A cluster of six respiratory cultures positive for Mycobacterium xenopi -Clinical characteristics and genomic characterization

Mycobacterium xenopi is a slow growing non-tuberculous mycobacterium (NTM) isolated from water systems and has been associated with pseudo-outbreaks and pulmonary infections in humans. We observed a cluster of six respiratory cultures positive for M. xenopi within a six-month period at our institution, approximately double our normal isolation rate of this organism. Only three of the six cases met clinical, radiographic, and microbiologic criteria for NTM infection. An investigation led by our hospital's Healthcare Epidemiology and Infection Program found no epidemiologic link between the six patients. Three isolates underwent whole-genome sequencing (WGS) and phylogenetic analysis confirmed they were non-clonal. In vitro susceptibility data found the isolates were sensitive to macrolides, moxifloxacin, and rifabutin. Our findings suggest that isolation of M. xenopi from pulmonary specimens may be increasing, further defines the genomic population structure of this potentially emerging infection, and establishes WGS as a useful tool for outbreak investigation strain typing.

Carbapenem-resistant Enterobacterales standardization across a large health care system

Carbapenem-resistant Enterobacterales (CRE) are multidrug resistant organisms that pose a significant risk in the health care setting. Standardized identification ensures prompt isolation and is imperative to maintain patient safety.

No difference in anti-spike antibody and surrogate viral neutralization following SARS-CoV-2 booster vaccination in persons with HIV compared to controls (CO-HIV Study)

Background

Understanding the immune response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination will enable accurate counseling and inform evolving vaccination strategies. Little is known about antibody response following booster vaccination in people living with HIV (PLWH).

Methods

We enrolled SARS-CoV-2 vaccinated PLWH and controls without HIV in similar proportions based on age and comorbidities. Participants completed surveys on prior SARS-CoV-2 infection, vaccination, and comorbidities, and provided self-collected dried blood spots (DBS). Quantitative anti-spike IgG and surrogate viral neutralization assays targeted wild-type (WT), Delta, and Omicron variants. We also measured quantitative anti-nucleocapsid IgG. The analysis population had received full SARS-CoV-2 vaccination plus one booster dose. Bivariate analyses for continuous outcomes utilized Wilcoxon tests and multivariate analysis used linear models.

Results

The analysis population comprised 140 PLWH and 75 controls with median age 58 and 55 years, males 95% and 43%, and DBS collection on 112 and 109 days after the last booster dose, respectively. Median CD4 count among PLWH was 760 cells/mm3 and 91% had an undetectable HIV-1 viral load. Considering WT, Delta, and Omicron variants, there was no significant difference in mean quantitative anti-spike IgG between PLWH (3.3, 2.9, 1.8) and controls (3.3, 2.9, 1.8), respectively (p-values=0. 771, 0.920, 0.708). Surrogate viral neutralization responses were similar in PLWH (1.0, 0.9, and 0.4) and controls (1.0, 0.9, 0.5), respectively (p-values=0.594, 0.436, 0.706).

Conclusions

PLWH whose CD4 counts are well preserved and persons without HIV have similar anti-spike IgG antibody levels and viral neutralization responses after a single SARS-CoV-2 booster vaccination.

Development of a workflow for the detection of vancomycin-resistant Enterococcus faecium and Enterococcus faecalis from rectal swabs using the spectra VRE medium

BACKGROUND

Spectra™ VRE agar (Remel, Lenexa, KS) is a chromogenic agar that is FDA approved for screening patients for VRE colonization. The package insert recommends confirming isolates with identification and susceptibility testing, but confirming every culture delays time to result. Given the agar's historic high specificity for E. faecium isolates, we theorized the agar could be utilized as a stand-alone screening to minimize reagents and time.

AIM

Our laboratory sought to develop a workflow to optimize the use of the medium.

METHODS

We plated 3,815 rectal swabs to the Spectra VRE agar and compared results to traditional identification and susceptibility testing.

RESULTS

Dark blue or purple colonies on the agar demonstrated a sensitivity of 98% and specificity of 85% for detection of VRE faecium, but light blue colonies were significantly less specific for E. faecalis.

CONCLUSIONS

We streamlined our workflow to accept dark blue or purple colonies as VRE faecium and plan to perform additional testing only on light blue colonies. Interestingly, higher quantity of growth increased the accuracy of the agar. In the future, growth quantity may be used to further streamline the workflow once more data is obtained.

1216. An Outbreak Investigation of Pulmonary Infections Caused by Mycobacterium abscessus in a Cardiothoracic Transplant Population

Background

Nontuberculous mycobacteria (NTM) are environmental organisms that can form biofilms in municipal water systems and as such are difficult to eliminate. Mycobacterium abscessus is a rapid-growing NTM that can cause skin and soft tissue, disseminated, and pulmonary infections. M. abscessus is difficult to treat, often requiring prolonged therapy with several antibiotics due to its intrinsic drug resistance. In 2021, our institution identified a significant increase in pulmonary infections caused by M. abscessus in the cardiothoracic transplant population.

Methods

All M. abscessus cases among inpatients at our institution were extracted from the electronic medical record (EMR) between January 2019 and September 2021. Clinical characteristics were determined through EMR review and included demographics, transplant status, specimen type, COVID-19 history, and patient care practices involving water. A multidisciplinary team conducted an investigation to identify possible variations in practice related to the source of water used for clinical care activities in this identified population.

Results

Between January 2021 and September 2021, there were 12 cases of M. abscessus among inpatients at our institution compared to 6 cases in 2019 and 5 in 2020 (Figure 1).

Between 2019 and 2020, post-heart and pre-/post-lung transplant patients comprised 9% of cases, 55% of cases were pulmonary infections, and none had a history of COVID-19 infection. In 2021, post-heart and pre-/post-lung transplant patients comprised 58% of cases, 83% of cases were pulmonary infections, and 33% of cases had a history of COVID-19 infection.

There were varying sources of water utilized for the clinical care activities in this identified population (Table 1). Figure 1Mycobacterium abscessus epidemic curveTable 1Patient Care Practices Involving Water

Conclusion

To investigate the potential outbreak, we are actively collecting water samples and swabs from water fixtures in both patient and nourishment rooms for water culturing. To mitigate a potential water-borne source, we will use sterile water for all clinical care practices involving water and for all patient water drinking needs in the post-heart and pre-/post-lung transplant population impacted by the outbreak. The only use of tap water is hand hygiene and patient bathing.

Disclosures

Asra Salim, MPH, CPH, FAPIC, IRhythym Technologies Inc: Stocks/Bonds Valentina Stosor, MD, DiaSorin: Advisor/Consultant|Eli Lilly and Company: Grant/Research Support|Med Learning Group: Honoraria Michael P. Angarone, DO, Abbvie: Advisor/Consultant Michael G. Ison, MD MS, GlaxoSmithKlein: Grant/Research Support|Pulmocide: Grant/Research Support|Viracor Eurfins: Advisor/Consultant Janna L. Williams, MD, Abbvie: COVID19 Infection Prevention Consultant.

2356. Standardizing a centralized allocation process for rarely used anti-infective medications across a health system

Background

Obtaining scarce antimicrobials used to treat infrequently encountered infections such as malaria have posed a challenge to clinicians and health systems. Due to the rare occurrence of these infections, maintaining adequate, readily available supply for emergent need is challenging, and a lack of clear procurement processes from regional storage sites may contribute to delays in care. In March 2021, artesunate became commercially available for the treatment of severe malaria, a disease for which prompt initiation of therapy is paramount. To improve access to this and other critical but seldom used medications, a centralized storage and allocation process was established throughout our 10-hospital health system. We aim to describe the structure of this program which may serve as a model for other institutions to expedite and streamline access to these lifesaving therapies.

Tracking document for therapy supply management.

Methods

In May 2021, a centralized process was created to track and store rare anti-infective medications for uncommon infections, including those caused by tropical, parasitic, and mycobacterial pathogens (Table 1). Supply is monitored and maintained by clinical pharmacists monthly to ensure adequate courses are available. When needed for a specific patient, infectious diseases physicians and pharmacists recommend therapy which triggers a request to the central pharmacy.

Results

Medications are stored at our large academic medical center (AMC), which manages these cases more routinely compared to other sites. Community hospitals within the health system are able to contact the AMC for prompt distribution of these medications when needed. Since its inception, we have deployed rare use anti-infective medications to partner facilities twice for severe malaria cases, which resulted in expedited receipt of therapy for critically ill patients. The time from therapy recommendation to medication administration was reduced from 12–24 hours to 2–6 hours.

Conclusion

This centralized rare anti-infective medication process expedites and streamlines access for critical therapeutic agents to ensure they are readily available when needed by limiting procurement/storage concerns. This model may be used as a framework for other medications for which timely administration, but rare use, is critical to patient care.

Disclosures

All Authors: No reported disclosures.

967. Safety Outcomes of a Hospital-wide β-Lactam Graded Challenge Allergy Protocol

Background

Up to 15% of hospitalized patients report a penicillin allergy, but most can tolerate a β-lactam (BL). Use of second-line non-BL antibiotics poses untoward health consequences. Assessing for true penicillin allergies includes penicillin skin testing (PST) and a direct amoxicillin challenge, each with their own limitations in hospitalized patients. For inpatients, a graded challenge (GC) with the desired BL offers a streamlined approach for many who could benefit from a first-line BL agent. In 2019, a hospital-wide BL Allergy Assessment and Clinical Pathway was implemented, including a BL GC protocol for low-risk patients. This study aimed to assess the safety of the GC procedure in hospitalized patients.

Methods

A retrospective, observational cohort study of adult inpatients who completed a GC procedure between Sept 2019 – Sept 2021 was conducted. Primary objective: determine the incidence of a significant hypersensitivity reaction from a BL GC. Secondary objectives: 1) determine the incidence of a non-hypersensitivity reaction and 2) identify antimicrobial stewardship interventions in those tolerant to a BL GC (e.g. antibiotic switch to a BL with completion of treatment course and de-labeling of penicillin allergy).

Results

Fifty-one patients completed a GC procedure. Most (78%) had a reported allergy to penicillin vs. other BL antibiotics. Reported allergic reactions stratified by risk for a serious hypersensitivity episode were: low/reaction > 10 yrs ago – 33%; moderate-high/reaction > 10 yrs ago – 47%, and moderate-high/reaction < 5 yrs ago – 19.6%. A PST prior to a GC was performed in 75% of patients. Common GC agents included amoxicillin-clavulanate (16%), amoxicillin (12%) and ceftriaxone (12%). One patient experienced a hypersensitivity reaction managed with diphenhydramine and another patient had a non-hypersensitivity reaction. Both cases were non-life-threatening. Of the 49 patients who tolerated a GC, 76% were switched to a BL with completion of treatment course, and 84% of documented penicillin allergies were de-labeled or clarified in the electronic medical record.

Conclusion

The GC procedure in our hospitalized patients was generally well tolerated including those with an allergy history concerning for a moderate-high risk hypersensitivity reaction.

Disclosures

Nathaniel J. Rhodes, PharmD, MSc, American Academy of Colleges of Pharmacy: Grant/Research Support|Paratek: Grant/Research Support|Third Pole Therapeutics: Advisor/Consultant Marc H. Scheetz, PharmD, MSc, Abbvie: Advisor/Consultant|Allecra: Grant/Research Support|Merck: Advisor/Consultant|Nevakar: Advisor/Consultant|Nevakar: Grant/Research Support|Premier Healthcare Solutions: Honoraria|Spero: Advisor/Consultant|SuperTrans Medical: Advisor/Consultant|SuperTrans Medical: Grant/Research Support|Takeda: Advisor/Consultant|Third Pole Therapeutics: Advisor/Consultant.

180. Impact of a two-step diagnostic bundle on hospital-onset Clostridioides difficile infection rates and treatment across a large health system

Background

Distinguishing true Clostridioides difficile Infection (CDI) from colonization is a challenge, with nearly 20% of hospitalized adults being carriers of C. difficile. Polymerase chain reaction (PCR) testing alone is not able to distinguish colonization from infection, leading to over-diagnosis and unnecessary treatment. Despite interventions including pre-approval by antimicrobial stewardship programs (ASP), reportable hospital-onset CDI (HO-CDI) rates across our health system remained high. In 2021, we implemented a C. difficile PCR with reflex toxin enzyme immunoassay (EIA) testing strategy to improve diagnostic accuracy and treatment outcomes. The purpose of this study was to evaluate the impact of this two-step testing algorithm bundled with education, ASP support and order set changes on HO-CDI rates and C. difficile treatment across our health system.

Methods

PCR with EIA testing algorithm was implemented between May and August 2021 across seven hospitals within the Northwestern Medicine Health System. Multifaceted education was delivered to leadership and clinicians in person and electronically to. ASP performed daily diagnostic prospective audit and management support. Clinical decision support (CDS) was incorporated into order sets to promote diagnostic stewardship (Table 1, Figure 1). Standardization of analyst-developed tracking reports allowed for longitudinal monitoring across the system and at each facility, including unit- and patient-level data. Figure 1

Molecular detection, not extended culture incubation, contributes to diagnosis of fungal infection

Background

Despite its low sensitivity, fungal culture remains one of the key methods for diagnosis and treatment of fungal infections, as it identifies the etiology at the genus and species level and affords the opportunity for susceptibility testing. The Manual of Clinical Microbiology recommends that fungal culture screening for all pathogens should routinely be held for 4 weeks to maximize the recovery of slow-growing species. Information on the optimal fungal culture time in this era of expansion of immunocompromised populations and availability of molecular diagnostics is lacking. We reviewed our experience with fungal culture to determine the optimal culture incubation time. In addition, our experience of broad-range ITS PCR for diagnosis of culture-negative fungal infections was also reviewed.

Methods

Fungal culture and ITS PCR results from January 1, 2013, to December 31, 2017, were reviewed.

Results

This study included 4234 non-duplicated positive cultures. Ninety-six percent (4058) of the positive cultures were detected in the first 7 days of incubation. During the second week of incubation, 111 (2.8%) positives were detected from day 8 to day 10, and 71 (1.7%) were detected from day 11 to day 14. Only 6 (0.1%) positive cultures were detected in the third week of incubation, and no positive culture was detected in the fourth week of incubation. No clinically significant fungal isolates were recovered after 14 days. Clinically significant pathogens were detected in 16 (0.2%) culture-negative samples by ITS PCR.

Conclusion

Extending culture incubation beyond 2 weeks did not generate clinically relevant results. When culture failed to make a laboratory diagnosis, broad-range internal transcribed spacer (ITS) rRNA gene PCR followed by sequencing produced clinically significant results.

The value of repeat patient testing for SARS-CoV-2: real-world experience during the first wave

INTRODUCTION

Reports of false-negative quantitative reverse transcription PCR (RT-qPCR) results from patients with high clinical suspension for coronavirus disease 2019 (COVID-19), suggested that a negative result produced by a nucleic acid amplification assays (NAAs) did not always exclude the possibility of COVID-19 infection. Repeat testing has been used by clinicians as a strategy in an to attempt to improve laboratory diagnosis of COVID-19 and overcome false-negative results in particular.

AIM

To investigate whether repeat testing is helpful for overcoming false-negative results.

METHODS

We retrospectively reviewed our experience with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) testing, focusing on the yield of repeat patient testing for improving SARS-CoV-2 detection by NAA.

RESULTS

We found that the yield from using repeat testing to identify false-negative patients was low. When the first test produced a negative result, only 6 % of patients tested positive by the second test. The yield decreased to 1.7 and then 0 % after the third and fourth tests, respectively. When comparing the results produced by three assays, the Centers for Disease Control and Prevention (CDC) SARS CoV-2 RT-qPCR panel, Xpert Xpress CoV-2 and ID NOW COVID-19, the ID NOW assay was associated with the highest number of patients who tested negative initially but positive on repeat testing. The CDC SARS CoV-2 RT-qPCR panel produced the highest number of indeterminate results. Repeat testing resolved more than 90 % of indeterminate/invalid results.

CONCLUSIONS

The yield from using repeat testing to identify false-negative patients was low. Repeat testing was best used for resolving indeterminate/invalid results.

Diagnostic performance of Ion 16S metagenomics kit and Ion reporter metagenomics workflow for bacterial pathogen detection in culture-negative clinical specimens from sterile sources

PCR-based deep sequencing of 16S rRNA gene allows for detection of a wide array of bacterial pathogens in culture-negative specimens. Ion 16S metagenomics kit and Ion Reporter metagenomics workflow (Ion 16S mNGS) provides an end-to-end solution with integrated workflow. Ninety-eight clinical samples with the diagnosis generated with 16S rRNA gene PCR/chain termination (Sanger) sequencing (16S CS) was used to assess the performance of Ion 16S mNGS. Compared to species level detection of 16S CS, the Ion 16S mNGS had 88% sensitivity and 76% specificity. When accounting for genus level of detection, the Ion 16S mNGS had 100% sensitivity. Notably, Ion 16S mNGS generated diagnosis in 13% of 16S CS and culture-negative samples. In addition, Ion 16S mNGS had the advantage of detecting more than 1 pathogen in 16S CS positive samples. We showed that the workflow had high reproducibility.

1674. Epidemiology of Urinary Tract Infections in the Renal Transplant Population in a Large Urban Midwestern Hospital: A Retrospective Cohort Study

Background

Renal Transplant Recipients (RTR) are at high risk for Urinary Tract Infections (UTIs). However, the best empiric option for treatment is not well defined, because the prevalence of extended spectrum beta-lactamase (ESBL) Enterobacteriaceae and carbapenem resistant Enterobacteriaceae (CRE). The primary purpose of this study was to describe the prevalence of multidrug resistant UTIs in a contemporary cohort of RTR at a large Midwestern tertiary care hospital. Secondary outcomes evaluated frequency of key symptoms and physical exam findings, as well as characteristics of patients who developed multidrug resistant organisms, morbidity, and mortality.

Methods

This was a single-center retrospective cohort study. Patients were included if they were 18 years or older and underwent their transplant between July 11, 2019 and November 26, 2018. Statistical analysis was performed using Fischer’s Exact T-test for comparison of the patients with and without ESBL UTIs. Significance was defined by a p< 0.05

Results

Two hundred fifty-two patients were evaluated (median age 54.4, 38.3% female gender), 36 patients developed UTIs, and no patients had CRE organisms. ESBL UTI prevalence was 7/252 (2.8%) among the total RTR population and 7/36 (19.4%) among the population of RTR who developed UTIs. Mortality rates did not differ significantly between patients with and without ESBL UTI (0% and 6.9%, respectively, p=.489) (Table 1). Additional clinical characteristics of the patients that developed ESBL UTIs were also obtained (Table 2).

Table 1: Comparison of non-ESBL UTI and ESBL UTI

Table 2: Characteristics of Patients with ESBL UTI

Conclusion

Prevalence of ESBL UTIs among RTR are low at a tertiary Midwestern hospital.

Disclosures

All Authors: No reported disclosures

864. Whole Genome Sequencing is Unable to Track Candida auris Transmission

Background

Candida auris (C. auris), an emerging yeast species, is often drug-resistant and has caused outbreaks in healthcare settings. Surging C. auris cases at our institution prompted whole genome sequencing (WGS) of patient and environmental specimens and comparison to local and international isolates.

Methods

WGS was performed on clinical and environmental isolates obtained from Northwestern Memorial Hospital (NMH) from June 2018 to December 2019. Genome sequences were compared against isolates from other institutions in the Chicagoland area obtained from a reference lab (ACL) and from the CDC. Two isolates underwent long-read sequencing on the Oxford Nanopore GridION platform to obtain closed genomes. WGS was performed on the remaining isolates with the Illumina MiSeq platform.

Results

Twenty isolates from NMH, five from ACL, and two from the CDC underwent WGS to yield 12.6 Mb genomes. Any two NMH isolates differed from each other by a maximum of 36 single nucleotide variants (SNV) (Figure 1). Two patients thought to be part of a transmission cluster (isolates CA06 and CA07), differed by 7 SNVs. No phylogenetic grouping between hospital systems across Chicagoland was observed. Isolates from room surfaces from a C. auris patient differed by 1-6 SNVs from each other and from 7-8 SNVs from the patient isolate. Samples taken from different body sites of another patient differed by 4-9 SNVs. Average SNV counts were lower among nosocomially acquired cases when compared to C. auris isolates present on admission (Figure 2). All NMH isolates were fluconazole sensitive, but a fluconazole resistant ACL isolate differed from a sensitive NMH isolate by only 4 SNVs.

Figure 1: Phylogenetic tree of all NMH and ACL isolates with fluconazole sensitivities

Figure 2: Observed pairwise SNP differences between nosocomial and POA strains

Conclusion

WGS of C. auris did not reveal identical isolates in any instance, even from the same patient or the patients and their environment. Generally, lower numbers of SNVs were observed for intra- versus inter-institutional isolates. More work is needed to determine the use, if any, of WGS in outbreak investigations.

Disclosures

All Authors: No reported disclosures

584. Ventricular assist device infections with Pseudomonas aeruginosa

Background

Infection is a leading cause of morbidity and mortality in ventricular assist device (VAD) recipients. Pseudomonas aeruginosa (PA) is the second most common organism implicated in VAD infections, occurring in 10–50% of infections. The epidemiology of VAD recipients with PA infection are poorly described.

Methods

We identified patients (pts) at Northwestern Memorial Hospital with a VAD-specific PA infection from January 1, 2012 to Dec 31, 2019. VADs included the Heartmate II, Heartmate 3, and Heartware HVAD devices. VAD-specific infections were defined according to the 2013 ISHLT Guidelines.

Results

Seventeen out of 91 (18.7%) VAD infections were due to PA. Infections of the driveline exit site (DLES) occurred most commonly (n=15, 88.2%), followed by pocket (n=2, 11.8%) and pump (n=2, 11.8%) infections. Median time to infection after VAD implantation was 295 days (IQR 154 – 440 days). Eight (47.1%) pt isolates were not fluoroquinolone (FQ) susceptible. Resistance to multiple antibiotic classes was observed in pts in whom serial cultures were obtained. Median antibiotic treatment was 107 days (IQR 55 – 183 days, maximum 775 days). Five (29.4%) pts received FQ monotherapy on initial diagnosis, 3 (60%) of whom required change to a different class for resistance. Surgical debridement and VAD exchange were performed in 5 (29.4%) and 3 (17.6%) pts respectively. Co-pathogens were identified in 9 (52.9%) pts, the most common being Staphylococcus aureus (n=2) and Enterococcus spp (n=2). A total of 5 (29.4%) pts went on to successful heart transplantation; one had recurrent PA infection at the prior DLES requiring prolonged antibiotics and removal of retained DL material. All cause 1-year mortality rate was 11.7% (n = 2), both of whom died from cerebrovascular accidents.

Conclusion

VAD-specific infections with PA occurred late after device implantation and required prolonged antibiotic courses. Antimicrobial resistance was high at diagnosis and worsened in pts on prolonged therapy. Morbidity and mortality in pts with PA VAD infections were high. The preponderance of DLES infections warrants further study and highlights the need for improvements in DLES care and infection prevention strategies.

Disclosures

All Authors: No reported disclosures

803. Risk factors associated with Clostridioides difficile infection in hospitalized patients with community-acquired pneumonia

Background

Patients with community-acquired pneumonia (CAP) who are hospitalized and treated with antibiotics may carry an increased risk for developing Clostridioides difficile infection (CDI). Accurate risk estimation tools are needed to guide monitoring and CDI mitigation efforts. We aimed to identify patient-specific risk factors associated with CDI among hospitalized patients with CAP.

Methods

Design: retrospective case-control study of hospitalized patients who received CAP-directed antibiotic therapy between 1/1/2014 and 5/29/2018. Cases were hospitalized CAP patients who developed CDI post-admission. Control patients did not develop CDI and were selected at random from CAP patients hospitalized during this period. Variables: comorbidities, laboratory results, vital signs, severity of illness, prior hospitalization, and past antibiotic use. Propensity-score weights: identified via structural decomposition analysis of pre-treatment variables. Analysis: weighted classification tree models that predicted any CDI, hospital-onset CDI, and any healthcare-associated CDI according to CAP antibiotic treatment. Performance: percent accuracy in classification (PAC) and weighted positive (PPV) and negative predictive values (NPV). Modeling: completed using the ODA package (v1.0.1.3) for R (v3.5.1).

Results

A total of 32 cases and 232 controls were identified. Sixty pre-treatment variables were screened. Structural decomposition analysis, completed in two stages, identified prior hospitalization (OR 6.56, 95% CI: 3.01-14.31; PAC: 80.3%) and BUN greater than 29 mg/dL (OR 11.67, 95% CI: 2.41-56.5; PAC: 80.8%) as propensity-score weights. With respect to CDI, receipt of broad-spectrum anti-pseudomonal antibiotics was significantly (all P’s< 0.05) associated with any CDI (NPV: 90.29%, PPV: 27.94%), hospital-onset CDI (NPV: 97.53%, PPV: 26.86%), and healthcare-associated CDI (NPV: 92.89%, PPV: 27.94%).

Conclusion

We identified risk factors available at hospital admission and empiric use of broad-spectrum Gram-negative antibiotics as being associated with the development of CDI. Model PPVs were over two-fold greater than our sample base rate. Increased monitoring and avoidance of overly broad antibiotic use in high-risk patients appears warranted.

Disclosures

All Authors: No reported disclosures

Development of a protocol for detection of SARS-CoV-2 in sputum and endotracheal aspirates using Cepheid Xpert Xpress SARS-CoV-2

Sputum and endotracheal aspirates (ETs) are not among the vendor-approved specimens for the Cepheid Xpert SARS-CoV-2 assay. However, they are the common lower respiratory tract specimens submitted for laboratory diagnosis. Testing for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in lower respiratory tract samples is required for the discharge of patients from coronavirus disease (COVID) units at some institutions. We developed a protocol used for testing unliquified viscous sputum or tracheal aspirate with the Cepheid Xpert SARS-CoV-2 assay.

2203. Patient-Specific Risk Stratification to Identify Patients at High and Low Risk for P. aeruginosa in Community-Acquired Pneumonia

Background

Pseudomonas aeruginosa (PsA) is an infrequent pathogen associated with poor outcomes in community-acquired pneumonia (CAP). Identifying patients at high and low-risk for PsA in CAP is necessary to reduce inappropriate and overly broad-spectrum antibiotic use. We evaluated the distribution of risk-factors in hospitalized CAP patients with and without PsA infection.

Methods

Design: retrospective, single-center, case–control study. Inclusion: hospitalized CAP patients admitted to the general medicine wards between January 1, 2014 and May 29, 2018. Exclusion: cystic fibrosis, ≥ 3 admissions within 30 days, CAP requiring ICU admission, and death within 48 hours of admission. Case patients had PsA in respiratory or blood cultures during the index CAP admission. Controls were randomly selected targeting a 3:1 ratio. Comorbidities, pneumonia severity index, and m-APACHE II were assessed. Gram-negative risk factors defined by Shindo et al. 2013 (PMID: 23855620) and validated by Kobayashi et al. (2018; PMID: 30349327) were scored for each patient. Stepwise logistic regression was used to identify covariates that distinguished cases from controls at a P < 0.2; these were then used to generate propensity weights (i.e., inverse-probability conditioned on covariates). Unadjusted and adjusted odds ratios for case status were estimated using logistic regression according to: the total number of risk factors present and threshold values, respectively. All analyses were conducted using IC Stata (v.14.2).

Results

54 cases and 152 controls were included. The distribution of the patient-specific sum of risk factors for PsA is shown in Figure 1. The univariate OR for case status was 4.29 (95% CI:1.55–11.9) at n = 3 risk factors, which was similar after propensity weight adjustment [aOR = 4.64 (95% CI: 1.32–16.3)]. The univariate OR of case status was 2.98 among patients with ≥ 3 risk factors (95% CI: 1.34–6.62), which was similar after propensity weight adjustment [aOR = 2.8 (95% CI: 1.02–7.72)], and correct classification was 73.8%.

Conclusion

At a threshold of ≥ 3 PsA risk factors, cases and controls were well classified, even after adjusting for propensity weights. The impact of patient-specific PsA risk-stratification on CAP outcomes and appropriate antibiotic use should be evaluated.

Disclosures

All authors: No reported disclosures.

2010. A Significant Reduction in Empiric Vancomycin Days of Therapy for Suspected MRSA Pneumonia in Adult Non-ICU Patients After Implementation of a Rapid MRSA Nasal PCR Test with Antimicrobial Stewardship Intervention

Background

Methicillin-resistant Staphylococcus aureus (MRSA), when implicated in respiratory tract infections, can be associated with significant morbidity and mortality. The prevalence of severe MRSA pneumonia may be as high as 10%; however, recent evidence suggests that MRSA is much less prevalent as a cause of community-acquired pneumonia (CAP) among community-dwelling patients and may be as low as 0.1%. Nonspecific features of pneumonia in non-ICU patients (viral co-infection, multi-lobar infiltrates) often lead clinicians to cautiously initiate empiric anti-MRSA therapy. Recommendations of when to safely de-escalate empiric treatment prior to known respiratory cultures are not established. To decrease anti-MRSA therapy in non-ICU pneumonia patients with a low probability of MRSA pneumonia, we employed a nasal screening paired with antimicrobial stewardship intervention.

Methods

A retrospective, single-center, pre-post interventional study was conducted at Northwestern Memorial Hospital (NMH), in Chicago, IL, to assess the duration of empiric vancomycin for suspected MRSA pneumonia in non-ICU patients before (January 2019) and after (March 2019) the implementation of a rapid MRSA nasal PCR test. During the post-implementation period, an NMH Antimicrobial Stewardship (AS) member identified and assessed the daily (M-F) use of empiric vancomycin for pneumonia in non-ICU patients. When vancomycin use criteria were not met, the AS pharmacist requested the team order a BD MRSA Nasal PCR test (NPV: 97.2%) to classify patients as either possible MRSA pneumonia or unlikely MRSA pneumonia. Results of a negative MRSA Nasal PCR with an ongoing clinical disposition not suggestive of MRSA pneumonia prompted the AS pharmacist to recommend de-escalation of vancomycin.

Results

See table.

Conclusion

The use of a rapid MRSA nasal PCR test with active antimicrobial stewardship intervention significantly reduced the duration of empiric vancomycin in hospitalized non-ICU patients with suspected MRSA pneumonia.

Disclosures

All authors: No reported disclosures.

1821. Understanding the Components and Calculation of the SAAR, Illustrative Data

Background

The standardized antimicrobial administration ratio (SAAR) compares each hospital’s observed to predicted days of antimicrobial therapy. However, confusion exists about how hospital-level, seasonal, and hospital-peer-based variations in antibiotic use might impact an institution’s SAAR. We characterized the impact of each of these three types of variation on predicted SAARs utilizing local NHSN data.

Methods

Analysis of antibiotic consumption data from an academic medical center in Chicago, IL was conducted. SAAR and antimicrobial days per 1,000 days present (AD/1,000DP) were compiled in monthly increments from 2014 to 2016.Antimicrobial consumption was aggregated and classified into agent categories according to NHSN criteria. Month-to-month changes in both the SAAR and AD/1,000DP were evaluated. Azithromycin AD/1,000DP from 2012 through 2017 were explored for seasonal variation as defined as >20% increase in AD/1,000DP from each quarter to the overall mean AD/1,000DP for all months. A simulation was performed to explore the potential effect of seasonality on the SAAR. Demographic covariates within the SAAR model were altered while holding constant observed antibiotic use; thus we were able to observe the potential impact of demographics. Finally, a simulation explored the effect of altered consumption at other hospitals on a local institution’s SAAR.

Results

Across all antibiotic agent categories for both ICU (n = 4) and general wards (n = 4), the average matched-month percent change in AD/1,000DP was highly predicted and correlated with the corresponding change in SAAR (Figure 1, Pearson’s r = 0.99). The monthly mean ± SD AD/1,000DP was 235.0 (range 47.2–661.5), and the mean ± SD SAAR was 1.09 ± 0.26 (range 0.79–1.09) across the NHSN antibiotic agent categories. Five quarters were found to have seasonal variation in AD/1000DP for azithromycin (Figure 2). Simulations demonstrated that changing antimicrobial usage at comparator hospitals does not impact the local SAAR, and seasonal variation may cause fluctuating SAARs.

Conclusion

Month-to-month changes in the SAAR mirror monthly changes in an institution’s AD/1,000DP. Seasonal variation can impact the SAAR, and the effect changing peer hospital antibiotic consumption is not currently captured by the SAAR methodology.

Disclosures

J. Liu, Merck: Grant fund from Merck, Research grant. D. Martin, Syneos Health: Employee, Salary. GlaxoSmithKline: Independent Contractor, Salary. M. H. Scheetz, Merck & Co., Inc.: Grant Investigator, Grant recipient. Bayer: Consultant, Consulting fee.

209. Impact of a Risk-based CAP Prescribing Guideline Paired with Antimicrobial Stewardship to Improve Antibiotic Prescribing for Patients at Low Risk for Drug-Resistant Pathogens

Background

Antimicrobial stewardship programs (ASPs) reduce the burden of multidrug-resistant organisms and improve antibiotic prescribing. Concerns about drug-resistant pathogens (DRPs) in community-acquired pneumonia (CAP) lead to over-prescribing of broad-spectrum antibiotics, and ASP interventions to improve CAP prescribing are not well defined. In 2017, our hospital implemented a CAP guideline for patients at low risk for DRPs along with ASP support. The purpose of this study was to evaluate the impact of the guideline with ASP support on CAP-specific antibiotic prescribing.

Methods

This was a pragmatic two-phase quasi-experimental analysis of CAP-specific antibiotic consumption before and after implementation of a CAP guideline evaluated according to each phase of implementation. The guideline provided Gram-positive and Gram-negative risk factors and guidance on oral fluoroquinolone (FQs) alternatives. ASP interventions were implemented in two phases: (A) prospective audit and feedback in July 2016 and (B) publication of guideline with education in March 2017. Impact of each intervention was evaluated by interrupted time series segmented-regression analysis. Univariate statistics were calculated using EpiInfo 7. Least-squares segmented regressions were completed in Microsoft Excel.

Results

CAP-specific antibiotic administrations were 782 over the entire study period, with 764, 771, and 928 administrations observed before phase A, after A, and after B, respectively. Macrolide consumption increased after the guideline (P = 0.029). We observed a significant step change decrease in FQ consumption was observed after phase A) (P = 0.039) and a positive upward trend in oral alternatives agents after phase B (P = 0.090), as shown in the figure. Consumption of broad Gram-negative agents and vancomycin/linezolid were not significantly different after the guideline.

Conclusion

Implementation of a CAP guideline with patient-specific and DRP risk factors was associated with significant changes in CAP-specific prescribing. Changes in prescribing were temporally associated with ASP interventions. Additional studies into the impact of this guideline on correct classification of Gram-negative resistance and clinical outcomes are needed.

Disclosures

D. Martin, GlaxoSmithKline: Independent Contractor, Salary

Syneos Health: Employee, Salary. R. G. Wunderink, Achaogen: Consultant, Consulting fee. Arsanis: Consultant, Consulting fee. Bayer: Consultant, Consulting fee. GlaxoSmithKline: Consultant, Consulting fee. KBP Biosciences: Consultant, Consulting fee. Meiji-Seiko: Consultant, Consulting fee. Merck: Consultant, Consulting fee. Nabriva: Consultant, Consulting fee. Polyphor: Consultant, Consulting fee. Roche/Genetech: Consultant, Consulting fee. Shionogi: Consultant, Consulting fee. The Medicines Company: Consultant, Consulting fee. Accelerate Diagnostics: Consultant, Consulting fee. Curetis: Consultant, Consulting fee. bioMerieux: Consultant, Consulting fee. M. H. Scheetz, Merck & Co., Inc.: Grant Investigator, Grant recipient. Bayer: Consultant, Consulting fee.

Clinical Outcomes of Carbapenem-Resistant Acinetobacter baumannii Bloodstream Infections: Study of a 2-State Monoclonal Outbreak

Objective.

To characterize the clinical outcomes of patients with bloodstream infection caused by carbapenem-resistant Acinetobacter baumannii during a 2-state monoclonal outbreak.

Design.

Multicenter observational study.

Setting.

Four tertiary care hospitals and 1 long-term acute care hospital.

Methods.

A retrospective medical chart review was conducted for all consecutive patients during the period January 1, 2005, through April 30, 2006, for whom 1 or more blood cultures yielded carbapenem-resistant A. baumannii.

Results.

We identified 86 patients from the 16-month study period. Their mortality rate was 41%; of the 35 patients who died, one-third (13) had positive blood culture results for carbapenem-resistant A. baumannii at the time of death. Risk factors associated with mortality were intensive care unit stay, malignancy, and presence of fever and/or hypotension at the time blood sample for culture was obtained. Only 5 patients received adequate empirical antibiotic treatment, but the choice of treatment did not affect mortality.

Fifty-seven patients (66.2%) had a single positive blood culture result for carbapenem-resistant A. baumannii; the only factor associated with a single positive blood culture result was the presence of decubitus ulcers. Interestingly, during the study period, a transition from single to multiple positive blood culture results was observed. Four patients, 3 of whom were in a burn intensive care unit, were bacteremic for more than 30 days (range, 36–86 days).

Conclusions.

To our knowledge, this is the first time a study has described 2 patterns of bloodstream infection with A. baumannii: single versus multiple positive blood culture results, as well as a subset of patients with prolonged bacteremia.

Contamination of Examination Gloves in patient Rooms and Implications for Transmission of Antimicrobial-Resistant Microorganisms

An assessment of bacterial contamination on examination gloves indicated that contaminated gloves may be a mechanism of indirect bacterial transmission from the hands of healthcare workers to patients. This mechanism is indicated by the recovery of identical Acinetobacter baumannii isolates from gloves and from the clinical cultures of a patient with invasive infection.

Outcomes of an enhanced surveillance program for carbapenem-resistant Enterobacteriaceae

Optimal surveillance strategies for identifying patients colonized with and at risk for transmitting carbapenem-resistant Enterobacteriaceae (CRE) are urgently needed. We instituted an enhanced surveillance program for CRE that identified unrecognized CRE-colonized patients but failed to identify possible CRE transmissions. We also identified risk factors associated with transmitting CRE.

Raccoon rabies virus variant transmission through solid organ transplantation

IMPORTANCE

The rabies virus causes a fatal encephalitis and can be transmitted through tissue or organ transplantation. In February 2013, a kidney recipient with no reported exposures to potentially rabid animals died from rabies 18 months after transplantation.

OBJECTIVES

To investigate whether organ transplantation was the source of rabies virus exposure in the kidney recipient, and to evaluate for and prevent rabies in other transplant recipients from the same donor.

DESIGN

Organ donor and all transplant recipient medical records were reviewed. Laboratory tests to detect rabies virus-specific binding antibodies, rabies virus neutralizing antibodies, and rabies virus antigens were conducted on available specimens, including serum, cerebrospinal fluid, and tissues from the donor and the recipients. Viral ribonucleic acid was extracted from tissues and amplified for nucleoprotein gene sequencing for phylogenetic comparisons.

MAIN OUTCOMES AND MEASURES

Determination of whether the donor died from undiagnosed rabies and whether other organ recipients developed rabies.

RESULTS

In retrospect, the donor's clinical presentation (which began with vomiting and upper extremity paresthesias and progressed to fever, seizures, dysphagia, autonomic dysfunction, and brain death) was consistent with rabies. Rabies virus antigen was detected in archived autopsy brain tissue collected from the donor. The rabies viruses infecting the donor and the deceased kidney recipient were consistent with the raccoon rabies virus variant and were more than 99.9% identical across the entire N gene (1349/1350 nucleotides), thus confirming organ transplantation as the route of transmission. The 3 other organ recipients remained asymptomatic, with rabies virus neutralizing antibodies detected in their serum after completion of postexposure prophylaxis (range, 0.3-40.8 IU/mL).

CONCLUSIONS AND RELEVANCE

Unlike the 2 previous clusters of rabies virus transmission through solid organ transplantation, there was a long incubation period in the recipient who developed rabies, and survival of 3 other recipients without pretransplant rabies vaccination. Rabies should be considered in patients with acute progressive encephalitis of unexplained etiology, especially for potential organ donors. A standard evaluation of potential donors who meet screening criteria for infectious encephalitis should be considered, and risks and benefits for recipients of organs from these donors should be evaluated.

Screening for extended-spectrum beta-lactamase-producing Enterobacteriaceae among high-risk patients and rates of subsequent bacteremia

BACKGROUND

Bloodstream infections due to extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae have been associated with increased hospital costs, length of stay, and patient mortality. However, the role of routine inpatient surveillance for ESBL colonization in predicting related infection is unclear.

METHODS

From 2000 through 2005, we screened 17,872 patients hospitalized in designated high-risk units for rectal colonization with vancomycin-resistant enterococci and ESBL-producing Enterobacteriaceae using a selective culture medium. In patients with a bloodstream infection due to ESBL-producing Enterobacteriaceae (ESBL-BI) during the study period, surveillance results were evaluated for evidence of antecedent ESBL-producing Enterobacteriaceae colonization.

RESULTS

The rate of ESBL-producing Enterobacteriaceae colonization doubled during the 6-year study period, increasing from 1.33% of high-risk patients in 2000 to 3.21% in 2005. Among patients with ESBL-producing Enterobacteriaceae colonization, 49.6% also carried vancomycin-resistant enterococci. The number of ESBL-BIs increased >4-fold in 5 years, from 9 cases in 2001 to 40 cases in 2005. Of 413 patients colonized with ESBL-producing Enterobacteriaceae, 35 (8.5%) developed a subsequent ESBL-BI. Of concern, more than one-half of all ESBL-BIs occurred in patients who were not screened. These 56 patients received a diagnosis of ESBL-BI in the emergency department, when hospitalized in low-risk medical units, or at transfer from an acute or long-term health care facility.

CONCLUSIONS

Colonization with ESBL-producing Enterobacteriaceae is increasing at a rapid rate, and routine rectal surveillance for ESBL-producing Enterobacteriaceae may have clinical implications. However, in our experience, over one-half of patients with an ESBL-BI did not undergo screening through our current surveillance measures. As a result, targeted screening for ESBL-producing Enterobacteriaceae among additional patient populations may be integral to future ESBL-BI prevention and management efforts.

Postpartum mastitis and community-acquired methicillin-resistant Staphylococcus aureus

This single-center, case-control study documents a relative increase in methicillin resistance among 48 cases of Staphylococcus aureus–associated postpartum mastitis during 1998–2005. Of 21 cases with methicillin resistance, 17 (81%) occurred in 2005. Twenty (95%) isolates contained the Staphylococcus cassette chromosome mec type IV gene; this suggests that the increase is due to community-acquired methicillin-resistant Staphylococcus aureus.

In vitro activities of various antimicrobials alone and in combination with tigecycline against carbapenem-intermediate or -resistant Acinetobacter baumannii

The activities of tigecycline alone and in combination with other antimicrobials are not well defined for carbapenem-intermediate or -resistant Acinetobacter baumannii (CIRA). Pharmacodynamic activity is even less well defined when clinically achievable serum concentrations are considered. Antimicrobial susceptibility testing of clinical CIRA isolates from 2001 to 2005 was performed by broth or agar dilution, as appropriate. Tigecycline concentrations were serially increased in time-kill studies with a representative of the most prevalent carbapenem-resistant clone (strain AA557; imipenem MIC, 64 mg/liter). The in vitro susceptibility of the strain was tested by time-kill studies in duplicate against the average free serum steady-state concentrations of tigecycline alone and in combination with various antimicrobials. Ninety-three CIRA isolates were tested and were found to have the following antimicrobial susceptibility profiles: tigecycline, MIC(50) of 1 mg/liter and MIC(90) of 2 mg/liter; minocycline, MIC(50) of 0.5 mg/liter and MIC(90) of 8 mg/liter; doxycycline, MIC(50) of 2 mg/liter and MIC(90) of > or =32 mg/liter; ampicillin-sulbactam, MIC(50) of 48 mg/liter and MIC(90) of 96 mg/liter; ciprofloxacin, MIC(50) of > or =16 mg/liter and MIC(90) of > or =16 mg/liter; rifampin, MIC(50) of 4 mg/liter and MIC(90) of 8 mg/liter; polymyxin B, MIC(50) of 1 mg/liter and MIC(90) of 1 mg/liter; amikacin, MIC(50) of 32 mg/liter and MIC(90) of > or =32 mg/liter; meropenem, MIC(50) of 16 mg/liter and MIC(90) of > or =128 mg/liter; and imipenem, MIC(50) of 4 mg/liter and MIC(90) of 64 mg/liter. Among the tetracyclines, the isolates were more susceptible to tigecycline than minocycline and doxycycline, according to FDA breakpoints (95%, 88%, and 71% of the isolates were susceptible to tigecycline, minocycline, and doxycycline, respectively). Concentration escalation studies with tigecycline revealed a maximal killing effect near the MIC, with no additional extent or rate of killing at concentrations 2x to 4x the MIC for tigecycline. Time-kill studies demonstrated indifference for tigecycline in combination with the antimicrobials tested. Polymyxin B, minocycline, and tigecycline are the most active antimicrobials in vitro against CIRA. Concentration escalation studies demonstrate that tigecycline may need to approach concentrations higher than those currently achieved in the bloodstream to adequately treat CIRA bloodstream infections. Future studies should evaluate these findings in vivo.

Voriconazole therapeutic drug monitoring in allogeneic hematopoietic stem cell transplant recipients

Voriconazole, a new antifungal agent, is increasingly being used after HSCT. The hepatic cytochrome P450 isoenzyme 2C19 plays a significant role in voriconazole metabolism. As CYP2C19 exhibits significant genetic polymorphism, some patients metabolize voriconazole poorly resulting in increased plasma drug levels. The clinical significance of this is unknown, and the utility of monitoring voriconazole levels is unclear. Steady-state trough plasma voriconazole levels were obtained in 25 allogeneic HSCT recipients using an HPLC assay. Patients had drug levels checked once (n=13), twice (n=10), or > or =3 times (n=2) 5-18 days (median 10) after starting voriconazole or dose modification. The 41 voriconazole levels were 0.2-6.8 microg/ml (median 1.6); 6 (15%) were <0.5 (possibly below the in vitro MIC90 for Aspergillus spp.). Voriconazole concentrations correlated with aspartate aminotranferase (AST) (r=0.5; P=0.0009) and alkaline phosphatase (r=0.34; P=0.03), but not with creatinine, bilirubin and alanine aminotransferase (ALT). Since liver dysfunction is common after HSCT, it was not possible to determine if elevated AST and alkaline phosphatase levels were the cause or the consequence of higher voriconazole levels. We conclude that trough voriconazole levels vary considerably between patients, and suggest monitoring levels in patients receiving voriconazole for confirmed fungal infections, and in those with elevated AST or alkaline phosphatase levels.

Yield of vancomycin-resistant enterococci and multidrug-resistant Enterobacteriaceae from stools submitted for Clostridium difficile testing compared to results from a focused surveillance program

It has been suggested that a method of performing surveillance for vancomycin-resistant enterococci (VRE) is to screen specimens submitted for Clostridium difficile testing. We compared this approach to our focused surveillance program of high-risk units during October 1997 to compare the yield of VRE and multidrug-resistant Enterobacteriaceae (MDRE) with both methods. Of the stools submitted for C. difficile testing, 14% were positive for VRE or MDRE, whereas rectal swabs from routine surveillance yielded 11% VRE- or MDRE-positive results. Although stools submitted for C. difficile testing resulted in a higher percentage of positive cultures, 14 VRE- and 2 MDRE-positive patients from our high-risk population were missed because many patients had no stool submitted for C. difficile testing. Therefore, while screening stools submitted for C. difficile testing cannot replace our focused surveillance program, it appears advantageous to assess these stools at various intervals to detect new patient reservoirs of drug-resistant organisms that may benefit from routine surveillance cultures.

Outbreak of nosocomial infections due to extended-spectrum beta-lactamase-producing strains of enteric group 137, a new member of the family Enterobacteriaceae closely related to Citrobacter farmeri and Citrobacter amalonaticus

A member of the Enterobacteriaceae initially identified as Kluyvera cryocrescens by the MicroScan Gram-Negative Combo 13 panel caused an outbreak of nosocomial infections in four patients (pneumonia, n = 2; urinary tract infection, n = 1; wound infection, n = 1) and urinary tract colonization in one patient. When the strains were tested by the Enteric Reference Laboratory of the Centers for Disease Control and Prevention, biochemical results were most compatible with Yersinia intermedia, Kluyvera cryocrescens, and Citrobacter farmeri but identification scores were low and test results were discrepant. However, when the biochemical test profile was placed in the computer database as a new organism, all strains were identified as the organism with high identification scores (0. 999968 to 0.999997) and no discrepant test results. By 16S rRNA sequence analysis the organism clustered most closely with, but was distinct from, Citrobacter farmeri and Citrobacter amalonaticus. Based on its unique biochemical profile and rRNA sequence, this organism is designated Enteric Group 137. Restriction endonuclease analysis and taxonomic antibiograms of strains causing the outbreak demonstrated a single clone of Enteric Group 137, and antibiotic susceptibility testing revealed the presence of extended-spectrum beta-lactamase (ESBL) resistance. Enteric Group 137 appears to be a new opportunistic pathogen that can serve as a source of ESBL resistance in the hospital.

Synergistic effect of gentamicin plus ampicillin on enterococci with differing sensitivity to gentamicin: a phenotypic assessment of NCCLS guidelines

Between December 1, 1993, and December 1, 1996, we tested 4,411 isolates of Enterococcus sp. at gentamicin concentrations of 500 micrograms/mL and 2000 micrograms/mL using agar dilution to phenotypically categorize them into 3 groups: those with a MIC < or = 500 micrograms/mL (n = 3,132; 71%); a MIC > 500, but < or = 2000 micrograms/mL (n = 441; 10%); and those with a MIC > 2000 micrograms/mL (n = 838; 19%). Ten unique strains of each phenotype were tested to determine which gentamicin concentration was the best in vitro predictor of synergy with ampicillin. Testing was done by a time-kill method using clinically achievable levels of ampicillin and gentamicin. We found that for the gentamicin MIC < or = 500 micrograms/mL group, 7 of 10 isolates demonstrated synergy with ampicillin as manifested by a > or = 2 log10 increase in killing versus the effect of ampicillin alone (at 1/2 the MIC for ampicillin). In the group sensitive to a gentamicin MIC range between > 500 and < or = 2,000 micrograms/mL, none of the 10 isolates demonstrated synergy. Absence of synergy was also found in the group resistant to 2,000 micrograms/mL of gentamicin. Assessment of eight additional enterococcal isolates with reduced sensitivity to ampicillin (MIC from 32-256 micrograms/mL) found no correlation between gentamicin sensitivity at 500 micrograms/mL and any in vitro test for synergy, nor with clinical therapeutic outcome. Gentamicin at 2 micrograms/mL combined with ampicillin was as effective in enhancing killing as a higher level of 4 micrograms/mL. These findings validate the current NCCLS guideline for predicting synergistic activity against enterococci in strains with usual susceptibility to ampicillin, and suggest that a therapeutic level less than maximal recommended dosing is sufficient when using gentamicin in this setting.

Epidemiology of infectious complications in cancer patients

Patients with underlying malignancies are at risk for a wide array of infectious diseases that cause significant morbidity and mortality. To develop a clear etiologic understanding of the infectious agents involved first requires a knowledge of the factors that predispose to infection. Neutropenia is clearly the single most important risk factor for infection in the cancer patient. However, a variety of both host and treatment-associated factors act together to predispose these patients to opportunistic infections. Approaching the individual malignancies with a knowledge of the underlying risk factors helps logically guide diagnosis and therapy. The astute clinician must also be aware of new and emerging infections in this patient population. As new pathogens are discovered and established pathogens become increasingly drug resistant, they will continue to present challenges for physicians caring for these patients in the years ahead.