β-Lctamase-Mediated Resistance in Gram-Negative Bacilli

Editor-in-Chief

ANTIMICROBICS AND

Charles W. Stratton, MD Vanderbilt University

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Volume 15, Number 5 May 1996

P-Lactamase-Mediated

Resistance in Gram-Negative

Charles W. Stratton

cilhn, and piperacillin, as well as with the development of ~-lactamase-resistant cephalosporins such as cefotaxime and ceftazidime. Finally, two new classes of p-lactam agents-monobactams (aztreonam) and carbapenems (imipenem)--were developed; both of these classes were resistant to the recognized types of P-lactamases. Despite the development of these late-generation ~4actan-1agents specifically designed to escape hydrolysis by the recognized types of p-lactamases, mutants of the TEM and SHV enzymes possessing activity against these extended-spectrum p-lactam agents quickly evolved. These extended-spectrum P-lactamases have been widely reported, particularly in Klebsiella spp. These mutants usually have fewer than five amino acid substitutions in comparison with their parent enzymes, yet they are able to hydrolyze these newer penicillins and cephalosporins. Some of these enzymes can hydrolyze monobactams whereas others (such as recent additions to the class A enzymes that have been classified as group 2f) possess significant rates of hydrolysis for carbapenems. Previously, only class B metallo-P-lactamases demonstrated such rates.

Vanderbilt University School of Medicine, Nashville, Tennessee

Introduction P-Lactamase-mediated resistance in Gram-negative bacilli is best understood by fiit reviewing the historical setting in which this resistance was clinically recognized. An understanding of this clinical setting is important because two different mechanisms of plactamase-mediated resistance in Gram-negative bacilli arose concomitantly. Because of this simultaneous occurrence in the clinical practice of medicine, these two types of p-lactamase-mediated resistance are often confused.

Plasmid-Mediated P-Lactamases New p-lactam agents with increased activity against Gram-negative bacteria were welcome additions in the 1960s and 1970s because of a shift in the etiology of nosocomial infections in which Gram-negative bacilli began to replace staphylococci as the predominant nosocomial pathogens. Unfortunately, p-lactamase-mediated resistance to these agents in Gram-negative bacilli quickly became a problem. This type of resistance was first noted by novel p-lactamases that were plasmid-mediated. These /34actamases were principally TEM and SHV types that disseminated first among the members of the Enterobacteriaceae,then to pseudomonads, and finally to members of the genera Haemophilusand Neisseria.The pharmaceutical industry responded with the development of extended-spectrum penicillins such as azlocillin, mezloAlDlEX

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Chromosomally Mediated f3-Lactamases In addition to this resistance caused by plasmid-mediated P-lactamases, there has been a less well-appreciated parallel emergence of resistance to these same extended-spectrum penicillins, broad-spectrum cephalosporins, monobactams, and even to carbaEkevier

Bacilli

penems (albeit with enzymes that to date possess only weak carbapenase activity) caused by quantitative changes in the expression of chromosomally encoded inducible p-lactamases. A clear understanding of the genetic control and expression of chromosomally mediated P-lactamases is necessary in order to understand their increasing clinical importance. Any bacterial genus that characteristically produces /3-lactarnase, by definition, possesses on its chromosome the genetic information needed for the production of this enzyme. These are collectively termed ampC genes. Induction of the ampC genes depends on a second, adjacent gene, designated ampR. In the absence of an inducer (i.e., a ~-la&am agent), ampRrepresses the synthesis of P-lactamase whereas in the presence of an inducer, this gene activates the synthesis of p-lactamase. Deletion mutants of ampRgenerate a noninducible phenotype that constitutively produces enzyme at a low level that is consistent with its dual re-

In This Issue P-Lactamase-Mediated Resistance in Gram-Negative Bacilli Charles W. Stratton

Enterobacter gergoviae Bacteremia Due to a Cholangitis: A Case Report Lawrence Cone, Anthony F. Torrnay, and Narinder K Midha 1069-417X/96SO.O0

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presser/activator role. The activator form of ampR is regulated by ampG. This gene appears to sense the disruption of peptidoglycan synthesis by a l3-lactam agent. It then converts the repressor form of ampR to the activator form. When ampR is in its activator mode, the expression of ampC is greatly stimulated, the result is increased production of P-lactamase. The repressor form of ampR is regulated by ampD, possibly in association with ampE. When the inducer is removed, ampD reverses the ampG-mediated activator form and restores ampR to its repressor form. This suppresses ampC and turns off the production of filactamase. An understanding of the genetic control and expression of chromosomally mediated b-lactamases clarifies the difference between high- and low-level constitutive production of enzyme. High-level constitutive production of group I fi-lactamase (i.e., stable derepression) is the consequence of the mutational loss of ampD which allows ampG to maintain ampR permanently in its activator form. Low-level constitutive production of group I l3-lactamase is the consequence of the mutational loss of ampR, which negates both repression and activation of am&. The clinical problem of resistance caused by chromosomally mediated plactamase was first appreciated with Pseudomonas aeruginosa. In 1986, Pedersen and colleagues reported an epidemic spread of resistant P. aeruginosa in a cystic fibrosis center. The in vivo development of resistance in these Pseudomonas strains was subsequently studied by Giwercman et al., who demonstrated that during a 2-week course of antipseudomonal therapy with fi-lactam with fl-lactam agents, the proportion of resistance caused by derepression of group I fHactamase increased markedly. By using a large inoculum, these investigators were able to

show that initial sputum cultures contained preexisting subpopulations of resistant strains in a high proportion; i.e., 10 to 20% of the total P. aeruginosa population in these pretreatment samples. They also showed that the majority of the resistant P. aeruginosa isolates present after therapy were stable, partially derepressed producers of group I ~lactarnase without any significant differences in the number and intensity of outer membrane protein bands. Induction and derepression of group I fl-lactamases appear to be associated with alterations of porin proteins as well as with the extracellular expression of enzyme via efflux pumps. In fact, it is possible that the efflux pumps involved may, with subtle mutations, become multidrug efflux pumps that can cause resistance to multiple agents. The association between the use of new cephalosporins and the development of resistance due to derepression of group I lNuAamase is clear. In vitro data and clinical experience have confirmed the original observations and predictions by Sanders and Sanders. Of the newer cephalosporins, ceftazidime appears to be the agent most often associated with the development of group I @f3amase. This may be related to the inherent stability of ceftazidime to this P-lactamase. The clinical importance of such resistance has been emphasized in a report by Chow et al., who found that the administration of a third-generation cephalosporin with 14 days of Enterobacfer spp. bacteremia was highly associated with a cephalosporin-resistant isolate in the initial positive blood culture (P < 0.001). Both univariant and multivariant analysis revealed that the presence of such a resistant strain was associated with a higher mortality rate than was seen with a sensitive strain. Bacteriologic failure during antimicrobial therapy was seen in 9% of patients overall,

with 3% developing a super-infection with a resistant strain of Enterobacter and 6% having emergence of resistance in the original isolate. Finally, emergence of resistance was found to occur more often with therapy that uses a third-generation cephalosporin (19%) than with therapy that uses other p-lactam agents (0%; P = 0.002) or aminoglycoside therapy (1%; P = 0.001). Resistance related to chromosomally mediated b-lactamase has been associated with the use of newer third-generation cephalosporins. Indeed, the ongoing antimicrobial pressure related the overuse of these cephalosporins is rapidly reducing their clinical effectiveness. Livermore has suggested that third-generation cephalosporins favor the selection of derepressed isolates because these cephalosporins are weak labile inducers. Those microorganisms that are enzyme-inducible fail to pro duce enough P-lactamase to protect themselves, whereas those preexisting microorganisms that are derepressed and constitutively producing large amounts of enzyme are protected and soon overrun the bacterial population at the infected site. In inducible populations of Gram-negative bacilli, derepressed mutants are always present, usually at frequencies of 10T7but occasionally at frequencies of up to 10e5. This type of selection has rarely been reported with ureidopenicillins and not at all with carbenicillin. More judicious use of newer cephalosporins may reduce the potential for resistance caused by the derepression of ~lactarnases. In addition, the use of extended-spectrum penicillins rather than cephalosporins offers another approach to this clinical problem because the use of these agents appears less likely to result in derepression of enzymes. The use of @lactamase inhibitors has not proven useful because these agents, with the exception of tazobactam, do not inhibit group I enzymes very well.

NOTE: No responsibility is assumed by the Publisher for any injury and/or damage to persons or pmperty as a matter of produds liability, negligence or otherwise, or fmm any use or opcntion of any methods, produds, instmctio~ or ideas contained in the material herein. No suggested test or pmccdwe should be carried out wkss, in the reader’s judgment, its risk is justified. Beause of rapid advances in medical scienas, we rccommcnd that the independent verification of diagnoses and drug dosages should be made. Discussions, views, and recommendations as to medical pmcedures, choice of drugs, and dmg dosages arc the responsibility of the authors. Anhicrobics and I~ectiour Diseaws Newslerrer (ISSN 1069417X) is issued monthly in one indexed volume per year by Elsevicr Science Inc., 655 Avenue of the Americas, New York, NY 10010. Forcus.tomerservicephone (212) 633-3950;TOLL-FREE forcostomers in tbeU.S.A andCanada: 1-8884ESINFO (l-8884374636) or fax: (212) 633-3680. Subscription price per year: $238.00; for orders outside theUnited States, Canada, and Mexico: $296.00. Periodical vge paid at New York, NY and at additional mailing c&ices. Postmaster: Sad address changes to Antimicrobics md1rJeciiou.s Diseaws Newsletter. Elsevier Science Inc., 655 Avenue of the Americas, New York, NY 10010.

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Combination therapy with an aminoglycoside has not prevented the emergence of resistance to B-lactam agents caused by derepression. Whether this will be the same for the addition of fluoroquinolones remains to be seen.

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Case Report

Enterobacter

gergoviae Bacteremia

Lawrence Cone, MD, DSC. Anthony F. Torrnay, MD Department of Medicine, Section of Infectious Diseases Gastroenterology and Medical Oncology Eisenhower Medical Center Ranch0 Mirage, California

Narinder K Midha, MS, M(ASCP)SM Assistant Director of the Clinical Microbiology Laboratory Vanderbilt University Medical Center Nashville, Tennessee 37323

Infections due to Enterobacter spp. have increased significantly in recent years. Often nosocomial in origin, these organisms are not infrequently multiply resistant to many classes of antimicrobials. Enterobacrer gergoviue hasbeen isolated in fewer than 100 patients from several sights including urine, sputum, wounds, and blood. The current case report describes an instance of E.geroviae bacteremia accompanying cholangitis due to neoplastic obstruction. Although E.geroviue has been previously isolated in four instances from blood cultures, the clinical aspects of the bacteremia and antimicrobial therapy were not detailed. Also note is made that of the 10 known species of Enterobactcr, only E.geroviue and E. sakazakii still remain highly sensitive to advanced penicillins, third-generation cephlosporins, monobactams, carbapenems, aminoglycosides, and quinolones. 32

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Due to a Cholangitis:

A Case Report

Introduction The genus Enterobacter, previously known asAerobucter has in recent

years evolved as a frequent and serious nosocomial pathogen. It is responsible for 11% of lower respiratory infections, 10% of post-operative wound infections, 6% of urinary tract infections and 5% of bacteremias. Ten species of Enterobacter are known, which include E. cloacae, E. uerogenes,E. ugglomerans, E. sakuzakii, E.geroviue, E. amnigenus,

E. taylorae, E. intermedium, E. asburiae and E. hormaechei. E.geroviue was first described by Richard and coworkers in 1976 from clinical and environmental cultures obtained in France and Africa, and named by Brenner et al. in 1980. E.geroviue is distinguished from E. cloacae and E. aerogenes, E. agglomerans and E. sakazakii by its positive urea reaction and negative reactions for KCN, D-sorbitol, mutate, beta-xylosidase, alphamethylglucoside and gelatinase. Fewer than 100 isolates of E.geroviue have been described, and only in four instances was the organism isolated from blood. Detailed antimicrobial therapy has also not been previously recorded. We report a patient with cholangitis and bacteremia successfully emdicated by combination antimicrobial therapy.

Case Report A 77-year-old white, male, retired Q 19% Elsevier

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gastroenterologist was found to have unresectable pancreatic cancer in April 1994. One and a half month following palliative and uncomplicated cholecystojejunostomy and gastroenterostomy, the patient became icteric and febrile. Of note was the perioperative use of cefotctan as prophylaxis. Physical examination was otherwise unremarkable except for minimal right upper quadrant tenderness. The patient was hospitalized and empirically started on vancomycin and aztreonam. Computerized tomographic scanning of the abdomen demonstrated a mass at the head of the pancreas and dilated biliary mdicles with air in the biliary tree. A chest X-ray was normal. The WBC was 9600 mm3 with an increase in polymorphonuclear leucocytes and band forms. Hb/Hct measured 13.7/39.5 and the platelets numbered 255,060 mm3. The bilirubin was 4.3 mg%, alkaline phosphatase 298 IU (upper limit of normal, 160), the GGTP 593 (upper limit, 85) SGGT 181 (upper limit, 65) and the SGPT 360 (upper limit, 45). The remaining chemistries were normal. A urine culture was sterile and blood cultures revealed growth of a Gram-negative bacillus, which was sensitive to ticamillin, piperacillin, cefotaxime, ceftazidime, netilmicin, amikacin, gentamicin, aztreonam, imipenem, and ciprofloxacin. Antimicmbics

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