The 2016 Garrod Lecture: The role of the healthcare epidemiologist in antimicrobial chemotherapy-a view from the USA

Antimicrobial chemotherapy now spans 80 years and four generations. The healthcare epidemiologist has an important role to play in this field. Efforts focus in three areas: (i) minimizing the transmission of antimicrobial-resistant bacteria in healthcare settings (infection control); (ii) optimizing use of currently available antibacterial drugs (antibiotic stewardship); and (iii) recognizing and responding to opportunities for new drug development. For each area, the epidemiologist provides data that address four practical questions-'What is the problem?', 'What should be done?', 'Is it being done?' and 'Is it working?'. A team approach is crucial to acting on the epidemiological data. Examples are presented to illustrate different roles of the epidemiologist, and tools and measures that have been developed to address some problems of current importance. Monitoring of quality, integrity and security of data remains a major focus. The epidemiologist will continue to have a key role in antimicrobial chemotherapy.

Antimicrobial Stewardship—the State of the Art in 2011 Focus on Outcome and Methods

Antimicrobial stewardship programs attempt to optimize prescribing of these drugs to benefit both current and future patients. Recent regulatory and other incentives have led to widespread adoption of such programs. Measurements of the success of these programs have focused primarily on process measures. However, evaluation of outcome measures will be needed to ensure sustainability of these efforts. Outcome efforts to date provide some evidence for improved care of individual patients, some evidence for minimizing emergence of resistance, and ample evidence for cost reduction. Attention to evaluation methods must be increased to provide convincing evidence for the continuation of such programs.

Methicillin-resistant Staphylococcus aureus (MRSA) nasal carriage in residents of Veterans Affairs long-term care facilities: role of antimicrobial exposure and MRSA acquisition

OBJECTIVE

To identify risk factors associated with methicillin-resistant Staphylococcus aureus (MRSA) acquisition in long-term care facility (LTCF) residents.

DESIGN

Multicenter, prospective cohort followed over 6 months.

SETTING

Three Veterans Affairs (VA) LTCFs.

PARTICIPANTS

All current and new residents except those with short stay (<2 weeks).

METHODS

MRSA carriage was assessed by serial nares cultures and classified into 3 groups: persistent (all cultures positive), intermittent (at least 1 but not all cultures positive), and noncarrier (no cultures positive). MRSA acquisition was defined by an initial negative culture followed by more than 2 positive cultures with no subsequent negative cultures. Epidemiologic data were collected to identify risk factors, and MRSA isolates were typed by pulsed-field gel electrophoresis (PFGE).

RESULTS

Among 412 residents at 3 LTCFs, overall MRSA prevalence was 58%, with similar distributions of carriage at all 3 facilities: 20% persistent, 39% intermittent, 41% noncarriers. Of 254 residents with an initial negative swab, 25 (10%) acquired MRSA over the 6 months; rates were similar at all 3 LTCFs, with no clusters evident. Multivariable analysis demonstrated that receipt of systemic antimicrobials during the study was the only significant risk factor for MRSA acquisition (odds ratio, 7.8 [95% confidence interval, 2.1-28.6]; P = .002). MRSA strains from acquisitions were related by PFGE to those from a roommate in 9/25 (36%) cases; 6 of these 9 roommate sources were persistent carriers.

CONCLUSIONS

MRSA colonization prevalence was high at 3 separate VA LTCFs. MRSA acquisition was strongly associated with antimicrobial exposure. Roommate sources were often persistent carriers, but transmission from roommates accounted for only approximately one-third of MRSA acquisitions.

Cooking fuel type, household ventilation, and the risk of acute lower respiratory illness in urban Bangladeshi children: a longitudinal study

Acute lower respiratory illnesses (ALRI) are the leading cause of death among children <5 years. Studies have found that biomass cooking fuels are an important risk factor for ALRI. However, few studies have evaluated the influence of natural household ventilation indicators on ALRI. The purpose of this study was to assess the association between cooking fuel, natural household ventilation, and ALRI. During October 17, 2004-September 30, 2005, children <5 years living in a low-income neighborhood of Dhaka, Bangladesh, were assessed weekly for ALRI and surveyed quarterly about biomass fuel use, electric fan ownership, and natural household ventilation (windows, ventilation grates, and presence of a gap between the wall and ceiling). Bivariate and multivariate analyses were performed using generalized estimating equations. Six thousand and seventy-nine children <5 years enrolled during the study period (99% participation) experienced 1291 ALRI. In the multivariate model, ≥2 windows [OR = 0.75, 95% CI = (0.58, 0.96)], ventilation grates [OR = 0.80, 95% CI = (0.65, 0.98)], and not owning an electric fan [OR = 1.50, 95% CI = (1.21, 1.88)] were associated with ALRI; gap presence and using biomass fuels were not associated with ALRI. Structural factors that might improve household air circulation and exchange were associated with decreased ALRI risk. Improved natural ventilation might reduce ALRI among children in low-income families.

PRACTICAL IMPLICATIONS

The World Health Organization has stated that controlling pneumonia is a priority for achieving the fourth Millennium Development Goal, which calls for a two-third reduction in mortality of children <5 years old compared to the 1990 baseline. Our study represents an important finding of a modifiable risk factor that might decrease the burden of respiratory illness among children living in Bangladesh and other low-income settings similar to our study site. We found that the existence of at least two windows in the child's sleeping room was associated with a 25% decreased ALRI risk. Increasing available natural ventilation within the household in similar settings has the potential to reduce childhood mortality because of acute lower respiratory illnesses.

Effect of setting a maximum milking time, from peak lactation, on production, milking time and udder health

AIM

To examine the effect of setting a maximum milking time, from peak lactation until drying-off, on production, duration of milking, and udder health of dairy cows.

METHODS

Forty cows were assigned in twin-pairs to be either milked until cups were removed at a milk flow-rate threshold of 0.35 kg/minute (Control), or until cups were removed at a milk flow-rate threshold of 0.35 kg/minute, or maximum time, whichever came first (MaxT). The maximum time was set by determining the milking time of the 70th percentile cow when ranked from fastest to slowest, irrespective of yield. The milking routine was typical of that practised on dairy farms in New Zealand, and involved no pre-milking preparation. The study began at peak lactation (68 (SD 7) days in milk; DIM) and continued for 26 weeks. Duration of milking and milk yield were measured for each milking. Composition of milk was determined from weekly herd tests, and milk quality from fortnightly somatic cell counts (SCC). Completeness of milking and teat condition were assessed during the study. The bacterial status of quarter milk samples was determined at the beginning and end of the study, and all treated cases of clinical mastitis recorded. ANOVA was used to examine the effect of treatment group on variables of interest.

RESULTS

Total milk, fat and protein yields during the study period did not differ between treatments. On average, 30.3% of the morning and 27.6% of the afternoon milkings of MaxT cows reached the maximum time at which cups were removed, and were therefore shortened. While the average milking time of the slowest-milking cow was longer for the Control compared with MaxT group in Weeks 1-18, the average milking time did not differ between treatments. There was no difference in overall SCC, and the incidence of clinical mastitis, or the percentage of infected quarters at drying-off, was similar for the MaxT and Control cows.

CONCLUSION

The results show that setting a maximum milking time can reduce the milking time of slower-milking cows in a herd without compromising overall herd production and udder health.

CLINICAL RELEVANCE

Although the numbers of cows in the study were small there was no evidence of a major increase in SCC, or subclinical or clinical mastitis when a maximum milking time was set for slower-milking cows.

Quantitative analysis and molecular fingerprinting of methicillin-resistant Staphylococcus aureus nasal colonization in different patient populations: a prospective, multicenter study

OBJECTIVES

To better understand the prevalence of methicillin-resistant Staphylococcus aureus (MRSA) colonization or infection in different patient populations, to perform quantitative analysis of MRSA in nasal cultures, and to characterize strains using molecular fingerprinting.

DESIGN

Prospective, multicenter study.

SETTING

Eleven different inpatient and outpatient healthcare facilities.

PARTICIPANTS

MRSA-positive inpatients identified in an active surveillance program; inpatients and outpatients receiving hemodialysis; inpatients and outpatients with human immunodeficiency virus (HIV) infection; patients requiring cardiac surgery; and elderly patients requiring long-term care. METHODS. Nasal swab samples were obtained from January 23, 2006, through July 27, 2007; MRSA strains were quantified and characterized by molecular fingerprinting.

RESULTS

A total of 444 nares swab specimens yielded MRSA (geometric mean quantity, 794 CFU per swab; range, 3-15,000,000 CFU per swab). MRSA prevalence was 20% for elderly residents of long-term care facilities (25 of 125 residents), 16% for HIV-infected outpatients (78 of 494 outpatients), 15% for outpatients receiving hemodialysis (31 of 208 outpatients), 14% for inpatients receiving hemodialysis (86 of 623 inpatients), 3% for HIV-infected inpatients (5 of 161 inpatients), and 3% for inpatients requiring cardiac surgery (6 of 199 inpatients). The highest geometric mean quantity of MRSA was for inpatients requiring cardiac surgery (11,500 CFU per swab). An association was found between HIV infection and colonization with the USA300 or USA500 strain of MRSA (P < or = .001). The Brazilian clone was found for the first time in the United States. Pulsed-field gel electrophoresis patterns for 11 isolates were not compatible with known USA types or clones.

CONCLUSION

Nasal swab specimens positive for MRSA had a geometric mean quantity of 794 CFU per swab, with great diversity in the quantity of MRSA at this anatomic site. Outpatient populations at high risk for MRSA carriage were elderly residents of long-term care facilities, HIV-infected outpatients, and outpatients receiving hemodialysis.

Hospitalization earlier than 1 year prior to admission as an additional risk factor for methicillin-resistant Staphylococcus aureus colonization

Our case-control study sought to identify risk factors for colonization with methicillin-resistant Staphylococcus aureus (MRSA) at hospital admission among patients with no known healthcare-related risk factors. We found that patients whose most recent hospitalization occurred greater than 1 year before their current hospital admission were more likely to have MRSA colonization. In addition, both the time that elapsed since the most recent hospitalization and the duration of that hospitalization affected risk.

Characterisation of a Staphylococcus aureus strain with progressive loss of susceptibility to vancomycin and daptomycin during therapy

Following an initial response to vancomycin therapy, a patient with meticillin-resistant Staphylococcus aureus (MRSA) bacteraemia developed endocarditis, failed a second course of vancomycin and then failed daptomycin therapy. An increase in the vancomycin minimum inhibitory concentrations of four consecutive MRSA blood isolates from 2 microg/mL to 8 microg/mL was shown by Etest. Population analysis of four successive blood culture isolates recovered over the 10-week period showed that the MRSA strain became progressively less susceptible to both vancomycin and daptomycin. Retrospectively, the macro Etest method using teicoplanin indicated a decrease in vancomycin susceptibility in the second blood isolate. The patient improved after treatment with various courses of trimethoprim/sulfamethoxazole, quinupristin/dalfopristin and linezolid. Early detection of vancomycin-heteroresistant S. aureus isolates, which appeared to have clinical significance in this case, continues to be a challenge for the clinical laboratory. Development of suitable practical methods for this should be given priority. Concurrent development of resistance to vancomycin and daptomycin, whilst rare, must be considered in a patient who is unresponsive to daptomycin following vancomycin therapy.

High rate of mobilization for blaCTX-Ms

The bla_CTX-Ms have been mobilized to plasmids more frequently than other class A β-lactamases.

We constructed a phylogenetic analysis of class A β-lactamases and found that the bla_CTX-Ms have been mobilized to plasmids ≈10 times more frequently than other class A β-lactamases. We also found that the bla_CTX-Ms are descended from a common ancestor that was incorporated in ancient times into the chromosome of the ancestor of Kluyvera species through horizontal transfer. Considerable sequence divergence has occurred among the descendents of that ancestral gene sequence since that gene was inserted. That divergence has mainly occurred in the presence of purifying selection, which indicates a slow rate of evolution for bla_CTX-Ms in the pre–antimicrobial drug era.

Antimicrobial resistance: international control strategies, with a focus on limited-resource settings

Microorganisms resistant to multiple anti-infective agents have increased worldwide. These organisms threaten both optimal care of patients with infection as well as the viability of current healthcare systems. In addition, antimicrobials are valuable resources that enhance both prevention and treatment of infections. As resistance diminishes this resource, it is a societal goal to minimise resistance and therefore to reduce forces that produce resistance. This review considers strategies for minimising resistance that are needed at several different levels of responsibility, ranging from the patient care provider to international agencies. It then describes responses that might be appropriate according to the resources available for control, focusing on limited-resource settings. Antimicrobial resistance represents an international concern. Response to this problem demands concerted efforts from multiple sectors both in developed and developing countries, as well as the strengthening of multinational/international partnerships and regulations. Both medical and public health agencies should be in the forefront of these efforts.

Cytomegalovirus seroprevalence and childhood sources of infection: A population-based study among pre-adolescents in the United States

BACKGROUND

Among pre-adolescents, the importance of different sources of cytomegalovirus (CMV) infection is unclear.

OBJECTIVE

To assess the importance of several CMV sources among pre-adolescent children.

STUDY DESIGN

We used data from a United States population-based sample conducted from 1988 to 1994: 4-10-year-old participants (n=3386) of the Third National Health and Nutrition Examination Survey. We tested available sera for CMV-specific-IgG and assessed CMV prevalence differences by surrogates for exposure to childhood CMV sources (maternal CMV serostatus, breast-feeding, older sibling CMV serostatus, and child care center attendance).

RESULTS

CMV infection was more prevalent (70%) among Mexican American children with foreign-born householders than among children with native-born householders (31% non-Hispanic White, 39% non-Hispanic Black, and 37% Mexican American children). Child's serostatus was associated with their mother's (prevalence difference range (PDR)=33-40%) and older sibling's serostatus (PDR=39-50%). Breast-feeding was associated with CMV in some racial/ethnic and householder groups (PDR=-5.1% to 22.7%). There was little difference in CMV seroprevalence by child care center attendance (PDR=-6.5% to -0.4%).

CONCLUSIONS

This study expands understanding of CMV by identifying the importance of householder nativity and demonstrating the importance of family transmission among the general population of pre-adolescents.

Importance of bacterial burden among methicillin-resistant Staphylococcus aureus carriers in a long-term care facility

OBJECTIVE

To evaluate the prevalence and transmission of methicillin-resistant Staphylococcus aureus (MRSA) nasal colonization, as well as risk factors associated with MRSA carriage, among residents of a long-term care facility (LTCF).

DESIGN

Prospective, longitudinal cohort study.

SETTING

A 100-bed Veterans Administration LTCF.

PARTICIPANTS

All current and newly admitted residents of the LTCF during an 8-week study period.

METHODS

Nasal swab samples were obtained weekly and cultured on MRSA-selective media, and the cultures were graded for growth on a semiquantitative scale from 0 (no growth) to 6 (heavy growth). Epidemiologic data for the periods before and during the study were collected to assess risk factors for MRSA carriage.

RESULTS

Of 83 LTCF residents, 49 (59%) had 1 or more nasal swab cultures that were positive for MRSA; 34 (41%) were consistently culture-negative (designated "noncarriers"). Of the 49 culture-positive residents, 30 (36% of the total of 83 residents) had all cultures positive for MRSA (designated "persistent carriers"), and 19 (23% of the 83 residents) had at least 1 culture, but not all cultures, positive for MRSA (designated "intermittent carriers"). Multivariate analysis showed that participants with at least 1 nasal swab culture positive for MRSA were likely to have had previous hospitalization (odds ratio, 3.9) or wounds (odds ratio, 8.2). Persistent carriers and intermittent carriers did not differ in epidemiologic characteristics but did differ in mean MRSA growth score (3.7 vs 0.7; P<.001).

CONCLUSIONS

Epidemiologic characteristics differed between noncarriers and subjects with at least 1 nasal swab culture positive for MRSA. However, in this LTCF population, only the degree of bacterial colonization (as reflected by mean MRSA growth score) distinguished persistent carriers from intermittent carriers. Understanding the burden of colonization may be important when determining future surveillance and control strategies.

Antimicrobial resistance prevention initiative--an update: proceedings of an expert panel on resistance

Antimicrobial resistance is a growing problem that complicates the treatment of important nosocomial and community-acquired infections. It is a worldwide problem that spans the range of human pathogens, including bacteria, fungi, and viruses. This update from the Antimicrobial Resistance Prevention Initiative (ARPI) provides a review of some important trends in antibiotic, antifungal, and antiviral resistance. Areas of focus include multidrug-resistant bacteria in the hospital setting; the growing problem of community-acquired methicillin-resistant Staphylococcus aureus; triazole and polyene resistance in nosocomial infections caused by non-Candida albicans or Aspergillus species, and the utility of in vitro susceptibility testing for these fungal infections; antiviral resistance in alpha- or beta-herpesviruses causing genital herpes or cytomegalovirus infection in immunocompromised hosts; and concerns about a possible pandemic involving avian influenza A and the importance of minimizing emergence of resistant strains of this highly pathogenic virus. The challenges in each area are different, but the general keys to addressing the growing problem of antimicrobial resistance continue to be responsible antimicrobial stewardship and the development of newer antimicrobial agents.

Accuracy of six antimicrobial susceptibility methods for testing linezolid against staphylococci and enterococci

A challenge panel of enterococci (n = 50) and staphylococci (n = 50), including 17 and 15 isolates that were nonsusceptible to linezolid, respectively, were tested with the Clinical and Laboratory Standards Institute broth microdilution and disk diffusion reference methods. In addition, all 100 isolates were tested in parallel by Etest (AB Biodisk, Solna, Sweden), MicroScan WalkAway (Dade, West Sacramento, CA), BD Phoenix (BD Diagnostic Systems, Sparks, MD), VITEK (bioMérieux, Durham, NC), and VITEK 2 (bioMérieux) by using the manufacturers' protocols. Compared to the results of the broth microdilution method for detecting linezolid-nonsusceptible staphylococci and enterococci, MicroScan results showed the highest category agreement (96.0%). The overall categorical agreement levels for VITEK 2, Etest, Phoenix, disk diffusion, and VITEK were 93.0%, 90.0%, 89.6%, 88.0%, and 85.9%, respectively. The essential agreement levels (results within +/-1 doubling dilution of the MIC determined by the reference method) for MicroScan, Phoenix, VITEK 2, Etest, and VITEK were 99.0%, 95.8%, 92.0%, 92.0%, and 85.9%, respectively. The very major error rates for staphylococci were the highest for VITEK (35.7%), Etest (40.0%), and disk diffusion (53.3%), although the total number of resistant isolates tested was small. The very major error rate for enterococci with VITEK was 20.0%. Three systems (MicroScan, VITEK, and VITEK 2) provided no interpretations of nonsusceptible results for staphylococci. These data, from a challenge panel of isolates, illustrate that the recent emergence of linezolid-nonsusceptible staphylococci and enterococci is providing a challenge for many susceptibility testing systems.

Antimicrobial resistance prevention initiative--an update: proceedings of an expert panel on resistance

Antimicrobial resistance is a growing problem that complicates the treatment of important nosocomial and community-acquired infections. It is a worldwide problem that spans the range of human pathogens, including bacteria, fungi, and viruses. This update from the Antimicrobial Resistance Prevention Initiative (ARPI) provides a review of some important trends in antibiotic, antifungal, and antiviral resistance. Areas of focus include multidrug-resistant bacteria in the hospital setting; the growing problem of community-acquired methicillin-resistant Staphylococcus aureus; triazole and polyene resistance in nosocomial infections caused by non-Candida albicans or Aspergillus species, and the utility of in vitro susceptibility testing for these fungal infections; antiviral resistance in alpha- or beta-herpesviruses causing genital herpes or cytomegalovirus infection in immunocompromised hosts; and concerns about a possible pandemic involving avian influenza A and the importance of minimizing emergence of resistant strains of this highly pathogenic virus. The challenges in each area are different, but the general keys to addressing the growing problem of antimicrobial resistance continue to be responsible antimicrobial stewardship and the development of newer antimicrobial agents.

Longitudinal analysis of community health workers' adherence to treatment guidelines, Siaya, Kenya, 1997-2002

OBJECTIVES

To investigate community health workers' (CHW) adherence over time to guidelines for treating ill children and to assess the effect of refresher training on adherence.

METHODS

Analysis of 7151 ill-child consultations performed by 114 CHWs in their communities from March 1997-May 2002. Adherence was assessed with a score (percentage of recommended treatments that were prescribed), calculated for each consultation. Recommended treatments were those that were indicated based on CHW assessments. We used piecewise regression models to evaluate adherence before and after training.

RESULTS

The average adherence score was 79.4%. Multivariable analyses indicate that immediately after the first refresher training, the mean adherence level improved for patients with a severe illness, but worsened for patients without severe illness. Adherence scores declined rapidly during the 6 months after the second refresher training.

CONCLUSIONS

The first refresher was partially effective, the second refresher had an effect contrary to that intended, and patient characteristics had a strong influence on adherence patterns. Longitudinal studies are useful for monitoring the dynamics of CHW performance and evaluating effects of quality improvement interventions.

Multiple-locus variable-number tandem-repeat assay analysis of methicillin-resistant Staphylococcus aureus strains

Our objective was to determine if a multiple-locus variable-number tandem-repeat assay (MLVA) for Staphylococcus aureus could predict pulsed-field gel electrophoresis (PFGE) types (i.e., USA types), thus allowing us to replace PFGE with a simpler and more rapid typing method. One hundred three well-characterized isolates representing 13 major lineages of S. aureus were tested by both PFGE and MLVA. MLVA was performed using a rapid DNA extraction technique and PCR primers for sdrCDE, clfA, clfB, sspA, and spa. PFGE was performed with genomic DNA fragments generated using SmaI, as per CDC protocols. Banding patterns were analyzed both visually and with BioNumerics software. All isolates were typeable with MLVA and PFGE. MLVA patterns were highly reproducible. PFGE separated the isolates into 13 types with 42 subtypes. Using any band difference to designate a novel MLVA type, MLVA produced 45 types, including 9 clusters containing multiple isolates. Using BioNumerics and a cutoff of >75% relatedness, MLVA produced 28 types, 11 of which contained >1 isolate. Epidemiologically related outbreak isolates of USA300-0114 from five states clustered in one MLVA pattern. USA100 isolates were present in several unrelated (<40%) MLVA types. A cutoff of >80% separated outbreak strains of USA300-0114 into three distinct MLVA types. MLVA did not differentiate community methicillin-resistant S. aureus (MRSA) lineages (USA300, USA400, USA1000, and USA1100) from health care MRSA lineages (USA100, USA200, or USA500). The ability of MLVA to differentiate among strains that are indistinguishable by PFGE may be of epidemiologic value and warrants further study.

Comparison of disk diffusion, VITEK 2, and broth microdilution antimicrobial susceptibility test results for unusual species of Enterobacteriaceae

We compared the antimicrobial susceptibility testing results generated by disk diffusion and the VITEK 2 automated system with the results of the Clinical and Laboratory Standards Institute (CLSI) broth microdilution (BMD) reference method for 61 isolates of unusual species of Enterobacteriaceae. The isolates represented 15 genera and 26 different species, including Buttiauxella, Cedecea, Kluyvera, Leminorella, and Yokenella. Antimicrobial agents included aminoglycosides, carbapenems, cephalosporins, fluoroquinolones, penicillins, and trimethoprim-sulfamethoxazole. CLSI interpretative criteria for Enterobacteriaceae were used. Of the 12 drugs tested by BMD and disk diffusion, 10 showed >95% categorical agreement (CA). CA was lower for ampicillin (80.3%) and cefazolin (77.0%). There were 3 very major errors (all with cefazolin), 1 major error (also with cefazolin), and 26 minor errors. Of the 40 isolates (representing 12 species) that could be identified with the VITEK 2 database, 36 were identified correctly to species level, 1 was identified to genus level only, and 3 were reported as unidentified. VITEK 2 generated MIC results for 42 (68.8%) of 61 isolates, but categorical interpretations (susceptible, intermediate, and resistant) were provided for only 22. For the 17 drugs tested by both BMD and VITEK 2, essential agreement ranged from 80.9 to 100% and CA ranged from 68.2% (ampicillin) to 100%; thirteen drugs exhibited 100% CA. In summary, disk diffusion provides a reliable alternative to BMD for testing of unusual Enterobacteriaceae, some of which cannot be tested, or produce incorrect results, by automated methods.

Multicenter evaluation of the Etest and disk diffusion methods for differentiating daptomycin-susceptible from non-daptomycin-susceptible Staphylococcus aureus isolates

Daptomycin is a novel cyclic lipopeptide that is approved by the U.S. Food and Drug Administration for the treatment of complicated skin and skin structure infections associated with Staphylococcus aureus and other gram-positive pathogens and also staphylococcal bacteremia, including right-sided endocarditis. The Clinical and Laboratory Standards Institute (CLSI) established "susceptible-only" interpretive criteria for broth microdilution (BMD) and disk diffusion (DD) testing of daptomycin in 2005. However, a series of S. aureus isolates have been recovered with daptomycin MICs in the nonsusceptible range (i.e., MICs of >1 microg/ml). The objective of this study was to determine the ability of the Etest and DD methods to differentiate daptomycin-susceptible from nonsusceptible isolates of S. aureus compared to the results of the CLSI BMD reference method. There was a good correlation between Etest MIC results and the results of BMD among laboratories (r = 0.86 to 0.88), with 95.3% of the Etest MICs within a +/-1 log(2) dilution of the BMD MIC result. A total of 92 of 102 (90.2%) non-daptomycin-susceptible isolates of S. aureus identified by BMD in two participating laboratories were also classified as nonsusceptible by Etest. However, the very major and major error rates reported by one of the participating laboratories were 13.5 and 4.0%, respectively, primarily due to the absence of an intermediate category. The DD method, however, did not reliably differentiate daptomycin-susceptible from non-daptomycin-susceptible isolates. In 2005, daptomycin disks were voluntarily removed from the market by Cubist Pharmaceuticals. The disk diffusion breakpoints were subsequently removed from the CLSI M100 standard in 2006.

Carbapenem resistance in Klebsiella pneumoniae not detected by automated susceptibility testing

Detecting β-lactamase–mediated carbapenem resistance among Klebsiella pneumoniae isolates and other Enterobacteriaceae is an emerging problem. In this study, 15 bla_KPC-positive Klebsiella pneumoniae that showed discrepant results for imipenem and meropenem from 4 New York City hospitals were characterized by isoelectric focusing; broth microdilution (BMD); disk diffusion (DD); and MicroScan, Phoenix, Sensititre, VITEK, and VITEK 2 automated systems. All 15 isolates were either intermediate or resistant to imipenem and meropenem by BMD; 1 was susceptible to imipenem by DD. MicroScan and Phoenix reported 1 (6.7%) and 2 (13.3%) isolates, respectively, as imipenem susceptible. VITEK and VITEK 2 reported 10 (67%) and 5 (33%) isolates, respectively, as imipenem susceptible. By Sensititre, 13 (87%) isolates were susceptible to imipenem, and 12 (80%) were susceptible to meropenem. The VITEK 2 Advanced Expert System changed 2 imipenem MIC results from >16 μg/mL to <2 μg/mL but kept the interpretation as resistant. The recognition of carbapenem-resistant K. pneumoniae continues to challenge automated susceptibility systems.

Effect of multiple interventions on community health workers' adherence to clinical guidelines in Siaya district, Kenya

Evaluation of a community health worker (CHW) programme in Siaya district, Kenya, showed CHWs commonly made errors in managing childhood illness. We assessed the effect of multiple interventions on CHW healthcare practices. A sample of 192 ill-child consultations performed by 114 CHWs in a hospital outpatient department between February and March 2001 were analysed. The mean percentage of assessment, classification and treatment procedures performed correctly for each child was 79.8% (range 13.3-100%). Of the 187 children who required at least one treatment or referral to a health facility, only 38.8% were prescribed all treatments (including referral) recommended by the guidelines. Multivariate analyses found no evidence that the intervention-related factors studied (refresher training, supervision, involvement of community women in the CHW selection process, adequacy of medicine supplies, and use of a guideline flipchart during consultations) were significantly associated with overall or treatment-specific guideline adherence. A multivariate linear regression analysis revealed that several non-intervention-related factors, such as patient characteristics, were significantly associated with overall guideline adherence. Given that our study was cross-sectional and our measurement of exposure to several interventions was based on CHW recall, the estimated effects of the interventions should be interpreted with caution. Despite these limitations, however, our results raise questions about the effectiveness, in the setting of Siaya district, of several interventions commonly used to improve the quality of care given by CHWs.

Adherence to infectious diseases society of America guidelines for empiric therapy for patients with community-acquired pneumonia in a commercially insured cohort

BACKGROUND

There is little published research addressing how the 2003 Infectious Diseases Society of America (IDSA) guidelines for empiric therapy of community-acquired pneumonia (CAP) are implemented in clinical practice.

OBJECTIVE

This study was designed to describe antibiotic treatment patterns among patients with CAP treated in ambulatory settings in light of the IDSA guidelines.

METHODS

Health insurance claims data from a large managed care organization with -30 million enrollees located in geographically diverse regions of the United States were analyzed. Patients > or =18 years of age with CAP who received a prescription for any antibiotic in an ambulatory setting during 2004 were identified via International Classification of Diseases, Ninth Revision, Clinical Modification codes for diagnosis (481-486). Recent antibiotic use was defined as receipt of any antibiotics <90 days before the date of diagnosis. Antibiotics were identified through National Drug Codes and from outpatient medical claims data with the use of J codes. Individuals were classified, per IDSA guidelines, as previously healthy without recent antibiotic use (group 1); previously healthy with recent antibiotic use (group 2); with comorbidities and without recent antibiotic use (group 3); and with comorbidities and recent antibiotic use (group 4). The guideline adherence was calculated using the number of patients receiving recommended treatment divided by the total number of patients in each group.

RESULTS

Of 34,342 patients identified, 76.5% had no reported comorbidities. Among group-1 patients, 52.0% received the recommended empiric therapy (macrolide or doxycycline). In group-2 patients, 42.5% received the recommended therapy (respiratory quinolone alone or advanced-generation macrolide plus amoxicillin or amoxicillin-clavulanate). A high rate of compliance with recommended empiric therapy (advanced-generation macrolides or respiratory quinolones) was observed in group-3 patients (81.5%). In group-4 patients, 43.4% received the recommended therapy (respiratory quinolone or advanced-generation macrolide plus ss-lactam). Patients whose therapy was adherent with the guidelines had fewer respiratory-infection-related hospital admissions within 30 days after initiation of antibiotic treatment (overall, relative risk = 0.81 [95% CI, 0.71-0.94]).

CONCLUSION

Although these data reflect a period shortly after the 2003 IDSA guidelines were published, they suggest that there is room for improvement with regard to choice of empiric antibiotic therapy among these patients with CAP treated in ambulatory settings.

Class 1 integrons in resistant Escherichia coli and Klebsiella spp., US hospitals

We examined Escherichia coli and Klebsiella spp. from US hospitals for class 1 integrons. Of 320 isolates, 181 (57%) were positive; association of integrons with resistance varied by drug and organism. Thus, determining integron epidemiology will improve understanding of how antibacterial resistance determinants spread in the United States.

Investigation of inducible clindamycin and telithromycin resistance in isolates of β-hemolytic streptococci

We evaluated the accuracy of an erythromycin-clindamycin double-disk test (D-zone test) and an erythromycin-telithromycin D-zone test for detection of inducible resistance in isolates of beta-hemolytic streptococci with erythromycin resistance. The results of these tests were compared to results of a broth microdilution (BMD) induction test using combinations of erythromycin and either clindamycin or telithromycin. Of 29 erythromycin-resistant, clindamycin-susceptible isolates, 16 were positive by the erythromycin-clindamycin D-zone test; all of these demonstrated inducible clindamycin resistance by BMD. Twelve isolates were D-zone test-negative, did not demonstrate inducible resistance by BMD, and were positive for a mef determinant. Of 39 erythromycin-resistant, telithromycin-susceptible isolates, 13 were erythromycin-telithromycin D-zone test-positive, 19 questionably positive (unclear blunting of the zone), and 7 were D-zone test-negative. The erythromycin-telithromycin D-zone test result did not correlate with inducible resistance by BMD or the presence of an erm or mef gene. These results demonstrate that the erythromycin-clindamycin D-zone and BMD induction tests accurately detect inducible clindamycin resistance, but the erythromycin-telithromycin D-zone test is not reliable for detecting inducible telithromycin resistance.

Resistance in nonfermenting gram-negative bacteria: multidrug resistance to the maximum

Nonfermenting gram-negative bacteria pose a particular difficulty for the healthcare community because they represent the problem of multidrug resistance to the maximum. Important members of the group in the United States include Pseudomonas aeruginosa, Acinetobacter baumannii, Stenotrophomonas maltophilia, and Burkholderia cepacia. These organisms are niche pathogens that primarily cause opportunistic healthcare-associated infections in patients who are critically ill or immunocompromised. Multidrug resistance is common and increasing among gram-negative nonfermenters, and a number of strains have now been identified that exhibit resistance to essentially all commonly used antibiotics, including antipseudomonal penicillins and cephalosporins, aminoglycosides, tetracyclines, fluoroquinolones, trimethoprim-sulfamethoxazole, and carbapenems. Polymyxins are the remaining antibiotic drug class with fairly consistent activity against multidrug-resistant strains of P aeruginosa, Acinetobacter spp, and S maltophilia. However, most multidrug-resistant B cepacia are not susceptible to polymyxins, and systemic polymyxins carry the risk of nephrotoxicity for all patients treated with these agents, the elderly in particular. A variety of resistance mechanisms have been identified in P aeruginosa and other gram-negative nonfermenters, including enzyme production, overexpression of efflux pumps, porin deficiencies, and target-site alterations. Multiple resistance genes frequently coexist in the same organism. Multidrug resistance in gram-negative nonfermenters makes treatment of infections caused by these pathogens both difficult and expensive. Improved methods for susceptibility testing are needed when dealing with these organisms, including emerging strains expressing metallo-beta-lactamases. Improved antibiotic stewardship and infection-control measures will be needed to prevent or slow the emergence and spread of multidrug-resistant, nonfermenting gram-negative bacilli in the healthcare setting.

Resistance in nonfermenting gram-negative bacteria: multidrug resistance to the maximum

Nonfermenting gram-negative bacteria pose a particular difficulty for the healthcare community because they represent the problem of multidrug resistance to the maximum. Important members of the group in the United States include Pseudomonas aeruginosa, Acinetobacter baumannii, Stenotrophomonas maltophilia, and Burkholderia cepacia. These organisms are niche pathogens that primarily cause opportunistic healthcare-associated infections in patients who are critically ill or immunocompromised. Multidrug resistance is common and increasing among gram-negative nonfermenters, and a number of strains have now been identified that exhibit resistance to essentially all commonly used antibiotics, including antipseudomonal penicillins and cephalosporins, aminoglycosides, tetracyclines, fluoroquinolones, trimethoprim-sulfamethoxazole, and carbapenems. Polymyxins are the remaining antibiotic drug class with fairly consistent activity against multidrug-resistant strains of P aeruginosa, Acinetobacter spp, and S maltophilia. However, most multidrug-resistant B cepacia are not susceptible to polymyxins, and systemic polymyxins carry the risk of nephrotoxicity for all patients treated with these agents, the elderly in particular. A variety of resistance mechanisms have been identified in P aeruginosa and other gram-negative nonfermenters, including enzyme production, overexpression of efflux pumps, porin deficiencies, and target-site alterations. Multiple resistance genes frequently coexist in the same organism. Multidrug resistance in gram-negative nonfermenters makes treatment of infections caused by these pathogens both difficult and expensive. Improved methods for susceptibility testing are needed when dealing with these organisms, including emerging strains expressing metallo-beta-lactamases. Improved antibiotic stewardship and infection-control measures will be needed to prevent or slow the emergence and spread of multidrug-resistant, nonfermenting gram-negative bacilli in the healthcare setting.

The influence of observation and setting on community health workers' practices

OBJECTIVE

To determine whether results from an evaluation that involved observation of community health workers while they performed patient consultations in a hospital reflected normal everyday practices.

DESIGN

Comparison of two samples of ill-child consultations: (i) consultations performed during an evaluation in which we observed community health workers in a hospital in-patient and outpatient department from February to March 2001 and (ii) consultations performed under no observation in villages and documented in clinical registers within the 90 days before the hospital evaluation.

SETTING

Siaya District Hospital and villages in Kenya.

STUDY PARTICIPANTS

Community health workers.

MAIN OUTCOME MEASURE

Treatment error indicator, defined as the percentage of consultations where at least one recommended treatment (where recommended treatments were those that were indicated based on community health worker assessments of the child's condition) was not prescribed.

RESULTS

We analyzed data on 1132 consultations (372 from the hospital evaluation and 760 from the community) performed by 103 community health workers. For all types of consultations combined, the difference between treatment error indicators (hospital minus community) was -16.4 [95% confidence interval (CI): -25.6, -7.1].

CONCLUSIONS

We found that community health workers made treatment errors less frequently when they were observed in a hospital in-patient or outpatient department than when they were not observed in the community. Evaluations that involve the observation of community health workers in a hospital setting might overestimate the quality of care that they normally give in their villages.

Testing for induction of clindamycin resistance in erythromycin-resistant isolates of Staphylococcus aureus

Disk diffusion and broth microdilution (BMD) were used to perform clindamycin (CLI) induction testing on 128 selected nonduplicate isolates of Staphylococcus aureus. Disk diffusion testing involved placing CLI and erythromycin (ERY) disks approximately 12 mm apart (measured edge to edge) on a Mueller-Hinton agar plate that had been inoculated with an S. aureus isolate; the plate was then incubated for 16 to 18 h. Two distinct induction phenotypes (labeled D and D(+)) and four noninduction phenotypes (designated as negative [Neg], hazy D zone [HD], resistant [R], and susceptible [S]) were observed in disk diffusion results. A clear, D-shaped zone of inhibition around the CLI disk was designated as the D phenotype and was observed for 21 isolates while a D-shaped zone containing inner colonies growing up to the CLI disk was designated as D(+) (17 isolates). In addition, 10 isolates were CLI susceptible and ERY resistant but were not inducible and showed no blunting of the CLI zone (Neg phenotype). Isolates that were CLI and ERY resistant (constitutive macrolide-lincosamide-streptogramin B resistance) demonstrated either a double zone of inhibition with an inner ring of reduced growth up to the edge of the disks (HD phenotype; 33 isolates) or solid growth around the CLI and ERY disks (R phenotype; 16 isolates). Finally, 31 isolates were susceptible by disk testing to both CLI and ERY (S phenotype). PCR results showed that isolates with a D phenotype harbored ermA, isolates with a D(+) phenotype contained either ermC (16 isolates) or ermA and ermC (one isolate), and all 10 isolates with a Neg phenotype contained msrA. All isolates with an HD or R phenotype harbored at least one erm gene. Isolates showing the D(+) phenotype by disk diffusion were also detected by BMD using a variety of CLI and ERY concentrations; however, isolates with the D phenotype were more difficult to detect by BMD and will likely require optimization of ERY and CLI concentrations in multilaboratory studies to ensure adequate sensitivity. Thus, at present, disk diffusion is the preferred method for testing S. aureus isolates for inducible CLI resistance.

Experimental prediction of the evolution of ceftazidime resistance in the CTX-M-2 extended-spectrum beta-lactamase

We applied in vitro evolution to an Escherichia coli strain containing bla(CTX-M-2) and obtained 10 independent mutant bla(CTX-M-2) alleles that confer elevated resistance to ceftazidime (MIC > or = 32 microg/ml) but lost the ability to confer resistance to cefepime. All alleles had a Pro-to-Ser substitution at position 167.

Patterns of health care resource utilization after macrolide treatment failure: Results from a large, population-based cohort with acute sinusitis, acute bronchitis, and community-acquired pneumonia

BACKGROUND

Macrolide antibiotics are used as first-line therapy for the treatment of respiratory tract infections. The recent emergence of macrolide-resistant pathogens is a major concern.

OBJECTIVE

This study quantifies the frequency of macrolide treatment failure in respiratory infections and examines its impact on health care use.

METHODS

Patients with respiratory infections treated with macrolides in outpatient clinics from January to December 2002 were identified from a health insurance claims database. Macrolide treatment failure was defined as the receipt of a second antibiotic, different from the first, within 4 weeks after the initial macrolide. The end points were numbers of hospitalizations and emergency department and office visits within 1 month after the initial macrolide. We examined diagnostic codes on claim forms for posttreatment hospitalizations and visits to identify those most likely to be related to treatment failure as opposed to other causes. Utilization data were analyzed by Poisson regression to control for confounding variables.

RESULTS

The patients were divided into acute sinusitis (n = 111,135), acute bronchitis (n = 157,360), and community-acquired pneumonia (n = 36,212). Of these respective groups, 11,285 (10.2%), 15,498 (9.9%), and 4144 (11.4%) received a second antibiotic within 4 weeks. This subgroup with macrolide treatment failure was older, included more women, and had used more medical care before the index visit compared with patients with treatment success. After adjustment for age, sex, and previous health care use, patients experiencing treatment failure were more likely to be admitted to the hospital or to use emergency department or outpatient care after the index visit. This association was strongest for admissions and visits pertaining to the care of respiratory infections.

CONCLUSIONS

By our definition, about 10% of patients with respiratory infections who were treated with macrolide antibiotics experienced treatment failure within 4 weeks. Macrolide treatment failure was associated with increased health care utilization.

Debate—Guidelines for control of glycopeptide-resistant enterococci (GRE) have not yet worked

Glycopeptide-resistant enterococci (GRE) have become a focus of concern in many countries because options for antimicrobial therapy of GRE infection are limited. Several guidelines for the control and prevention of GRE colonization and infection have been developed for healthcare settings, and occasional journal articles now report "control" (usually relative reduction of incidence or prevalence rate rather than elimination) of GRE infections. Yet, rates of infection and colonization with GREcontinue to climb in many parts of the world, showing that true control has not been achieved. Programmes to control GRE will be effective only when they (1) are less expensive to implement; (2) are shown to be cost-effective despite the fact that they merely reduce prevalence levels rather than eradicating the problem; (3) do not require almost perfect implementation to be effective; (4) are shown to be sustainable; (5) are shown to work in acute-care settings other than selected academic centres; and (6) are shown to work in non-acute care settings. Until then, it is clear that guidelines for control of GRE have not worked. New guidelines that truly control GRE must be developed, and this must be done quickly.

Initial and follow-up costs by treatment outcome for children with respiratory infections

OBJECTIVE

To estimate the initial and follow-up costs of treatment of respiratory infections among pediatric patients and the relationship between costs and outcomes.

METHODS

A total of 3677 episodes of care from 2328 patients who were <17 years of age and had respiratory or ear infections that were treated with an antibiotic initially. The sample was drawn from the Medical Expenditure Panel Surveys for the years 1996 through 1999. Treatment failure was defined as the receipt of a second antibiotic, different from the first, in a 4-week window. We compared follow-up costs by outcome (treatment failure vs success) using a 2-part model of medical costs. We also performed a paired analysis by selecting 2 episodes, one in which the outcome was failure and the other in which the outcome was success, for patients with at least 1 of each type and computing the difference in costs.

RESULTS

Follow-up costs for provider visits for episodes for which the patient experienced treatment failure were 216 dollars versus 53 dollars for episodes for which the patient did not experience treatment failure. Follow-up drug costs, including the cost of the second antibiotic, were 75 dollars for children who experienced treatment failure versus 23 dollars, respectively. Cost estimates from the paired analysis were similar, confirming that results are not biased by unobserved time-invariant patient characteristics.

CONCLUSION

Children who have respiratory infections and experience treatment failure incur substantially higher costs.

Utility of NCCLS guidelines for identifying extended-spectrum beta-lactamases in non-Escherichia coli and Non-Klebsiella spp. of Enterobacteriaceae

NCCLS screening and confirmation methods for detecting extended-spectrum beta-lactamases (ESBLs) apply only to Escherichia coli and Klebsiella spp., yet ESBLs have been found in other members of the family Enterobacteriaceae. We evaluated the effectiveness of NCCLS methods for detecting ESBLs in 690 gram-negative isolates of Enterobacteriaceae that excluded E. coli, Klebsiella pneumoniae, and Klebsiella oxytoca. Isolates were collected between January 1996 and June 1999 from 53 U.S. hospitals participating in Project ICARE (Intensive Care Antimicrobial Resistance Epidemiology). The antimicrobial susceptibility patterns of the isolates were determined by using the NCCLS broth microdilution method (BMD), and those isolates for which the MIC of ceftazidime, cefotaxime, ceftriaxone, or aztreonam was >or=2 microg/ml or the MIC of cefpodoxime was >or=8 microg/ml (positive ESBL screen test) were further tested for a clavulanic acid (CA) effect by BMD and the disk diffusion method (confirmation tests). Although 355 (51.4%) of the isolates were ESBL screen test positive, only 15 (2.2%) showed a CA effect. Since 3 of the 15 isolates were already highly resistant to the five NCCLS indicator drugs, ESBL detection would have an impact on the reporting of only 1.7% of the isolates in the study. Only 6 of the 15 isolates that showed a CA effect contained a bla(TEM), bla(SHV), bla(CTX-M), or bla(OXA) beta-lactamase gene as determined by PCR (with a corresponding isoelectric focusing pattern). Extension of the NCCLS guidelines for ESBL detection to Enterobacteriaceae other than E. coli and Klebsiella spp. does not appear to be warranted in the United States at present, since the test has poor specificity for this population and would result in changes in categorical interpretations for only 1.7% of Enterobacteriaceae tested.

In VitroActivities of Daptomycin, Linezolid, and Quinupristin-Dalfopristin against a Challenge Panel of Staphylococci and Enterococci, Including Vancomycin-IntermediateStaphylococcus aureusand Vancomycin-ResistantEnterococcus faecium

We assessed the in vitro activities of daptomycin, linezolid, and quinupristin-dalfopristin (QD) against a contemporary challenge panel of 88 staphylococcal and 90 enterococcal isolates. The staphylococci selected included vancomycin-intermediate Staphylococcus aureus (VISA), methicillin-resistant S. aureus, and coagulasenegative staphylococci. Enterococcal isolates included vancomycin-resistant Enterococcus faecium (VREF) containing either vanA, vanB1, or vanD. The MICs of daptomycin, linezolid, and QD were determined using commercial broth microdilution panels. All three VISA isolates were susceptible to daptomycin, linezolid, and QD. QD was the most active agent against staphylococcal isolates (MIC50 < or = 0.5 microg/ml and MIC90 = 1 microg/ml), including those with decreased susceptibility to vancomycin. QD was also the most active agent against VREF (MIC90 < or = 0.5 microg/ml). No differences were seen for susceptibility of vanA, vanB1, and vanD VREF strains for daptomycin, linezolid, or QD. Daptomycin was the most effective against E. faecalis. On the basis of manufacturer-suggested interpretive criteria, 92% of isolates were susceptible (MIC90 = 4 microg/ml). All isolates tested were susceptible to at least one antimicrobial agent for which interpretive criteria have been defined. Population analysis of three S. aureus isolates for which the daptomycin MICs were 8 microg/ml showed a pattern of homogeneous resistance.

Evaluation of the NCCLS extended-spectrum beta-lactamase confirmation methods for Escherichia coli with isolates collected during Project ICARE

To determine whether confirmatory tests for extended-spectrum beta-lactamase (ESBL) production in Escherichia coli are necessary, we selected 131 E. coli isolates that met the National Committee for Clinical Laboratory Standards (NCCLS) screening criteria for potential ESBL production from the Project ICARE (Intensive Care Antimicrobial Resistance Epidemiology) strain collection. For all 131 isolates, the broth microdilution (BMD) MIC of at least one extended-spectrum cephalosporin was >/=2 micro g/ml. For 21 of 131 (16%) isolates, the ESBL confirmatory test was positive; i.e., the BMD MICs of ceftazidime or cefotaxime decreased by >/=3 doubling dilutions in the presence of clavulanic acid (CA) or the disk diffusion zone diameters increased by >/=5 mm around ceftazidime or cefotaxime disks in the presence of CA. All 21 isolates were shown by PCR to contain at least one of the genes bla(TEM), bla(SHV), and bla(OXA), and in isoelectric focusing (IEF) tests, all isolates demonstrated at least one beta-lactamase band consistent with a TEM, SHV, or OXA enzyme. Of the 21 isolates, 3 showed a CA effect for cefotaxime by BMD but not by disk diffusion testing. A total of 59 (45%) of the 131 isolates demonstrated decreased susceptibility to cefpodoxime alone (MIC = 2 to 4 micro g/ml), and none had a positive ESBL confirmatory test result. These were classified as false positives according to ESBL screen test results. For the remaining 51 (39%) isolates, the cefpodoxime MICs ranged from 16 to >128 micro g/ml and the MICs for the other extended-spectrum cephalosporins were highly variable. All 51 isolates gave negative ESBL confirmatory test results. Most showed IEF profiles consistent with production of both a TEM and an AmpC beta-lactamase, and representative isolates of several phenotypic groups showed changes in porin profiles; these 51 isolates were considered true negatives. In all, only 16% of 131 E. coli isolates identified as potential ESBL producers by the current NCCLS screening criteria were confirmed as ESBL producers. Thus, changing the interpretation of extended-spectrum cephalosporins and aztreonam results from the susceptible to the resistant category without confirming the presence of an ESBL phenotype would lead to a large percentage of false resistance results and is not recommended. However, by increasing the cefpodoxime MIC screening breakpoint to >/=8 micro g/ml, 45% of the false-positive results could be eliminated. NCCLS has incorporated this change in the cefpodoxime screening breakpoint in its recent documents.

Optimization of computer software settings improves accuracy of pulsed-field gel electrophoresis macrorestriction fragment pattern analysis

Computer-assisted analysis of pulsed-field gel electrophoresis (PFGE) libraries can facilitate comparisons of fragment patterns present on multiple gels. We evaluated the ability of the Advanced Analysis (version 4.01) and Database (version 1.12) modules of the Phoretix gel analysis software package (Nonlinear USA, Inc., Durham, N.C.) to accurately match DNA fragment patterns. Two gels containing 38 lanes of SmaI-digested Enterococcus faecalis OG1RF DNA were analyzed to assess the impact of (i) varying the lane position of the standards, (ii) using gel plugs made at different times, and (iii) normalizing the fragment patterns by using molecular weight (MW) algorithms versus retardation factor (R(f)) algorithms. Two sets of PFGE libraries (one containing SmaI restriction patterns from 62 Enterococcus faecium isolates and the other containing SmaI restriction patterns of 89 Staphylococcus aureus isolates) were analyzed to assess the impact of varying the matching tolerance algorithm (designated as the vector box setting [VBS]) in the Phoretix software. Varying the lane position of standards on a gel and using gel plugs made on different days resulted in different VBSs, although it was not possible to judge whether those differences were statistically significant. Normalization of E. faecalis OG1RF fragment patterns by R(f) and MW methodology yielded no statistically significant differences in variability between the same fragment on different lanes. Suboptimal VBSs decreased the specificity with which related isolates were grouped together in dendrograms. The optimal VBS for analysis of PFGE fragment patterns from E. faecalis isolates differed from that for S. aureus isolates and sometimes was not that recommended by the manufacturer. Thus, computer-assisted analysis of PFGE patterns seemed to compensate for the intra- and intergel variation evaluated in the present study, and optimizing the software for the species to be tested was a critical preliminary step before further PFGE library analysis.

Antimicrobial susceptibility testing of carbapenems: multicenter validity testing and accuracy levels of five antimicrobial test methods for detecting resistance in Enterobacteriaceae and Pseudomonas aeruginosa isolates

From January 1996 to May 1999, Project ICARE (Intensive Care Antimicrobial Resistance Epidemiology) received 448 nonduplicate clinical isolates of Enterobacteriaceae and Pseudomonas aeruginosa that were reported to be imipenem intermediate or resistant. However, broth microdilution (BMD) confirmatory testing at the Project ICARE central laboratory confirmed this result in only 11 of 123 (8.9%) Enterobacteriaceae isolates and 241 of 325 (74.2%) P. aeruginosa isolates. To investigate this overdetection of imipenem resistance, we tested 204 selected isolates from the Project ICARE collection plus five imipenem-resistant challenge strains at the Centers for Disease Control and Prevention against imipenem and meropenem by agar dilution, disk diffusion, Etest (AB BIODISK North America, Inc., Piscataway, N.J.), two MicroScan WalkAway conventional panels (Neg MIC Plus 3 and Neg Urine Combo 3) (Dade MicroScan, Inc., West Sacramento, Calif.), and two Vitek cards (GNS-116 containing meropenem and GNS-F7 containing imipenem) (bioMérieux Vitek, Inc., Durham, N.C.). The results of each test method were compared to the results of BMD testing using in-house-prepared panels. Seven imipenem-resistant and five meropenem-resistant isolates of Enterobacteriaceae and 43 imipenem-resistant and 21 meropenem-resistant isolates of P. aeruginosa were identified by BMD. For Enterobacteriaceae, the imipenem and meropenem test methods produced low numbers of very major and major errors. All test systems in the study produced low numbers of very major and major errors when P. aeruginosa was tested against imipenem and meropenem, except for Vitek testing (major error rate for imipenem, 20%). Further testing conducted in 11 of the participating ICARE hospital laboratories failed to pinpoint the factors responsible for the initial overdetection of imipenem resistance. However, this study demonstrated that carbapenem testing difficulties do exist and that laboratories should consider using a second, independent antimicrobial susceptibility testing method to validate carbapenem-intermediate and -resistant results.

Mechanisms of decreased susceptibility to cefpodoxime in Escherichia coli

Cefpodoxime is one of five antimicrobial agents recommended by the National Committee for Clinical Laboratory Standards for screening isolates of Klebsiella spp. and Escherichia coli for extended-spectrum beta-lactamase (ESBL) production. In a prior study, we noted that among 131 E. coli isolates for which the MIC of at least one extended-spectrum cephalosporin (ESC) or aztreonam was > or =2 micro g/ml (suggesting the presence of ESBL production), there were 59 isolates (45.0%) for which the MIC of cefpodoxime was 2 to 4 micro g/ml (i.e., a positive ESBL screening test), but the MICs of ceftazidime, cefotaxime, and ceftriaxone were < or =1 micro g/ml (below the ESBL screening breakpoint). Thus, the results appeared to be false-positive ESBL screening tests. These 59 isolates were divided into five phenotypic groups based on the susceptibility patterns of the organisms to a variety of beta-lactam agents and further characterized. The first group (32 isolates) all produced a TEM-1 beta-lactamase, and changes in the major outer membrane proteins were detected in representative strains. The second group (18 isolates) lacked bla(TEM) but showed a number of porin changes; some also showed a modest elevation in production of the AmpC chromosomal beta-lactamase. In the third phenotypic group (seven isolates) all expressed an OXA-30 beta-lactamase. Some also harbored altered porins. The two remaining phenotypes each had a distinct pattern of porin changes with or without beta-lactamase production. These data indicate that several factors are associated with decreased susceptibility to cefpodoxime in E. coli, but none of the mechanisms are related to ESBL production. Current screening methods produced false-positive ESBL results for these isolates. Such isolates should not be classified as containing ESBLs, nor should interpretations of ESCs or aztreonam susceptibility be changed to resistant on test reports for these isolates.

Carbapenem resistance in a clinical isolate of Enterobacter aerogenes is associated with decreased expression of OmpF and OmpC porin analogs

We investigated the mechanism of imipenem resistance in Enterobacter aerogenes strain 810, a clinical isolate from the United States for which the imipenem MIC was 16 micro g/ml and the meropenem MIC was 8 micro g/ml. An imipenem-susceptible revertant, strain 810-REV, was obtained after multiple passages of the strain on nonselective media. For the revertant, the imipenem MIC was /=128 micro g/ml), cefoxitin (>/=32 micro g/ml), and cefotaxime (>/=64 micro g/ml) remained the same. The beta-lactamase and porin profiles of the parent, the revertant, and carbapenem-susceptible type strain E. aerogenes ATCC 13048 were determined. Strains 810 and 810-REV each produced two beta-lactamases with pIs of 8.2 and 5.4. The beta-lactamase activities of the parent and revertant were similar, even after induction with subinhibitory concentrations of imipenem. While 810-REV produced two major outer membrane proteins of 42 and 39 kDa that corresponded to Escherichia coli porins OmpC and OmpF, respectively, the parent strain appeared to produce similar quantities of the 39-kDa protein (OmpF) but decreased amounts of the 42-kDa protein (OmpC). When the parent strain was grown in the presence of imipenem, the 42-kDa protein was not detectable by gel electrophoresis. However, Western blot analysis of the outer membrane proteins of the parent and revertant with polyclonal antisera raised to the OmpC and OmpF analogs of Klebsiella pneumoniae (anti-OmpK36 and anti-OmpK35, respectively) showed that strain 810 expressed only the 42-kDa OmpC analog in the absence of imipenem (the 39-kDa protein was not recognized by the anti-OmpF antisera) and neither the OmpC nor the OmpF analog in the presence of imipenem. The OmpC analog is apparently down-regulated in the presence of imipenem; however, 810-REV expressed both OmpC and OmpF analogs. These data suggest that imipenem resistance in E. aerogenes 810 is primarily associated with the lack of expression of the analogs of the OmpC (42-kDa) and OmpF (39-kDa) outer membrane proteins, which also results in decreased susceptibility to meropenem and cefepime.

Does antimicrobial resistance cluster in individual hospitals?

Factors that affect the resistance rates for an organism-drug combination in a given hospital also might influence resistance rates for other organism-drug combinations. We examined correlations between resistance prevalence in non-intensive care inpatient areas of 41 hospitals participating in phase 3 (1998-1999) of Project ICARE (Intensive Care Antimicrobial Resistance Epidemiology). We focused on statistically significant (P<.05) Pearson correlation coefficients for methicillin-resistant Staphylococcus aureus, coagulase-negative staphylococci, vancomycin-resistant enterococci, and resistance to third-generation cephalosporins, imipenem, and fluoroquinolones in Escherichia coli, Klebsiella pneumoniae, Enterobacter species, and Pseudomonas aeruginosa. Resistance prevalence rates in individual hospitals were not strongly correlated among gram-positive organisms, and few correlations were seen between rates in gram-positive and gram-negative organisms. More frequent significant associations were found among resistance rates for gram-negative organisms. Resistance to third-generation cephalosporins in K. pneumoniae was significantly correlated with the majority of other sentinel antimicrobial-resistant organisms. High prevalence of this organism may serve as a marker for more generalized resistance problems in hospital inpatient areas.

Temporal changes in prevalence of antimicrobial resistance in 23 US hospitals

Antimicrobial resistance is increasing in nearly all health-care-associated pathogens. We examined changes in resistance prevalence during 1996-1999 in 23 hospitals by using two statistical methods. When the traditional chi-square test of pooled mean resistance prevalence was used, most organisms appear to have increased in prevalence. However, when a more conservative test that accounts for changes within individual hospitals was used, significant increases in prevalence of resistance were consistently observed only for oxacillin-resistant Staphylococcus aureus, ciprofloxacin-resistant Pseudomonas aeruginosa, and ciprofloxacin- or ofloxacin-resistant Escherichia coli. These increases were significant only in isolates from patients outside intensive-care units (ICU). The increases seen are of concern; differences in factors present outside ICUs, such as excessive quinolone use or inadequate infection-control practices, may explain the observed trends.

Monitoring antimicrobial use and resistance: comparison with a national benchmark on reducing vancomycin use and vancomycin-resistant enterococci

To determine if local monitoring data on vancomycin use directed quality improvement and decreased vancomycin use or vancomycin-resistant enterococci (VRE), we analyzed data from 50 intensive-care units (ICUs) at 20 U.S. hospitals reporting data on antimicrobial-resistant organisms and antimicrobial agent use. We compared local data with national benchmark data (aggregated from all study hospitals). After data were adjusted for changes in prevalence of methicillin-resistant Staphylococcus aureus, changes in specific prescriber practice at ICUs were associated with significant decreases in vancomycin use (mean decrease -48 defined daily doses per 1,000 patient days, p<0.001). These ICUs also reported significant decreases in VRE prevalence compared with those not using unit-specific changes in practice (mean decrease of 7.5% compared with mean increase of 5.7%, p<0.001). In this study, practice changes focused towards specific ICUs were associated with decreases in ICU vancomycin use and VRE prevalence.

Validation of Vitek version 7.01 software for testing staphylococci against vancomycin

We tested 143 isolates of staphylococci with vancomycin by the National Committee for Clinical Laboratory Standards broth microdilution (BMD) reference method and compared the results to those generated using the Vitek automated system (GPS-105 and GPS-107 cards and version 7.01 software). For ten isolates, the vancomycin MICs by BMD were 8 microg/ml. By Vitek, the vancomycin MICs ranged from 2 to 16 microg/ml. Vancomycin MICs of > or =32 microg/ml were reported for two additional isolates by Vitek; however, the MICs decreased to < or =0.5 microg/ml on retesting. By BMD, the vancomycin MICs for both isolates were 1 microg/ml. While the modal vancomycin MIC results by BMD for S. aureus and coagulase-negative staphylococci (CoNS) were both 1 microg/ml, Vitek results showed a mode of < or =0.5 microg/ml for S. aureus, and a mode of 2 microg/ml for CoNS. Vitek did not report vancomycin MICs of 1 or 4 microg/ml for any of the isolates tested. While the sensitivity of detecting staphylococci with reduced susceptibility to vancomycin appears to be improved with Vitek version 7.01 software, when compared to earlier software versions, laboratories may notice an overall shift in MIC data toward higher vancomycin MICs, although for the most part, this does not affect the categorical interpretations of the results.

Risk factors for methicillin-resistance among patients with Staphylococcus aureus bacteremia at the time of hospital admission

BACKGROUND

Community-acquired infections caused by methicillin-resistant Staphylococcus aureus (MRSA) seem to be increasing. Characteristics permitting recognition of patients with such strains would aid infection control efforts and choice of empiric therapy pending culture and susceptibility results.

METHODS

Retrospective review of medical records for all adults seen in the Emergency Care Center at Grady Memorial Hospital, Atlanta, Georgia, whose blood cultures taken within 24 hours of entry yielded S. aureus. Risk factors for the presence of methicillin resistance in S. aureus isolates recovered from patients with staphylococcal bacteremia were assessed.

RESULTS

S. aureus isolates from 118 (40%) of 297 study patients with bacteremia at the time of admission were methicillin-resistant. Multivariate analysis identified hospitalization in the 6 months preceding admission [odds ratio (OR) = 4.4; 95% CI, 2.0-9.8], receipt of antimicrobial agents in the past 3 months (OR = 5.6; 95% CI, 2.6-11.9], presence of indwelling urinary catheter (OR = 7.3; CI, 2.5-20.9), and nursing home residence (OR = 9.9; 95% CI, 3.9-25.6) to be independently associated with the presence of methicillin resistance. All but 4 of the 118 patients with methicillin-resistant strains had at least 1 of these factors and the proportion of resistant isolates progressively increased as more of these features were present.

CONCLUSIONS

The presence of these risk factors should be considered when making decisions about isolation and other infection control procedures as well as empiric antimicrobial therapy with vancomycin for patients with suspected staphylococcal infection at the time of hospital admission. Similar studies could guide practices for dealing with such patients in other centers, because the occurrence of MRSA infections at the time of admission may vary widely by geographic area.

Epidemiology and Microbiology of Hospital-Acquired Pneumonia

Hospital-acquired pneumonia (HAP) is the second most common nosocomial infection in the critically ill patient and is associated with the greatest mortality and increased morbidity and cost of care. The major risk factor for the development of HAP in intensive care is the occurrence of intubation and mechanical ventilation, giving rise to the term ventilator-associated pneumonia (VAP). Incidence of VAP varies in different populations of critically ill patients and generally ranges from 9 to 20%, with an overall rate of 10 to 15 cases per 1,000 ventilator days. The cumulative risk of developing VAP is ~1% per day of mechanical ventilation (MV). The crude mortality rate of VAP is 60% and the estimates of attributable risk range from 27 to 43%. Mortality from VAP is influenced by host factors, the virulence of the pathogens, and the adequacy of initial antimicrobial therapy. The etiologic agents for VAP differ according to the population studied, duration of hospital stay, time after intubation, and prior antimicrobial therapy. Risk factors include nonmodifiable factors like age, chronic obstructive pulmonary disease, severe head trauma, and multiple trauma, and modifiable factors like large volume gastric aspiration, duration of MV, elevated gastric pH, histamine type 2 blocker therapy, ventilator circuit change frequency, self-extubation, and reintubation. The impact that diagnosis using invasive diagnostic techniques may have on the epidemiological characteristics of VAP are unknown, but may potentially reduce problems resulting from misclassification of this entity.