The mosquito Culex pipiens is the primary vector of Rift Valley fever, West Nile, encephalitis, and Zika viruses, and periodic lymphatic filariasis. Developing insecticide resistance in mosquitoes demands the development of new approaches to fight these diseases. Paratransgenesis and RNAi approaches by using engineered bacteria have been shown to reduce mosquito vector competence. Serratia-AS1 is a bacterium found in mosquitoes and was genetically modified for expression of antimalaria effector molecules that repress development of malaria parasites in mosquitoes. The aim of this study was to determine how a genetically marked Serratia strain expressing the mCherry fluorescent protein (mCherry-Serratia) affects the colonization potential, life span, blood feeding behavior, fecundity, and fertility of Cx. pipiens. mCherry-Serratia bacteria disseminated into larvae, pupae, and newly emerged adults and dramatically increased in numbers following a blood meal. The bacterium was transmitted to progeny, showing that it can extend horizontally, transstadially, and vertically through the mosquito population.The presence of mCherry-Serratia did not affect blood feeding behavior, survival rate, fecundity, and fertility of Culex mosquitoes. This is the first study to evaluate the effects of an engineered bacteria on the fitness of Cx. pipiens. Although challenges remain, such as producing engineered bacteria to secrete anti-pathogens associated with Cx. pipiens, introducing such bacteria into mosquito populations, our findings of minimal fitness cost caused by Serratia-AS1 bode well for the development of paratransgenesis and RNAi approaches.