Genome-wide screen for genes required for smooth lipopolysaccharide production in Escherichia coli K-12

Lipopolysaccharide (LPS) is a major component of the outer membrane of Gram-negative bacteria, contributing to membrane integrity and environmental interactions. Genome-wide studies defining bacterial gene functions have been extensively performed in the model strain Escherichia coli K-12 which lacks O-antigen (OAg), and therefore does not produce smooth LPS (S-LPS). Consequently, the genetic requirements for S-LPS production in this model system remain incompletely defined. Here, a functional wbbL gene was introduced into the E. coli K-12 KEIO single-gene deletion mutant library to restore OAg synthesis, enabling genome-wide analysis of S-LPS production by screening with colicin E2 (ColE2) and validated with LPS silver staining. This identified 319 mutants with increased sensitivity to ColE2 in the presence of OAg, suggesting broader envelope-associated effects during screening. In addition, 27 mutants showed defects in S-LPS production, corresponding to genes involved in OAg biosynthesis, LPS core and sugar precursor synthesis, OAg ligation and regulation, and enterobacterial common antigen biosynthesis. A further 18 mutants initially appeared defective in S-LPS production but could not be validated upon reconstruction, and whole-genome sequencing revealed secondary mutations responsible for the observed phenotypes. This study provides a validated genetic framework for S-LPS production in E. coli K-12 and highlights the importance of rigorous validation in genome-wide screening approaches.