A 58-year-old man with liver metastases from rectal cancer underwent laparoscopic rectal resection, sigmoid-rectal anastomosis, lymphadenectomy, temporary ileostomy, and segmentectomy. On the 3rd day after surgery, fever up to 38°C with cough and yellow sputum appeared, CT showed possible bilateral pneumonia and metastases. Later, the fever rose to 40.6°C with chills, abdominal pain, distension, a nonhealing wound with pus, and elevated inflammatory markers such as C-reactive protein and procalcitonin. Blood culture was positive for Clostridium innocuum identified by MALDI-TOF spectrometry after 24 hours and Eggerthella lenta after 72 hours. Vancomycin 1g + piperacillin/tazobactam 4.5g every 12 hours for 7 days were given, fever subsided and blood culture was negative. Further treatment with cefoperazone sulbactam and levofloxacin lasted 9 days, the patient was discharged in an improved condition. These anaerobic bacteria can cause bloodstream infection after colorectal surgery and can be quickly identified by the MALDI-TOF method.