Carbapenemases- Definition, Types, Significance, Bacteria

Carbapenemases are the β-lactamase enzymes that hydrolyze most of the β-lactam antibiotics, including carbapenems. They are the most potent β- lactamases having a very wide spectrum of activity against penicillins, cephalosporins, monobactams, and carbapenems.

• They induce resistance to carbapenems by hydrolyzing them using catalytic substrate serine or zinc.
• According to Amber’s classification of β- lactamases, the carbapenems currently known fall into the molecular class A, B, and D β- lactamases. 
• They are encoded chromosomally as well as by plasmid genes also. Due to this, there are wide variations among known carbapenemases. Plasmid-encoded carbapenemase-producing genes can be horizontally transferred in between related bacterial genera; hence dissemination of carbapenemase producers is rapid. Till now, mostly opportunistic pathogens associated with hospital-acquired infections and a very small portion of pathogens associated with community-acquired infections are found to produce these enzymes. However, there is a very high chance of dissemination of carbapenemase producers both in the community as well as in clinical settings. And, if this happens, there will be a great threat to public health and the infection treatment system. 

Types/Classification of Carbapenemase

There are several schemes for the classification of carbapenemases, but the widely accepted classification method is based on the molecular basis or Amber’s classification method. According to this method, carbapenemases can be grouped into three classes; Molecular Class A Carbapenemases, Molecular Class B Carbapenemases, and Molecular Class D Carbapenemases. Each class contains several enzyme families and their sub-families. 

Among these different enzymes, five of them are clinically very important and are called “the Big Five Carbapenemases”. They are IMP, VIM, KPC, NDM, and OXA carbapenemases.  

Molecular Class A Carbapenemases

• They are the carbapenemases that require serine for their hydrolytic activities, hence are in the group called the serine-carbapenemases. Functionally classifying, they fall under the functional group 2f. They can hydrolyze penicillins, cephalosporins, aztreonam (monobactams), and carbapenems. They are not inhibited by EDTA but are susceptible to inhibitors like clavulanate and tazobactam.  
• They are commonly reported in bacteria like Klebsiella pneumoniae, E. coli, Enterobacter cloacae, Serratia marcescens, and Pseudomonas aeruginosa.

This class further is classified into six different carbapenemase families/groups, each with multiple variants; 

• (Guiana extended-spectrum β – Lactamase) GES, 
• (Klebsiella pneumoniae carbapenemase) KPC, 
• (Serratia marcescens enzyme) SME, and 
• (Imipenem hydrolyzing β – Lactamase/nonmetallo enzyme carbapenemase) IMI/NMC is an important and widely studied group, while
• (Sulfhydryl Reagent Variable – 38) SHV-38 and (Serratia fonticola carbapenemase – 1) SFC-1 are rarer and new members. 

SME, IMI/NMC, SHV-38, and SFC-1 are chromosomal-encoded enzymes, while, KPC and GES are plasmid-encoded.

Molecular Class B Carbapenemases

• They are Metallo-β-lactamase (MBLs) enzymes that require Zinc to hydrolyze the β- lactam rings. Functionally classifying, they fall under functional group 3. They can hydrolyze penicillins, cephalosporins, and carbapenems but are unable to hydrolyze monobactams. As they are Metallo-beta-lactamases, they are inhibited by metal chelators like EDTA. EDTA can chelate Zn⁺² of the enzymes and make them inactive. Other β-lactam inhibitors like clavulanate and tazobactam are resistant to class B carbapenemases. 
• They are mainly reported in pathogens like Bacillus cereus, Klebsiella pneumoniae, Serratia marcescens, Pseudomonas aeruginosa, Citrobacter freundii, Bacteroides fragilis, and Stenotrophomonas maltophilia.
• Currently, this class contains more than ten different carbapenemase families, each with multiple variants.

Some of the common types are: 

• (New Delhi Metallo-β-lactamases) NDM
• (Imipenemases) IMP
• (Verona integron-encoded Metallo-beta-lactamases) VIM
• (German Imipenemases) GIM
• (Seoul Imipenemases) SIM
• (Australian Imipenemases)AIM
• (Sao Paulo Metallo-beta-lactamases) SPM
• (Kyorin University Hospital Metallo-beta-lactamases) KHM
• CcrA Metallo-beta-lactamase
• (Bacillus cereus carbapenemase II) BcII
• CphA Carbapenemase of Aeromonas spp. 
• L1 Carbapenemases

Clinically, the most important Metallo-beta-lactamases are NDM, VIM, IMP, GIM, and SIM. 

Molecular Class D Carbapenemases

• They are also called Class D serine carbapenemase, and as the name suggests they are serine-based β- lactamases that require serine for their hydrolytic activities. As they are serine-based, EDTA is not able to inhibit them effectively; similarly, clavulanate and tazobactam also show poor inhibition against them. 
• They are mainly reported in Enterobacteriaceae (mainly Klebsiella pneumoniae) and Pseudomonas aeruginosa. 
• This group contains a sole member “OXA (Oxacillinase) carbapenemase”. OXA contains more than 489 variants and 37 of them are considered important carbapenemases.OXA-48 and OXA-48-like enzymes are the most important carbapenemases. 

The Big Five Carbapenemases

There are five clinically most relevant carbapenemases which are reported more frequently than any other carbapenemases in clinical isolates, called ‘the Big Five’. This group includes KPC, IMP, VIM, NDM, and OXA carbapenemases.

KPC (Klebsiella pneumoniae carbapenemase)

KPC is the most commonly reported and clinically the most important family in Class A carbapenemases. Since its first report from North Carolina, USA, in 1996 in an isolate of Klebsiella pneumoniae, it has been isolated and identified in different countries of South East Asia, the Middle East, Europe, Latin America, Central, and South America, and Pacific Regions. It is endemic in the USA, China, Israel, Greece, Colombia, Italy, and Puerto Rico. 

24 different KPC coding genes (bla_KPC) have been reported to produce 24 different variants of KPC enzymes. Of them, KPC-2 and KPC-3 are the most common ones. It has been extensively reported in K. pneumoniae and other Enterobacterales (like E. coli, Enterobacter spp., Serratia marcescens, K. oxytoca, Salmonella enterica, Proteus mirabilis, Citrobacter freundii, etc.), and Pseudomonas aeruginosa.      

IMP (Imipenemases)

Also called IMP-type Metallo-beta-lactamase is a family in Class B carbapenemases that require Zn⁺² for their hydrolytic activities. It mainly confers resistance against imipenem and meropenem antibiotics. It (IMP-1) was first identified in Japan in 1991. Currently, there are 88 variants of IMP: most of which are most prevalent in the Asian region. They are plasmid-encoded carbapenemases mostly reported in Pseudomonas aeruginosa. Besides, different variants of IMP are also reported in Serratia marcescens and other some Enterobacteriaceae members.   

VIM (Verona integron-encoded Metallo-beta-lactamases)

VIM is another family in Class B carbapenemases encoded by integron bla_VIM genes of a plasmid. VIM carbapenemases are the most prevalent types of MBLs. VIM types of carbapenemases can hydrolyze all penicillins, cephalosporins, carbapenems, and cephamycins, but aren’t sensitive to monobactams. They are mainly produced by Pseudomonas aeruginosa, K. pneumoniae, and E. coli. VIM was first discovered in Verona Italy in 1999. Now it is reported in the Far East, USA, Europe, and South America. More than 40 variants of VIM are reported to date, and among them, VIM-1 and VIM-2 are the most common ones. 

NDM (New Delhi Metallo-β-lactamases)

NDM is a new family in Class B carbapenemases. It is an acquired MBL family first identified in 2008 in Sweden from a Swedish patient of Indian origin who was previously treated in a hospital in New Delhi, India. NDM-1 was the first member which was first reported in Klebsiella pneumoniae. Now it is common in K. pneumoniae and E. coli. They can be transferred easily via horizontal gene transfer; hence it is of high concern. It is now endemic in India and nearby Asian countries like Pakistan. They are also reported in the USA, UK, Japan, Egypt, and Canada. NDM carbapenemases can hydrolyze all β- lactam antibiotics except aztreonam.     

OXA (Oxacillinases)

It is a family of serine carbapenemase in Class D carbapenemases. It was first reported in 2003 in Turkey. It is a very diverse type of carbapenemases. Plasmid-encoded OXA-48 is the most important variant in the OXA family.  It is mainly reported in A. baumannii, Pseudomonas aeruginosa, and members of Enterobacteriaceae (mainly E. coli and Klebsiella pneumoniae). They are endemic in Indian regions, North African countries, and some European countries. 

Significance of Carbapenemases

Render resistance against carbapenem antibiotics, ‘antibiotics of the last resort’.

Makes treatment of infections by carbapenemase-producing pathogens very difficult and even impossible in some cases. This increases morbidity, mortality, hospital stay, antibiotic administration time, medical/treatment cost, and the chance of secondary infections and immune compromisation.

Common Pathogenic Bacteria Producing Carbapenemase

Carbapenemase production is seen in diverse bacterial species, including a few Gram-positive and multiple Gram-negative ones. However, clinically, carbapenemase production in Gram-negative bacteria is considered very important. The emergence of plasmid-encoded carbapenemases and their ability to horizontally transfer in other related bacteria helps in the wide dissemination of carbapenemases in diverse bacterial genera and strains. 

Some common carbapenemase-producing pathogens are:

Among these pathogens, the most important bacteria that are even indicated by the US CDC and the WHO as a very serious threat to public health are:

1. Carbapenem-resistant Acinetobacter baumannii (CRAB)
2. Carbapenem-resistant Klebsiella pneumoniae (CRKP)
3. Carbapenem-resistant Pseudomonas aeruginosa (CRPA)

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