Molecular Bases of Three Characteristic Phenotypes of Pneumococcus: Optochin-Sensitivity,Coumarin-Sensitivity,and Quinolone-Resistance

Streptococcus pneumoniae

is uniquely sensitive to amino alcohol antimalarials in the erythro configuration, such as optochin, quinine, and quinidine. The protein responsible for the optochin (quinine)-sensitive (Opt^s, Qin^s) phenotype of pneumococcus is the proteolipid c subunit of the F₀F₁ H⁺-ATPase. Opt^R/Qin^R isolates arose by point mutations in the atpC gene and produce different amino acid changes in one of the two transmembrane α-helices of the c subunit. In addition, comparison of the sequence of the atpCAB genes of S. pneumoniae R6 (Opt^s) and M222 (an Opt^R strain produced by interspecies recombination between pneumococcus and S. oralis), and S. oralis (Opt^R) revealed that, in M222, an interchange of atpC and atpA had ocurred. We also demonstrate that optochin, quinine, and related compounds specifically inhibited the membrane-bound ATPase activity. Equivalent differences between Opt^s/Qin^s and Opt^R/Qin^R strains, both in growth inhibition and in membrane ATPase resistance, were found.

Pneumococci also show a characteristic sensitivity to coumarin drugs, and a relatively high level of resistance to most quinolones. We have cloned and sequenced the gyrB gene, and characterized novobiocin resistant mutants. The same amino acid substitution (Ser-127 to Leu) confers novobiocin resistance on four isolates. This residue position is equivalent to Val-120 of Escherichia coli ryGB, a residue that lies inside the ATP-binding domain but is not involved in novobiocin binding in E. coli, as revealed by crystallographic data. In addition, the genes encoding the ParC and ParE subunits of topoisomerase IV, together with the region encoding amino acids 46 to 172 (residue numbers as in E. coli) of the pneumococcal ryGA subunit, were characterized in respect to fluoroquinolone resistance. The gyrA gene maps to a physical location distant from the gyrB and parEC loci on the chromosome. Ciprofloxacin-resistant (Cp^R) clinical isolates had mutations affecting amino acid residues of the quinolone resistance-determining region of ParC (low-level Cp^R), or in both resistance-determining regions of ParC and GyrA (high-level Cp^R). Mutations were found in residue positions equivalent to Ser-83 and Asp-87 of the E. coli GyrA subunit. Transformation experiments demonstrated that topoisomerase IV is the primary target of ciprofloxacin, DNA gyrase being a secondary one.