Spontaneously occurring coaggregation-defective (COG-) mutants of oral actinomycetes and streptococci were isolated and used to study interactions between cells of these two kinds of bacteria. COGmutants of each kind of bacteria were isolated by a simple enrichment scheme. Parent strains were mixed with a coaggregating partner strain, coaggregated cells were removed by lowspeed centrifugation, and non-coaggregated cells were recycled by the addition of more partner strain cells. COGmutants

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... ituted up to 10% of the parent strain cell type in the final enriched cell suspension. Unlike their respective parent strains, COGmutants of Actinomyces viscosus T14V and Actinomncwes naeslundii ATCC 12104, and A. naeslundii I exhibited no lactose-reversible coaggregation with certain oral Streptococcus sanguis strains. However, these COGmutants were not altered in their coaggregations with another S. sangilis strain, Hi, a member of a streptococcal coaggregation group that exhibits only lactosenonreversible coaggregations with oral actinomycetes. Although all coaggregations between S. sanguis Hi and these actinomycetes appear to be alike, examination of a COGmutant of S. sanguis Hi revealed that, like its parent, it coaggregated with A. viscosus T14V and its COGmutants, but unlike its parent, it did not coaggregate with the two A. naeslundii strains or their COGmutants. Thus, it was concluded that at least two types of surface components are important in mediating coaggregation between S. sanguiis HI and actinomycetes. The COGmutant of S. sanguis allowed detection of these components, which were indistinguishable in previous studies. 1200 on May 5, 2020 by guest http://iai.asm.org/ Downloaded from INFECT. IMMUN. 1204 KOLENBRANDER on May 5, 2020 by guest http://iai.asm.org/ Downloaded from