Scientists studied how the bacteria that cause pneumonia react to the human antimicrobial peptide LL‑37 and a chemokine called CXCL10. They found that the bacteria are killed easily in a simple lab solution, but become more resistant in a richer medium. A specific bacterial transporter called Opp makes the bacteria more vulnerable to CXCL10, while changes in another protein (FtsX) don’t affect resistance.

Antimicrobial peptides (AMPs), including chemokines, are produced during infections to kill pathogenic bacteria. To fill in gaps in knowledge about the sensitivities of <i>Streptococcus pneumoniae</i> and related <i>Streptococcus</i> species to chemokines and AMPs, we performed a systematic, quantitative study of inhibition by chemokine CXCL10 and the AMPs LL-37 and nisin. In a standard Tris-glucose buffer (TGS), all strains assayed lacked metabolic activity, as determined by resazurin (alamarBlue) reduction, and were extremely sensitive to CXCL10 and AMPs (50% inhibitory concentration [IC₅₀], &#x223c;0.04 &#x3bc;M). In TGS, changes in sensitivities caused by mutations were undetectable. In contrast, strains that retained reductive metabolic activity in a different assay buffer (NPB [10 mM sodium phosphate {pH 7.4}, 1% {vol/vol} brain heart infusion {BHI} broth]) were less sensitive to CXCL10 and AMPs than in TGS. In NPB, mutants known to respond to AMPs, such as &#x394;<i>dlt</i> mutants lacking d-alanylation of teichoic acids, exhibited the expected increased sensitivity. <i>S. pneumoniae</i> serotype 2 strain D39 was much (&#x223c;10-fold) less sensitive to CXCL10 killing in NPB than serotype 4 strain TIGR4, and the sensitivity of TIGR4 was unaffected by the absence of capsule. Candidate screening of strain D39 revealed that mutants lacking Opp (&#x394;<i>amiACDEF</i>) oligopeptide permease were significantly more resistant to CXCL10 than the wild-type strain. This increased resistance could indicate that Opp is a target for CXCL10 binding or that it transports CXCL10 into cells. Finally, &#x394;<i>ftsX</i> or &#x394;<i>ftsE</i> mutants of <i>Bacillus subtilis</i> or amino acid changes that interfere with FtsX function in <i>S. pneumoniae</i> did not impart resistance to CXCL10, in contrast to previous results for <i>Bacillus anthracis</i>, indicating that FtsX is not a general target for CXCL10 binding.<b>IMPORTANCE</b><i>S. pneumoniae</i> (pneumococcus) is a human commensal bacterium and major opportunistic respiratory pathogen that causes serious invasive diseases, killing millions of people worldwide annually. Because of its increasing antibiotic resistance, <i>S. pneumoniae</i> is now listed as a "superbug" for which new antibiotics are urgently needed. This report fills in knowledge gaps and resolves inconsistencies in the scientific literature about the sensitivity of <i>S. pneumoniae</i> and related <i>Streptococcus</i> pathogens to chemokines and AMPs. It also reveals a new mechanism by which <i>S. pneumoniae</i> can acquire resistance to chemokine CXCL10. This mechanism involves the Opp (AmiACDEF) oligopeptide transporter, which plays additional pleiotropic roles in pneumococcal physiology, quorum sensing, and virulence. Taking the results together, this work provides new information about the way chemokines kill pneumococcal cells.