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Background Next-generation sequencing has shown the cystic fibrosis (CF) lung microbiome to be a complex polymicrobial community. Anaerobic bacteria decrease in relative abundance with older age and disease progression, and may impact host inflammatory state. Persons with CF suffer from recurrent pulmonary exacerbations (PEs) that are treated with broad-spectrum antibiotics. Our objectives were to examine the effect of broad-spectrum (BS) vs narrow-spectrum (NS) anaerobic antibacterial treatment on bacterial diversity, and on pulmonary function recovery. Methods Pulmonary function tests (PFTs) and respiratory samples were collected as part of a prospective 18 month longitudinal study in CF patients at 4 time points, baseline (B), pulmonary exacerbation (E), end of exacerbation treatment (T), and follow-up (F). Treatment antibiotics were classified as broad or narrow based on anaerobic activity. 16S rRNA sequencing generated operational taxonomic units for analysis. Alpha diversity (relative abundance) was calculated via Shannon Index and Inverse Simpson formulas and β diversity (similarity in community composition) by Morisita-Horn (MH). Differences in diversity indices and PFTs were compared with regard to BS vs NS anaerobic activity, and statistical significance determined by GLS regression. Results Changes in alpha diversity for BS vs NS were not significantly different (P &gt; 0.05) (Figures 1 and 2). Community composition measured by MH was consistently more similar for NS than BS (T-B: 0.66 vs. 0.45, P = 0.04; F-B: 0.75 vs. 0.47, P &lt; 0.01) (Figure 3). Recovery of forced expiratory volume in 1 second (FEV1) from E to F was significantly higher in the NS group (25.4 vs. 21.0, P &lt; 0.01) (Figure 4). Conclusion While antibiotic spectrum did not influence bacterial abundance, BS therapy led to higher changes in community composition from B and E onset following antibiotic administration compared with NS therapy. The differences in β diversity suggest BS therapy can have a lasting impact on community composition. As those receiving NS therapy had similar or better recovery of pulmonary function than those with BS, there is no indication that NS therapy leads to worse clinical outcomes. A limitation may be that children receiving BS therapy tended to have more severe disease. Disclosures All authors: No reported disclosures.

2573. Impact of Anaerobic Antibacterial Spectrum on Cystic Fibrosis Lung Microbiome Diversity and Pulmonary Function