title: High-level azithromycin resistance occurs in Neisseria gonorrhoeae as a result of a single point mutation in the 23S rRNA genes.
abstract: High-level azithromycin resistance (AZM-HR), defined as a MIC of > or = 256 mg/liter, emerged in Neisseria gonorrhoeae in the United Kingdom in 2004. To determine the mechanism of this novel phenotype, isolates from the United Kingdom that were AZM-HR (n, 19), moderately AZM resistant (MICs, 2 to 8 mg/liter) (n, 26), or sensitive (MICs, 0.12 to 0.25 mg/liter) (n, 4) were screened for methylase (erm) genes and for mutations in the mtrR promoter region, associated with efflux pump upregulation. All AZM-resistant isolates and 12 sensitive isolates were screened for mutations in domain V of each 23S rRNA allele. All AZM-HR isolates contained the A2059G mutation (Escherichia coli numbering) in three (3 isolates) or four (16 isolates) 23S rRNA alleles. Most (22/26) moderately AZM resistant isolates contained the C2611T mutation in at least 3/4 alleles. The remainder contained four wild-type alleles, as did 8/12 sensitive isolates, while one allele was mutated in the remaining four sensitive isolates. Serial passage of AZM-sensitive colonies on an erythromycin-containing medium selected AZM-HR if the parent strain already contained mutation A2059G in one 23S rRNA allele. The resultant AZM-HR strains contained four mutated alleles. Eight isolates (five moderately AZM resistant and three AZM-HR) contained mutations in the mtrR promoter. No methylase genes were detected. This is the first evidence that AZM-HR in gonococci may result from a single point mutation (A2059G) in the peptidyltransferase loop in domain V of the 23S rRNA gene. Mutation of a single allele is insufficient to confer AZM-HR, but AZM-HR can develop under selection pressure. The description of a novel resistance mechanism will aid in screening for the AZM-HR phenotype.
authors.affiliation: Sexually Transmitted Bacteria Reference Laboratory, Centre for Infections, Health Protection Agency, 61 Colindale Avenue, Colindale, London NW9 5HT, United Kingdom. stephanie.chisholm@hpa.org.uk
authors.forename: Stephanie A
authors.initials: SA
authors.lastname: Chisholm
authors.forename: Jayshree
authors.initials: J
authors.lastname: Dave
authors.forename: Catherine A
authors.initials: CA
authors.lastname: Ison
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D017963:Azithromycin / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics; D024881:Drug Resistance, Bacterial / Q000235:genetics; D009344:Neisseria gonorrhoeae / Q000187:drug effects* / Q000235:genetics*; D017354:Point Mutation / Q000235:genetics*; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic / Q000235:genetics; D012338:RNA, Ribosomal, 23S / Q000235:genetics*; D012097:Repressor Proteins / Q000235:genetics
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D012338:RNA, Ribosomal, 23S; D012097:Repressor Proteins; C087450:mtrR protein, Neisseria gonorrhoeae; D017963:Azithromycin