β-lactamases

Background

• Includes a spectrum of molecules that hydrolyze β-lactams, from penicillins to carbapenems
  • See also extended-spectrum β-lactamases and carbapenemases

Ambler Classification

• Classification based on amino acid sequences rather than function

Class	Binding Site	Examples	Inhibitors
A	serine	TEM, SHV, KPC, CTX-M, GES	clavulanic acid, tazobactam, avibactam, vaborbactam, relebactam
B	metallo	VIM, NDM, IMP
C	serine	AmpC, P99	avibactam, vaborbactam, relebactam
D	serine	OXA (oxacillinase) enzymes	avibactam (OXA-48), ±clavulanic aciid

Serine β-lactamases

• Amber classes A, B, and C are the serine β-lactamases
• Contain a serine residue at the active site
• Class A: inhibited by clavulanic acid or tazobactam
  • Constitutively expressed plasmid
  • Most common ESBL in Gram-negative bacteria
  • Resistance to 2nd and 3rd generation cephalosporins
  • Common in E. coli, Klebsiella, and Proteus spp.
  • Examples include:
    • Penicillinases: TEM-1 (common in GNBs), SHV-1
    • ESBLs: CTX-M, TEM-3
    • Carbapenemases: K. pneumoniae carbapenemase (KPC)
• Class C: not inhibited by clavulanic acid or EDTA, resistant to cefoxitin, inhibited by cloxicillin in vitro
  • AmpC = chromosomal
  • Often an inducible AmpC gene present in the genome
  • Common in Citrobacter, Serratia, and Enterobacter
• Class D: not inhibited by EDTA, variably inhibited by clavulanic acid; hard to identify
  • Common in Pseudomonas
  • Difficult to detect with routine screening
  • Examples include:
    • ESBLs: OXA-11
    • Carbapenemases: OXA-23, OXA-48

Metallo-β-lactamases

• Ambler Class B are the metallo-β-lactamases
• Contain a zinc ion at the active site
• Inhibited by EDTA, not inhibited by clavulanic acid
• Examples include:
  • Carbapenemases:
    • New Delhi metallo-beta-lactamase (NDM-1)
    • Imipenemases (IMP)
    • Verona integron-encoded metallo-β-lactamases (VIM)
    • L1 β-lactamase, present in the Stenotrophomonas maltophilia chromosome

Bush-Jacoby Classification

Group	Ambler	Substrates	Inhibitors		Definition	Examples
			CA/TZB	EDTA
Group 1: Cephalosporinases
1	C	cephalosporins	—	—	hydrolyzes cephalosporins better than benzylpenicillin, and hydrolyzes cephamycins	E. coli AmpC, P99, ACT-1, CMY-2, FOX-1, MIR-1
		cephalosporins	—	—	increased hydrolysis of ceftazidime and other oxyimino-β-lactams	GC1, CMY-37
Group 2: β-Lactamases
2a	A	penicillins	yes	—	hydrolyzes benzylpenicillin better than cephalosporins	PC1
2b		penicillins and early cephalosporins	yes	—	hydrolyzes benzylpenicillin similar to cephalosporins	TEM-1, TEM-2, SHV-1
2be		extended-spectrum cephalosporins, monobactams	yes	—	increased hydrolysis of oxyimino-β-lactams (third-generation plus monobactams)	TEM-3, SHV-2, CTX-M-15, PER-1, VEB-1
2br		penicillins	—	—	resistance to clavulanic acid, sulbactam, and tazobactam	TEM-30, SHV-10
2ber		extended-spectrum cephalosporins, monobactams	—	—	increased hydrolysis of oxyimino-β-lactams plus resistance to clavulanic acid, sulbactam, and tazobactam	TEM-50
2c		carbenicillin	yes	—	increased hydrolysis of carbenicillin	PSE-1, CARB-3
2ce		carbenicillin, cefepime	yes	—	increased hydrolysis of carbenicillin, cefepime, and cefpirome	RTG-4
2d	D	cloxacillin	variable	—	increased hydrolysis of cloxacillin or oxacillin	OXA-1, OXA-10
2de		extended-spectrum cephalosporins	variable	—	hydrolyzes cloxacillin or oxacillin and oxyimino-β-lactams	OXA-11, OXA-15
2df		carbapenems	variable	—	hydrolyzes cloxacillin or oxacillin and carbapenems	OXA-23, OXA-48
2e	A	extended-spectrum cephalosporins	yes	—	hydrolyzes cephalosporins, and inhibited by clavulanic acid but not aztreonem	CepA
2f		carbapenems	variable	—	increased hydrolysis of carbapenems, oxyimino-β-lactams, cephamycins	KPC-2, IMI-1, SME-1
Group 3: Carbapenemases
3a	B	carbapenems	—	yes	broad-spectrum hydrolysis including carbapenems but not monobactams	IMP-1, VIM-1, CcrA, IND-1, L1, CAU-1, GOB-1, FEZ-1
3b		carbapenems	—	yes	preferential hydrolysis of carbapenems	CphA, Sfh-1

Epidemiology

• The most common β-lactamase is TEM-1
• The most common carbapenemases in the US are KPCs, followed by NDM and OXA-48-like carbapenemases

Common β-Lactamases

β-lactamase1	AMX	AMC	TIC	TIM	PIP	TZP	CFZ	FOX	CRO
TEM-1	R	S	R	S	I/R	S	S/I/R	S	S
TEM-1 hyperproduction	R	I/R	R	I/R	R	S/I/R	I/R	S	S
OXA-1	R	I/R	R	I/R	R	I/R	R	S	S
IRT type	R	I/R	I/R	I/R	S/I/R	S/I/R	S	S	S
CMT type	R	R	R	I/R	R	I/R	I/R	S	I/R
ESBL type	R	S/I	R	S	I/R	S/I	R	S	R
AmpC hyperproduction	R	R	I/R	I/R	I/R	I/R	R	I/R	S/I/R

Management

• Antibiotic therapy tailored to the resistance pattern
• Carbapenems, aminoglycosides, fluoroquinolones, and TMP-SMX typically work well
• See also Carbapenem-resistant organisms

Further Reading

• Updated Functional Classification of β-Lactamases. Antimicrob Agents Chemother. 2010;54(3):969-976. doi: 10.1128/AAC.01009-09