Phenotypic heterogeneity affects Stenotrophomonas maltophilia K279a colony morphotypes and ß-lactamase expression

Stenotrophomonas maltophilia K279a diverges into subpopulations with distinct but reversible phenotypes of small and big colonies when challenged with ampicillin. This observation is consistent with the formation of long cell chains during exponential growth phase and the occurrence of mainly coccoid– or rod-shaped cells in liquid media. Further, scanning electron micrographs of SMK279a revealed that cells formed gigantic outer membrane vesicles in response to ß-lactam treatment. RNA-seq analysis of small vs. big colonies unveiled that cells regulate at least seven genes differentially among colony morphotypes. Among those were the blaL1 and blaL2 genes the most strongly regulated ones with an eleven- and six-fold increased transcription, respectively. Further studies with promoter fusions of blaL1 and blaL2 genes implied that expression of both genes is also subject to high levels of phenotypic heterogeneous expression on a single cell level. Additional RNA-seq analysis of this homogenously versus heterogeneously blaL2 expressing cells identified comE homologue as differentially expressed, in which by the expression of extra copies of comE in S. maltophilia K279a reduced the level of those cells that were in a blaL2-ON model to 1% or lower. Together with genome-wide sequence analysis of cells from the different colony morphotypes, the data presented here suggests that phenotypic heterogeneity in S. maltophilia K279a is a result of non-genetic variations within isogenic populations and also polymorphisms in this strain do not influence ß-lactamase resistance phenotype. Overall design: In total 10 samples were analyzed (5 different conditions with two independent samples), condition K279a small colonies: S. maltophilia K279a small colonies from LB agar plates grown on ampicillin for 48 h, condition K279a big colonies: S. maltophilia K279a big colonies from LB agar plates grown on ampicillin for 48 h, condition K279a without ampicillin: S. maltophilia K279a colonies from LB agar plates grown without ampicillin for 48 h as a control, condition K279a liquid culture 27 h: S. maltophilia K279a grown in liquid medium for 27 h, condition K279a liquid culture 32 h: S. maltophilia K279a grown in liquid medium for 32 h