EVALUATION OF HORSE BLOOD AGAR (HBA) SUPPLEMENTED WITH PYRIDOXAL

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The use of a lysis-centrifugation blood culture method prompted us to explore the use of a supplemented blood agar medium to permit the growth of nutritionally deficient streptococci (NDS) which do not grow on traditional blood or chocolate agars. These NDS can cause endocarditis and bacteremia so are important to detect. If a suitable medium could be developed, in addition to use as a blood culture medium, it could also be used more routinely for abscesses and other sterile fluid cultures, which on occasion may also contain NDS as significant pathogens. Our traditional formula of horse blood agar was supplemented with differing concentrations of PXL initially and tested with control strains of NDS in order to determine the lowest concentration of PXL necessary to obtain good growth of NDS. This HBA + PXL was then tested with 35 strains of a wide variety of bacteria to check for either inhibition of growth or significant changes in morphology that the PXL might result in. These tests showed comparable growth between HBA and HBA + PXL. In addition to these 35 species, 296 strains of Group A streptococci were also tested in parallel due to a previous report which had suggested the possibility of inhibition of Group A streptococci by PXL. We found no inhibition and in fact found that may isolates grew better in the presence of PXL. HBA with 0.0005% PXL is now used as a routine plating medium in our laboratory for both blood cultures as well as other specimens, such as wounds, abscesses and sterile body fluids. We have had 2 blood isolates of NDS, the recognition of which was facilitated by the use of this medium, as well as at least one neck wound isolate which was judged to be clinically significant.