Sulphonamide Resistance in Neisseria meningitidis and Commensal Neisseria Species

Sammanfattning: Extensive use of the sulphonamide drugs against the bacterium Neisseria meningitidis has resulted in drug resistance development. Sulphonamide resistance in N. meningitidis is caused by alterations in the chromosomal folP gene, coding for DHPS (dihydropteroate synthase). One type of resistant DHPS has high sequence divergence compared to DHPS from susceptible strains. This divergent DHPS has a duplication of two amino acids, crucial for resistance, and an altered amino acid in position 68, important for both resistance and substrate binding. When introduced into a susceptible DHPS, these two alterations did not incur resistance and resulted in abnormal substrate binding properties. This indicated that the divergent DHPS was not directly developed by mutations, but rather had been acquired by horizontal transfer of folP from another species.Commensal Neisseria species are implied as the origin of the horizontally transferred resistance. Sulphonamide-resistant commensal Neisseria isolates were detected in throat swabs from healthy individuals not exposed to these drugs; however, transformation of resistance from these commensals to N. meningitidis was restricted in the laboratory. A comparison of the genomic region surrounding folP revealed differences in gene organisation and in the DNA uptake sequence between N. meningitidis and distantly related commensals. These differences are likely to restrict transformation between distantly related Neisseria species.DHPS participates in the folate biosynthesis pathway. The enzyme preceding DHPS in the pathway, HPPK (hydroxymethyl-dihydropterin pyrophosphokinase), from N. meningitidis was characterised and a method for studying substrate channelling from HPPK to DHPS was developed. The information gained could be exploited in the search for new antibiotics.In conclusion, well-adapted sulphonamide-resistant strains of N. meningitidis and commensal Neisseria are established in the bacterial population and resistance can be horizontally spread by natural transformation. This may explain the abundance of sulphonamide-resistant N. meningitidis, although these drugs are no longer used against this bacterium.