Streptococcal Macrolide Resistance Mechanisms

Importance of Macrolide Resistance of Streptococci

Macrolide resistance of S. pneumoniae is a major problem worldwide, with very high rates in some European countries and Asia, and somewhat lower rates in the Americas. The practical implication of this for the treatment of pneumococcal pneumonia is debated in the literature, but there are several case reports of failures of macrolide antimicrobials in the treatment of such infections. Treatment of upper respiratory tract infections, especially of anatomic sites where drug concentrations may be low, such as the respiratory sinuses and middle ear, may be more affected by this resistance than pneumococcal pneumonia.

Prevalence of Erythromycin Resistance of S. pneumoniae in Various Regions

Macrolide resistance of S. pyogenes appears to be relatively infrequent in North America, but emergence of more widespread resistance has been reported in Europe and other regions. The implications of this for the eradication of this bacterium from the pharynx are not clear cut; in one study the eradication rate for erythromycin treatment was 80% if the bacterium was erythromycin susceptible and 60% if it was not (Clin Infect Dis 1999;29:869).

Erythromycin susceptibility of S. pyogenes in various geographic regions

The resistance mechanisms can be desribed phenotypically and by genotype

Phenotypic Description

There are two major recognized resistance phenotypes, "MLSB" and "M"

MLS_B - Resistant to macrolides, lincosamides, and streptogramin B. Conferred by methylation of a single adenine in the bacterial 50s ribosome that binds to erythryomycin, erm. Can be either inducible (iMLS_B) or constitutive (cMLS_B)

M -Resistant to macrolides, but not lincosamides or streptograminB. Conferred by macrolide efflux pump, mef.

Very recently, two novel mechanisms of macrolide resistance of pneumococci have been described, which complicate the inference of the genetic mechanism of resistance based on the phenotype. These two mechanisms are mutations in Domain V (Roman numeral V, not the letter "V") of the 23s rRNA, and mutations of two different ribosomal proteins that constitute part of the assembled 50s rRNA, ribosomal protein L4 and ribosomal protein L22. These mutations appear to result in two different phenotypes, ML and MSB, which can be confused with the phenotypes produced by erm and mef, respectively. These phenotypes can be differentiated from the M and MLSB phenotypes by inclusion of a streptogramin B and clindamycin in the testing.

The table below gives an example of typical susceptibility patterns for the various phenotypes

Genes Encoding for Macrolide Resistance

The genes classes that encode for the phenotypes are also of two main types, "erm" and "mef"

erm - The bacterial gene class coding for erythromycin ribosomal methylase, which methylates a single adenine in 23S rRNA, itself a component of 50S rRNA. Multiple erm gene types are recognized, and the nomenclature is very confusing. However a new nomenclature system may help (Antimicrob. Agents Chemother 1999;43:2823-2830)

The erm classes that are important for resistance of streptococci are class A and class B. Most authors refer to erm(TR) and erm(AM) for designating streptococcal erm

To make matters even more confusing there can be both constitutive (always on) and inducible variants of erm, the phenotypes of which are designated as either cMLS_B or iMLS_B, respectively. erm gene regulation is complex and may involve more than one mechanism.

mef macrolide efflux pump. There are two recognized streptococcal mef types, mef(A) and mef(E), the former described in S. pyogenes and the latter in S. pneumoniae. mef(A) and mef(E) are 90% homologous, and it has been proposed that both be referred to as mef(A). mef actively pumps out macrolides (14 and 15 ring , but not 16 ring), but not lincosamides, streptogramins, or ketolides.

rRNA and ribosomal protein mutations The rRNA mutations are all in Domain 5 ("V") of 23S rRNA, very close to, or at the same location as the adenine that gets methylated by ermA ("A2058"), either at position 2058, 2059, or 2611, and are all adenine to guanine mutations. This region of Domain V is at the active translation site (peptidyl transferase region) of the 50S rRNA subunit (see Science 2000;289:905, or this link for more on ribosomal structure). There are four copies of rRNA in pneumococci, and these mutations have to occur in at least two of the ribsomes; the degree of resistance probably depends on whether the mutations are in 2, 3, or all 4 rRNAs. The ribosomal protein mutations have been mutations in either the L4 or L22 proteins, with a variety of changes, including substitutions, deletions, and additions (AAC 2002;46:371; AAC 2000;44:3395; AAC 2000;44:2118).

macrolide - erythromycin (more properly erythromycin A), clarithromycin, azithromycin (more properly an azalide) [all of these are either 14 or 15 ring compounds]. Rokitamycin is a 16 ring macrolide.

lincosamide - lincomycin and clindamycin

streptogramin B - quinupristin, a component of dalfopristin/quinupristin, or Synercid. Dalfopristin is a streptogramin A compound. Also pristinimycin I_A is a streptogramin B compound, while pristinamycin II_A is a streptogramin A compound, both of which are components of the combination antimicrobial agent, pristinamycin .

ketolide - a macrolide which has been chemically modified. Telithromycin is one example of a ketolide.

Testing of streptococcal isolates with erythromycin and clindamycin disks applied close together can often yield phenotypic information, although it is not always possible to differentiate between phenotypes using this method. See J. Clin. Microbiol 2001;39:1311-1315 for details of this method for phenotypic analysis of S. pneumoniae. The figures below are taken from that paper, with some major modifications made to simplify this discussion; see the paper for the real and more complicated story.

iMLSB	cMLSB	M
iMLSB- the clindamycin zone is blunted towards the erythromycin because the erythromycin induces clindamycin resistance cMLSB- no zone around either erythromycin or clindamycin because erm is fully expressed at all times M- no change in the clindamycin zone induced by erythromycin because mef does not pump out clindamycin regardless of erythromycin presence
A= clindamycin and B=erythromycin

Examples of triple disk testing of rRNA or ribosomal protein mutants. Disk on left is clindamycin, middle is erythromycin, and right is streptogramin B. All bacteria are S. pneumoniae. Plate A shows ML (rRNA mutation), plates B and C show MSB (Both L4 mutations). Taken from AAC 2000;44:3395.

Prevalence of the Various Phenotypes/Genotypes By Geographic Region for S. pneumoniae

Practical Implications of the S. pneumoniae Macrolide Resistance Phenotype

• If the phenotype is "M" - clindamycin, telithromycin (not yet marketed), or quinupristin/dalfopristin should retain their activities, as should the 16 ring macrolides (none marketed in the US). This could be useful in the outpatient setting, and probably may be of most importance in upper respiratory tract infections such as sinusitis and otitis media. I'm not aware of any clinical trials demonstrating this however.
• If the phenotype is "ML" or "MSB", then telithromycin should retain activity. Note that this mechanism of resistance is so far very rare in most countries, but appears to be a problem in some central and eastern European countries.
• If the phenotype is iMLSB - ask your lab to double check, as this phenotype appears very unlikely for pneumococcus
• If the phenotype is cMLSB - then telithromycin almost always(>95%) remains active, but the susceptibility to this drug should be checked specifically. Note that this differs from S. pyogenes, in which telithromycin is much less active for cMLSB producers.

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Written by Paul H. Edelstein 1/25/02.Modified 4/29/03. If you spot any errors, please let me know.