Differential virulence contributions of the efflux transporter MexAB-OprM in Pseudomonas syringae infecting a variety of host plants

Abstract

AbstractEfflux transporters such as MexAB-OprM contribute to bacterial resistance to diverse antimicrobial compounds. Here, we show that MexB contributes to epiphytic and apoplastic growth of Pseudomonas syringae strain B728a, as well as lesion formation in common bean (Phaseolus vulgaris). While a mexB deletion mutant formed fewer lesions after topical application to common bean, these lesions contain the same number of cells (105 to 107 cells) as those caused by the wild-type strain. The internalized population size of both the WT and the MexB mutant within small segments of surface-sterilized asymptomatic portions of the leaves varied from undetectably low to as high as 105 cells/cm2. Localized populations of bacteria within the leaf must apparently exceed a threshold size of about 105 cells/cm2 in order for a visible lesion to form. Strain B728a was capable of moderate to extensive apoplastic growth in diverse host plants including lima bean (P. lunatus), fava bean (Vicia faba), pepper (Capsicum annuum), Nicotiana benthamiana, sunflower (Helianthus annuus), and tomato (Solanum lycopersicum). MexB was not required for growth in a subset of these plant species, and in which the duration of growth after inoculation was longer, indicating apparent variation in onset time or magnitude of plant chemical defenses among those hosts. The use of a hyper-susceptible efflux pump mutant strain is an informative strategy to explore the diversity of host chemical immune responses.