Rare Spiroplasma turonicum Infection Reached the Bloodstream, Joint, and Skin
A 61-year-old woman in southeast France arrived with 10 days of left knee pain, but the illness proved far broader than isolated septic arthritis. Blood and joint samples both yielded Spiroplasma turonicum, an intracellular gram-negative bacterium usually associated with plants, insects, and ticks rather than human disease. The case adds a new clinical pattern to a genus already known for rare human infections: systemic spread with bacteremia, arthritis, and transient annular erythema in a severely immunocompromised host.
The patient’s immune status framed the entire presentation. She had IgA lambda multiple myeloma diagnosed in 2017, had undergone autograft, and had been receiving teclistamab since 2024. At admission, inflammatory markers were elevated, with C-reactive protein at 83 mg/L, and serum protein electrophoresis showed severe hypogammaglobulinemia at 5.6 g/L, with undetectable IgA and IgM. Profound loss of humoral defense matters here because organisms that rarely trouble immunocompetent hosts can move beyond the usual local barriers and seed the bloodstream, synovium, and skin.
Joint Fluid Culture Pointed to an Unusual Gram-Negative Spiral Bacterium
The left knee was inflamed, painful, and functionally limiting. Positron emission tomography showed hyperfixation in the popliteal region of the left leg, and knee aspiration revealed inflammatory fluid with 16,800 white cells/mL, most of them polynuclear neutrophils. That pattern fit a bacterial joint infection, but the microbiology quickly became unusual.
Knee aspirate was cultured in blood culture bottles and incubated at 35°C in a BacT/ALERT 3D system. After 48 hours, the culture flagged positive. Gram stain showed tiny gram-negative spiral-shaped bacteria, and subculture on blood agar produced several small zones of β-hemolysis without visible colony growth. MALDI Biotyper mass spectrometry did not identify the organism. Blood cultures drawn at the same time grew the same gram-negative spiral-shaped bacteria, confirming systemic spread rather than a localized joint-only process.
A second clue came from tissue. Cytologic analysis of samples collected during arthroscopic lavage and debridement showed numerous neutrophils without infiltration by cancer cells. PCR testing for Campylobacter spp. and Helicobacter spp. was negative, narrowing the differential while emphasizing how easily rare Mollicutes-like organisms can escape standard identification workflows.
Carbapenem Therapy Did Not Resolve the Inflammation
Initial management included arthroscopic lavage and debridement of the popliteal nodular lesion, followed by empirical carbapenem therapy. Blood inflammatory markers improved only partially, yet the local inflammatory signs in the knee persisted. Three weeks later, a second lavage again produced inflammatory joint fluid with the same spiral-shaped gram-negative bacteria, still not identified by MALDI-TOF.
Then the disease widened clinically. After surgery, several large asymptomatic annular erythematous lesions appeared on the trunk, arms, and legs and resolved within 48 hours. In this setting, the skin findings were not cosmetic background noise. They likely reflected systemic dissemination and an immune-mediated cutaneous response, especially because related Spiroplasma and Mycoplasma organisms have been associated with erythematous syndromes. The abrupt appearance and short duration of the rash also fit a transient inflammatory phenomenon rather than a primary dermatologic disorder.
16S rRNA Sequencing Identified the Pathogen After Conventional Methods Failed
The diagnosis was finally established through 16S rRNA gene sequencing of the joint fluid, which identified the bacterium as Spiroplasma turonicum. That step was decisive. In a laboratory where conventional culture morphology and MALDI-TOF failed to yield a species-level answer, sequencing provided the only reliable route to organism identification.
Once the pathogen was known, treatment changed to doxycycline plus levofloxacin. Blood markers and local inflammatory signs then improved progressively. After 6 days of the dual regimen and better laboratory values, the patient was discharged and completed a 6-week antibiotic course. Four months later, outpatient follow-up showed blood inflammatory markers within reference ranges and no clinical relapse.
Horsefly Exposure Fits the Ecology of the Organism
The ecology of S. turonicum offers a plausible route of acquisition. Horseflies are the main known vector, so clinicians asked about exposure history after the organism was identified. The patient recalled a short stay in the inland region of Nice in southeast France, including a visit to a horse ranch. Even brief contact with an environment where arthropod exposure is plausible can matter when the host is highly vulnerable and the pathogen is rarely considered in routine workups.
This case also enlarges the known human disease spectrum for Spiroplasma species. Previous human infections have been reported only rarely and with different species, including infections after tick exposure and another disseminated case in France involving a species associated with bees. Human S. turonicum infection had already been described once in Spain, but the current case ties the organism to bacteremia, arthritis, and skin erythema in a host with profound immune compromise.
Teclistamab and Hypogammaglobulinemia Likely Shaped the Risk
The patient’s myeloma treatment likely created the conditions for this infection to become invasive. Teclistamab targets B-cell maturation antigen and depletes both malignant and normal long-lived plasma cells, leading to prolonged hypogammaglobulinemia and a heavy infection burden. In practical terms, the loss of antibody-mediated defense reduces the host’s ability to contain unusual extracellular and intracellular organisms once exposure occurs. Infections that might otherwise remain localized can present as bloodstream infection with metastatic inflammation in joints and skin.
The joint involvement itself is notable. Arthritis due to Spiroplasma spp. had not previously been described, although Mycoplasma pneumoniae has been isolated from joint fluid in reported cases. The similarity between these organisms is biologically relevant because both belong to the same family and may share inflammatory pathways that drive mucocutaneous or synovial injury. The appearance of annular erythema in this case strengthens that hypothesis, even if the precise mechanism remains unproven.
A Rare Pathogen That Demands Sequencing, Not Assumptions
This case demonstrates how easily an unusual zoonotic or arthropod-associated organism can evade standard diagnostic algorithms. Culture showed tiny spiral-shaped gram-negative bacteria, MALDI-TOF failed, and the organism remained unidentified until 16S rRNA sequencing was performed. In immunocompromised patients with septic arthritis, bacteremia, and unexplained rash, that sequence of events should prompt a wider microbiologic search rather than repeated reliance on routine panels.
The bedside lesson is concrete: severe immunosuppression can convert a rare environmental organism into systemic disease, and species-level diagnosis may require molecular methods before targeted therapy can work. The infection resolved only after doxycycline and levofloxacin were started, after surgical source control had already failed to fully contain the process. Even so, the report remains limited to a single patient, leaving the true frequency of human S. turonicum infection, the best antimicrobial regimen, and the full range of organ involvement unresolved.