A 16-year-old male patient with acute lymphoblastic leukemia presents to his oncologist with pain and swelling of the left knee. He recently received a course of chemotherapy and radiotherapy, and he is taking oral steroids. Straw-colored fluid with 21 white blood cells is aspirated from his knee. No microorganisms are seen on the smear, and none grow in culture. Unfortunately, only a few drops of the fluid are cultured on plate media. Over the next 6 months, the patient is in and out of the hospital, receiving antibiotics and having more cultures done, with no positive findings to explain his pain and swelling. He is admitted to the hospital, where an arthroscopic procedure is performed to evaluate the problem. Widespread synovitis is seen. Culture samples obtained from the surgery grow a yellowpigmented, gram-negative rod on blood agar, but no growth is observed on MacConkey agar. Indole and urease testing are negative, but the oxidase test and wet mount motility are positive. The bacterium is identified as Sphingomonas paucimobilis. The patient is treated with a 6-week course of intravenous amikacin and ceftazidime. Despite the effectiveness of treatment, the patient is left with residual knee pain and stiffness because of articular cartilage destruction.

Questions

1. Which microorganisms are in the differential diagnosis for the patient?

2. What tests can be done to provide differential evidence for bacterial identification?

3. What method or methods should be used to test for susceptibility of the pathogens identified in this case?