محاضرة 5 مقاومة

• β-lactamases


  Enzymes capable of hydrolysing the β-lactam rings (the active site) of β-lactam antibiotics, thereby deactivating their antibacterial properties
• Classification of β-lactamases


  • Molecular structure classification of Ambler
  • Functional classification of Bush-Jacobi-Medeiros
• Ambler classification


  • β-lactamases are grouped into four class A, B, C, and D according to motifs composed of primary sequences constituting the protein molecules
  • Classes A, C, and D use a serine as an enzyme active center
  • Class B use the metal zinc
• Bush-Jacobi-Medeiros functional classification


  • β-lactamases are classified into groups 1 to 3 depending on the degradation of β-lactam substrates and the effects of inhibitors
  • Group 1 comprises cephalosporinases
  • Group 2 comprises β-lactamases other than those in group 1 having serine in the active center
  • Group 3 comprises metallo-β-lactamases (MBLs)
• The appearance of Extended Spectrum Beta-Lactamases (ESBLs) in organisms such as Klebsiella and E. coli indicates that β-lactamase-producing genes, such as TEM-type and SHV-type genes, encoded on a plasmid (R plasmid), have broadened the range of target drugs by genetic mutation
• Because the resistance genes are derived from the plasmid, they are easily transmitted among different bacterial species belonging to the same genera, such as the Enterobacteriaceae
• The repeated mutation of ESBLs has contributed to the emergence of new β-lactamases, namely carbapenemases, which hydrolyze carbapenem antibiotics
• Class A-ESBL (Extended spectrum β-lactamases)


  • Organisms producing ESBL enzymes are resistant to all Penicillins and 1st, 2nd and 3rd generation cephalosporins and monobactams, however remain sensitive to carbapenems and cephamycins
  • Resistance can be overcome by β-lactam + β-lactamase inhibitor (e.g. sulbactum or clavulanic acid)
  • Detected by Combination disk test (Ceftazidime and cetftazidime + clavulanic acid), Three dimensional test (best method)
• Carbapenemases


  • Characterized by susceptibility to inhibition by β-lactamase inhibitors such as clavulanic acid and tazobactam
  • The KPC-type is transmitted via a plasmid and is the most representative example of a class A carbapenemase, it also plays an important role in controlling the characteristics of carbapenem-resistant Enterobacteriaceae bacteria
• Class B-MBL (Metallo beta-lactamase)


  • These organisms are resistant to all those antibiotics to which AmpC beta-lactamase producers are resistant plus they are resistant to carbapenems
  • Resistance cannot be overcome by β-lactam + β-lactamase inhibitor combination
  • Detected by EDTA disk synergy test, modified Hodge test
  • Degrades all β-lactam agents except monobactams
  • Enzymatic activity is suppressed by a chelating agent, such as ethylenediaminetetraacetic acid (EDTA)
  • The MBL gene can reside on an integron, transposon, plasmid, chromosome, or various other genetic molecules
  • Examples: (IMP) Verona integrated-encoded MBL (VIM), Sao Paulo MBL (SPM) , Germany imipenemase (GIM), New Delhi MBL (NDM)
• Class C- AmpC beta-lactamase


  • These organisms are resistant to all those antibiotics to which ESBL producers are resistant plus they are resistant to cephamycins (e.g. cefoxitin and cefotetan)
  • But they are sensitive to carbapenems
  • Resistance cannot be overcome by β-lactam + β-lactamase inhibitor combination
  • Detected by AmpC disk test using cefoxitin disk
  • The β-lactamase belonging to class C is derived from the ampC gene carried on the genome of many members of the Enterobacteria genus of the Enterobacteriaceae and is functionally a cephalosporinase classified into Bush-Jacoby functional group 1
  • It is resistant to clavulanic acid, but sensitive to cephamycins, such as cefoxitin and ceftazidime
• Class D- oxacillinase


  • Resistance can be overcome by β-lactam + β-lactamase inhibitor combination
  • Class D ESBL oxacillinases (OXAs) β-lactamases of Ambler class D, known as OXA enzymes, which include OXA-1 and OXA-10, possess an active serine site similar to class A and C β-lactamases
  • These β-lactamases show cloxacillin- and oxacillin-hydrolyzing activity and are classified into Bush-Jacoby functional group 2d
• The Ambler method of molecular structure classification is simple and effective at organizing the various ESBLs and carbapenemases, but functional classification using the Bush-Jacobi-Medeiros method is also important for clinicians faced with treating patients in a critical condition due to ESBL- and carbapenem-resistant bacterial infections