Seventy-eight clinical isolates and four control strains of viridans streptococci were tested in parallel for arginine hydrolysis by five different methods. These comprised two commercial systems, the API20 STREP and Vitek GPI card, two published methods, one based on ammonia production and one on alkalisation of Møeller's decarboxylase medium, and a method based on alkalisation of a phenol-red broth medium dispensed into microtitration plates. The clinical isolates were speciated by their biochemical reactions in the API20 STREP and API20 ZYM systems. One strain produced only a weak reaction for arginine hydrolysis in the medium by ammonia production, but otherwise the results with this medium, the API20 STREP and the microtitration plate method were identical. Tests with the Moeller decarboxylase medium and Vitek GPI card gave negative results with isolates that were positive by other methods. Inoculum size was shown to influence arginine hydrolysis obtained with NCTC 7863 and 10713.


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