Methylparaben dampens virulence and transmissibility of the Drosophila pathogen Pseudomonas entomophila

In the last 20 years, Pseudomonas entomophila (Pe) has emerged as a model to explore insect immunity to bacterial intestinal pathogens. Laboratory studies evidenced multiple detrimental effects of Pe on Drosophila melanogaster. However, these effects require that the bacteria are ingested in extremely high concentrations of 1010 - 1011 CFU per mL (OD600 20 - 200), questioning the relevance of this pathogen in nature. Here, we tested whether the need for such high doses may be due to protective effects of the food preservative methylparaben (Nipagin), a standard ingredient of artificial Drosophila diets. While significant mortality in flies fed diet containing standard methylparaben concentration required doses of >1010 CFU per mL, when methylparaben was absent we could observe mortality using 500,000x lower doses. Consistent with these results, we demonstrated strong bactericidal properties of methylparaben on Pe in vitro. In the absence of methylparaben even the smallest inocula (105 CFU per mL) led to high bacterial loads (106 CFU per fly) after several days, indicating the ability of Pe to grow and overcome the flies defenses. We also demonstrate that in the absence of methylparaben, infected flies could easily transmit the pathogen to other adults and to offspring, resulting in high mortality and thus highlighting the potential of Pe as a pathogen of Drosophila in nature. Our study also underscores that careful consideration should be given to food additives used in standard diets in laboratory research on host-pathogen interaction. ImportanceAccurate characterization of pathogen infections requires appropriate experimental methodologies. Infections of insects with Pe are frequently studied using fruit flies as a model organism, with laboratory cultures typically maintained on artificial media containing various food preservatives. In this study, we show that one commonly used preservative, methylparaben, significantly influences the outcome of oral infections with Pe. We found that minimal infection doses, far below the standards of the field, could be still lethal to flies raised on media without methylparaben. This increased virulence was also associated with increased transmission of the pathogen, both from infected adult flies to their offspring and to uninfected adults. Our findings show how subtle variations in experimental conditions can profoundly affect how we perceive pathogenic threats.