Down Regulation of Virulence Factors of Pseudomonas aeruginosa by Salicylic Acid Attenuates Its Virulence on Arabidopsis thaliana and Caenorhabditis elegans

Abstract

Salicylic acid (SA) is a phenolic metabolite produced by plants and is known to play an important role in several physiological processes, such as the induction of plant defense responses against pathogen attack. Here, using the Arabidopsis thaliana-Pseudomonas aeruginosa pathosystem, we provide evidence that SA acts directly on the pathogen, down regulating fitness and virulence factor production of the bacteria. Pseudomonas aeruginosa PA14 showed reduced attachment and biofilm formation on the roots of the Arabidopsis mutants lox2 and cpr5-2, which produce elevated amounts of SA, as well as on wild-type Arabidopsis plants primed with exogenous SA, a treatment known to enhance endogenous SA concentration. Salicylic acid at a concentration that did not inhibit PA14 growth was sufficient to significantly affect the ability of the bacteria to attach and form biofilm communities on abiotic surfaces. Furthermore, SA down regulated three known virulence factors of PA14: pyocyanin, protease, and elastase. Interestingly, P. aeruginosa produced more pyocyanin when infiltrated into leaves of the Arabidopsis transgenic line NahG, which accumulates less SA than wild-type plants. This finding suggests that endogenous SA plays a role in down regulating the synthesis and secretion of pyocyanin in vivo. To further test if SA directly affects the virulence of P. aeruginosa, we used the Caenorhabiditis elegans-P. aeruginosa infection model. The addition of SA to P. aeruginosa lawns significantly diminished the bacterium's ability to kill the worms, without affecting the accumulation of bacteria inside the nematodes' guts, suggesting that SA negatively affects factors that influence the virulence of P. aeruginosa. We employed microarray technology to identify SA target genes. These analyses showed that SA treatment affected expression of 331 genes. It selectively repressed transcription of exoproteins and other virulence factors, while it had no effect on expression of housekeeping genes. Our results indicate that in addition to its role as a signal molecule in plant defense responses, SA works as an anti-infective compound by affecting the physiology of P. aeruginosa and ultimately attenuating its virulence.Pseudomonas aeruginosa is a prevalent opportunistic pathogen in humans, causing chronic lung infections in cystic fibrosis patients, burn victims, and other immunocompromised people (61, 62). Pathogenesis of P. aeruginosa is mediated by a suite of cell-associated and excreted virulence factors. The cell-associated factors include flagella that aid in motility, systems that are involved in the delivery of effector proteins into the host cells (67), and lipopolysaccharide that suppresses host immune responses as well as being involved in the establishment of persistent infections (13). Secreted factors such as elastase and protease cause the degradation of host proteins such as elastin, collagen, and transferrins, destroying the integrity of the host tissues (2, 34), while low-molecular-weight toxins such as pyocyanin affect mu...

The plant signal salicylic acid shuts down expression of the vir regulon and activates quormone-quenching genes in Agrobacterium

“…However, some evidence suggests that SA may also directly affect bacteria. SA was shown to down-regulate fitness and virulence factor production in Pseudomonas aeruginosa PA14 (7). At concentrations that did not inhibit growth, SA also affected bacterial attachment and biofilm formation in this organism (7).…”

“…None of the chromosomally encoded chv genes has a vir box in its upstream sequences, nor are they regulated by the VirA/G system. Salicylic acid (SA), a plant phenolic metabolite, is a key signal molecule in regulating plant defense in response to a wide variety of pathogens (7)(8)(9). Upon infection, SA triggers either a localized or systemic acquired resistance response in which the plant gains long-lived resistance to pathogens (10).…”

Spatio Temporal Influence of Isoflavonoids on Bacterial Diversity in the Soybean Rhizosphere

“…An effective approach is to examine microbial associations with plant mutants deficient in the biosynthesis and rhizodeposition of specific groups of compounds (Prithiviraj et al 2005;Rudrappa et al 2008). It is worth noting that composition of rhizodeposits varies substantially among different plant species (Czarnota et al 2003;Warembourg et al 2003).…”

Concentration of Salicylic Acid in Tomato Leaves after Foliar Aspersions of This Compound

“…The authors found out that even in this situation, the SA showed a high concentration in cells for up to 24 hours when applying 1.0 mM, whereas for concentrations of 0.1 and 0.01 mM the permanence time of the compound in the cells was only one hour. The presence of high SA concentrations in leaves subjected to biotic or abiotic stress is attributed to its role as a signaling compound in the induction of acquired systemic resistance [21] [33] [34]. In the absence of an evident stress to induce the SA synthesis, and in the presence of exogenous SA applications made in this work, the observed concentrations could be attributed to the balance between the residual life and the transformation, or degradation, of applied SA.…”