Symbiotic microorganisms have been associated with several physiological processes in insects, including digestion. With the development of new molecular tools, more endosymbionts have been identified and linked to other developmental processes. Furthermore, some are able to manipulate insect reproduction, development and even provide defense against parasitoids and pathogens. In this study we isolated, cultured and studied bacteria living inside the midgut of the Red Imported Fire Ant Solenopsis invicta) fourth instar larvae. Images captured using Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) unveiled the internal anatomy of their digestive track and its associated microorganisms. The small-subunit ribosomal RNA gene was amplified from bacterial genomic DNA using the Polymerase Chain Reaction (PCR) and consensus sequence primers. Restriction Fragment Length Polymorphism (RFLP) analysis and NCBI database revealed at least ten different bacterial strains. Three of these strains were genetically modified with the plasmid vector pZeoDsRed and successfully reintroduced into fire ant colonies. Strong fluorescence was detected throughout the larval stage up to seven days after introduction. Although transformed bacteria were rescued after pupal emergence; most of them were excreted in the meconium. We further demonstrated that nurses contributed to the spread of the engineered bacteria within the colony by feeding the meconium to naive larvae. The role of these bacteria is being investigated by measuring the effects on the larvae and colony following antibiotic treatment. Culture independent methods and molecular tools are also being used to determine the abundance and diversity of bacteria in samples from different counties and states.