Autologous antimicrobials affect transposon library composition

Abstract

Genome-wide transposon insertion sequencing (Tn-seq) is a powerful tool to measure the importance of genes for growth. In this study, we applied Tn-seq to the Gram-positive model system Bacillus subtilis, and found that after growth in liquid medium the transposon library lacked transposon insertions in several genes related to lipoteichoic acid biosynthesis and cell wall teichoic acid modification. This was unexpected since these genes are not essential for normal growth. By growing the transposon library as a confluent layer of cells, and as discrete colonies, we found that these genes are only important when the transposon library is grown as a confluent layer. Thus, growing the transposon library as a mixed population reduces the fitness of teichoic acid mutants. We provide further evidence showing that this anomaly is caused by the production of multiple autologous antimicrobials and/or toxins in the growth medium. These findings show that growing genome-wide mutant libraries as mixed cultures can influence the mutant library composition.