Strong adherence of bacteria, yeast, erythrocytes, leukocytes, platelets, spores, and polystyrene spheres to membrane filter materials was noted during filtration through membranes with pore size diameters much larger than the particles themselves. Quantitative recovery on the membrane filters of these particles from low-concentration suspensions was achieved during gravity- or vacuum-assisted filtration through membranes with pore diameters as much as 30 times that of the filtered particles. Mechanical sieving was not responsible. The phenomenon was judged to be electrostatic. It could be partially blocked by pretreating the filter with a nonionic surfactant (Tween 20), and elution of adherent particles was achieved with 0.05% Tween 20. Gram-positive cocci were removed from suspension more efficiently than gram-negative rods. The commonly used cellulose membranes adsorbed more bacteria, blood cells, and other particles than did polycarbonate filters. Of lesser adsorptive capacity were vinyl acetate, nylon, acrylic, and Teflon membranes. Backwashing with saline, serum, 6% NaCl, dextran solutions, or phosphate buffers of varying molality and pH removed only a fraction of adherent particles. Tween 20 (0.05%) eluted up to 45% of adherent particles in a single back-filtration. Selected filters quantitatively removed the particles tested, which then could be washed and subjected to reagents for a variety of purposes. It is important to anticipate the removal of particles during membrane filtration, since it is not a simple mechanical event.

Thirteen representative pathogenic bacterial species were used to create septicemia in rabbits, by injecting 10(6) colony-forming units into the marginal ear vein. At a selected time, usually 30 to 60 min after injection, heart blood was drawn into heparin and dispensed in 5.0-,0.5-, and 0.1-ml volumes into duplicate bottles of commercial brain heart infusion broth with sodium polyanetholesulfonate, and into duplicate bottles of a newly developed blood-lysing solution. Lysed blood was filtered, and the filter membranes were cultured in brain heart infusion broth. At the 5.0-ml blood inoculum level, of 126 total culture bottles (63 rabbits) for each system, 83 conventional cultures versus 109 lysis-filtration cultures were positive. At the 0.5-ml blood inoculum, 20 of 126 conventional culture bottles were positive, versus 66 of 126 lysis-filtration cultures. At the 0.1-ml blood inoculum, 2 of 126 conventional culture bottles were positive, versus 30 of 126 lysis-filtration cultures. Overall, 105 of 378 conventional cultures and 205 of 378 lysis-filtration cultures were positive. The advantage of the lysis-filtration system was striking for both gram-positive and gram-negative organisms at all inoculum concentrations, but was greater for gram-positive organisms. Most significant was the rate of recovery by this new system, when the number of bacteria in the blood was reduced to the point where recovery by conventional culture was unlikely. It is postulated that the superiority of lysis-filtration culture may be due to release of bacteria by lysis of phagocytes, preventing continued loss of pathogens by intracellular destruction during the first hours of blood culture.

Evaluation of a commercially available lysis-centrifugation blood culture system (Isolator, DuPont Co., Wilmington, Del.) and a lysis-filtration blood culture system for 3,111 cultures showed that both methods had comparable recoveries (73 and 68%, respectively) of significant aerobic and facultatively anaerobic isolates. The unvented conventional blood culture bottle had a recovery rate of 59%. Although the lysis-centrifugation and lysis-filtration systems had comparable recoveries of pathogens, the lysis-centrifugation system had the advantage of having colonies immediately available for further testing. The contamination rate with the lysiscentrifugation system was 3%, compared with 6% with the lysis-filtration system and 0.4% with brain heart infusion.

A blood-lysing solution is described that is noninjurious to fastidious microbial pathogens. Lysed blood is filtered, and the filter with collected microbes is cultured.

A new blood lysis-filtration culture technique revealed a high incidence of Staphylococcus epidermidis in the blood of patients and of healthy people. Of 2,004 blood cultures from patients, the blood lysis method grew S. epidermidis in 233 (11.6%), whereas a conventional two-bottle culture system grew this organism in 48 (2.4%). To determine the incidence deriving from the skin, 100 mock blood cultures by each technique were performed. The antecubital fossa was prepared as for a phlebotomy. The needle was inserted through the skin but not into the vein. Needles were cultured by conventional and lysis-filtration culture. A total of 1 conventional culture of 100 (1%) and 2 lysis-filtration cultures of 100 (2%) grew S. epidermidis. Of 100 lysis-filtration and conventional control cultures with broth in place of blood, no cultures were positive. Blood samples from 8 of 117 (6.8%) healthy blood donors were positive for S. epidermidis by lysis-filtration, whereas no matching conventional cultures were positive. Phage typing patterns of skin and blood strains from selected individuals were the same. S. epidermidis isolates were often concomitant with isolates of bona fide pathogens. I conclude that intermittent, transient, asymptomatic S. epidermidis bacteremia occurs frequently in patients and in healthy humans.