Microbiology Case Study

A 37 12 months previous African American male was transferred from an out of doors hospital attributable to mitral valve endocarditis. His previous medical historical past was vital for diabetes mellitus kind II and finish stage renal illness requiring long-term dialysis. 4 months prior, he was bacteremic with methicillin resistant Staphylococcus aureus and obtained IV vancomycin remedy. At admission, his temperature was 102.1°F and labs revealed a white blood cell rely of 25.three TH/cm2 with 95% neutrophils and a left shift, a normocytic anemia, and a creatinine of 5.39 mg/dL. Bodily examination revealed a extreme mitral valve regurgitation. Varied imaging modalities confirmed vegetations on the mitral valve, full occlusion of the distal infrarenal stomach aorta, a number of subacute infarcts within the mind, a number of websites of osteomyelitis of the backbone, and a pelvic bone abscess. After accumulating preliminary blood cultures, vancomycin and cefepime had been began.

Laboratory Identification

A number of blood cultures had been optimistic and Staphylococcus aureus was recognized by MALDI-TOF mass spectrometry (Picture 1). Susceptibility testing of the isolate confirmed resistance to cefoxitin and oxacillin, per methicillin resistant S. aureus (MRSA). Attributable to a vancomycin MIC of two ug/ml by broth microdilution, an Etest was arrange and outcomes confirmed an MIC of three ug/ml (Picture 2). Rounded as much as subsequent doubling dilution, Scientific Laboratory Instruments manufacturer this resulted in a MIC of Four ug/dl, which was regarding for a vancomycin intermediate S. aureus(VISA). Repeat identification and susceptibility testing confirmed these findings. For that reason, the isolate was despatched to the division of well being for confirmatory testing.


 Vancomycin is the primary line agent for treating infections attributable to methicillin resistant S. aureus (MRSA). When MRSA isolates present diminished susceptibility outcomes to vancomycin, they’re categorized as vancomycin intermediate S. aureus (VISA) or vancomycin resistant S. aureus (VRSA). This phenomenon is regarding, because it leaves clinicians with comparatively few therapeutic choices. Broad spectrum antibiotics reminiscent of daptomycin, linezolid, or 5th technology cephalosporins (ceftaroline & ceftobiprobe) are potential therapy picks in these instances.

The Medical and Laboratory Requirements Institute (CLSI) has set the next MIC breakpoints for vancomycin in relation to S. aureus: ≤2 ug/ml vulnerable, Four-Eight ug/ml intermediate, and ≥16 resistant. If elevated vancomycin MICs are encountered within the laboratory, the isolate ought to be checked for purity, the organism identification ought to be confirmed, and susceptibility testing repeated. The laboratory ought to notify the state well being division and hospital an infection prevention crew if a VISA or VRSA is suspected. Additional testing of the isolate by the well being division and/or CDC is required. Applicable an infection management measures, reminiscent of sporting robes & gloves and adherence at hand hygiene, ought to be taken to lower the unfold of VISA/VRSA, as therapy choices are restricted.

Whereas not absolutely understood, the diminished vancomycin susceptibility in VISA isolates is considered attributable to an abnormally thickened peptidoglycan cell wall that makes it tougher for vancomycin to achieve the cell membrane and inhibit cell development. Alternatively, VRSA isolates mostly purchase the vanA vancomycin resistance gene from Enterococcus faecium, which confers high-level vancomycin resistance. Often these with VRSA infections beforehand have been contaminated with each VRE and MRSA. This co-infection permits for the vanA gene to be transferred by a plasmid or transposon from the VRE to the MRSA isolate, leading to a S. aureus isolate that’s now proof against vancomycin.

Within the case of our affected person, additional testing by the well being division confirmed his MRSA isolate had a vancomycin MIC of two ug/ml by broth microdilution (vulnerable) and an MIC three ug/ml by Etest. It was famous these outcomes had been inside a single doubling dilution. As broth micro dilution is the reference technique, his isolate was thought-about vulnerable to vancomycin. Nonetheless, the elevated MICs by each strategies recommend the isolate is creating diminished susceptibility to vancomycin. Attributable to septic emboli in numerous organs, he was not a surgical candidate and was managed medically with ceftaroline and linezolid.