Microbial Ecology Soil Bacteria are Differentially Affected by the Resin of the Medicinal Plant Pseudognaphalium vira vira and Its Main Component Kaurenoic Acid F. Gil 1 , R. De la Iglesia 2,3 , L. Mendoza 4 , B. Gonza ´lez 2,3 and M. Wilkens 1 (1) Departamento de Biologı ´a, Facultad de Quı ´mica y Biologı ´a, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile (2) Departamento de Gene ´tica Molecular y Microbiologı ´a, Facultad de Ciencias Biolo ´ gicas, P. Universidad Cato ´ lica de Chile, Santiago, Chile (3) Center for Advanced Studies in Ecology and Biodiversity, Facultad de Ciencias Biolo ´ gicas, P. Universidad Cato ´ lica de Chile, Santiago, Chile (4) Departamento de Quı ´mica de los Materiales, Facultad de Quı ´mica y Biologı ´a, Universidad de Santiago de Chile, Santiago, Chile Received: 27 May 2005 / Accepted: 14 June 2005 / Online publication: 10 June 2006 Abstract The diterpenoid kaurenoic acid is the main component of the resin from the medicinal plant Pseudognaphalium vira vira. As some diterpenoids have antimicrobial properties, the effect of this resin and the kaurenoic acid on soil bacteria was studied. The resin of P. vira vira and purified kaurenoic acid were two to four times more effective as antibacterial agents with Gram-positive than with Gram-negative soil isolates. The chemical stability of kaurenoic acid and the antibacterial activity of both the resin and the diterpenoid were studied in micro- cosms containing plant-associated soil. After 15 days of incubation, the diterpenoid was stable, as determined by 1 H nuclear magnetic resonance and thin-layer chroma- tography, and soil extracts still exhibited antibacterial activity. However, after 30 days of incubation, loss of antibacterial activity of soil extracts correlated with removal or chemical modification of kaurenoic acid. The effect of the resin or this diterpenoid on the soil bacteria community was analyzed by the terminal restriction fragment length polymorphisms technique. After 15 days of incubation, the resin and the pure compound caused significant changes in the soil bacterial community. The relative abundance of specific bacterial groups was differentially affected by the resin compo- nents, being the effects with the resin stronger than with the kaurenoic acid. After 30 days of incubation, these changes mostly reverted. These results indicate that a plant resin containing diterpenoid compounds plays a significant role controlling specific groups of micro- organisms in the soil associated with the plant. Introduction Plants, especially those used in the folklore medicine, are a potential source of antibacterial compounds. Some plants exudate resinous material containing diterpenoids, flavonoids, and phenolic compounds. The production of resin is a response to stress, protecting the leaves from water loss, incrementing reflectance to solar radiation, and decreasing the internal temperature. Also, the sticky nature of the resin would help as a defensive mechanism to phytophagous. The specific function of plant resin compounds is largely unknown. Flavonoids may have an antioxidant function, and diterpenoids have been asso- ciated with antifungal, antiviral, cytotoxic, and antibac- terial activities [5, 18, 19]. The release of antimicrobial plant compounds to the soil should affect the soil microorganisms. It is known that agrochemicals affect the genetic diversity of the soil microbial community [23], and that plants can influence the bacterial rhizosphere [11]. Few studies have been conducted to determine the effect of naturally produced antimicrobial compounds on the diversity of soil microbial communities. The influence of antimicrobials on soil microbial communities may be a key component to understand factors that influence the structure and diversity of these communities. We studied the effect of the resin from Pseudognaphalium vira vira and its major component, the diterpenoid kaurenoic acid, on the soil bacteria associated with this plant. P. vira vira is a medicinal plant used in folk medicine as wound-healing antiseptic [20], in the treatment of cold and flu, and different bronchial illnesses [8]. This plant produces a resin [16, 20] that is mainly composed (about 6% dry weight) of diterpenoid kaurenoic acid whose structure is shown in Fig. 1. In studies using pure cultures, the resin Correspondence to: M. Wilkens; E-mail: mwilkens@lauca.usach.cl DOI: 10.1007/s00248-006-9107-z & Volume 52, 10–18 (2006) & * Springer Science+Business Media, Inc. 2006 10