In 1913, 25 years before the release of penicillin, Ehrlich correctly assessed the capacity of bacteria to develop resistance to antibiotics. Since Ehrlich’s initial observations in 1913, the challenge of predicting and minimizing the scope of bacterial resistance has been a goal of modern medicine. Hearing Ehrlich’s advice, Alexander Fleming himself predicted not only how useful antibacterial drugs would be but also how dangerous a world without them could be. In an interview shortly after winning the Nobel Prize in 1945 for discovering penicillin, Fleming said: "The thoughtless person playing with penicillin treatment is morally responsible for the death of the man who succumbs to infection with the penicillin-resistant organism." In 1940, Abraham and Chain described the first beta-lactamase enzyme (penicillinase) isolated from Bacillus (Escherichia) coli, well before the clinical use of penicillin . Ironically, the enzyme was initially thought to present few clinical concerns because penicillin, largely developed to treat staphylococcal disease, was not found to be associated with beta-lactamase enzymes . The global dissemination of methicillin-resistant Staphylococcus aureus, at its height from 1995–2005, provides perspective for the relevance of contemporary concerns over CRE. Today, over 2000 unique protein sequences coding for beta-lactamases have been in both Gram-negative and Gram-positive organisms .
Nomenclature for beta-lactamases are initially based on amino acid sequences, defined as the Ambler classification, which divides beta-lactamases into 4 major classes A–D .
A second classification system, first proposed by Richmond and Sykes , classified beta-lactamases based on functional characteristics. Further functional classification of key beta-lactamases was proposed by Bush, Jacoby, and Medeiros, and relies on substrate and inhibitor activity profiles to organize enzymes in a 1–4 system that can be readily identified with their phenotype in clinical isolates . Table 1 outlines the classification of carbapenemase enzymes.
Class A Beta-Lactamases
Until recent concerns about carbapenemase enzymes, Enterobacterales were generalizable for their susceptibility to common beta-lactamase inhibitors, clavulanic acid, tazobactam, and sulbactam. In 1965, a class A beta-lactamase reported from E. coli described the first plasmid-mediated beta-lactamase from a patient from Greece, which now bears the name “TEM” from the index patient in which it was reported . Additional class A beta-lactamases in the form of SHV (sulfhydryl reagent variable) variants are now commonly found in beta-lactamase-resistant isolates of E. coli and Klebsiella pneumoniae associated with bloodstream infections, as well as hospital-acquired respiratory and urinary tract infections . Within 2 years of the introduction of cefotaxime and ceftazidime, novel extended spectrum β-lactamases (ESBLs) were reported in clinical isolates . Mutations in pre-existing blaTEM-1 and blaSHV-1 genes gave rise to the emergence of ESBLs capable of hydrolyzing extended-spectrum cephalosporins with an oxyimino side chain. These cephalosporins include cefotaxime, ceftriaxone, and ceftazidime, as well as the oxyimino-monobactam aztreonam. Cefotaximases, characterized by CTX-M genes arose by plasmid transfer from pre-existing chromosomal ESBL genes from the bacteria Kluyvera spp.,  CTX-M ESBLs now represent a large proportion of global ESBL strains and are the dominant ESBLs in the US, particularly among urinary tract infections [10, 10].
Class A Serine Carbapenemases
The non-metallo beta-lactamases can hydrolyze carbapenems as well as cephalosporins, penicillin, and aztreonam , marking the evolution of beta-lactamase activity from primarily targeting penicillins and cephalosporins to enzymes that hydrolyze carbapenems. Representative of class A serine carbapenemases include the enzymes NMC-A, IMI, SME, and KPC. Table 1 outlines the classification of beta-lactamases capable of hydrolyzing carbapenem antibiotics.
KPC enzymes have emerged as the most well-known, and well-described of the class A serine carbapenemases, which, while initially described in K. pneumoniae, are now readily detected in many members of the Enterobacteriaceae family. They confer resistance to all beta-lactams (excluding, carbapenems, and monobactams). Gram-negative bacteria harboring blaKPC genes can be carried on mobile genetic elements such as transposons (e.g., Tn4401b) and multiple plasmid (IncFII, IncL/M, and IncN) types . Organisms expressing KPC genes are often concurrently resistant to other classes of antibiotics, such as quinolones and aminoglycosides, thus creating highly MDR organisms .
Class B Beta-Lactamases
Metallo-beta-lactamases (MBLs) have been isolated from Acinetobacter spp. and Pseudomonas spp., and are increasingly associated with members of the Enterobacterales. such as K. pneumoniae, K. oxytoca, E. coli, and Enterobacter spp. MBLs are commonly associated with the Verona integron–encoded metallo-β-lactamase (VIM), Pseudomonas (IMP)-type and the New Delhi MBL (NDM) [13, 13] (52). MBLs are commonly expressed from mobile genetic elements such as integrons, plasmids and transposons, which have contributed to the spread of MBLs. MBLs effectively hydrolyze beta-lactamases and are not inhibited by beta-lactamase inhibitors (clavulanic acid, tazobactam, vaborbactam). They also differ from KPCs (serine carbapenemases) because they contain a bivalent metal ion, commonly zinc (Zn2) ions which coordinate histidine/cytosine/asparagine residues at the active site . Genetic insertion of entire sequences upstream of the blaNDM-1 gene among bacteria in the Enterobacterales seems to suggest that NDM variants likely evolved from Acinetobacter baumannii . Currently, > 20 NDM types have been identified , yet expansion of NDMs do not appear to be due to proliferation of a single dominant clone. NDMs have been found in several epidemic clones, including K. pneumoniae ST11 and ST147 and E. coli ST131 and ST101, which are known to harbor other β-lactamase genes and antibiotic resistance determinants . IMP-type metallo-β-lactamases are among the most common families of acquired carbapenemases detected from Enterobacteriaceae and have been reported mainly in East Asia, including Japan . Although IMP-type metallo-β-lactamases are class B beta-lactamases, their hydrolytic properties for carbapenems are weak, often resulting in elevated resistance to meropenem while retaining imipenem susceptibility [19, 19]. This feature has been implicated in the spread of blaIMP harboring plasmids in Japan, as determined after screening the antibiotic susceptibility of infectious bacteria to imipenem as a representative carbapenem .
Class C, Serine-Based Cephalosporinases
AmpC β-lactamase expressors are common to members of the family are commonly associated with Serratia spp., Pseudomonas aeruginosa, Providencia spp., indole-positive Proteus mirabilis/vulgaris, Citrobacter spp., and Enterobacter spp., which collectively are often referred to as “SPICE” organisms. AmpC β-lactamase are generally resistant to penicillins, β-lactamase inhibitors, and cephalosporins such as cefoxitin, cefotetan, ceftriaxone and cefotaxime. In many Enterobacterales, AmpC expression is low but inducible in response to β-lactam exposure, and resistance occurs through two primary mechanisms. Disruption of peptidoglycan (murein) synthesis by beta-lactam agents leads to an accumulation of N-acetylglucosamine-1,6-anhydro-N-acetylmuramic acid oligopeptides. Displacement of the UDP-N-acetylmuramic acid peptides signals a conformational change in AmpR, which activates the transcription of ampC regulatory genes. Additionally, many Enterobacterales contain a cytoplasmic N-acetyl-muramyl-l-alanine amidase enzyme (AmpD) which removes peptides from the 1,6-anhydro-N-acetylmuramic acid and N-acetylglucosamine-1,6-anhydro-N-acetylmuramic acid oligopeptide derivatives. Under wild-type expression, AmpD aids in reducing their concentrations of the aforementioned enzymes, preventing the overexpression of AmpC. The most common cause of AmpC overexpression in clinical isolates is a mutation in ampD leading to AmpC hyper-inducibility and/or constitutive hyperproduction . AmpR mutations are less common but can also result in high-constitutive or hyper-inducible phenotypes. Infrequently, AmpC induction can also occur via mutations in AmpG, an inner membrane permease enzyme, involved in cell wall turn-over and AmpC regulation into the cytosol [23, 23]. AmpC expression can often combine with other mechanisms of resistance often related to permeability such as porin loss and/or efflux pumps, resulting in clinically significant resistance .
Class D Serine Oxacillinases
Class D beta-lactamases, initially characterized as oxacillinases because of their hydrolytic activity against β-lactams have recently become a clinical concern. Class D beta-lactamases, and OXA variants, possess variable but significant carbapenemase activity and are generally not inhibited by clavulanic acid, tazobactam, and sulbactam (with some exceptions; e.g., OXA-2 and OXA-32 are inhibited by tazobactam but not by sulbactam and clavulanate, and OXA-53 is inhibited by clavulanate) . Nearly one-third of OXA variants hydrolyze carbapenems. OXA-carbapenemases are predominantly isolated in Acinetobacter spp.; however, the OXA-48 carbapenemase was described in a MDR isolate of K. pneumoniae in 2004 in a patient with a UTI . The initial cases of an OXA-48 producing K. pneumoniae presented as a highly drug-resistant strain because it co-expressed other beta-lactamases (class A extended-spectrum β-lactamase SHV-2a and the narrow-spectrum β-lactamases TEM-1 and OXA-47) as well as defects in outer membrane proteins . While OXA-48 is a class D β-lactamase with the highest known catalytic efficiency for imipenem [28, 28], organisms containing the plasmid-mediated blaOXA-4 gene effectively hydrolyze penicillins, but hydrolyze carbapenems at low levels and show weak activity to extended-spectrum cephalosporins, and poor hydrolytic activity against ceftazidime and cefepime. Recently, OXA-48-positive organisms have spread to over 30 different countries and within various bacteria of the Enterobacterales [27, 27, 27].
This review will focus on enzymatic forms of CRE, attributed to carbapenemase-producing bacteria. However, it should be noted that carbapenem-resistance is often regarded as a phenotypic designation and encompasses non-enzymatic forms of carbapenem resistance. cell wall differences, external decreased membrane permeability, efflux pumps, and the presence of various broad-spectrum β-lactamases (e.g., AmpC cephalosporinase), often manifest in carbapenem resistance when measured phenotypically in laboratories.