The Brief Case: Staphylococcus intermedius Group—Look What the Dog Dragged In

CASE A46-year-old man with a history of diabetes mellitus (type I), hypertension, congestive heart failure, and coronary artery disease presented to his primary care physician with a 3-week history of ulceration on his left, great toe. The patient reported poor compliance with blood glucose level checks and insulin use. Upon examination, the physician noted a 1by 1.5-cm ulcer on the underside of the patient’s toe. The ulcer was debrided, and a specimen for microbiological culture was collected on a flocked swab. As part of this routine work-up, the physician took a detailed history and noted that the patient lived alone with his two dogs. A specimen Gram stain revealed no polymorphonuclear leukocytes but abundant Gram-positive cocci. The specimen was plated to sheep blood agar (SBA), chocolate agar, and MacConkey agar. The next day, abundant white beta-hemolytic colonies resembling Staphylococcus spp. were noted on the blood agar plate (BAP) and white colonies were noted on the chocolate plate. No other organisms grew in this culture. No growth was observed on the MacConkey agar plate. Upon further testing, the isolate was positive for catalase, coagulase (StaphTEX Blue; Hardy Diagnostics, Santa Maria, CA), and pyrrolidonyl arylamidase (PYR [Oxoid biochemical identification system]; Thermo Scientific, Hampshire, United Kingdom). A rapid immunochromatographic assay for penicillin-binding protein 2a was positive (PBP2a culture colony test; Alere Inc., Waltham, MA). The isolate was identified by matrix-assisted laser desorption– ionization time of flight (MALDI-TOF) mass spectrometry (MS) as Staphylococcus intermedius, a member of the S. intermedius group (SIG), with a score of 2.022. Susceptibility testing was notable for discrepant results for methicillin resistance testing by KirbyBauer disk diffusion: the isolate was oxacillin resistant (zone size of 17 mm) and cefoxitin susceptible (zone size of 35 mm). Otherwise, the isolate exhibited intermediate resistance to trimethoprim-sulfamethoxazole and was susceptible to vancomycin, linezolid, doxycycline, clindamycin, and erythromycin, as per Clinical and Laboratory Standards Institute document M100 (1). The patient was treated empirically with oral doxycycline and topical gentamicin cream and recovered well.