Simultaneous loss of bacteriophage receptor and coaggregation mediator activities in Actinomyces viscosus MG-1

Actinomyces bacteriophages were used as tools to study coaggregation between actinomyces and streptococci. Four bacteriophage isolates, phages AV-1, AV-2, AV-3, and 1281, bound to coaggregation group A Actinomyces viscosus and to group E A. naeslundii. No binding to groups B, C, D, or F was observed. Only A. viscosus MG-1 was capable of supporting a productive infection by these phages. Spontaneously occurring bacteriophage-resistant mutants of A. viscosus MG-1 were isolated and were shown to fall into two classes. Class I mutants were resistant to all four phages, whereas class II mutants were resistant only to phage AV-3. In each case, strains resistant to a particular phage were unable to bind that phage, suggesting that a loss or alteration of the cell surface phage receptor had occurred. Both classes of mutants were unable to coaggregate with streptococci representing coaggregation group 1 and had also lost the ability to mediate one type of coaggregation with group 4 streptococci. Class II mutants also were unable to coaggregate with group 2 streptococci. Lactose-inhibitable interactions with other streptococci (groups 3 and 4) were unchanged in the mutants. The simultaneous loss of sensitivity to phage AV-3 and the ability to coaggregate with coaggregation group 1 streptococci suggests the possibility of a relationship between these two cell surface structures.