The organism: Staphylococcus aureus grows as gram-positive cocci in clusters. It shows beta-hemolysis on sheep blood agar, and is coagulase and catalase positive. The emergence of penicillin-resistant species, due to a plasmid-based, inducible and secreted form of penicillinase, was almost complete in the late 1980s. During the next decade, strains resistant to methicillin emerged (MRSA). The resistance is conveyed by the mecA gene, which encodes a variant penicillin binding protein (PBP2a), which has a low affinity for the antibiotic. The organism described in the current case is the first isolate of VRSA with vancomycin MIC of 128 mgrams/mL (as of last month, eight clinical infections with organisms of intermediate resistance, MIC = 8 mgrams/mL, have been confirmed). The strain contained the enterococcal vanA gene, which encodes a depsipeptide precursor protein able to bypass the interaction of vancomycin with the d-alanyl-d-alanyl terminus of peptidoglycan. The vanA gene expression results in the highest MICs, occasionally as high as 500 mgrams/mL, unlike the others (B though G) which can be as low as 12 mgrams/mL. Therefore it is reasonable to hypothesize that the transfer of genetic material occurred in this patient between the VRE to the MRSA (the S. aureus strain also contained the mecA gene). It is important to note that in cases of intermediate resistant strains of S. aureus (VISA), the resistance mechanisms is not due to vanA-G genes, but potentially due to overexpression of peptidoglycan precursors.

Laboratory diagnosis: At JHH, the organism is typically identified on sheep blood agar plates, which show beta-hemolysis. After gram-stain screening, colonies should be positive for catalase and clumping observed on glass slide tests. S. aureus also secretes a soluble coagulase (tube-test), and thermostable endonuclease. Upon presumptive diagnosis, antibiotic susceptibility is set up using agar dilution procedure (a standard amount of broth culture is inoculated on agar plates containing various antibiotics in standardized concentrations. If a presumptive S. aureus is found to have vancomycin MIC equal or greater than 4 mgrams/mL, the agar susceptibility tests is repeated, and if confirmed the susceptibility is further verified using E-test antibiotic strips. Although there is a high degree of correlation between the two methods, the MICs as detected by Etest can be discrepant by a dilution step. If testing reveals MIC equal to 4 mgrams/mL, it is confirmed as VISA, and it is sent to CDC for further testing and to rule out VRSA. Often with the acquisition of vancomycin resistance, S. aureus develops an increased susceptibility to oxacillin; in a recent case at JHH this was not accompanied by a loss of mecA).  In successive cultures of S. aureus from an individual receiving vancomycin, it was also observed that the organism became negative for thermostable nuclease, and slide coagulase, while maintaining positivity for the secreted coagulase (tube test).

Copyright © 2002 THE JOHNS HOPKINS UNIVERSITY