Obesity is one of the largest current public health problems. Recent studies suggest that persistent changes in the intestinal microbiota (dysbiosis) can eventually lead to obesity. A stable core of intestinal microbiota exists, primarily composed of the phyla Firmicutes and Bacteroidetes, but their proportions can be altered by antibiotics. Such changes appear to not only alter host energy consumption but also modify host satiety mechanisms. Our study evaluated possible changes in the gut microbiota caused by oral administration of tetracycline, both alone or in combination with Lactobacillus gasseri in rats. Zoometric analyses were conducted and qPCR of fecal samples were analyzed to allow comparison before and during treatment regarding Firmicutes Bacteroidetes proportions. The results showed increased weight and body mass index (BMI) in animals treated with tetracycline alone (P < 0.05) when compared to the group that received tetracycline with probiotic, except for BMI in phase two when there was no statistical significance. Molecular analysis showed that after animals were treated with tetracycline, Firmicutes predominated over Bacteroidetes bacteria, which was coincident with increased weight and BMI. Probiotic addition may have minimized tetracycline dysbiosis, preventing excessive weight gain. Changes in microbiota caused by antibiotics have been shown to be an important factor related to childhood obesity. Microbiological manipulation of microbiota can play an important role in weight control, especially with antibiotic acting microbiota. More studies are needed to elucidate this mechanism.