Streptobacillus moniliformis bacteraemia: A case report

Dear Editor:

Streptobacillus moniliformis is a fastidious, pleomorphic, Gram-negative rod that is part of rodents’ upper respiratory tract microbiota. S. moniliformis as well as Spirillium minus cause “rat-bite fever” (RBF) disease.¹

Here we report a case of bacteraemia and possible infectious endocarditis due to S. moniliformis. A 31-year-old man was admitted to the emergency department after a two-week history of fever and unspecific cutaneous lesions in some fingers and feet. He had little dotted lesions in both hands located only on the fingers but not on the palms. There were also other unspecific lesions on the sole of the feet. Blood samples were collected to perform further serologic studies. Considering that the patient general condition was good, he was discharged under paracetamol therapy and he was referred to the infectious diseases outpatient department. One week later, the patient returned to the hospital because of persistent fever as well as the appearance of arthralgia and additional skin lesions. After examining the patient, one and two millimetre-sized petechial and purpuric lesions were found on both hands (fingers and palms) and feet (the right toe and the left heel). Some of them were slightly bigger and similar to Osler's nodes. Neither cellulitis nor oedema was observed. Empiric therapy was started with intravenous (IV) ceftriaxone 2 g/24 h due to infectious endocarditis suspicion.

The initial peripheral blood analysis (obtained one week before) and the new analysis results demonstrated normocytic anaemia, thrombocytosis, C-reactive protein value of 15 mg/L, erythrocyte sedimentation rate of 40 mm/h and normal values of rheumatoid factor, and serologic studies were negative (HIV, hepatitis, toxoplasma and Treponema pallidum serologies). Histopathological studies of skin lesions found small thrombi causing occlusive vasculopathy. Neither the ophtalmoscopic exam nor the transthoracic echocardiogram revealed any abnormalities.

Gram stain of the blood cultures (positive after 20 h incubation) showed thin and long Gram-negative bacilli. Microorganism identification directly from blood culture using MALDI-TOF mass spectrometry was not achieved. After a 72-h incubation under capnophilic atmosphere, small and greyish colonies grew on sheep blood agar subculture. Those colonies were identified using MALDI-TOF analysis as S. moniliformis with a 1.95 score. Identification by 16S rRNA PCR and sequencing yield a 709 bp amplified fragment that shared 99% identity with S. moniliformis ATCC49940 (acc. num. KP657489.1). Susceptibility testing was performed by the gradient diffusion (Etest) and disk-diffusion methods on sheep blood agar plates. The isolate was susceptible to penicillin (MIC: 0.03 mg/L), cefotaxime (MIC: 0.012 mg/L), imipenem (MIC: 0.012 mg/L), tetracycline (MIC: 0.38 mg/L) and ciprofloxacin (MIC 0.19 mg/L) and was resistant to aminoglycosides (MICs: tobramycin 8 mg/L, amikacin 32 mg/L and gentamycin 4 mg/L), colistin (MIC: 32 mg/L) and co-trimoxazole (MIC: >32 mg/L), in agreement with previous reports.^1–3

Since penicillin-resistance in S. moniloformis is extremely rare, the recommended treatment is penicillin.^1,2,5 In our case, the empirical treatment with IV ceftriaxone 2 g/day was prolonged for four weeks due to its proved clinical efficacy against S. moniliformis, and the patient cured.^4,5 Without proper treatment, the mortality associated to this disease is approximately 10%.^1,3,4 In this case the patient could have presented a typical RBF onset (fever, polyarthralgias and polymorphic cutaneous lesions in both hands and feet^1,3–5); however, it was also difficult to distinguish between RBF and possible endocarditis.⁶ Indeed, positive results of 2 separate blood cultures with S. moniliformis (a microorganism with the ability to cause infective endocarditis) as major criteria, and the presence of fever, vascular (determined by histopathology) and immunological phenomena (Osler nodes-like) as minor criteria, posed concerns for the existence of possible infective endocarditis.

After knowing the blood culture results, the patient was asked about animal contact. He explained he had four dogs, two cats and two domestic rats, but he did not remember being bitten recently. Rats and other rodents are the main reservoir of S. moniliformis and its bite is directly related with RBF. Moreover, It can also be transmitted by contact with its saliva, excrements or through other colonized pets.^1,3,4 No history of animal bites, which is the case of our patient, have also been reported by nearly 30% of the patients.^2,5 Although, the incidence of RBF in Spain is unknown, several cases have been reported (three cases of bacteraemia and other cases of arthritis, abscesses and wound infection) after rodents’ bite.² Overall, this case emphasizes the need for a high clinical suspicion and a well-addressed anamnesis, which was indeed performed inaccurately here, in order to diagnose this infectious disease.

S. moniliformis is a fastidious bacillus that requires microaerobic atmosphere (5–10% CO₂), long incubation period (2–7 days) and culture media with blood for its growth. In blood cultures it is inhibited by the presence of sodium polyanethol sulfonate (used as an anticoagulant in some blood cultures) but it is not affected by the presence of resins in the media.^1–5 The incorporation of MALDI-TOF analysis in clinical Microbiology departments has reduced the time needed for optimal bacterial identification in such a fastidious microorganism.³ If there is a high clinical suspicion of RBF and difficulties in culturing samples, diagnosis carried out by 16S rRNA PCR analysis can be done.^1,3,5

In conclusion, infections caused by S. moniliformis in Spain are rare; ours is the fourth case of bacteraemia reported. History of rodent contact and high clinical suspicion are essential in order to adapt laboratory protocols and to improve microbiological diagnosis and patient outcome.

Acknowledgements

We thank all the staff of the Microbiology and Infectious Diseases Departments who with their daily work contributed to the improvement of the patient's care.