The Ambler Scheme and Carbapenemases

The Ambler classification comprises 4 groups viz. A,B, C and D. Group A, C and D enzymes are serine enzymes while those of group B are metallo-enzymes. Carbapenemase mainly belong to three  groups of beta-lactamases,  A, B and D.

Classification of Carbapenemase under the Ambler Scheme

1. Ambler Class A: This constitute the penicillinase. Class A enzymes are characterized by an active-site serine with a molecular mass of approximately 29,000 Da [3]. The most common is the KPC (Klebsiella pneumoniae  Carbapenemase). Other examples falling under the same  group are SME (Serratia marcesens), NMC (non-metallo carbapenemase), IMI (imipenemase), GES (Guyana extended-spectrum-lactamase), etc. These were first identified in 1980 and these enzymes have ability to confer resistance against the carbapenem, often called as the drug of last resort. They remain inhibited, at least in vitro by β-lactamase inhibitors, especially clavulanic acid. 

2. Ambler Class B: These consists of metallo-proteins which hydrolyze all antibiotics except aztreonam. NDM-1(New Delhi Metallo-beta-lactamases-1) falls under this class. NDM-1 has also been reported to be the common carbapenemase to be encountered in all Enterobacteriaceae including Acinetobacter baumannii [7,8,9]. Besides NDM-1, other carbapenemases falling under this category includes IMP (reported first time from Japan in 1991) [5], VIM ( reported first time in Verona, Italy in 1997) [6], GIM and SIM.  

3. Ambler Class D: This constitute the oxacillinases (OXA-48 derivatives) and their genes are found both on plasmids and in the chromosome [4]. The OXA-23, OXA-24 and OXA-58 are found  mainly found in A. baumannii, whereas OXA-48  is  reported mostly  in K. pneumoniae. They are capable of hydrolyzing penicillins, 1st generation cephalosporins and carbapenems. They are resistant to beta-lactamase inhibitors and weakly active against 2nd and 3rd generation cephalosporins (cefotaxime or ceftazidime) and cause partly hydrolysis of carbapenem.

Carbapenemases are  group of β-lactamases which have the ability to hydrolyze carbapenem group  along with penicillins, cephalosporins and monobactams. Carbapenemases producing organism include  Pseudomonas aeruginosa and Acinetobacter spp.

Distribution of Carbapenemase [4]
S.No	Type	Encoded	Organism	Reported
1	SME	Chromosomally	Serratia marcescens	Rarely
2	NMC	Chromosomally	Enterobacter cloacae	Rarely
3	IMI	Chromosomally	Enterobacter cloacae	Rarely
4	KPC	Plasmid	K. pneumoniae (mainly), P. aeruginosa, and A. baumannii	Highly Prevalent
5	GES	Plasmid (Integrons)	P. aeruginosa and Klesiella pneumoniae

The Klebsiella pneumoniae carbapenemase (KPC) enzyme (encoded by the blaKPC gene) is one of the five major carbapenemase families including  the VIM, IMP and NDM metallo-beta-lactamases, and the OXA-48-like oxacillinases [11].

Detection of bacteria producing carbapenemases in clinical samples can be done by CDC screening methods and Chromogenic plate methods. Likewise, detection of carbapenemase activity in cultured bacteria can be done by screening for carbapenemase producers, utilizing inhibitor-based approach and detection based on carbapenem hydrolysis. Inhibitor-based approach can be done by methods like Combination disk test (CDT), double disk synergy test (DDSTs) and Gradient disffusion strips. Carbapenem hydrolysis can be done by several methods including Cloverleaf method often called as modified Hodge test (MHT), colorimetric assays, Carbapenem inhibition method (CIM), Starch-iodine assay, Spectrometry and Electrochemical assay[10]. Immunochromatography can also be performed.  

REFERENCES:

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