Transient restructuring of the active oral resistome during probiotic Streptococcus salivarius K12 colonization in a...

The oral cavity harbours a complex and transcriptionally active antibiotic resistance gene (ARG) reservoir shaped by polymicrobial biofilm ecology. Whether probiotic-mediated ecological modulation can remodel the active resistome without promoting horizontal gene transfer remains poorly understood.

To investigate the impact of Streptococcus salivarius K12 (Ssk12) colonisation on active resistome dynamics within saliva derived polymicrobial biofilms and determine whether probiotic driven ecological restructuring transiently alters resistance-associated transcriptional signatures.

Saliva-derived polymicrobial biofilms were established on three-dimensional melt electrowritten poly(ε-caprolactone) (MEW-mPCL) scaffolds and exposed to Ssk12. Metatranscriptomic profiling was performed across four time points (Baseline, Day 4, Day 7, and Day 10), complemented by quantitative PCR validation and ARG-mobile genetic element (MGE) co-localisation analysis to characterise resistome restructuring during probiotic colonisation and decolonisation.

Baseline biofilms contained 27 ARGs spanning 16 antibiotic classes, predominantly ermB, tet(M), and tet(W). During peak Ssk12 colonisation (Days 4-7), total ARG abundance declined to approximately 17% of baseline levels, with marked reductions in efflux-associated and β-lactam/fluoroquinolone resistance-associated transcripts. Partial resistome recovery occurred by Day 10 (~32% of baseline), indicating reversible ecological modulation rather than permanent dysbiotic restructuring. ARG dynamics were primarily reshaped by ARG-bearing taxa rather than enrichment of high-confidence putatively mobile resistance determinants.

S. salivarius K12 transiently remodelled the transcriptionally active oral resistome within structured polymicrobial biofilms without evidence of enhanced putative horizontal resistance gene mobilisation. These findings support a proof-of-concept model in which probiotic driven ecological restructuring may create a transient resistome state potentially associated with altered responsiveness to selected antibiotic classes.