The inoculum size effect in the dimorphic fungus Candida albicans results from production of an extracellular quorum-sensing molecule (QSM). This molecule prevents mycelial development in both a growth morphology assay and a differentiation assay using three chemically distinct triggers for germ tube formation (GTF): L-proline, N-acetylglucosamine, and serum (either pig or fetal bovine). In all cases, the presence of QSM prevents the yeast-to-mycelium conversion, resulting in actively budding yeasts without influencing cellular growth rates. QSM exhibits general cross-reactivity within C. albicans in that supernatants from strain A72 are active on five other strains of C. albicans and vice versa. The QSM excreted by C. albicans is farnesol (C 15 H 26 O; molecular weight, 222.37). QSM is extracellular, and is produced continuously during growth and over a temperature range from 23 to 43°C, in amounts roughly proportional to the CFU/milliliter. Production is not dependent on the type of carbon source nor nitrogen source or on the chemical nature of the growth medium. Both commercial mixed isomer and (E,E)-farnesol exhibited QSM activity (the ability to prevent GTF) at a level sufficient to account for all the QSM activity present in C. albicans supernatants, i.e., 50% GTF at ca. 30 to 35 M. Nerolidol was ca. two times less active than farnesol. Neither geraniol (C 10 ), geranylgeraniol (C 20 ), nor farnesyl pyrophosphate had any QSM activity.The dimorphic fungus Candida albicans is one of the most important fungi in medicine (26). It is a member of the normal flora residing in the intestinal tract of humans and other animals and is thought to be acquired during passage through the birth canal (26). C. albicans is also the model system for studying the basic biology of dimorphic fungi. Because of its medical importance, molecular tools are available with C. albicans that are unavailable for other dimorphic fungi (3). One unresolved problem in fungal biology is the dependence of cell morphology on initial cell density. For fungi exhibiting yeast-mycelium dimorphism, this phenomenon has been termed the inoculum size effect (19). Under otherwise identical conditions, budding yeasts are produced following inoculation at Ն10 6 cells/ml, whereas germ tubes and mycelia are produced with inocula of Ͻ10 6 cells/ml. We believe the inoculum size effect is a general phenomenon for all dimorphic fungi. This effect has been especially well documented for C. albicans. Cell density is listed by Odds (26) as 1 of 11 general factors favoring the filamentous form.In this study we isolate and characterize the extracellular quorum-sensing molecule (QSM) which is responsible for the inoculum size effect in C. albicans. Quorum sensing is a wellknown phenomenon in prokaryotes, but it has as yet only been hinted at in eukaryotes (18). Furthermore, since quorum sensing uses extracellular signal molecules, it is poised to mediate interactions of the producing fungus with its chemical and physical environment as well as with other bacteria an...

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Two bacteriophage collections were examined with regard to their ability to form plaques on multiple bacterial host species. Nine of 10 phages studied were found to be broad-host-range bacteriophages. These phages fell into two groups. Group 1, the SN series, was isolated from sewage treatment plant samples with Sphaerotilus natansATCC 13338 as a host. The DNAs of these bacteriophages contained modified bases and were insensitive to cleavage by type I and II restriction endonucleases. The efficiency of plating of these bacteriophages was changed only slightly on the alternate host. Group 2, the BHR series, was isolated by a two-host enrichment protocol. These bacteriophages were sensitive to restriction, and their efficiency of plating was dramatically reduced on the alternate host. Our results suggest that a multiple-host enrichment protocol may be more effective for the isolation of broad-host-range bacteriophages by avoiding the selection bias inherent in single-host methods. At least two of the broad-host-range bacteriophages mediated generalized transduction. We suggest that broad-host-range bacteriophages play a key role in phage ecology and gene transfer in nature.