Lysis and filtration of blood culture specimens were combined with impedance detection of bacterial growth to facilitate the diagnosis of bacteremia. A blood lysis-filtration technique (Zierdt et al., 1976) was coupled to a simple, inexpensive automated detection system. The practical and technical aspects of the impedance detection system are discussed. This new blood culturing system was compared to a conventional system for 264 aerobic blood cultures. A 30-ml sample of the blood-broth mixture was withdrawn from the conventional aerobic blood culture bottle and processed in parallel. Excluding the isolation of the commonly recognized contaminants, the detection efficiency was 36% greater in the new system. A total of 53 blood cultures from 107 patients were positive by one or both methods. The new system detected 92% of the total number of positive cultures, compared with 56% detected by the routine method. The explanation of the differences is discussed.

A lysed-blood culture system that quickly lyses patients' blood near neutrality and is relatively noninjurious to more delicate pathogens such as Haemophilus influenzae and Bacteroides fragilis is reported. The lysing solution includes culture medium, 0.004 M sodium carbonate and bicarbonate, 0.04% Triton X-100,and 0.6% Rhozyme (a mixture of proteases). Most of the pathogens tested multiplied in the lysing solution. The lysed blood normally is immediately filtered. The membrane is transferred to culture broth. The greatest advantage realized from this blood culture technique is separation of pathogens from antibiotics, bactericidal antibodies, complement, opsonins, and phagocytic systems. Another advantage is the concentration of organisms into a small volume of clear medium for faster growth and visualization of growth. It was observed that both gram-negative and -positive organisms were attracted during filtration to the filter material and were not removed from it by backwashing with buffer. Thus, filter membranes with porosities much larger than would nominally be expected to retain bacteria retained all or part of light and heavy Escherichia coli and Staphylococcus aureus suspensions. Advantage may be taken of this phenomenon to use filters with larger pore sizes and avoid filter clogging by poorly lysed specimens. Porr lysis may result from addition of too much blood to the lysing solution, blood with elevated numbers of erythrocytes or leukocytes, or blood from some people whose blood is naturally more resistant to lysis.

Two cases of endocarditis caused by Cardiobacterium hominis are reported. In both instances infection was subacute and characterized by (i) implantation on abnormal valves, (ii) chronic course lasting weeks to months before recognition, and (iii) rapid clinical and bacteriological response to penicillin, as well as other antibiotics commonly used to treat infections caused by gram-negative bacilli. Our isolates of C. hominis are compared with strains in the National Institutes of Health culture collection. Optimal growth requires yeast extract and incubation at 37 degrees C with increased humidity and supplemental CO2. The production of indole, a positive oxidase reaction, and characteristic sugar fermentation distinguish C. hominis from other slow-growing, gram-negative bacilli.