Louis B. Rice

Multidrug-resistant, extensively drug-resistant and pandrug-resistant bacteria: an international expert proposal for interim standard definitions for acquired resistance

Many different definitions for multidrug-resistant (MDR), extensively drug-resistant (XDR) and pandrug-resistant (PDR) bacteria are being used in the medical literature to characterize the different patterns of resistance found in healthcare-associated, antimicrobial-resistant bacteria. A group of international experts came together through a joint initiative by the European Centre for Disease Prevention and Control (ECDC) and the Centers for Disease Control and Prevention (CDC), to create a standardized international terminology with which to describe acquired resistance profiles in Staphylococcus aureus, Enterococcus spp., Enterobacteriaceae (other than Salmonella and Shigella), Pseudomonas aeruginosa and Acinetobacter spp., all bacteria often responsible for healthcare-associated infections and prone to multidrug resistance. Epidemiologically significant antimicrobial categories were constructed for each bacterium. Lists of antimicrobial categories proposed for antimicrobial susceptibility testing were created using documents and breakpoints from the Clinical Laboratory Standards Institute (CLSI), the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the United States Food and Drug Administration (FDA). MDR was defined as acquired non-susceptibility to at least one agent in three or more antimicrobial categories, XDR was defined as non-susceptibility to at least one agent in all but two or fewer antimicrobial categories (i.e. bacterial isolates remain susceptible to only one or two categories) and PDR was defined as non-susceptibility to all agents in all antimicrobial categories. To ensure correct application of these definitions, bacterial isolates should be tested against all or nearly all of the antimicrobial agents within the antimicrobial categories and selective reporting and suppression of results should be avoided.

Effect of antibiotic therapy on the density of vancomycin-resistant enterococci in the stool of colonized patients.

BACKGROUND Colonization and infection with vancomycin-resistant enterococci have been associated with exposure to antibiotics that are active against anaerobes. In mice that have intestinal colonization with vancomycin-resistant enterococci, these agents promote high-density colonization, whereas antibiotics with minimal antianaerobic activity do not. METHODS We conducted a seven-month prospective study of 51 patients who were colonized with vancomycin-resistant enterococci, as evidenced by the presence of the bacteria in stool. We examined the density of vancomycin-resistant enterococci in stool during and after therapy with antibiotic regimens and compared the effect on this density of antianaerobic agents and agents with minimal antianaerobic activity. In a subgroup of 10 patients, cultures of environmental specimens (e.g., from bedding and clothing) were obtained. RESULTS During treatment with 40 of 42 antianaerobic-antibiotic regimens (95 percent), high-density colonization with vancomycin-resistant enterococci was maintained (mean [+/-SD] number of organisms, 7.8+/-1.5 log per gram of stool). The density of colonization decreased after these regimens were discontinued. Among patients who had not received antianaerobic antibiotics for at least one week, 10 of 13 patients who began such regimens had an increase in the number of organisms of more than 1.0 log per gram (mean increase, 2.2 log per gram), whereas among 10 patients who began regimens of antibiotics with minimal antianaerobic activity, there was a mean decrease in the number of enterococci of 0.6 log per gram (P=0.006 for the difference between groups). When the density of vancomycin-resistant enterococci in stool was at least 4 log per gram, 10 of 12 sets of cultures of environmental specimens had at least one positive sample, as compared with 1 of 9 sets from patients with a mean number of organisms in stool of less than 4 log per gram (P=0.002). CONCLUSIONS For patients with vancomycin-resistant enterococci in stool, treatment with antianaerobic antibiotics promotes high-density colonization. Limiting the use of such agents in these patients may help decrease the spread of vancomycin-resistant enterococci.

Outcome of Cephalosporin Treatment for Serious Infections Due to Apparently Susceptible Organisms Producing Extended-Spectrum β-Lactamases: Implications for the Clinical Microbiology Laboratory

ABSTRACT Although extended-spectrum beta-lactamases (ESBLs) hydrolyze cephalosporin antibiotics, some ESBL-producing organisms are not resistant to all cephalosporins when tested in vitro. Some authors have suggested that screening klebsiellae or Escherichia colifor ESBL production is not clinically necessary, and when most recently surveyed the majority of American clinical microbiology laboratories did not make efforts to detect ESBLs. We performed a prospective, multinational study of Klebsiella pneumoniae bacteremia and identified 10 patients who were treated for ESBL-producingK. pneumoniae bacteremia with cephalosporins and whose infecting organisms were not resistant in vitro to the utilized cephalosporin. In addition, we reviewed 26 similar cases of severe infections which had previously been reported. Of these 36 patients, 4 had to be excluded from analysis. Of the remaining 32 patients, 100% (4 of 4) patients experienced clinical failure when MICs of the cephalosporin used for treatment were in the intermediate range and 54% (15 of 28) experienced failure when MICs of the cephalosporin used for treatment were in the susceptible range. Thus, it is clinically important to detect ESBL production by klebsiellae or E. coli even when cephalosporin MICs are in the susceptible range (≤ 8 μg/ml) and to report ESBL-producing organisms as resistant to aztreonam and all cephalosporins (with the exception of cephamycins).

International Prospective Study of Klebsiella pneumoniae Bacteremia: Implications of Extended-Spectrum β-Lactamase Production in Nosocomial Infections

Context Worldwide prevalence of extended-spectrum -lactamase (ESBL)producing organisms is of great concern because of their broad antibiotic resistance. Contribution Analysis of consecutive cases of Klebsiella pneumoniae bacteremia at 12 hospitals on 6 continents shows that although incidence varies widely among institutions, almost one third of cases of nosocomial bacteremia and almost one half of intensive care unitbased infections were caused by ESBL-producing organisms. Patient-to-patient spread is common, and prevention requires careful attention to routine infection control measures. Cautions Specific antibiotic exposures before K. pneumoniae infection cannot be confirmed as risk factors for ESBL-related infections on the basis of this study. The Editors Since the discovery that resistance of Staphylococcus aureus to penicillin is mediated by a -lactamase, much effort has been made to create -lactam antibiotics that are stable to common -lactamases. Cephalosporin antibiotics containing an oxyimino side-chain represent a major advance in antibiotic development. The merger of the oxyimino chain and a 2-amino-5-thiazolyl nucleus (in such antibiotics as ceftriaxone, cefotaxime, and ceftazidime) resulted in stability to the effects of the common TEM-1 and SHV-1 -lactamases produced by gram-negative bacilli, such as Escherichia coli and Klebsiella pneumoniae. However, within a few years of the commercial release of these antibiotics, gram-negative bacilli (especially K. pneumoniae) that harbored mutated versions of the parent TEM and SHV enzymes were detected. These and other newly detected -lactamases (for example, the functionally similar CTX-M types) hydrolyze -lactam antibiotics containing the oxyimino side-chain. Genes encoding these extended-spectrum -lactamases (ESBLs) were carried on transferable plasmids. These plasmids frequently carried determinants of resistance to other classes of antibiotics, particularly the aminoglycosides (1, 2). Thus, ESBL-producing gram-negative bacilli were found to truly be multiresistant pathogens: The majority of these strains were resistant to all -lactam antibiotics (with the exception of cephamycins and carbapenems), most aminoglycosides, trimethoprimsulfamethoxazole, and sometimes the fluoroquinolones. Although the prevalence of ESBL production among gram-negative bacilli varies geographically (and may even vary from hospital to hospital within a city), widespread recognition of the advent of these -lactamases has been lacking (3). Previous studies of the epidemiology of ESBL-producing organisms have been largely limited to single institutions (4, 5). Because substantial information on the epidemiology of ESBL-producing K. pneumoniae or other gram-negative bacilli is lacking, we established a collaboration of researchers from 7 countries on 6 continents to prospectively enroll consecutive patients with K. pneumoniae bacteremia. Methods Study Design We performed a prospective observational study of 440 consecutive, sequentially encountered patients with K. pneumoniae bacteremia at 12 hospitals in South Africa, Taiwan, Australia, Argentina, the United States, Belgium, and Turkey. No patient was excluded from analysis. Patients were enrolled from 1 January 1996 to 31 December 1997. Patients were older than 16 years of age and had positive blood cultures for K. pneumoniae. The investigators completed a 188-item study form on each episode of K. pneumoniae bacteremia. Patients were followed for 1 month after the onset of bacteremia to assess clinical outcome, including death and infectious complications. The study was observational in that administration of antimicrobial agents and other therapeutic management was controlled by the patients physician rather than the investigators (6). The study was approved by institutional review boards as required by local hospital policy at the time of the study. Definitions All study definitions were established before data analysis. Nosocomial bacteremia was defined as K. pneumoniae bacteremia occurring more than 48 hours after admission to hospital. An episode of bacteremia was defined as the period of 14 days from the time of collection of the first blood culture positive for K. pneumoniae. Severity of acute illness was assessed at the time of the positive blood cultures by using the Pitt bacteremia score, a previously validated scoring system that is based on mental status, vital signs, requirement for mechanical ventilation, and recent cardiac arrest (Table 1) (7, 8). Severity of illness in patients in an intensive care unit at the time of onset of bacteremia was assessed by using the Acute Physiology and Chronic Health Evaluation-3 score (9). Site of infection was determined to be pneumonia, urinary tract infection, meningitis, incisional wound infection, other soft-tissue infection, intra-abdominal infection, or primary bloodstream infection according to Centers for Disease Control and Prevention definitions (10). Previous antibiotic therapy was defined as antibiotics given for at least 2 days within the 14 days before an episode of K. pneumoniae bacteremia (11). Antibiotic therapy for the episode of K. pneumoniae bacteremia was the receipt of antibiotics that are active in vitro against the blood culture isolate (that is, susceptible according to 1999 NCCLS breakpoints [12], given for at least 2 days within the first 5 days of collection of the first positive blood culture. Mortality was death from any cause within 14 days from the date of the first positive blood culture for K. pneumoniae. Table 1. The Pitt Bacteremia Score* Microbiological Analysis Production of ESBL was phenotypically determined by broth dilution using the NCCLS performance standards current as of January 1999 (12). A 3 twofold decrease in the minimal inhibitory concentration (MIC) for cefotaxime or ceftazidime tested in combination with clavulanic acid compared with its MIC when tested alone was considered phenotypic confirmation of ESBL production. For example, an isolate with a ceftazidime MIC of 8 g/mL and a ceftazidimeclavulanic acid MIC of 1 g/mL fulfills this definition of an ESBL-producing organism (12). The MICs of antibiotics commonly used in the treatment of gram-negative sepsis were determined for the ESBL-producing isolates by using the gradient diffusion method (Etest, AB Biodisk, Solna, Sweden). Pulsed-field gel electrophoresis was used to establish the genotypic relationships of ESBL-producing isolates from each hospital (11). Statistical Analysis All statistical comparisons were made by using PROPHET Statistics software, version 6.0 (ABTech-BBN Corp., Charlottesville, Virginia) or Stata software, version 7.0 (Stata Corp., College Station, Texas). For bivariate comparisons, the chi-square or Fisher exact test was used to compare categorical variables. Continuous variables were compared by using the t-test or the MannWhitney test. Length of stay before positive blood culture was missing for 10 (4%) patients, all of whom had been readmitted within 1 week of discharge. Length of stay for these 10 patients was therefore estimated by using a predictive model that assumed missing at random. Because previous receipt of antibiotics was not random, a propensity score model was used to further evaluate the effect of receipt of antibiotics containing an oxyimino group as a risk for ESBL production. The score was calculated by using a logistic model in which receipt of -lactam antibiotics containing an oxyimino group (cefuroxime, ceftriaxone, cefotaxime, ceftazidime, or aztreonam) was the dependent variable (scored as yes or no) and predictors were all available factors hypothesized to influence the receipt of antibiotic therapy (underlying patient conditions), with adjustment for center by using the indicator variables for site. Factors included in the calculation of this score were age, sex, admission from nursing home, underlying diseases (cancer, HIV infection, diabetes, or renal and liver disease), previous surgery, use of corticosteroids, presence of a central line, mechanical ventilatory support, presence of a nasogastric tube, and the indicator variables for site. A logistic model clustered on the patient that used receipt of antibiotics containing an oxyimino group and the quintile stratified score was then used to evaluate the risk for ESBL production. Role of the Funding Source Merck and Company provided support for laboratory studies but played no role in study design, conduct of the study, interpretation of the results, or approval of the study before publication. Results During the study period, 455 episodes of K. pneumoniae bacteremia occurred in 440 patients; of these, 202 (44.4%) episodes in 196 patients were community acquired and 253 (55.6%) episodes in 244 patients were nosocomially acquired. Table 2 shows the characteristics of the participants. Table 2. Characteristics of 244 Patients with Nosocomial Klebsiella pneumoniae Bacteremia Of the episodes of K. pneumoniae bacteremia, 18.7% (85 of 455) were due to ESBL-producing organisms and 81.1% (369 of 455) were due to nonESBL-producing organisms. One additional episode involved an isolate that showed a markedly elevated MIC for the oxyimino -lactam antibiotics but no decrease in MIC with the addition of clavulanic acid. This isolate had an MIC for cephamycin antibiotics in the resistant range and may have possessed an AmpC-like enzyme. This episode was excluded from further analysis. Seventy-eight of 253 (30.8%) episodes of nosocomial bacteremia were due to ESBL-producing organisms. Table 3 shows the sites of infection associated with nosocomial ESBL-producing K. pneumoniae bacteremia. Fewer episodes of community-acquired K. pneumoniae bacteremia (3.5% [7 of 202]) were due to ESBL-producing organisms (P <0.001) (risk ratio, 8.9 [95% CI, 4.2 to 18.8]). Of the 253 episodes of nosocomial bacteremia, 69 were acquired in the intensive care unit. Of these 69 episodes, 30 (43.5%) episodes o

Antibiotic Therapy for Klebsiella pneumoniae Bacteremia: Implications of Production of Extended-Spectrum β-Lactamases

The prevalence of extended-spectrum beta -lactamase (ESBL) production by Klebsiella pneumonia approaches 50% in some countries, with particularly high rates in eastern Europe and Latin America. No randomized trials have ever been performed on treatment of bacteremia due to ESBL-producing organisms; existing data comes only from retrospective, single-institution studies. In a prospective study of 455 consecutive episodes of Klebsiella pneumoniae bacteremia in 12 hospitals in 7 countries, 85 episodes were due to an ESBL-producing organism. Failure to use an antibiotic active against ESBL-producing K. pneumoniae was associated with extremely high mortality. Use of a carbapenem (primarily imipenem) was associated with a significantly lower 14-day mortality than was use of other antibiotics active in vitro. Multivariate analysis including other predictors of mortality showed that use of a carbapenem during the 5-day period after onset of bacteremia due to an ESBL-producing organism was independently associated with lower mortality. Antibiotic choice is particularly important in seriously ill patients with infections due to ESBL-producing K. pneumoniae.

Outbreak of ceftazidime resistance caused by extended-spectrum beta-lactamases at a Massachusetts chronic-care facility.

During a 4-month period in late 1988, we isolated ceftazidime-resistant strains of Klebsiella pneumoniae and other members of the family Enterobacteriaceae from 29 patients at a chronic-care facility in Massachusetts. Ceftazidime resistance resulted from two distinct extended-spectrum beta-lactamases of the TEM type which efficiently hydrolyzed the cephalosporin: YOU-1 with a pI of 5.57 and YOU-2 with a pI of 5.2. Genes encoding these enzymes were present on different but closely related high-molecular-weight, multiple antibiotic resistance plasmids of the H12 incompatibility group and were transferable by conjugation in vitro. Agarose gel electrophoresis of extracts from clinical isolates indicated that this outbreak arose from plasmid transmission among different strains of the family Enterobacteriaceae rather than from dissemination of a single resistant isolate. Isolation rates of ceftazidime-resistant organisms transiently decreased after use of this drug was restricted, but resistant isolates continued to be recovered 7 months after empiric use of ceftazidime ceased. Images

Extended-Spectrum β-Lactamases in Klebsiella pneumoniae Bloodstream Isolates from Seven Countries: Dominance and Widespread Prevalence of SHV- and CTX-M-Type β-Lactamases

ABSTRACT A huge variety of extended-spectrum β-lactamases (ESBLs) have been detected during the last 20 years. The majority of these have been of the TEM or SHV lineage. We have assessed ESBLs occurring among a collection of 455 bloodstream isolates of Klebsiella pneumoniae, collected from 12 hospitals in seven countries. Multiple β-lactamases were produced by isolates with phenotypic evidence of ESBL production (mean of 2.7 β-lactamases per isolate; range, 1 to 5). SHV-type ESBLs were the most common ESBL, occurring in 67.1% (49 of 73) of isolates with phenotypic evidence of ESBL production. In contrast, TEM-type ESBLs (TEM-10 type, -12 type, -26 type, and -63 type) were found in just 16.4% (12 of 73) of isolates. The finding of TEM-10 type and TEM-12 type represents the first detection of a TEM-type ESBL in South America. PER (for Pseudomonas extended resistance)-type β-lactamases were detected in five of the nine isolates from Turkey and were found with SHV-2-type and SHV-5-type ESBLs in two of the isolates. CTX-M-type ESBLs (blaCTX-M-2 type and blaCTX-M-3 type) were found in 23.3% (17 of 73) of isolates and were found in all study countries except for the United States. We also detected CTX-M-type ESBLs in four countries where they have previously not been described—Australia, Belgium, Turkey, and South Africa. The widespread emergence and proliferation of CTX-M-type ESBLs is particularly noteworthy and may have important implications for clinical microbiology laboratories and for physicians treating patients with serious K. pneumoniae infections.

A potential virulence gene, hylEfm, predominates in Enterococcus faecium of clinical origin.

An open reading frame (hyl(Efm)) with homologies to previously described hyaluronidase genes has been identified in nonstool isolates of Enterococcus faecium. E. faecium isolates (n=577) from diverse sources were screened for the presence of hyl(Efm) and esp(Efm), a putative virulence gene associated with epidemic E. faecium strains. The presence of esp(Efm) was roughly twice that of hyl(Efm), but both were found primarily in vancomycin-resistant E. faecium isolates in nonstool cultures obtained from patients hospitalized in the United States. These data suggest that specific E. faecium strains may be enriched in determinants that make them more likely to cause clinical infections. Differences in the prevalence of these strains may help explain variations in the clinical importance of multiresistant E. faecium across different continents.

The clinical consequences of antimicrobial resistance.

The continued evolution of antimicrobial resistance in the hospital and more recently in the community threatens to seriously compromise our ability to treat serious infections. The major success of the seven-valent Streptococcus pneumoniae vaccine at reducing both infection and resistance has been followed by the emergence of previously minor serotypes that express multiresistance. The almost universal activity of cephalosporins and fluoroquinolones against community Escherichia coli strains has been compromised by the spread of CTX-M beta-lactamase-producing, fluoroquinolone-resistant strains, and the emergence of community-onset methicillin-resistant Staphylococcus aureus, particularly in the United States, has forced us to re-think our empirical treatment guidelines for skin and soft-tissue infections. Finally, our most potent and reliable class of antibiotics, the carbapenems, is compromised by the growth, primarily in intensive care units, of multiresistant Klebsiella pneumoniae, Acinetobacter baumanni, and Pseudomonas aeruginosa. The lack of a robust pipeline of new agents, particularly against resistant Gram-negative bacteria, emphasizes the importance of optimizing our use of current antimicrobials and promoting strict adherence to established infection control practices.

Gene Dosage and Linezolid Resistance in Enterococcus faecium and Enterococcus faecalis

ABSTRACT Resistance to linezolid has been associated with a G2576U mutation in domain V of the 23S rRNA. We analyzed nine clinical isolates of linezolid-resistant enterococci and showed a clear association between the number of 23S rRNA genes containing this mutation and the level of linezolid resistance expressed.