Antibiotic treatment has repeatedly been in the spotlight as a determinant factor for the creation of a disrupted gastrointestinal microbiota that may persist for an extended period. In humans, antibiotic-induced dysbiosis during childhood has been implicated in the pathogenesis of a wide range of diseases, including obesity and gastrointestinal and allergic conditions. Currently, there are no studies investigating the impact of antibiotic treatment on the composition of the gastrointestinal microbiota or the microbial function in kittens. The aim of this study is a detailed description and comparison of the fecal microbiota and the microbiota-host co-metabolites in serum of kittens receiving antibiotics (n=30) and control kittens not receiving antibiotics (n=15) over a time period of 10 months. A second goal of this study is to describe the normal fecal microbiota of kittens, as well as the microbiome-associated metabolome, and how both of these change during development (from 2 months to one year of age). A third goal is to investigate whether antibiotic-induced dysbiosis at a very young age is associated with pathologic conditions (such as obesity or diarrhea) during the first year of life. Antibiotic-treated kittens with upper respiratory tract disease (URTD) will be enrolled and receive either amoxicillin-clavulanic acid (n=15) or doxycycline (n=15) as part for the standard treatment for URTD. Bacterial DNA will be isolated from fecal samples and the fecal microbiome will be analyzed by MiSeq (Illumina). Serum microbial metabolites will be evaluated by performing untargeted metabolomic analysis on a lego Pegasus IV mass spectrometer. Associations between microbiota or metabolic shifts with clinical conditions such as obesity or episodes of diarrhea will also be investigated.