As a consequence of the prolific use of antibiotics, bacterial resistance to these compounds has become epidemic. In response to evolutionary pressures pathogenic bacteria have learned to evade antimicrobial agents through a variety of mechanisms.
Note that the specific mechanism employed by resistant bacteria is dependent on the class of antibiotics used.
The antibiotic resistance mechanisms are predominantly performed by proteins which are encoded on plasmids. These plasmids can be readily shared among bacteria, even very different bacteria, explaining the rapid dissemination of antibiotic resistance. Since a given bacterium can possess more than one plasmid, it is not uncommon that bacteria are capable of utilizing several different resistance mechanisms. Bacteria which harbour multiple resistance mechanisms are thus multi-drug resistant, and they are frequently referred to as "Super-bugs", in the popular press .
Aminoglycosides are a class of antibiotics which are clinically used to treat a wide variety of bacterial infections. For example, Gentamicin and Tobramycin are used in the treatment of pneumonia and meningitis caused by gram-negative bacilli, and in the treatment of gram-positive nosocomial bacterial infections. Amikacin has found widespread use in the treatment of serious nosocomial gram-negative bacterial infections and mycobacterial infections in AIDS patients.
Clinical Resistance against Aminoglycosides
The predominant mechanism of resistance against aminoglycoside antibiotics is "altering the antibiotic so as to decrease its toxicity". Three classes of enzymes have been found in pathogenic bacteria which are responsible for this resistance mechanism:
Collectively these enzymes are referred to as the Aminoglycoside Modifying enzymes (AME’s). The AME’s are capable of conferring resistance by chemically modifying the aminoglycoside antibiotics, thus dramatically reducing the effectiveness of these compounds.
Reversing Antibiotic Resistance
If one could deactivate the AME’s, than bacteria, which were previously resistant, would again become susceptible against aminoglycoside antibiotics. This is precisely the strategy pursued in the Berghuis Lab. By determining the three-dimensional structures for AME’s, and developing specific inhibitors, based on these structures, we aim to develop compounds that are capable of reversing aminoglycoside antibiotic resistance.
Progress Made (see Publications)
The crystal structure for a representative member of the APH class of enzymes, APH(3’)-IIIa has been determined. Remarkably, this structure revealed stunning similarity to Eukaryotic Protein kinases, a class of enzymes which have been extensively studied because of their role in cancer.
[See also our Gallery for figures of APH(3')-IIIa]
The three-dimensional structure of AAC(6’)-Ii has been determined in our lab. The fold of this enzyme reveals that it is similar to enzymes involved in gene regulation (histone acetyltransferases).
[See also our Gallery for figures of AAC(6')-Ii]
Current Research and Future Directions
The Berghuis Lab is pursing the structure determination of several aminoglycoside modifying enzymes, belonging to each of the three classes. Furthermore, for those enzymes whose structure has been determined, a variety of computational methods are used to exploit the structural data towards the design and development of compounds which have the potential of reversing aminoglycoside antibiotic resistance.