Gene Expression in Pseudomonas aeruginosa : Evidence of Iron Override Effects on Quorum Sensing and Biofilm-Specific Gene Regulation

Abstract

Prior studies established that the Pseudomonas aeruginosa oxidative stress response is influenced by iron availability, whereas more recent evidence demonstrated that it was also controlled by quorum sensing (QS) regulatory circuitry. In the present study, sodA (encoding manganese-cofactored superoxide dismutase [Mn-SOD]) and Mn-SOD were used as a reporter gene and endogenous reporter enzyme, respectively, to reexamine control mechanisms that govern the oxidative stress response and to better understand how QS and a nutrient stress response interact or overlap in this bacterium. In cells grown in Trypticase soy broth (TSB), Mn-SOD was found in wild-type stationary-phase planktonic cells but not in a lasI or lasR mutant. However, Mn-SOD activity was completely suppressed in the wild-type strain when TSB was supplemented with iron. Reporter gene studies indicated that sodA transcription could be variably induced in iron-starved cells of all three strains, depending on growth stage. Iron starvation induction of sodA was greatest in the wild-type strain and least in the lasR mutant and was maximal in stationary-phase cells. Reporter experiments in the wild-type strain showed increased lasI::lacZ transcription in response to iron limitation, whereas the expression level in the las mutants was minimal and iron starvation induction of lasI::lacZ did not occur. Studies comparing Mn-SOD activity in P. aeruginosa biofilms and planktonic cultures were also initiated. In wild-type biofilms, Mn-SOD was not detected until after 6 days, although in iron-limited wild-type biofilms Mn-SOD was detected within the initial 24 h of biofilm establishment and formation. Unlike planktonic bacteria, Mn-SOD was constitutive in the lasI and lasR mutant biofilms but could be suppressed if the growth medium was amended with 25 M ferric chloride. This study demonstrated that (i) the nutritional status of the cell must be taken into account when one is evaluating QS-based gene expression; (ii) in the biofilm mode of growth, QS may also have negative regulatory functions; (iii) QS-based gene regulation models based on studies with planktonic cells must be modified in order to explain biofilm gene expression behavior; and (iv) gene expression in biofilms is dynamic.

Delays in Pseudomonas aeruginosa quorum-controlled gene expression are conditional

“…However, P. aeruginosa has a particularly high-affinity iron-acquisition system (34), and medium conditioned by pregrowth with this species might have lower levels of iron than media conditioned by other bacteria. Iron is known to be a repressor of a battery of P. aeruginosa genes, including some quorum-controlled genes (35). We observed early production of pyocyanin, a quorumcontrolled pigment involved in iron metabolism (36), when cultures were grown in conditioned medium with acyl-HSL signals.…”

Phenotypic Characterization ofStreptococcus pneumoniaeBiofilm Development

“…The increase in the number of detectable proteins may be best explained by protein modification, changes in the growth conditions, and/or adaptation to the sessile mode of growth. Environmental factors such as iron, oxygen, and lack of carbon sources have been shown not only to influence protein production within biofilms but also to profoundly impact biofilm formation by P. aeruginosa and other biofilm-forming bacteria (1,11,37,62,69,72).…”

Messing with Bacterial Quorum Sensing

“…It is maximally produced under iron-deficient conditions and repressed when Fe 3ϩ is provided in excess (71). Cross-linking of quorum sensing with nutrient availability has been observed for other bacterial systems (8). A preliminary analysis suggested that bradyoxetin-like signal molecules were present in the extracts of various Rhizobium strains as well as other species of the ␣-proteobacterial group (71).…”