Flocculation in Schizosaccharomyces pombe

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Abstract
Each containing 10 ml. malt-extract broth (Oxoid, 2 %, w/v). The inoculum consisted of 5 x 106 stationary-phase yeast cells from a 24 h. static culture in the same medium in a tightly capped McCartney bottle. Cultures were shaken on a rotatory shaker at 150 rev./min. at 32 "; the generation time was 120 min. RESULTS AND DISCUSSION Flocculation, operationally defined as the formation of aggregates consisting of more than 10 organisms, occurred only during the stationary growth phase. Observed with a microscope, the first flocs appeared 2 to 3 h. after the end of logarithmic growth. Maximum flocculation was attained within 5 to 10 h., when flocs contained 25 to 35 % of the population. The largest floc contained up to 106 organisms. Static cultures, such as those used for inocula, did not flocculate even when incubated for weeks, provided they remained largely anaerobic. Organisms grown in a medium containing (per 1.) 5 g. yeast extract (Difco) + 30 g. glucose, static or shaken, never flocculated. Formation of flocs was not due to failure of daughter organisms to separate after fission. An hour or two before flocculation, aggregates consisting of more than 10 organisms were never observed. Moreover, separation of pairs and larger aggregates by ultrasonic treatment at the end of logarithmic growth did not prevent subsequent flocculation. Flocs were stable to dilution in deionized water and could be separated from free (non-flocculated) organisms by differential sedimentation. A flocculated culture was transferred to a 15 ml. tapered centrifuge tube and allowed to stand undisturbed for 5 min. at room temperature. The flocs settled to the bottom of the tube, whereas free organisms remained in the supernatant fluid which could then be decanted from the flocs or sampled in situ for counting with a haemocytometer. Flocs were purified by repeated sedimentation in deionized water. Free organisms separated from flocs during the time of maximum flocculation could not be induced to flocculate among themselves when shaken in their own medium and thus were considered to be non- induced or incompetent. That flocs were stable in deionized water indicates that no special component of spent medium sustained floc formation. Addition of uninoculated medium, or of

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