ESBL and Classification of β-lactamases
ESBL stands for extended-spectrum beta-lactamases, i.e. a group of enzymes produced by both Gram-negative and Gram-positive bacteria which can hydrolyze all β- lactam agents except carbapenems and cephamycins. ESBLs belong to group 2be or group 2d (OXA-type) of the Bush-Jacoby-Medeiros classification scheme.
Transmission of ESBL
Being plasmid-mediated, genes encoding ESBL are easily exchanged not only among the same group but also in different bacterial species. For eg: ESBL genes from E.coli can be transferred either to Klebsiella, Pseudomonas or even to gram-positive bacteria.
In a clinical or community setting, such ESBL producing organisms are transmitted either by direct contact with an infected person’s bodily fluids (blood, drainage from a wound, urine, bowel movements, or phlegm) or contact with equipment or surfaces that have been contaminated with such organisms (as in case of catheters or surgical instruments) or contaminated hands.
Why is ESBL producing bacteria a problem?
ESBL producing bacteria may cause a wide range of infections leading from uncomplicated urinary tract infection (UTI), diarrhea, and skin infections to life-threatening bloodstream infections (such as septic shock) pneumonia, gastroenteritis, nosocomial infections, etc. Plasmids responsible for ESBL production also carry genes encoding resistance to other classes of antibiotics (for example, aminoglycosides, quinolones, etc). This limits the treatment options of ESBL-producing organisms and poses a threat to successful treatment.
Classification of Beta lactamases
The Ambler molecular classification and the Bush-Jacoby-Medeiros functional classification are the two most commonly used classification systems for β-lactamases.
The most widely used classification of β-lactamases is the Ambler classification that divides β lactamases into four classes (A, C, D, and B) based on their amino acid sequences. Ambler originally specified two classes (A and B) but later added C and D as new β-lactamases bore no resemblance to the existing class.
Class A, C, and D enzymes utilize serine for β-lactam hydrolysis, and class B metalloenzymes require divalent zinc ions for substrate hydrolysis.
contained the active-site serine β-lactamases
a member of serine β-lactamases, known as the ‘AmpC’ β-lactamases.
Another class of serine β-lactamases, commonly known as the OXA β-lactamases that had no resemblance to either class A or class C.
the metallo-β-lactamases that require a bivalent metal ion, usually zinc for activity.
Bush-Jacoby-Medeiros classification scheme
The Bush-Jacoby-Medeiros classification scheme groups β-lactamases according to functional similarities (substrate and inhibitor profile). There were 4 groups described by Jacob and bush but the fourth group has now been omitted as the enzyme properties were similar to that of the previous 3 groups, so currently, there are three main groups and multiple subgroups in this system.
Group I cephalosporinases
Ambler Class C beta-lactamases (also known as AmpC enzymes) fall in this group. Cephalosporin is the substrate. Distinctive characters are
- This group is not inhibited by beta-lactamase inhibitors like clavulanic acid or EDTA.
- Greater hydrolysis of cephalosporin than penicillinase hydrolyzes cephamycins
- Often chromosomal enzymes in gram-negatives but some are plasmid-coded
- In this class, the enzyme is inducible. Thus any exposure of bacteria to beta-lactam antibiotics leads to an increase in enzyme production.
- The Group I producer beta-lactamases are resistant to beta-lactam/beta-lactamase inhibitor combinations, penicillins, cephamycins, as well as 1st, 2nd, and 3rd generation cephalosporins but sensitive to cefepime and carbapenems
- The enzymes in group I are found in the Enterobacteriaceae family as well as Pseudomonas aeruginosa. Examples include E. coli AmpC, P99, ACT-1, CMY-2, FOX-1, MIR-1 enzymes.
Group I contains a subgroup 1e that can hydrolyze ceftazidime and often other oxyimino-β-Lactams. Examples include GC1, CMY-37
Group II serine β-lactamases
Functional group 2 β-lactamases, including molecular classes A and D (ambler classification), represent the largest group of β-lactamases. There are various subgroups each with a different property.
|Sub group||Substrate||Defining character||Examples|
|2a||Penicillin||1. Predominant penicillinase in Staphylococci and enterococci |
2. Preferentially hydrolyze benzylpenicillin and many penicillin derivatives, with poor hydrolysis of cephalosporins, carbapenems, or monobactams except nitrocefin hydrolysis
3. Are inhibited by clavulanic acid and tazobactam
4. Majority are chromosomal, although some staphylococcal penicillinases are plasmid-encoded
|2b||Penicillins and early cephalosporins||1. Readily hydrolyze penicillins and early cephalosporins, such as cephaloridine and cephalothin |
2. Strongly inhibited by clavulanic acid and tazobactam
3. Most common plasmid-mediated β lactamases
|TEM-1, TEM-2, SHV-1|
|2be*||Extended-spectrum cephalosporins, monobactams||1. Increased hydrolysis of oxyimino-β-lactams (cefotaxime, ceftazidime, ceftriaxone, cefepime, aztreonam) |
2. Are sensitive to inhibition by clavulanic acid, a feature used in their detection by clinical laboratories
|TEM-3, SHV-2, CTX-M-15|
|2br||Penicillin||Have acquired resistance to clavulanic acid, sulbactam and tazobactam||TEM-30,SHV-10|
|1. Increased hydrolysis of oxyimino-β-lactams combined with resistance to clavulanic acid, sulbactam and tazobactam |
2. Also known as CMT (complex mutant TEM) β-lactamases
|2c||Carbenicillin||Ability to hydrolyze carbenicillin or ticarcillin Easily inhibited by clavulanic acid or tazobactam||PSE-1, CARB-3|
|2ce||Extended-spectrum cephalosporins||Increased hydrolysis of carbenicillin, cefepime, and cefpirome Inhibited by clavulanic acid or tazobactam||RTG-4 (CARB-10)|
|2d||Cloxacillin||Hydrolyze cloxacillin or oxacillin, also carbenicillin hence are termed OXA enzymes OXA-related enzymes now comprise the second largest family of β-lactamases||OXA-1 OXA-10|
|2df||Carbapenems||1. Hydrolyze cloxacillin or oxacillin and carbapenems 2. The enzymes, and their producing organisms, are typically unresponsive to inhibition by clavulanic acid||OXA-23 OXA-48|
|2e||Extended-spectrum cephalosporins||1. Hydrolyze cephalosporins. |
2. Inhibited by clavulanic acid but not aztreonam
|2f||Carbapenems||Increased hydrolysis of carbapenems, oxyimino-β lactams, cephamycins||KPC-2, IMI-1, SME-1|
Group III MBLs
Group III β lactamases include Metallo-β -lactamases (MBLs), included in class B of Amblers classification. They differ structurally from the other β-lactamases by their requirement for a zinc ion at the active site.
Distinguishing characters include
- Ability to hydrolyze carbapenems, but not monobactams.
- Are not inhibited by clavulanic acid or tazobactam, but by metal ion chelators such as EDTA, dipicolinic acid, etc.
- Originally were identified as chromosomal enzymes in Gram-positive or occasional Gram-negative bacilli, such as Bacteroides fragilis or Stenotrophomonas maltophilia but now are plasmid-mediated hence can be detected on a wide variety of bacteria.
Only two functional subgroups are described.
- Subgroup 3a includes the major plasmid-encoded MBL families, such as the IMP and VIM enzymes that have appeared globally, most frequently in non-fermentative bacteria but also in Enterobacteriaceae
- Subgroup 3b contains a smaller group of MBLs that preferentially hydrolyze carbapenems in contrast to penicillins and cephalosporins. Examples include L1, CAU-1, GOB-1, FEZ-1 enzymes.
References and further readings
- Hall B G. and Barlow M. Revised Ambler classification of β- lactamases. Journal of Antimicrobial Chemotherapy doi:10.1093/jac/dki130 Advance Access publication 4 May 2005.
- Bush K, Jacoby GA. Updated functional classification of beta-lactamases. Antimicrob Agents Chemother. 2010;54(3):969-976. doi:10.1128/AAC.01009-09
- Rawat D, Nair D. Extended-spectrum β-lactamases in Gram Negative Bacteria. J Glob Infect Dis. 2010;2(3):263-274. doi:10.4103/0974-777X.68531