Overall contamination (on-plus off-streak) of the Isolator (Du Pont Co.) blood culture tube (23%) was greater than that of a conventional broth blood culture bottle (0.6%) or that of a biphasic blood culture bottle (1.3%). To determine the source of this contamination, Isolator cultures of blood from 59 healthy volunteers and of sterile broth from 60 vials were made. A total of 37% of the blood cultures and 22% of the broth cultures were contaminated (P = 0.06). Staphylococcus epidermidis-contaminated cultures represented 31 and 10% of the blood and broth cultures, respectively (P = 0.06). Contamination of plates processed on a bench top, in front of horizontal laminar flow, and in a biological safety cabinet with vertical laminar flow were compared. Processing plates in a biological safety cabinet resulted in a significant reduction in the number of contaminated plates (P < 0.05). The contamination rate for 7,874 Isolator blood cultures processed in the biological safety cabinet was significantly decreased to 6.7% on-streak (9.3% on-plus offstreak). Contamination of Isolator-processed blood cultures originated from the laboratory and the patient. The former can be reduced by inoculating plates in a vertical laminar flow biological safety cabinet and by maintaining adequate quality control of media. The latter may be unavoidable.
The SeroSTAT latex agglutination test (Scott Laboratories, Inc., Fiskesville, R.I.) for identification of group D streptococci was compared with bile-esculin agar and Lancefield grouping by using 110 clinical isolates of group D streptococci (S. faecalis, 74; S. bovis, 24; S. durans, 3; S. faecium, 9) and 65 viridans streptococci (S. anginosus-constellatus, 2; Streptococcus MG, 15; S. sanguis II, 14; S. mitis, 7; S. mutans, 5; S. salivarius, 8; S. sanguis I, 8; S. acidominimus, 1; S. morbillorum, 2; S. pneumoniae, 3). All strains of group D streptococci were bile-esculin positive. SeroSTAT reactions were falsely positive with 2 strains of S. sanguisII and 1 strain of Streptococcus MG (4.6% of all viridans streptococci tested) and falsely negative with 10 S. bovis, 8 S. faecalis, 1 S. durans, and 6 S. faecium strains (22.7% of all group D streptococci tested). When considered with colonial morphology and hemolytic reaction, SeroSTAT is a rapid (60-s) and useful test for recognition of group D streptococci.
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