Gonococcal aggregation causes upregulation of genes involved in antibiotic tolerance

Abstract

Aggregation and biofilm formation can increase the tolerance of bacteria to external stressors, including antibiotic treatment. While resistant bacteria grow at an elevated drug dose, tolerant bacteria survive longer-term treatment. The mechanisms by which aggregation confers tolerance are insufficiently characterized for most organisms, including the human pathogen Neisseria gonorrhoeae. We hypothesize that bacterial aggregation causes upregulation of genes involved in tolerance and that deletion of these genes increases killing rates during antibiotic treatment. To test this hypothesis and identify genes involved in gonococcal tolerance, we compared the transcriptome of aggregating and planktonic N. gonorrhoeae strains. In general, the transcriptome analysis shows that aggregation causes a strong upregulation of prophage-related genes and a shift towards anaerobic respiration. We generated deletion strains for the twenty most upregulated genes and measured their killing kinetics during treatment with the clinically relevant antibiotics ceftriaxone or ciprofloxacin. We identified five genes and one multigene segment that are involved in gonococcal antibiotic tolerance. These include prophage genes whose deletion affects tolerance differently in aggregating and planktonic strains. Furthermore, deletion of genes encoding a putative multi-drug efflux pump, an alcohol dehydrogenase, and a DNA repair protein reduces tolerance. In summary, we have identified multiple genes that affect antibiotic tolerance and are upregulated in response to aggregation. Author summaryOften bacterial infections recur after antibiotic treatment because not all of the bacteria were killed. The ability to survive treatment by bactericidal drugs is termed tolerance. It is well established that aggregation can increase tolerance by reducing growth and metabolism. However, the genes involved in tolerance are not well characterized, especially in the human pathogen Neisseria gonorrhoeae. Here, we aim to identify such genes by following the hypothesis that aggregation upregulates genes that cross-protect N. gonorrhoeae from antibiotic treatment. We show that prophage-associated genes are strongly upregulated in aggregates and that deletion of various phage genes affects tolerance to the currently administered drug, ceftriaxone. We identify three additional genes belonging to different functional classes whose deletion reduces tolerance to ciprofloxacin. Our study is an important step towards understanding the molecular mechanisms of gonococcal antibiotic tolerance. In particular, we propose that prophages could serve as a target for the treatment of tolerant gonococcal infections.