The insect- and plant-associated lifestyles of Pseudomonas protegens CHA0 are preserved following serial passage through insect larvae.

The plant-beneficial bacterium Pseudomonas protegens CHA0 (CHA0) is widely studied for the biological control of soil-borne plant diseases. Beyond its root-colonising capabilities, CHA0 can also infect and kill insect larvae and thus exhibits a multi-host lifestyle shared with other plant- and insect-colonising bacteria. To better understand the robustness of this multi-host lifestyle, we subjected CHA0 to ten consecutive passages through larvae of the pest insect Plutella xylostella via repeated cycles of insect colonisation and killing forcing it into an insect-only lifestyle. Overall, serial passaging did not result in consistent changes in insect killing speed, larval or root colonisation, plant protection efficiency, microbial antagonism or in vitro growth. This suggests that its multi-host lifestyle was conserved following serial passage. Nonetheless, a few independently passaged lines showed an increase in larval killing speed, which in one case might be linked to choline uptake. To disentangle changes specific to the insect host from those arising due to the experimental system itself, we conducted parallel serial passages through the same system while omitting the insect host. In some of these lines, exposure to the background of the system led to changes in microbial antagonism and in in vitro growth, which likely are associated with mutations in regions encoding for regulatory systems. Our findings indicate that P. protegens CHA0 remains phenotypically stable in complex environments such as an insect host, suggesting that the multi-host lifestyle might also be conserved when applied in the field and supporting CHA0's potential for reliable biocontrol performance against both plant diseases and insect pests.