Point Mutations in the folP Gene Partly Explain Sulfonamide Resistance of Streptococcus mutans
Comparing sulfamethoxazole minimal inhibitory concentrations (MICs) for folP knockout E. coli cells transformed with pUC19 plasmid carrying differing chromosomal folP genes of streptococci. To determine the effect plasmid encoded mutant folP has on the sulfamethoxazole resistance of transformed C600ΔfolP E. coli bacteria, growths (in SMX containing media) were compared for folP-deficient E. coli transformed with pUC19 carrying triple-mutant folP (residues 37, 172, and 193: bar A), double-mutant folP (bars B, C, and D), single-mutant folP (residue 193: bar E), and wild-type folP (bar F) from S. mutans isolate 8. The sulfamethoxazole resistance of transformed folP deficient cells was notably increased by transformation with plasmid encoding triple-mutated folP (wildtype folP MIC = 20 μM, triple-mutant folP MIC = 50 μM). Note: the MIC (sulfamethoxazole) for chromosome-encoded folP in S. mutans isolate 8 was 4 mM (see Table 1). Controls comprising C600ΔfolP E. coli transformed with pUC19 encoding either mutant folP from S. mutans isolates 797 (bar G) and 135 (bar H) or wild-type folP from S. sobrinus isolate 7 (bar I) or S. downei isolate 477 (bar J) showed basal or less sulfamethoxazole resistance (MICs = 20–30 μM). *: S. mutans isolate 8 mutant folP; **: S. mutans isolate 797 mutant folP; ***: S. mutans isolate 135 mutant folP.