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Background Aminoglycoside-containing regimens may be an effective treatment option for infections caused by carbapenem-resistant Klebsiella pneumoniae (CR-Kp), but aminoglycoside-resistance genes are common in these strains. The relationship between the aminoglycoside-resistance genes and aminoglycoside MICs remains poorly defined. Objectives To identify genotypic signatures capable of predicting aminoglycoside MICs for CR-Kp. Methods Clinical CR-Kp isolates (n = 158) underwent WGS to detect aminoglycoside-resistance genes. MICs of amikacin, gentamicin, plazomicin and tobramycin were determined by broth microdilution (BMD). Principal component analysis was used to initially separate isolates based on genotype. Multiple linear regression was then used to generate models that predict aminoglycoside MICs based on the aminoglycoside-resistance genes. Last, the performance of the predictive models was tested against a validation cohort of 29 CR-Kp isolates. Results Among the original 158 CR-Kp isolates, 91.77% (145/158) had at least one clinically relevant aminoglycoside-resistance gene. As a group, 99.37%, 84.81%, 82.28% and 10.76% of the CR-Kp isolates were susceptible to plazomicin, amikacin, gentamicin and tobramycin, respectively. The first two principal components explained 72.23% of the total variance in aminoglycoside MICs and separated isolates into four groups with aac(6′)-Ib, aac(6′)-Ib′, aac(6′)-Ib+aac(6′)-Ib′ or no clinically relevant aminoglycoside-resistance genes. Regression models predicted aminoglycoside MICs with adjusted R2 values of 56%–99%. Within the validation cohort, the categorical agreement when comparing the observed BMD MICs with the predicated MICs was 96.55%, 89.66%, 86.21% and 82.76% for plazomicin, gentamicin, amikacin and tobramycin, respectively. Conclusions Susceptibility to each aminoglycoside varies in CR-Kp. Detection of aminoglycoside-resistance genes may be useful to predict aminoglycoside MICs for CR-Kp.

Background Recently, we described a collection of ST298 Pseudomonas aeruginosa (PA) isolates that caused a prolonged epidemic of XDR infections. Many of these contain derivatives of a new plasmid, pPABL048, that harbors an MDR integron, in1697. In1697 contains a series of antimicrobial resistance (AMR) genes, one of which is the class D β-lactamase blaOXA-10. Variants of blaOXA-10 have been described that confer both extended-spectrum β-lactamase (ESBL) and carbapenemase activity. Methods Of all ST298 isolates, three were resistant to ceftazidime (CTZ). Genomic comparison of in1697 in CTZ-resistant and CTZ-sensitive strains revealed that all three strains harbored a blaOXA-10 allele with two single nucleotide variations resulting in amino acid changes at positions 153 (F153S) and 157 (G157D). Using the NCBI database, we identified this allele as unique and defined this β-lactamase as OXA-935. OXA-935 shares the G157D variation with OXA-14 which is known to confer resistance to ceftazidime. We sought to characterize the function of OXA-935 and to determine the crystal structures of OXA-14 and OXA-935. Results Deletion of blaOXA-935 phenotypically converted all three strains to CTZ-susceptible. Expression of blaOXA-14 and blaOXA-935 conferred CTZ-resistance to laboratory PA strains PA01 and PA14. Determination of the crystal structures of OXA-14 (PDB code 7L5R) and OXA-935 (PDB code 7L5V) revealed that the F153S variant resulted in increased flexibility in the enzyme’s Ω loop. Conformational changes in the Ω loop likely contributed to the lack of carbamylation at lysine-70 (K70) observed in OXA-935. Carbamylation of K70 is known to be critical for enzymatic activity of class D β-lactamases. Conclusion OXA-935 is very similar to OXA-14; however, comparison revealed that the F153S variant has unique structural features and is functionally distinct. Despite these differences, both enzymes confer high-level CTZ resistance. As we increasingly rely on β-lactam antimicrobial therapy (e.g. ceftazidime, cefepime) and combination (e.g. ceftazidime-avibactam) therapy to treat MDR PA infections, it is critical that we continue to explore the mechanistic basis of β-lactam AMR in an effort to preserve existing treatments and design novel ones. Disclosures All Authors: No reported disclosures

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

Background The rapid spread of SARS-CoV-2, the causative agent of Coronavirus disease 2019 (COVID-19), has been accompanied by the emergence of distinct viral clades, although their clinical significance has yet to be fully elucidated. While whole genome sequencing efforts have identified viral diversity over time, less is known about the clinical significance of this diversity. This study assessed the nasopharyngeal viral loads within patients over time to determine if these changes affect clinical parameters. Methods Samples were collected from patients presenting to Northwestern Memorial Hospital in Chicago, IL with a positive SARS-CoV-2 RT-PCR from nasopharyngeal swabs. Cycle threshold (Ct) values less than 35 were considered positive, and whole genome sequencing was performed by reverse transcription, multiplex PCR, and Nanopore sequencing. Phylogenetic analysis was conducted on sequenced isolates and compared with publicly available global sequences. Sequence characteristics and viral loads were correlated with each clade. Results 177 samples were analyzed from March 14, 2020, through May 1, 2020. Most of the sequences (92.6%) clustered in three main clades [Figure 1]. Clade IDs were ordered by relative abundance as Clades 1 (n=122, 68.9%), 2 (n=34, 19.2%), and 3 (n=8, 4.5%). Over this time, Clade 1 viruses have been increasing in incidence across the USA and globally while Clade 2 viruses were uniquely predominant in Illinois with limited global distribution. Ct values were compared across clades [Figure 2]. Significantly lower average Ct values (higher viral loads) were observed in Clade 1 relative to both Clade 2 (p=0.0002) and Clade 3 (p=0.0011). These findings were independent of time from symptom onset to specimen collection. Phylogenetic Analysis of SARS-CoV-2 Isolates with Number of Clades and Clade Distribution Associations Between Viral Clade and Ct Value Conclusion These data suggest that SARS-CoV-2 genotype may impact viral load in the upper airways. It remains to be determined whether this difference in clades may impact transmission potential and overall viral spread. Further longitudinal studies with more specimens and associated clinical data are needed. Disclosures Michael G. Ison, MD MS, AlloVir (Consultant)