Plants and animals share functionally common bacterial virulence factors

Abstract

By exploiting the ability of Pseudomonas aeruginosa to infect a variety of vertebrate and nonvertebrate hosts, we have developed model systems that use plants and nematodes as adjuncts to mammalian models to help elucidate the molecular basis of P. aeruginosa pathogenesis. Our studies reveal a remarkable degree of conservation in the virulence mechanisms used by P. aeruginosa to infect hosts of divergent evolutionary origins.

Figure 1

Macroscopic and microscopic symptoms elicited by P. aeruginosa strain UCBPP-PA14 infiltrated into Arabidopsis leaves. (A) The upper part of an Arabidopsis leaf (ecotype LL-O) was infiltrated with bacterial suspensions at a titer of 10³ cfu/cm² and photographed 2 days postinfection. A chlorotic zone (white arrow) surrounds the soft-rot symptoms. (B and C) Arabidopsis leaves infiltrated with bacterial suspension and stained with trypan blue 2 days postinfection. Whole leaves were examined by using a Zeiss Axioskop and photographed. Bacterial movement was observed along leaf veins (arrows indicate vessel parenchyma filled with bacteria). Bacteria are absent in xylem vessels (arrowheads).

Figure 2

P. aeruginosa entering the mesophylic compartment of an Arabidopsis leaf through a stomatal opening. Detached Arabidopsis leaves were soaked in a bacterial suspension at a titer of 10³ cfu/ml. At 24 h postinfection, the leaves were fixed in osmium tetroxide, followed by dehydration in ethanol. The leaves then were coated in Technics Sputter Coater Hummer II and subsequently examined and photographed by using an Amray (Bedford, MA) 1000 scanning electron microscope. Bacteria are shown to be primarily concentrated on the surface of guard cells, above the stoma.

Figure 3

(A) Strategy used to screen for P. aeruginosa virulence-associated genes. (B) Midribs of lettuce leaves were inoculated with 10 μl of a bacterial suspension at a titer of 10³ cfu/ml and photographed 4 days postinfection. Bottom leaf was inoculated with the wild-type strain PA14 (black arrowhead) and top leaf was inoculated with the PA14-toxA mutant. Brown spots (white arrowheads) on the midrib of the top leaf reveal inoculation sites.

Figure 4

Histologic section of mouse liver and kidney taken 36 h postinoculation with P. aeruginosa strain PA14. (A) Massive perivascular bacterial infiltration and associated necrosis of the surrounding hepatocytes (arrow) are visible in the liver section. Brown and Hopps (B&H) Gram stain at ×40 magnification. (B) Section of kidney from the same animal showing large bacterial infiltrates within the tubules of the renal medulla (arrow). B&H Gram stain at ×40 magnification.