The use of bacteria for the enhancement of construction materials has been a focus of study in the past few years. Microbial cells have been applied to consolidate the surface of different materials. Earthen plasters are eco-efficient building products that can be applied on new buildings but also used to protect cultural heritage structures, with several advantages. They are based on abundant, affordable and easy to obtain materials, but are vulnerable to water. New eco-efficient bioproducts were developed based on E. coli cultures supplemented with iron, that can be obtained as an industrial by-product. They were applied in laboratory on an earth plaster surface and the biotreatment efficacy was assessed. The main objective was to improve the durability of the plastering mortar when exposed to water. An optimized E. coli-iron based surface biotreatment strongly increased the plaster resistance to water with the application of only 0.0625 mL/cm2 of E. coli++Fe leading to a significant increase of water absorption time in comparison with similar plasters where the same amount of water was applied. Moreover, all biotreatments produced an increase of resistance to water absorption creating a slight waterproofing effect, ensuring compatibility. Their loss of efficacy with time guarantees reversibility of the intervention. The results show the interest to perform an in-depth study on the microstructure of biotreated earth mortars to understand the biotreatment effect.