bThe erm(41) gene confers inducible macrolide resistance in Mycobacterium abscessus subsp. abscessus, calling into question the usefulness of macrolides for treating M. abscessus subsp. abscessus infections. With an extended incubation (14 days), isolates with MICs of >8 g/ml are considered macrolide resistant by current CLSI guidelines. Our goals were to determine the incidence of macrolide susceptibility in U.S. isolates, the validity of currently accepted MIC breakpoints, and the erm(41) sequences associated with susceptibility. Of 349 isolates (excluding those with 23S rRNA gene mutations), 85 (24%) had clarithromycin MICs of <8 g/ml. Sequencing of the erm(41) genes from these isolates, as well as from isolates with MICs of >16 g/ml, including ATCC 19977 T , revealed 10 sequevars. The sequence in ATCC 19977 T was designated sequevar (type) 1; most macrolide-resistant isolates were of this type. Seven sequevars contained isolates with MICs of >16 g/ml. The T28C substitution in erm(41), previously associated with macrolide susceptibility, was identified in 62 isolates (18%) comprising three sequevars, with MICs of <2 (80%), 4 (10%), and 8 (10%) g/ml. No other nucleotide substitution was associated with macrolide susceptibility. We recommend that clarithromycin susceptibility breakpoints for M. abscessus subsp. abscessus be changed from <2 to <4 g/ml and that isolates with an MIC of 8 g/ml have repeat MIC testing or erm sequencing performed. Our studies suggest that macrolides are useful for treating approximately 20% of U.S. isolates of M. abscessus subsp. abscessus. Sequencing of the erm gene of M. abscessus subsp. abscessus will predict inducible macrolide susceptibility.T hree closely related taxa of rapidly growing mycobacteria (RGM) with a controversial species/subspecies status, i.e., Mycobacterium abscessus subsp. abscessus, "M. abscessus subsp. massiliense," and M. abscessus subsp. bolletii, comprise the heterogeneous M. abscessus group (1, 2). A 2013 phylogenetic analysis based on genomic sequencing corroborated previous recommendations that the three taxa should be separated into three subspecies (3). Of these three subspecies, M. abscessus subsp. abscessus and M. abscessus subsp. bolletii, a relatively rare subspecies in the United States, demonstrate the presence of inducible macrolide resistance conferred by a novel erm gene, erm(41) (4).In contrast, the erm(41) gene is generally present in isolates of M. abscessus subsp. massiliense but contains a large, 397-bp deletion (that includes position 28T) that results in a nonfunctional erm(41) gene, and thus isolates do not show inducible macrolide resistance (2).In 2009, Nash and colleagues described two strains (MAB30 and MC1028) of M. abscessus subsp. abscessus which contained a nonfunctional erm gene (4); the strains were not inducibly resistant to clarithromycin, and even after extended incubation, they demonstrated susceptible clarithromycin MICs. The loss of function of the erm genes was associated with a T-to-C substitution at position 28 of th...

The phosphine compounds PPh 2 (G-1) and PPh 2 (G-2), where G-1 is 3,5-bis(benzyloxy)benzyl and G-2 is 3,5bis((3,5-bis(benzyloxy)oxy)benzyl, were synthesized. The complexes, trans-Ir(CO)Cl(PPh 2 (G-1)) 2 , 1, and trans-Ir(CO)Cl(PPh 2 (G-2)) 2 , 2, show reversible binding with C 60 . Thermodynamic data on the reversible binding were obtained in chlorobenzene by line width analysis of the 31 P{ 1 H} NMR spectra. The activation parameters ∆G 265 q (kcal mol -1 ), ∆H q (kcal mol -1 ), and ∆S q (cal mol -1 K -1 ) were calculated as 1.3, 18, and 20 for 1‚C 60 and 1.3, 24, and 42 for 2‚C 60 . The thermodynamic quantities ∆G 265 °(kcal mol -1 ), ∆H°(kcal mol -1 ), and ∆S°(cal mol -1 K -1 ) were determined to be -2.8, -25, and -105 for 1‚C 60 and -3.0, -18, and -57 for 2‚C 60 . The rates of reaction with O 2 for 1 and 2 were measured and are comparable to that for trans-Ir(CO)Cl(PPh 3 ) 2 under similar conditions. The compound PdI 2 (PPh 2 (G-1)) 2 , 3, does not show reversible binding with C 60 . Crystal structures were obtained for 1 (a ) 10.218( 1) Å, b ) 12.731(3) Å, c ) 13.116(2) Å, R ) 112.05(1)°, β ) 108.30(1)°, γ ) 102.22(1)°, V ) 1392.2(4) Å 3 ) and 3 (a ) 10.243(2) Å, b ) 12.689(2) Å, c ) 13.146(2) Å, R ) 112.33(1)°, β ) 108.96(1)°, γ ) 101.23(1)°, V ) 1392.4(4) Å 3 ).

cMacrolide resistance has been linked to the presence of a functional erythromycin ribosomal methylase (erm) gene in most species of pathogenic rapidly growing mycobacteria (RGM). For these Mycobacterium isolates, extended incubation in clarithromycin is necessary to determine macrolide susceptibility. In contrast, the absence of a detectable erm gene in isolates of M. chelonae, M. senegalense, and M. peregrinum and a nonfunctional erm gene in M. abscessus subsp. massiliense and 15% to 20% of M. abscessus subsp. abscessus isolates renders these species intrinsically macrolide susceptible. Not all RGM species have been screened for the presence of an erm gene, including the Mycobacterium mucogenicum group (M. mucogenicum, M. phocaicum, and M. aubagnense) and Mycobacterium immunogenum. A total of 356 isolates of these two pathogenic RGM taxa from two reference laboratories (A.R.U.P. Reference Laboratories and the Mycobacteria/Nocardia Laboratory at the University of Texas Health Science Center at Tyler) underwent clarithromycin susceptibility testing with readings at 3 to 5 days and 14 days. Only 13 of the 356 isolates had resistant clarithromycin MICs at initial extended MIC readings, and repeat values on all available isolates were <2 g/ml. These studies suggest that these two additional RGM groups do not harbor functional erm genes and, like M. chelonae, do not require extended clarithromycin susceptibility testing. We propose to the Clinical Laboratory and Standards Institute that isolates belonging to these above-mentioned six rapidly growing mycobacterial groups based on molecular identification with no known functional erm genes undergo only 3 to 5 days of susceptibility testing (to exclude mutational resistance).T he discovery of a family of erythromycin ribosomal methylase (erm) genes [erm(38), erm(39), erm(40), and erm(41)] that confer inducible macrolide resistance to rapidly growing mycobacteria (RGM) changed the Clinical and Laboratory Standards Institute (CLSI) recommendations for susceptibility testing of macrolides for RGM (1-4). Previously, susceptibility to clarithromycin (the class drug for macrolides and azalides) was reported only after 3 to 5 days of incubation. However, molecular studies have shown that inducible macrolide resistance in several of these genes is detected only following extended incubation of the isolates in the presence of clarithromycin (3). Because of this relatively slow inducible resistance, the interpretation of clarithromycin MICs with rapidly growing species that contain functional erm genes requires up to 14 days to reach a final MIC result.Previous studies of the erm gene in RGM have shown the importance of detecting inducible macrolide resistance following extended incubation of Mycobacterium isolates in clarithromycin (3). Isolates of M. chelonae, M. peregrinum, and M. senegalense do not appear to contain an erm gene, while isolates of M. abscessus subsp. massiliense have a large deletion in their erm(41) gene that renders them nonfunctional (2-6). A recent study ...

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