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 10 ¹⁰ – 10 ¹¹ CFU per mL (OD ₆₀₀ 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 >10 ¹⁰ CFU per mL, when methylparaben was absent we could observe mortality using 500,000× 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 (10 ⁵ CFU per mL) led to high bacterial loads (10 ⁶ 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.