal mutants (those with low-level uninducible beta-lactamase) were susceptible to these two compounds. Mezlocillin, cefotaxime, ceftriaxone, and (usually) carbenicillin were almost equally active against beta-lactamase-inducible organisms and their basal mutants, but were less active against stably derepressed mutants. This behavior reflected the beta-lactamase lability of these drugs, coupled with their weak inducer activity below the MIC. Carbenicillin was a labile strong inducer for a single P. vulgaris strain, and inducible enzyme was protective against the drug in this atypical organism. Cefoxitin and imipenem, both strong inducers below the MIC, were almost equally active against beta-lactamase-inducible organisms and their basal and stably derepressed mutants.

Susceptibility to beta-lactam antibiotics and gentamicin of gram-negative bacilli isolated from hospitalized patients: a Swedish multicenter study

A total of 952 blood and 1543 urine isolates of gram-negative bacilli from hospitalized patients in 1986-1987 were consecutively collected by 10 Swedish laboratories and tested for susceptibility to 8 beta-lactam antibiotics and to gentamicin. The isolates were mostly Escherichia coli (58% and 44%, respectively) and Klebsiella sp. (17% and 18%). Resistance to ampicillin in blood and urine isolates was found in 35% and 45%, respectively, to piperacillin in 5% and 6%, to cephalothin in 26% and 34%, to cefuroxime in 12% and 22%, to cefotaxime in 3% and 5%, to ceftazidime in 1% and 1%, to imipenem in 0.5% and 0.1%, to aztreonam in 3% and 2%, and to gentamicin in 0.8% and 0%. Resistance of clinically important gram-negative bacilli to new beta-lactam antibiotics and to gentamicin is infrequent in Sweden.

Clinical importance of inducible beta-lactamases in gram-negative bacteria

The clinical problems caused by inducible beta-lactamases in certain gram-negative bacteria are being recognized with increasing frequency. These problems include the rapid emergence of multiple beta-lactam resistance during therapy with many of the newer beta-lactam antibiotics. Such multiply resistant organisms are now spreading within the hospital and have become important nosocomial pathogens. This has been a particularly difficult problem for intensive care units, cystic fibrosis centers and burn units where there are clusters of patients who are highly susceptible to infections with organisms like Enterobacter spp., Serratia spp. and Pseudomonas aeruginosa, which possess inducible beta-lactamases. Only through an awareness of these problems, their cause, and restriction of the use of certain newer beta-lactam antibiotics can these problems be controlled.