The Role of Antibiotic-Target-Modifying and Antibiotic-Modifying Enzymes in Mycobacterium abscessus Drug Resistance

Abstract

The incidence and prevalence of non-tuberculous mycobacterial (NTM) infections have been increasing worldwide and lately led to an emerging public health problem. Among rapidly growing NTM, Mycobacterium abscessus is the most pathogenic and drug resistant opportunistic germ, responsible for disease manifestations ranging from "curable" skin infections to only "manageable" pulmonary disease. Challenges in M. abscessus treatment stem from the bacteria's high-level innate resistance and comprise long, costly and non-standardized administration of antimicrobial agents, poor treatment outcomes often related to adverse effects and drug toxicities, and high relapse rates. Drug resistance in M. abscessus is conferred by an assortment of mechanisms. Clinically acquired drug resistance is normally conferred by mutations in the target genes. Intrinsic resistance is attributed to low permeability of M. abscessus cell envelope as well as to (multi)drug export systems. However, expression of numerous enzymes by M. abscessus, which can modify either the drug-target or the drug itself, is the key factor for the pathogen's phenomenal resistance to most classes of antibiotics used for treatment of other moderate to severe infectious diseases, like macrolides, aminoglycosides, rifamycins, β-lactams and tetracyclines. In 2009, when M. abscessus genome sequence became available, several research groups worldwide started studying M. abscessus antibiotic resistance mechanisms. At first, lack of tools for M. abscessus genetic manipulation severely delayed research endeavors. Nevertheless, the last 5 years, significant progress has been made towards the development of conditional expression and homologous recombination systems for M. abscessus. As a result of recent research efforts, an erythromycin ribosome methyltransferase, two aminoglycoside acetyltransferases, an aminoglycoside phosphotransferase, a rifamycin ADP-ribosyltransferase, a β-lactamase and a monooxygenase were identified to frame the complex and multifaceted intrinsic resistome of M. abscessus, which clearly contributes to complications in treatment of this highly resistant pathogen. Better knowledge of the underlying mechanisms of drug resistance in M. abscessus could improve selection of more effective chemotherapeutic regimen and promote development of novel antimicrobials which can overwhelm the existing resistance mechanisms. This article reviews the currently elucidated molecular mechanisms of antibiotic resistance in M. abscessus, with a focus on its drug-target-modifying and drug-modifying enzymes.

Keywords: Mycobacterium abscessus; antibiotic; antibiotic-modifying enzymes; antibiotic-target-modifying enzymes; drug resistance; non-tuberculous mycobacteria; resistance genes.

FIGURE 1

Enzyme-mediated antibiotic resistance in Mycobacterium abscessus. M. abscessus is intrinsically resistant to a large and diverse array of antimicrobial agents. The occurrence of several enzymes that can modify and/or degrade antibiotics or alter their targets in M. abscessus enables it to resist the action of multiple classes of antibiotics. This figure provides an overview of the well-studied as well as some unexplored enzyme-mediated resistance mechanisms in M. abscessus. Methylation of 23S rRNA by Erm(41), an erythromycin ribosome methylase, lowers the binding affinity of macrolides for the ribosome exit tunnel and confers macrolide resistance. The expression of erm(41) gene in M. abscessus is induced upon exposure to macrolides, therefore this type of resistance by target modification is known as inducible macrolide resistance. The addition of different chemical groups including acyl and phosphate groups to vulnerable sites on the aminoglycoside molecule by M. abscessus enzymes can prevent the binding of aminoglycoside to its ribosomal target due to steric hindrance and result in resistance. A diverse group of enzymes that differ in the aminoglycosides that they can modify as well as in the region of the antibiotic that is modified, are found in M. abscessus. These include two putative acetyltransferases - AAC(2′) and Eis2 and one putative phosphotransferase - APH(3″). The substrate aminoglycosides for each enzyme are shown. In addition to aminoglycosides, Eis2 can also modify other ribosome-targeting antibiotics like capreomycin. Resistance to rifampicin and other rifamycin antibiotics in M. abscessus involves covalent modification and drug inactivation by an ADP-ribosyltransferase, Arr_Mab. Apart from ADP-ribosylation, two additional group transfer mechanisms of rifampicin inactivation including glycosylation, phosphorylation as well as decomposition of rifampicin by monooxygenation, are widespread in environmental bacteria but have not been explored in mycobacteria. Being a saprophyte in soil, exposure to antibiotic producing actinomycetes that utilize a variety of rifamycin inactivating mechanisms in its natural habitat might have favored the selection of an unknown reservoir of rifamycin resistance genes in M. abscessus, for example, those encoding a rifamycin glycosyltransferase (Rgt_Mab) and/or a rifamycin monooxygenase (Rox_Mab) (dashed lines). M. abscessus also produces a tetracycline-degrading flavin monooxygenase, MabTetX that activates molecular oxygen to hydroxylate and destabilize the antibiotic, which subsequently undergoes non-enzymatic decomposition, thereby conferring resistance. Resistance to β-lactams in M. abscessus is afforded by the production of a hydrolytic β-lactamase enzyme, Bla_Mab with the ability to degrade a broad range of β-lactams including extended-spectrum cephalosporins and carbapenems. RNAP, RNA polymerase; PBPs, penicillin-binding proteins; AAC(2′), 2′-N-acetyltransferase; Eis2, enhanced intracellular survival protein 2; APH(3″), aminoglycoside 3″-O-phosphotransferase.