Basic study of Beutin's washed sheep blood agar plate used for selective screening of verocytotoxin-producing/enterohemorrhagic Escherichia coli (VTEC/EHEC)

Abstract

Enterohemolysin production of Beutin's washed sheep blood agar plate has been used as an epidemiological marker for selective screening of Verocytotoxin-producing/Enterohemorrhagic Escherichia coli (VTEC/EHEC). In this study, we examined about the component of Beutin's washed sheep blood agar for further improvement of hemolysin production media. The following items were studied: the numbers of washings of sheep blood with phosphate buffered saline (PBS) (pH 7.2), concentration of sheep blood, kind and concentration of divalent metal ion (Ca2+, Mg2+) and basal medium. Twenty-seven strains of VT-producing E. coli of 7 different O-serotypes, 74 strains of VT-nonproducing E. coli of 24 different O-serotypes and one strain of O157 coded Escherichia hermanii were used for this basic study. In comparison of washing times of sheep blood with PBS, 5 times washing was better than 3 times, the original. In sheep blood concentration, supplement with 4% sheep blood was best for hemolysis observation. In experiment of addition of 2 divalent metal ions, Ca2+ and Mg2+, supplement with Ca2+ was more suitable than Mg2+ for hemolysis, and the supplement with 10 mM CaCl2, the original, was the best concentration. On the basal medium used in Beutin's sheep washed blood agar, 4 kinds of media were compared. In addition to Soybean-Casein Digest (SCD) agar, the original, Nutrient agar, Heart Infusion (HI) agar and Brain Heart Infusion (BHI) agar were examined, HI agar was the best blood agar base in the four media. As the above experimental results, the composition of the better Beutin's washed sheep blood agar may be summarize as follows: Heart Infusion agar 'Eiken' used for blood agar base, supplemented with 10 mM CaCl2 and 4% defibrinated sheep blood (Japan biosupp. Center) washed five times in phosphate-buffered saline, pH 7.2.