In critically ill patients, anti-anaerobic antibiotics increase risk of adverse clinical outcomes.

"data availability: the dataset supporting the results of this article has been posted to the nih sequence read archive (accession number prjna633879"

"operational taxonomic unit tables taxonomy classification tables and metadata tables are available at https://github com/rishichanderraj/rectalswab_bacterialdensity ."

"Conflict of interest: All authors have nothing to disclose."

"Support statement: This work was supported by the National Institutes of Health (R01HL144599 to R.P. Dickson, T32HL007749 to R. Chanderraj and R.P. Dickson, 1F31HL158033 to J.M. Baker, R01LM013325 and K01HL136687 to M.W. Sjoding, and R01AI143852 to R.J. Woods). This manuscript does not represent the views of the Department of Veterans Affairs or the US Government. This material is the result of work supported with resources and use of facilities at the Ann Arbor VA Medical Center (Ann Arbor, MI, USA). Funding information for this article has been deposited with the Crossref Funder Registry."