A 70-year-old female was admitted as an outpatient to our orthopedics clinic with a painful, warm, and swollen right knee. Her medical history revealed that she had undergone knee replacement 15 months ago due to longstanding osteoarthritis. In addition, she had been struck by a ball 5 months previously, resulting in a cerebrospinal fluid leak requiring a minor repair surgery. Her comorbidities included impaired fasting glucose, hypertension, and grade 1 diastolic dysfunction for which she was prescribed metformin, losartan/hydrochlorothiazide, and metoprolol, respectively. She occasionally took paracetamol for knee pain. Her social history was negative for alcohol, tobacco, and drug use. The physical examination was unremarkable apart from the unilateral enlarged right knee joint, accompanied by signs of inflammation. As fever or other signs of systemic infection were absent, only baseline laboratory tests were carried out at the administration. Results of blood tests revealed slight anemia with elevated acute-phase reactants, while leukocytosis was not noted. A white blood cell count of 8,76 x 103/µl (normal 4,06–10,6 x 103/µl), a hemoglobin level of 11,3 g/dL (normal 11,9–14,9 g/dL), a C- Reactive Protein level of 1,85 mg/dL (normal < 0,5 mg/dL), and an Erythrocyte Sedimentation Rate of 55 mm/h (normal < 30 mm/h) was noted. A joint fluid aspiration was performed and the material was sent for microscopic evaluation, cell count with differential, culture, and leukocyte esterase test. The cell count of the material displayed 12450 leukocytes/mm3. Leukocyte esterase, a recently established diagnostic marker for acute bacterial arthritis, tested positive at a high level (+++) in the aspirate. Gram-stained microscopic examination of the joint fluid revealed 20–30 leukocytes and 40–50 erythrocytes per field, while no microorganism was observed. After two days of incubation, colonies with a yellowish pigment were observed on a 5% sheep blood agar plate where the joint fluid was inoculated. The isolated strain was identified as Neisseria macacae using Matrix-assisted laser desorption ionization–time-of-flight mass spectrometry (MALDI-TOF MS, Bruker, Germany). After bacterial DNA isolation by using a commercial kit based on its protocol, the sample was sequenced by using Next Generation Sequencing technology, Illumina HiSeq2000/2500 platforms. Raw reads were assembled into coting’s to be used for annotation by using blast algorithm and NCBI reference database. Based on the annotation, we identified the bacterial species as Neisseria macacae with 100% similarity. All the sequence information was added to the GenBank BankIt submission with 2748532 submission ID. Since there is no specific recommendation for N. macacae at either Clinical and Laboratory Standards Institute (CLSI) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST), antibiotic susceptibility testing was performed using the Kirby-Bauer disc diffusion method, and results were evaluated according to CLSI breakpoint tables for Neisseria menengitides. The isolate was found to be susceptible to ceftriaxone, meropenem, and ciprofloxacin.
The patient was started on intravenous ceftriaxone and, after her clinical condition had stabilized, was continued on an outpatient clinic basis with close follow up. Dental, gynecological, and cardiological investigations were normal as regards the source of infection. Both the PCR and the throat culture were negative. Clinical improvement was achieved and follow-up laboratory tests were within the normal range. No adverse outcome was observed throughout a 1-year follow-up.