A 25 year old female presented to the ED with a mild fever and a blister on her hand, which arose from a rat bite. She stated that she worked with different kinds of animals including rodents. The patient was discharged with clindamycin after two sets of blood cultures were drawn. Anaerobic bottles became positive 48 hours after incubation. Gram stain showed unusual gram negative bacilli (Figure 1). Rapid Multiplex PCR Blood culture identification (FilmArray BCID-2) panel was negative. Poor growth on Blood agar was observed after 48 hours of sub-culture. The organism was identified as Streptobacillus monoliformis by Bruker Biotyper MALDI-ToF.
S. monoliformis appears as pleomorphic gram negative rods, with its classic characteristics of “bulges” or swollen-rods in tangled chains and filaments (Image 1). Sodium polyanethol sulfonate (SPS) inhibits S. monoliformis growth. As aerobic blood culture bottles made by certain manufacturers contain SPS, the growth is mostly seen in anaerobic bottles devoid of SPS. In liquid broth, S. monoliformis grows as “puff balls.”
Since S. moniliformis is a fastidious organism, the Gram stain from direct smears, such as positive blood culture bottles, in the absence of growth on solid agar media provides an adequate preliminary diagnosis. In such circumstances, 16srRNA sequencing provides a definitive identification. When it grows on agar media, it takes from 2-3 days to as long as 7 days and may appear as “fried-egg” colonies.
Wild rodents as well as laboratory rats carry S. moniliformis in their upper respiratory tract. Cats or dogs preying on rodents also carry the organism. Hence, rat bite fever (RBF) typically occurs upon animal bites.
RBF is a systemic disease wherein patients present with fever, vomiting, headache, and muscle pain or joint swelling. While S. monoliformis is the only known cause of RBF reported in the United States, RBF in Asia is caused by Spirillum minus. Signs and symptoms of the RBF caused by these two organisms slightly differ in addition to the fact that S. minus cannot be cultured in vitro. Several cases of endocarditis and septic arthritis due to S. moniliformis have been reported in the United States.
The notion of rat bite fever as a rare infection is likely due to the fact that 1) S. moniliformis is not a reportable disease and 2) the challenges associated with recovery and identification of this organism from culture media. Thus its occurrence maybe underestimated in the past decades. However, the advancement in technology, such as 16srRNA sequencing and MALDI-ToF, has allowed better diagnosis of RBF in recent years. S. moniliformis is generally responsive to penicillin, cephalosporins, carbapenems, aztreonam, clindamycin, clarithromycin, and tetracycline while it may have intermediate susceptibility to aminoglycoside and fluroquinolones. Since there are no definitive minimal inhibitory concentration (MIC) breakpoint guidelines for this organism, clinical laboratories do not generally perform antimicrobial susceptibility testing.