Prospective addition of beta-lactamase to blood culture medium

The value of adding beta-lactamase to bottles of blood-culture medium before their distribution to wards was investigated. Significantly more bottles containing beta-lactamase were culture-positive than those without (p less than 0.002). In another series, when the enzyme was added to both bottles in each set there was no significant difference in isolation rates between the two bottles. The groups of organisms which were isolated more readily when beta-lactamase was present were staphylococci and streptococci. Storage of beta-lactamase (Genzyme broad-spectrum mixture) in blood-culture medium at room temperature resulted in rapid loss of cephalosporinase activity, whereas little decline in penicillinase activity was noted over a period of 118 days.