Effects of carbazole‐degradative plasmid pCAR1 on biofilm morphology in Pseudomonas putida   KT 2440

Bacteria typically form biofilms under natural conditions. To elucidate the effect of the carriage of carbazole‐degradative plasmid pCAR1 on biofilm formation by host bacteria, we compared the biofilm morphology, using confocal laser scanning microscopy, of three pCAR1 ‐free and pCAR1 ‐carrying P seudomonas hosts: P . putida KT 2440, P . aeruginosa PAO1 and P . fluorescens P f0–1. Although pCAR1 did not significantly affect biofilm formation by PAO1 or P f0–1, pCAR1 ‐carrying KT 2440 became filamentous and formed flat biofilms, whereas pCAR1 ‐free KT 2440 formed mushroom‐like biofilms. pCAR1 contains three genes encoding nucleoid‐associated proteins ( NAPs ), namely, P mr, P nd and P hu. The enhanced filamentous morphology was observed in two double mutants [ KT 2440( pCAR1 Δ pmr Δ pnd ) and KT 2440( pCAR1 Δ pmr Δ phu )], suggesting that these NAPs are involved in modulating the filamentous phenotype. Transcriptome analyses of the double mutants identified 32 candidate genes that may be involved in filamentation of KT 2440. Overexpression of PP _2193 in KT 2440 induced filamentation and overexpression of PP _0308 or PP _0309 in KT 2440( pCAR1 ) enhanced filamentation of cells over time. This suggests that pCAR1 induces development of an abnormal filamentous morphology by KT 2440 via a process involving overexpression of several genes, such as PP _2193. In addition, pCAR1 ‐encoded NAPs partly suppress too much filamentation of KT 2440( pCAR1 ) by repressing transcription of some genes, such as PP _0308 and PP _0309.