® CONCISE REVIEWS OF PEDIATRIC INFECTIOUS DISEASES CONTENTS Macrolide-Inducible Resistance to Clindamycin and the D-Test EDITORIAL BOARD Co-Editors: Margaret C. Fisher, MD, and Gary D. Overturf, MD Editors for this Issue: Michael Capello, MD, and Leonard Krilov, MD Board Members Michael Cappello, MD Geoffrey A. Weinberg, MD Charles T. Leach, MD Ellen G. Chadwick, MD Leonard Weiner, MD Kathleen McGann, MD Janet A. Englund, MD Charles R. Woods, MD Jennifer Read, MD Leonard R. Krilov, MD Jeffrey R. Starke, MD Macrolide-Inducible Resistance to Clindamycin and the D-Test Charles R. Woods, MD, MS Key Words: D-test, macrolide resistance, lates resistant to clindamycin were first rec- on”) erm expression, initial in vitro suscep- clindamycin ognized in 1968.9 Relapse of S. aureus in- tibility tests show resistance to all 3 antibi- (Pediatr Infect Dis J 2009;28: 1115–1118) fection in a rabbit model of endocarditis otic classes. When the inducible genotype during clindamycin therapy was observed in (iMLSB) is present, in vitro tests show resis- the early 1970s.10 Clinical and bacteriologic tance to 14- and 15-membered ring macro- lindamycin has been used to treat seri- relapse in a patient with S. aureus endocar- lides listed above, but susceptibility to clin- Cous infections caused by susceptible ditis during the fourth week of clindamycin damycin and 16-membered ring macrolides Staphylococcus aureus strains in children for therapy after initial improvement was re- (available in some countries other than the 1 more than 30 years. It remains effective for ported in 1976.11 The initial isolate was US) is retained—these agents do not induce many infections caused by community-ac- susceptible to erythromycin and clindamycin resistance to themselves.12–14 quired methicillin-resistant S. aureus (CA- while that from the relapse was resistant to The inducible methylase system is 2,3 MRSA). Clindamycin also is useful for both. This led to abandonment of clindamy- regulated at the level of mRNA translation selected infections caused by pneumococci, cin for treatment of endocarditis. (translational attenuation) rather than gene group A streptococci, and a number of other The mechanism by which resistance transcription. The gene is transcribed to microbes. Absorption after oral administra- to clindamycin can emerge during therapy, mRNA constitutively, but the mRNA cannot tion is nearly complete, yielding serum con- the D-test used to detect it, and the clinical be read into protein without the presence of a centrations that approximate those from in- implications are discussed in this review. translational inducer. In the inducible erm 4 travenous (IV) administration. This permits cassette, the open reading frame of the meth- early transition to outpatient management of ylase gene and its dedicated ribosomal bind- susceptible infections without the complica- INDUCIBLE MACROLIDE 5 ing site is preceded by DNA sequences that tions of continued IV access. RESISTANCE encode another ribosomal binding site, a Clindamycin resistance is common The clinical implications of a posi- short 19 amino-acid peptide with a stop among health care-associated MRSA strains. tive D-test begin with an understanding of codon, and an additional sequence that in- Most CA-MRSA remain susceptible to date, 15–17 2 cross-resistance for 3 antibiotic families cludes 2 inverted repeats (IRs). but resistance rates vary by region. Pneu- that share a common binding site—macro- In the attenuated state, the secondary mococcal resistance to clindamycin may ex- 6 lides (eg, erythromycin, clarithromycin, structure of the transcribed mRNA has 2-stem ceed 30% in some areas of the US, while azithromcyin), lincosamides (eg, clinda- loops (Fig. 1 panel A). The first involves the about 4% of group A streptococcal isolates 7 mycin), and group B streptogrammins (eg, last 8 codons of the short peptide (functionally, are resistant. Macrolide-inducible resistance quinupristin). This cross-resistance, called IR1) and an adjacent IR2. The second consists to clindamycin was first recognized in the 8 the MLSB phenotype, results from enzymatic of an IR3 sequence (which follows IR2) that laboratory in the early 1960s. Clinical iso- dimethylation of an adenine residue in these binds to the sequences encoding the ribosomal antibiotics’ binding site in the 23S rRNA binding site and start codon for the methylase component of the 50S ribosomal subunit. gene (functionally, IR4). This effectively From the Department of Pediatrics, University of “hides” the ribosomal binding site and start Louisville School of Medicine, Louisville, KY. The methylase enzyme is encoded by a mul- Address for correspondence: Charles R. Woods, MD, tiallele plasmid-borne gene erm, which oc- codon for the methylase in the second (IR3– MS, Pediatric Infectious Diseases, 571 S. Floyd curs predominantly as variants erm(C) or IR4) stem-loop, such that the methylase en- St., Suite 321, Louisville, KY 40202. E-mail: erm(A) in staphylococci (other variants oc- zyme cannot be translated.15 [email protected]. When macrolide molecule capable of Copyright © 2009 by Lippincott Williams & Wilkins cur in pneumococci, hemolytic streptococci, ISSN: 0891-3668/09/2812-1115 and enterococci). When the MLSB pheno- inducing the methylase translation binds to DOI: 10.1097/INF.0b013e3181c35cc5 type is the result of constitutive (“always its target site in a ribosome, the “inhibited” The Pediatric Infectious Disease Journal • Volume 28, Number 12, December 2009 www.pidj.com | 1115 Concise Reviews The Pediatric Infectious Disease Journal • Volume 28, Number 12, December 2009 translation—clindamycin does not induce resistance to itself, and in vitro testing of isolates with the iMLSB genotype demon- strates clindamycin susceptibility. However, the macrolide-inducible DNA sequences that precede the erm(C) methlyase open reading frame undergo mutations, substitutions or de- letions that generate readily translatable (now constitutive MLSB) secondary mRNA struc- tures in about 1 in 2 million replications13,14 (Fig. 1 panel C). Many infections, especially when purulent collections are present, have microbial burdens that are exceed the denom- inator of this mutation rate by 10-fold or more, such that small numbers of clindamycin-resis- tant microbes are likely common in infections ϩ caused by such iMLSB strains. During the past 10 years, treatment failure has reported in handfuls of adult and pediatric cases when clindamycin was used for MRSA infections caused by strains that initially appeared susceptible to clindamycin but resistant to macrolides. Clindamycin re- sistance was evident upon retesting of the recurring isolates.3,12,18 The common theme of these cases has been initial improvement on clindamycin with subsequent recrudes- cence days to weeks into or after completion FIGURE 1. A, Shows the schematic representation of the secondary structure of the of therapy. This is consistent with the above inducible methylase cassette in the absence of macrolides. Two stem loops are low frequency mutation rate from iMLS formed by the interactions of inverted repeat 1 (IR1) with IR2 and IR3 with IR4. IR4 B genotype to constitutive MLS erm expres- consists of the ribosomal binding site and initial codons of the methylase mRNA. B sion, where a resistant subpopulation sur- Ribsomes are unable to complex with the methylase binding site in this conforma- vives the innate and early adaptive host im- tion. B, Shows the secondary structure after a macrolide-bound ribosome com- mune responses and emerges to cause new or plexes with the first few codons of the short peptide and stalls. Other ribosomes worsening of existing signs and symptoms. not yet inhibited by macrolides then bind to and translate the methylase, which protects remaining ribosomes from further macrolide binding. Panel C shows an example of mutation of the “inducible” mRNA sequences that can lead to consti- THE D-TEST tutive methylase translation. The peptide and IR2 are deleted, and all mature ribo- The combination of resistance to somes are methylated. Adapted from Mol Cell. 2008;30:190–202. erythromycin with susceptibility to clinda- mycin in S. aureus (and other gram-positive microbes) can be due to the iMLSB genotype ribosome is still able to bind to and start the mature ribosomes extant in the microbial or efflux pumps that remove macrolides but translation of the mRNA sequence of the cytoplasm. Most ribosomes remain macroli- not clindamycin from the microbe. The D- short peptide. The ribosome stalls after read- de-free (uninhibited) and readily able to as- test, based on disk diffusion susceptibility ing several codons but remains complexed sociate with the newly exposed erm ribo- testing, is recommended to determine if the 19 with the mRNA strand (macrolides work by somal binding site. The mRNA then is iMLSB genotype is present. In the D-test, inhibiting elongation of peptide chains be- translated, producing sufficient methylase to disks containing erythromycin (15 ␮g) and ␮ yond 3 or 4 residues). This attached stalling modify the MLSB binding sites of the ribo- clindamcyin (2 g) are placed 15 to 20 mm leads to a conformational change in the sec- somes that remain uninhibited. The microbe apart on an agar plate that has been inocu- ondary structure of the mRNA: the induc- thus is protected against further ribosomal lated with the clinical isolate (Fig. 2). A ible-state stem loops are opened up, a new inactivation despite increasing intracellular clindamycin-susceptible, erythromycin-re- single