Aminoglycoside modifying enzymes

Abstract

Aminoglycosides have been an essential component of the armamentarium in the treatment of life-threatening infections. Unfortunately, their efficacy has been reduced by the surge and dissemination of resistance. In some cases the levels of resistance reached the point that rendered them virtually useless. Among many known mechanisms of resistance to aminoglycosides, enzymatic modification is the most prevalent in the clinical setting. Aminoglycoside modifying enzymes catalyze the modification at different -OH or -NH₂ groups of the 2-deoxystreptamine nucleus or the sugar moieties and can be nucleotidyltransferases, phosphotransferases, or acetyltransferases. The number of aminoglycoside modifying enzymes identified to date as well as the genetic environments where the coding genes are located is impressive and there is virtually no bacteria that is unable to support enzymatic resistance to aminoglycosides. Aside from the development of new aminoglycosides refractory to as many as possible modifying enzymes there are currently two main strategies being pursued to overcome the action of aminoglycoside modifying enzymes. Their successful development would extend the useful life of existing antibiotics that have proven effective in the treatment of infections. These strategies consist of the development of inhibitors of the enzymatic action or of the expression of the modifying enzymes.

Fig. 1

Representative aminoglycosides and modification sites by AAC, ANT, and APH enzymes. An example of each kind of modification is shown on one of the substrates. The square and oval on positions 2′ and 6″ in paromomycin I indicate that although this molecule is preferentially acetylated at the position 1, 1,2′-di-N-acetylparomomycin and 1,6″-di-N-acetylparomomycin are also found as products of the enzymatic reaction (Sunada et al., 1999). AAC(3)-X can catalyze acetylation at the 3″-amino group in arbekacin and amikacin (Hotta et al. 1998).

Fig. 2

A. Genetic map of the Tn1331 transposon with the region including genes aac(6′)-Ib, aadA1 and bla_OXA-9 amplified. Circles and ovals represent attC and attI1* loci respectively. For clarity the points of potential crossover reactions are not indicated but they can be found in Ramirez et al. (Ramirez et al., 2008). Regions with a gene cassette structure are indicated below the genetic map by bars of different patterns. Their functionality as determined in recombination assays in the presence of IntI1 expressed from a recombinant clone harboring intI1 under the control of the P_tac promoter is shown. Directly repeated regions are shown as gray boxes on the sequences. B. Genetic maps of Tn1331, Tn1331.2, Tn1332, and the KQ element. Shadowed areas show the fragments inserted within the Tn1331 sequence that generated the other three genetic elements. C. Model for generation of a circular molecule containing aac(6′)-Ib (Zong et al., 2009). The white box indicates the DNA region that is found upstream of the gene in P. mirabilis JIE273. GC, gene cassette. Circular molecules are not drawn to scale.