Differential Virulence Contributions of the Efflux Transporter MexAB-OprM in Pseudomonas syringae Infecting a Variety of Host Plants.

Efflux transporters such as MexAB-OprM contribute to bacterial resistance to diverse antimicrobial compounds. Here, we show that MexB contributes to epiphytic and late-stage apoplastic growth of Pseudomonas syringae strain B728a, as well as lesion formation in common bean ( Phaseolus vulgaris ). Although a ∆ mexB mutant formed fewer lesions after topical application to common bean, these lesions contain the same number of cells (10 ⁵ to 10 ⁷ cells) as those caused by the wild-type strain. The internalized population size of both the wild-type and the ∆ mexB mutant within small samples of surface-sterilized asymptomatic portions of leaves varied from undetectably low to as high as 10 ⁵ cells/cm ² . Localized bacterial populations within individual lesions consistently exceeded 10 ⁵ cells/cm ² . 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 ), but MexB was not required for growth in a subset of these plant species. A model is proposed that MexB provides resistance to as-yet-unidentified antimicrobials that differ between plant species. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.