We monitored bacterial compositional changes during in vitro food oil degradation experiments, wherein activated sludge (AS) from different sources was employed as seed. Two AS samples were collected from different aeration processes at the same plant and one sample was collected from a different plant; these were used for the degradation of synthetic wastewater containing commercial food oil as the sole carbon source. A 16S rRNA gene-based next generation sequencing technique was applied to study the bacterial community in the seed AS samples and resultant water after in vitro degradation for two days. Out of the total assigned genera of 319, 153 were present in all three AS seed samples, while the remaining 166 genera, observed in one or two AS seed samples, accounted for only 5% of the total operational taxonomic units. We found that different bacterial groups of each AS capable of oil degradation were present in the samples after two days' incubation. This suggests that the oil degradation process differently affected the key microbial groups from different sources. Therefore, monitoring of the AS seed and optimizing the degradation condition may be required to achieve efficient degradation performance.