Comparative Susceptibility Landscapes Among Oral Streptococci: A Phenotypic Framework for Producer-Associated Inhibitory Activity

Bacteriocin research has traditionally emphasised the organisms that produce inhibitory peptides, whereas comparatively less attention has been given to the distribution of susceptibility to those inhibitory activities within microbial communities. Here, we examined comparative susceptibility among S. anginosus, S. mitis and S. pyogenes using a defined panel of ten historically characterised bacteriocin-producing streptococci and 46 indicator strains. Deferred-antagonism assays were performed independently on two experimental occasions, with all 460 producer–indicator combinations examined in each run. Susceptibility was scored using an ordinal four-category system based on inhibition-zone characteristics. The resulting susceptibility patterns were highly reproducible, with 93.7% exact categorical agreement between experimental runs and 97.8% agreement within one ordinal category. The isolates examined displayed selective combinations of susceptibility and reduced susceptibility across the producer panel rather than a simple continuum of susceptibility. In particular, the 12 S. anginosus isolates showed no inhibition by either of the two SalA-associated producer activities represented in the panel (0/12 for each in both runs) while remaining susceptible to several alternative producer-associated activities. We use the term Comparative Susceptibility Landscape (CSL) to describe this multidimensional, panel- and assay-dependent susceptibility phenotype. A CSL does not by itself identify a molecular immunity mechanism or demonstrate an ecological interaction. Rather, it provides a structured phenotypic framework from which mechanistic and ecological hypotheses can be developed.