Low temperature alters bacterial growth and surface-linked behaviors; however, the genetic role of the associated pilus systems in cold adaptation remains unclear. Here, we used the psychrotolerant tundra isolate Pseudomonas fragi D12 as a model to investigate the transcriptional responses and functional divergence of three fimbrial genes, fimA, fimC, and fimD, through a combination of transcriptome analysis and gene knockout/overexpression assays. RNA-seq analysis revealed that extreme cold stress (4°C) triggered a robust induction of the fim cluster and an adjacent regulatory module comprising an Arc-family DNA-binding protein and an EAL-domain phosphodiesterase. qRT-PCR confirmed the RNA-seq trends. Functional assays demonstrated distinct ecological roles; deletion of fimA increased swimming but reduced swarming, whereas overexpression of fimA led to an increase in swarming. fimC overexpression enhanced swimming, whereas fimC deletion decreased swarming. fimD deletion increased swimming and reduced swarming, while fimD overexpression suppressed swarming. Temperature-gradient experiments further showed that across the three temperatures examined (4°C, 15°C, and 30°C), motility and biofilm formation were the highest at 15°C. Transmission electron microscopy associated these behavioral changes with altered fimbrial density and organization, and growth-curve analysis indicated no major defects in planktonic proliferation. In combination, the data point to a fimbrial apparatus that is transcriptionally responsive to cold and may mechanically modulate the coupling of a single polar flagellum to liquid and solid interfaces, while the genomic context of fimACD remains compatible with local modulation of cyclic di-GMP signaling that has yet to be examined directly.IMPORTANCELow-temperature environments are widespread in nature; however, the genetic contributions of bacterial surface appendages to cold-associated behavioral adaptation remain poorly understood. Our work, using the psychrotolerant tundra isolate Pseudomonas fragi D12, offers a tractable example in which a single chaperone-usher fimbrial operon exerts a marked influence on how cells move and form biofilms across the three temperatures examined (4°C, 15°C, and 30°C). By combining transcriptomics, defined genetic changes, and imaging, we connect cold-inducible expression of the fimACD locus with altered fimbrial architecture, motility behavior, and biofilm robustness, while separating these effects from bulk growth. The results support a view in which fimbriae in a psychrotolerant bacterium operate as adjustable elements that influence when cells favor long-range swimming versus surface-associated growth. Such information may provide direct genetic and phenotypic evidence for functional specialization of the fimbrial system under cold stress, offering new insight into the molecular strategies that enable microbial survival in low-temperature habitats.
Li et al. (Mon,) studied this question.