Background and objectives: Gut microbes have been implicated in human weight gain and involve a few species of the Enterobacteriaceae family such as Klebsiella pneumoniae. We have tried to explore the effect of early colonization of the K. pneumoniae and subsequent eradication through bacteriophage therapy in rat pups on weight gain and loss. Materials and methods: Three pairs of rats selected for mating were grouped separately. Group I having five pups were kept on a sterile diet. Five pups each belonging to group II and III were fed with K. pneumoniae. At the end of 10th week, the pups belonging to the group III were fed with K. pneumoniae-specific phages for 8 weeks. At the end of 30th week, group III were again fed with the bacterium, while group II received bacteriophage therapy for the next 8 weeks. The weight of each of the pups was noted every Monday of the week till the completion of the study. Results: There was significantly higher weight gain (p <0.001) in the rats colonized by the bacterium (50% higher) than those without the colonization by K. pneumoniae by the end of the seventh week. When the bacterium was eradicated using a specific bacteriophage cocktail orally, the mean weight decreased and became almost similar to that of the control rats in about 12 weeks. Conclusion: The bacterial species K. pneumoniae, which is a saprophyte with voracious metabolic activities, may lead to more harvesting of energy from the food and in turn lead to obesity.