Investigation of Acyl Homoserine Lactone Quorum Sensing in Nitrospira inopinata, a Complete Ammonia Oxidizer

Abstract

Quorum sensing is an important system of regulation in microorganisms. However, quorum sensing has been better studied in heterotroph model organisms compared to chemolithoautotrophs, such as nitrifying microorganisms (nitrifiers). In Nitrospira inopinata ENR4, a complete ammonia oxidizer (comammox), only the presence of putative quorum sensing gene homologs to lasI and lasR which encode for acyl-homoserine lactone synthase and transcriptional regulator LasR , respectively, have been discovered. We sought to confirm the function of the lasI and lasR homologs in N. inopinata through heterogeneous expression of these genes in Pseudomonas putida F1. Three acyl homoserine lactones (3-OH-C10-HSL, 3-OH-C12-HSL, and 3-OH-C14-HSL) were found to be produced by the putative AHL synthase in P. putida F1. Only two of the AHLs (3-OH-C10-HSL and 3-OH-C14-HSL) were detected in N. inopinata cultures. The addition of these three acyl homoserine lactones into N. inopinata cultures resulted in slower growth and utilization of ammonia and nitrite. Whole-cell transcriptomics analysis of 3-OH-C12-HSL addition cultures revealed downregulation of energy production pathways and the fatty acid biosynthesis pathway compared to no-AHL addition controls. Our study confirms a functional quorum sensing system in N. inopinata that is mediated via three acyl homoserine lactones and opens future research pathways into the quorum sensing regulation of comammox Nitrospira and other nitrifiers.