The study looked at how a common mouth bacteria (Streptococcus mitis-oralis) becomes resistant to the antibiotic daptomycin and whether that resistance also makes it less vulnerable to the human immune peptide LL‑37. While the resistant strain still binds daptomycin, it shows a small group of cells that take up a lot more of the drug. Both the normal and resistant bacteria react similarly to LL‑37, meaning the resistance to daptomycin doesn’t automatically make the bacteria tougher for LL‑37 to kill.

<i>Streptococcus mitis-oralis (S. mitis-oralis)</i> infections are increasingly prevalent in specific populations, including neutropenic cancer and endocarditis patients. <i>S. mitis-oralis</i> strains have a propensity to evolve rapid, high-level and durable resistance to daptomycin (DAP-R) in vitro and in vivo, although the mechanism(s) involved remain incompletely defined. We examined mechanisms of DAP-R versus cross-resistance to cationic host defense peptides (HDPs), using an isogenic <i>S. mitis-oralis</i> strain-pair: (i) DAP-susceptible (DAP-S) parental 351-WT (DAP MIC = 0.5 &#xb5;g/mL), and its (ii) DAP-R variant 351-D10 (DAP MIC &gt; 256 &#xb5;g/mL). DAP binding was quantified by flow cytometry, in-parallel with temporal (1-4 h) killing by either DAP or comparative prototypic cationic HDPs (hNP-1; LL-37). Multicolor flow cytometry was used to determine kinetic cell responses associated with resistance or susceptibility to these molecules. While overall DAP binding was similar between strains, a significant subpopulation of 351-D10 cells hyper-accumulated DAP (&gt;2-4-fold vs. 351-WT). Further, both DAP and hNP-1 induced cell membrane (CM) hyper-polarization in 351-WT, corresponding to significantly greater temporal DAP-killing (vs. 351-D10). No strain-specific differences in CM permeabilization, lipid turnover or regulated cell death were observed post-exposure to DAP, hNP-1 or LL-37. Thus, the adaptive energetics of the CM appear coupled to the outcomes of interactions of S. <i>mitis-oralis</i> with DAP and selected HDPs. In contrast, altered CM permeabilization, proposed as a major mechanism of action of both DAP and HDPs, did not differentiate DAP-S vs. DAP-R phenotypes in this <i>S. mitis-oralis</i> strain-pair.