Isolation and characterization of unsaturated fatty acid auxotrophs of Streptococcus pneumoniae and Streptococcus mutans

Unsaturated fatty acid (UFA) biosynthesis is essential for the maintenance of membrane structure and function in many groups of anaerobic bacteria. Like Escherichia coli, the human pathogen Streptococcus pneumoniae produces straight-chain saturated fatty acids (SFA) and monounsaturated fatty acids. In E. coli UFA synthesis requires the action of two gene products, the essential isomerase/dehydratase encoded by fabA and an elongation condensing enzyme encoded by fabB. S. pneumoniae lacks both genes and instead employs a single enzyme with only an isomerase function encoded by the fabM gene. In this paper we report the construction and characterization of an S. pneumoniae 708 fabM mutant. This mutant failed to grow in complex medium, and the defect was overcome by addition of UFAs to the growth medium. S. pneumoniae fabM mutants did not produce detectable levels of monounsaturated fatty acids as determined by gas chromatography-mass spectrometry and thin-layer chromatography analysis of the radiolabelled phospholipids. We also demonstrate that a fabM null mutant of the cariogenic organism Streptococcus mutants is a UFA auxotroph, indicating that FabM is the only enzyme involved in the control of membrane fluidity in streptococci. Finally we report that the fabN gene of Enterococcus faecalis, coding for a dehydratase/isomerase, complements the growth of S. pneumoniae fabM mutants. Taken together, these results suggest that FabM is a potential target for chemotherapeutic agents against streptococci and that S. pneumoniae UFA auxotrophs could help identify novel genes encoding enzymes involved in UFA biosynthesis.