The 2012 ASHS Annual Conference

Xylella fastidiosa is a xylem-limited bacterial plant pathogen, and is the causal agent of Pierce's disease of grapevine, phony peach disease, plums leaf scald and scorch diseases of almond and many other fruit crop, ornamental and forest tree species. The disease symptoms are putatively due to blocking of the transpiration stream by the formation of biofilms (bacteria and exopolysaccharides) and the formation of plant-generated tylosis. The dissolved oxygen concentration in the xylem environment during the growing season can reach hypoxic levels (20 to 60 umol·L^-1). Wells et al., reported that X. fastidiosa was an obligate aerobe, and did not grow in an anoxic environment. Xanthomonas campestris, the closest genetic relative to X. fastidiosa, has been reported to be an obligate aerobe. The growth and biofilm formation of three strains of X. fastidiosa, X. campestris pv campestris and Erwinia sp. (facultative anaerobe) was compared in vitro in the presence of 0%, 0.21%, 2.1%, and 21% of O₂. The pattern of growth for X. fastidiosa closely resembled that of the facultative anaerobe (Erwinia sp.), and not the obligate anaerobe (X. campestris). X. fastidiosa was capable of growing in an anoxic environment in nutrient broth, and in two chemically-defined media, CHARDS and XDM2-PR. Planktonic growth and biofilm formation increased more in the anoxic treatment in the XDM2-PR medium. X. fastidiosa also produced a different organic acid profile under low oxygen conditions indicative of anaerobic fermentation.