Spontaneous Bacteroides fragilis spondylodiscitis in an elderly lady

Abstract

A 76-year-old woman presented following two episodes of unexplained falls at home. Blood cultures were positive for Bacteroides fragilis and following investigations she was diagnosed with L4/L5 spondylodiscitis confirmed on spine MRI. She was initially treated with intravenous metronidazole and flucloxacillin prior to switching to ceftriaxone with good results. No primary cause of B. fragilis bacteraemia was found in this case. B. fragilis is a rare cause of spondylodiscitis.

Background

Vertebral osteomyelitis, also known as spondylodiscitis, is a rare infection of the vertebral body. In approximately 50% of cases, it is caused by Staphylococcus aureus, followed by coaguloase-negative Staphylococci, Streptococci and Enterobacteriaceae.¹ Most commonly, spinal cord abscesses occur secondary to haematogenous spread from a cardiopulmonary source,^{2 3} with 28% occurring from direct transmission from the mediastinum, peritoneum and retroperitoneal spaces.⁴ Anaerobic bacteria rarely cause spondylodiscitis, accounting for less than 3% of all spinal infections.^{5 6} Despite Bacteroides being an important component of the microbiome of the gut, and producing a variety of infections, it is rarely a causative agent of spondylodiscitis, with 22 cases reported in the English literature to date.^{4 5 7} Most cases of Bacteroides fragilis spondylodiscitis are attributed to local spread from synchronous infections of the gut, such as pelvic abscesses or haematogenous spread from invasive procedures such as anal dilatation, sigmoidoscopy or trauma.⁵

B. fragilis spondylodiscitis typically presents insidiously and has a vague clinical course that makes diagnosis difficult, which is usually by positive blood culture.⁸ Diagnosis must be made promptly to prevent neurological impairment and structural bone damage that can lead to debilitating chronic pain syndromes.

Case presentation

A 76-year-old woman presented to our emergency department following two episodes of unexplained and unwitnessed falls at home. On presentation she was apyrexic, with a temperature of 36.9℃, but tachycardic with a heart rate of 108 bpm. She also reported a progressive functional deterioration now requiring a stick for mobilisation while usually independent.

The patient denied the presence of any new rash, headaches, neck stiffness, cough, weight loss, diarrhoea, dysuria or haematuria, but has noted having intermittent rigours over the past weeks. She had no significant previous medical history other than a previous uncomplicated laparoscopic ovarian cyst excision 31 years earlier with normal recovery. On examination she complained of tenderness on midline palpation of her lower lumbar spine. She was neurologically intact with no reduction in power or sensation, and no bladder or bowel symptoms.

Investigations

The patient’s leucocyte count was 21.4×10⁹/L (neutrophils 8.9, lymphocytes 1.7), haemoglobin 108 g/L and the platelet count was 298×10⁹/L. The C-reactive protein (CRP) was 107 (mg/L). Urinalysis was normal, with no growth on culture. Chest X-ray was clear without any visible bony abnormalities. Blood cultures were taken and the patient was started on piperacillin-tazobactam for sepsis of unknown origin per our local guidelines.

After 24 hours of incubation the blood cultures identified growth of B. fragilis, sensitive to metronidazole in the anaerobic sample. A contrast-enhanced CT of the patient’s abdomen and pelvis was performed to investigate for a potential abdominal source of infection; however, it incidentally revealed sclerosis of L4/L5 vertebra with subtle loss of cortical outlines. The rest of the lumbar discs showed a normal vacuum phenomenon. This was suggestive of L4/L5 spondylodiscitis. There were no other positive findings in the abdominal cavity.

On further questioning, the patient revealed that she had an 8-week history of worsening back pain that started while helping a neighbour carry her bin bags. It was initially managed conservatively by her general practitioner with oral naproxen, but no further investigations had been performed.

The patient then underwent an MRI scan confirming the diagnosis (figure 1). This revealed features suggestive of spondylodiscitis of L4/L5 with bone marrow oedema in the vertebral bodies, as well as a 1 cm width epidural abscess extending posterior to the body of the L4 vertebra, causing thecal sac compression in the left parasagittal location. A smaller right paravertebral abscess was also noted, and no psoas abscesses were found.

Figure 1.

MRI spine with features suggestive of discitis with bone marrow oedema in the vertebral bodies at L4/5.

Differential diagnosis

The differential diagnosis for acute onset back pain is broad. Common causes in the elderly include musculoskeletal causes such as compression fracture, degenerative disc disease or facet arthropathy, and muscular strain. The differential when associated with systemic symptoms and elevated inflammatory markers includes inflammatory causes such as sacroiliitis, infectious causes such as vertebral osteomyelitis, and malignant causes such as primary or metastatic malignancy. History and examination help to guide investigation. When neurological deficits are present, urgent MRI scan is imperative to rule out cord compression. A needle biopsy is the gold standard for diagnosis of spondylodiscitis.

Treatment

A provisional diagnosis of vertebral osteomyelitis secondary to B. fragilis was made. Antibiotic regimen was changed to oral metronidazole, as per culture sensitivities, and intravenous flucloxacillin as per our local guidelines for discitis while piperacillin-tazobactam was discontinued. Following 24 hours of treatment on the new regimen the CRP dropped to 31 (mg/L), and eventually down to 14 (mg/L) after 7 days. Treatment with both antibiotics was continued to cover for potential concurrent S. aureus and Streptococcus infections as no definitive tissue diagnosis for bacterial growth was available. Given the dramatic improvements in symptoms and blood results, it was deemed that, although desirable, performing a vertebral biopsy would not affect the patient’s management. Following a discussion with the patient, a decision was made to proceed without a biopsy.

Unfortunately the patient was unable to tolerate bed rest while in hospital and mobilised gently throughout admission, preferring to sit at the bedside rather than lay in the bed as advised by the spinal team.

Outcome and follow-up

All further blood cultures taken were negative to growth with 5 days of incubation. The patient was eventually discharged 34 days following initial admission with a 3-week course of once daily intravenous ceftriaxone continued in the community by our outreach programme.

The patient attended an outpatient clinic biweekly post discharge with repeat inflammatory markers. She completed her antibiotic course uneventfully and her inflammatory markers returned to normal with a C-reactive protein <5 mg/L and white cell count 6.0×10⁹/L. She did not have any further pain on weight bearing, and no longer required a walking aid for mobilisation, her neurological examination remained unremarkable. Two months after the completion of antibiotics the patient remained well without recurrence of symptoms, no further imaging studies were performed.

Discussion

B. fragilis has been documented as the cause of osteomyelitis in various locations,⁹ but was only first reported as a cause vertebral osteomyelitis in 1979,¹⁰ with few cases reported since (table 1).¹¹ Most cases, like the one discussed in 1979 were found to be secondary to abdominal pathologies including diverticulosis, recent colonoscopy or laparoscopic surgeries.¹¹ In a review of the literature via MEDLINE search of English case reports of B. fragilis spondylodiscitis, 8 of 22 cases (36.4%) published were found to have no identifiable source of infection.^{4 12–16} This is in keeping with a 2007 literature review by Elgouhari et al (3 of 12 cases).⁶ Risk factors for anaerobic vertebral osteomyelitis include diabetes mellitus, chronic liver disease, Gaucher disease, intravenous drug use, alcoholism, renal failure, malignancy, sickle cell anaemia and rheumatoid arthritis,^{5 6 15} none of which were present in our patient.

Table 1.

Summary of previous Bacteroides fragillis discitis reports to date

Reference	Age sex	Presenting complaint	Comorbidities	Diagnostic culture modality	Imaging modality	Treatment	Outcome	Presumed source
Sapico and Montgomerie (1979)10	18 M	Gunshot, postoperative complications: nephrectomy, persistent draining sinus, urinary tract infection, and pneumothorax	NS	Craig needle biopsy	Gallium scan	Intravenous clindamycin and gentamicin for 6 weeks; debridement of L3–L4	Complete recovery (follow-up for 6 months)	Gunshot wound, anaerobic organisms introduced at time of bullet penetration
Merine et al12	30 M	Back pain, fever and sepsis	Crohn’s disease	Blood culture sterile and CT-guided biopsy	CT scan	Antibiotics NS, abscess drainage	NS	No source identified
Merine et al12	35 M	Back pain, nausea and fever	Crohn’s disease	CT-guided biopsy	CT scan.	Antibiotics NS, abscess drainage	NS	No source identified
Feng and Austin11	68 M	Fever, intermittent chills and right upper quadrant pain. Back pain complaint 3-day postpresentation	NS	Fine-needle aspiration	CT scan	3-week intravenous cefazolin and metronidazole, then 3-month PO co-amoxiclav	Clinical resolution (no imaging)	Unclear : query Ogelvie syndrome
Bilgrami et al13	65 M	Fatigue, pain in right shoulder and anaemia	Metastatic prostate cancer	Blood culture	CT scan (no MRI)	Intravenous metronidazole 4 weeks	Died 4 months postdischarge from prostate cancer	No source identified
Chazan et al18	17 M	1-month history of lower back pain	NS	Tissue biopsy of involved disc space	Plain XR, CT scan, then MRI	Oral metronidazole 8 weeks	Complete recovery on follow-up (2 months)	Repeated anal dilatation
Doita et al19	60 M	1-month history of increased difficulty walking	Chronic lower back pain 6 months	CT-guided needle biopsy	Plain XR, CT scan	NS	Complete recovery.	Unclear: contained rupture of aneyrism of common iliac artery as cause of discitis or caused by discitis
Tsuji et al20	74 M	Increasing chronic back pain and difficulty walking	NS	Tissue biopsy of mycotic aneurysmal wall and vertebrae	Plain XR, CT scan, then MRI	Intravenous amipcillin-sulbactam for 1 week and PO tosufloxacin for 4 weeks	Complete recovery (follow-up for 3 years)	Mycotic aneurysm
Boutoille et al14	70 M	Fever and back pain	Diabetes mellitus	Blood culture	CT scan, then MRI	Intravenous metronidazole and clindamycin for 6 weeks followed by clindamycin alone for 4 weeks.	Complete recovery (follow-up 9 months)	No source identified
Lechice et al17	70 M	Fever neck pain and hand paresthesia	NS	Blood culture	MRI	Co-amoxiclav and metronidazole for 6 months.	6 months MRI showed only degenerative C2-C3 discitis	Colonoscopy with biopsy 9 days earlier
Lechice et al17	58 M	Septic shock: fever, hypotension, ascites, confusion and lumbar pain	Tuberculosis, alcoholic and viral hepatitis, cirrhosis	Blood culture, postmortem ribonucleic acid sequencing principal component analysis (RNA PCA) in infected discus	CT scan, then MRI	Co-amoxiclav, clindamycin and anti-tuberculous treatment (rifampicin, isoniazid, ethambutol).	Death secondary to candida albicans catheter-related septicaemia.	Oesophageal ulcer
Crema et al4	66 M	Back pain, fatigue, fever and leg weakness	NS	Blood culture, biopsy of intervertebral disc	MRI	6-week metronidazole and clindamycin.	MRI thoracic spine 6 weeks after initiation of treatment showing resolution of epidural and intramedullar spinal cord abscesses. Clinically improved but unable to ambulate independently.	No source identified.
Elgouhari et al6	58 M	Back pain	Diabetes mellitus, end-stage renal failure on haemodialysis	Paraspinous fluid culture	Plain XR then MRI	PO metronidazole 6 weeks.	Clinical resolution, resolution of inflammatory markers.	Incision and drainage of perianal abscess 9 months prior.
Al-Tawfiq16	18 F	Lower back pain, fever, chills, nausea and vomiting	Sickle cell	Blood culture	CT scan, then MRI	NS	NS	No source identified.
de Goeij et al15	82 M	Fever and lumbar back pain	Cerebral vascular disease, abdominal aortic aneurysm, atrial fibrillation and ischaemic cardiomyopathy	Blood culture positive, perioperative tissue culture negative	CT scan then MRI	Intravenous cefuroxime and metronidazole 5 days; 3-week intravenous clindamycin; 5-week PO metronidazole. Laminectomy	4-month postop clinically improved	No source identified
de Goeij et al15	67 M	Fever, thoracic pain and treated as pneumonia	NS	Blood culture positive, perioperative tissue culture negative	CT scan, then MRI	Intravenous clindamycin and surgical laminectomy followed by PO clindamycin	3 months of follow-up, low inflammatory markers with sustained dorsal pain	Unclear source; one single colic adenomatous polyp endoscopically removed following investigation for source
Asnani et al21	30 M	Tender painful lump over right loin and high fever	Sickle cell, splenectomy and penicillin allergy	Blood culture sterile, aspirate of psoas abscess	Plain XR, then MRI	Intravenous metronidazole and clindamycin for 8 weeks	At 6 months and 1 year, no recurrences or complications	No source identified
Kawakami et al1	77 M	Fever and shivering	NS	Blood culture, lumbar puncture revealing high protein and decreased glucose	CT scan, then MRI	Intravenous meropenem 14 days, intravenous ampicillin-sulbactam and metronidazole 7 days, PO co-amoxiclav 28 days	Radiological improvement (unspecified)	Splenic abscess
Han IH et al5	38 F	2-week history of increased back pain	Chronic low back pain 5 years	Open biopsy	MRI	Open biopsy and curettage, intravenous then PO metronidazole 5 weeks	Complete resolution on follow-up MRI	Recent percutaneous epidural adhesiolysis
Apostolis and Heiselman7	66F	Loin pain and fever	Uterine prolapse, multiple tooth extractions 12 days prior	Blood culture	CT scan unremarkable, MRI abdomen-pelvis	Intravenous meropenem and vancomycin then intravenous ertapenem, micafungin, daptomycin and PO metronidazole. Surgical laminectomy and debridement and removal of mesh from sacral promontory	6-month postop no recurrence of back pain	Admitted 1-week postlaparoscopic supracervical hysterectomy, bilateral salpingo-oophorectomy, sacrolpopexy with monofilament polypropylene Y mesh, transvaginal tape-obturator sling and cystoscopy with mesh. However, source of infection deemed secondary to dental procedures 12 days prior to urogenital surgery
Feng et al22	64 F	Minor fall and back pain	NS	Spinal aspiration	Plain XR, then MRI	Intravenous ertapenem 6 weeks and removal of possible infected mesh	1-month follow-up postop, minimal pain	Urovaginal prolapse with robotic-assisted hypsterectomy, sacrocolpopexy and urethral sling 1 month prior
Kierzkowska et al23	62M	Purulent discharge within a postoperative (transpedicular stabilisation of lumbar spine) scar and afebrile	Liver transplant, recurrent hepatitis C, hepatitis B, diabetes mellitus and invasive candidiasis	Tissue culture during debridement and removal of fixation device	NS	Intravenous clindamycin for 1 week, then PO for 4 weeks. Followed by surgical debridement with exchange of fixation device, then metronidazole and ampicillin-sulbactam for 4 weeks	Improvement of pain; no recurrence of infection on 2-year follow-up	Implant-related lumbar spine infection following recent transpedicular stabilisation of lumbar spine

NS, not specified; PO, oral; XR, X-ray.

Presentation of spondylodiscitis varies and can often be vague, although the most common clinical symptom on initial presentation is back pain, as reported in the literature as between 77%¹⁵ and 92%⁶ of cases, followed by fever.¹⁵ A rise in inflammatory markers is only moderately sensitive and was found to be elevated at presentation in 27%–88% of patients with vertebral osteomyelitis.⁶ The clinical and MRI presentations are similar in aerobic or anaerobic bacteria.¹⁷ Our case diagnosis of vertebral osteomyelitis was only achieved after initial investigation for an abdominal cause of bacteraemia via CT scan before a formal MRI was performed. Similar investigation pattern with secondary discovery of spinal pathology due to unusual presentation was seen in 7 of 22 cases.1 7 11 15–17 It is also noteworthy that like in our patient, 7 of 22 cases had no definitive tissue biopsy performed before diagnosis.7 13 14 16 17

Minimal data are available regarding the optimal antibiotic therapy for vertebral osteomyelitis, regarding the route of administration or duration of therapy. A minimum of 6 weeks of antimicrobial therapy is usually recommended, but duration of treatment should be guided by clinical improvement and inflammatory marker response.¹⁵ In treatment of B. fragilis spondylodiscitis, treatment with metronidazole is suggested due to B. fragilis’ beta lactamase making it resistant to penicillin,^{5 15} and is found to be effective in more than 95% of cases.⁶ For spondylodiscitis associated with neurologic deficits, urgent surgical decompression with laminectomy, with or without debridement is recommended.¹⁵

Learning points.

• Bacteroides fragilis spondylodiscitis can present as vague symptoms, with back pain being the most common symptom.

• Blood cultures and MRI can help in the diagnosis of spondylodiscitis, however tissue vertebral biopsy with culture is needed for confirmation of pathogenic origin.

• More than 95% of B. fragilis infections are susceptible to metronidazole. Flucloxacillin should be administered to cover for possible concurrent Staphylococcus aureus and Streptococcus infections if no definitive tissue diagnosis for bacterial growth is available.

• Patients presenting with vertebral osteomyelitis with known abdominal pathologies such as diverticulosis, recent colonoscopy or abdominal surgeries should be investigated for B. fragilis as a causative pathogen.