Major defense of the Gram negative bacteria against the β-
These antibiotics have a common four-atom ring known as a
Hydrolysis of the β-lactam ring & decarboxylation of the intermediate
The lactamase enzyme breaks the β-lactam ring
open, deactivating the molecule's antibacterial properties
Co-evolved with the β-lactam antibiotics since they came
into clinical use.
Earlier increased in microbes in which they were
uncommon (e.g. Staph. aureus) and then spread to
pathogens that had previously lacked them ( e.g. H.
influenzae and N. gonorrhea)
Introduction of the newer “resistant” β-lactam antibiotics
like cephamycins, oxyimino-cephalosporins,
carbepenems & monobactams 20 yrs back, resulted in
the emergence of a newer set of ESBLs, AmpC enzymes
Classification of β-lactamases
Ambler Molecular Classification
AA similarity and 10 structure.
Classes A, C & D have serine residues at their active
Class B are metallo-β-lactamases that require Zn & Ca
ions for their actions.
Classes A (TEM/SHV) & D (OXA) were plasmid
Class B & C were chromosomal mediated.
Based on functional similarities i.e. substrate and
4 main groups (1 to 4) and multiple subgroups.
More of immediate relevance to clinicians and
μbiologists because it considers β-lactamase
inhibitors and substrates that are clinically
Class A β-lactamases
Broad and Expanded spectrum β-lactamases
Include TEM, SHV, CTX-M types commonly.
BES, GES/IBC family, PER, SFO, TLA & VEB are members
Hydrolyze all classes of penicillins, Ist, IInd, IIIrd, IVth
gen. cephalosporins & monobactams.
Co-resistant to quinolones, aminoglycosides &
Cephamycins and carbapenems are not
affected them, though the latter are the
most consistent in their action.
Inhibited by β-lactamase inhibitors
ESBL producing organisms may lose their
outer membrane proteins, not related to β-
lactamase production & become resistant
to cephamycins too.
TEM type β-lactamases
TEM-1, TEM-2 are broad spectrum against which
IIIrd, IVth gen. cephalosporins & monobactams were
TEM-10, TEM-12 & TEM-26 are ESBLs, in majority
now, active against the above, though rarely
accompanied by resistance to inhibitors too.
Useful to follow the spread of individual resistance
SHV Type β-lactamases
SHV = sulphydryl variable
SHV-1 is a broad spectrum
β-lactamase similar to TEM-1.
Most commonly found in K.pneumoniae.
Are the most common clinically isolated in Europe and
Within 15 yrs, SHV-2 were found in every
inhabited continent, implying that selection
pressure from third-generation cephalosporins
in the first decade of their use was responsible.
SHV ESBLs have been detected in a wide range
of Enterobacteriaceae & outbreaks of SHV-
producing P. aeruginosa & Acinetobacter spp.
have now been reported.
SHV-5 and SHV-12 are among the most
Most common group of ESBLs not belonging to the
previous two were termed CTX-M to highlight their
greater activity against cefotaxime than cefepime or
Belying their name, they now hydrolyse ceftazidime
more than cefotaxime.
Mainly been found in strains of Salmonella enterica
serovar Typhimurium and E. coli.
Widespread findings of CTX-M-type ESBLs in
China and India, is speculated that CTX-M-type
ESBLs are now actually the most frequent ESBL
Tazobactam exhibits an almost 10-fold greater
inhibitory activity than clavulanic acid against
CTX-M-15 are the most widespread in
India, Middle East, Europe and the USA.
Other Class A ESBLs
Uncommon, mostly in P. aeruginosa & at limited
VEB 1 & 2 in South East Asia.
Others are PER-1, GES-1 & 2, IBC-2, PES-1 and may
be found in Enterobacteriaceae too.
BES-1, IBC-1, SFO-1 & TLA-1 found only in
Include some carbapenemases as well.
Class B β-lactamases
Also called the Metallo β lactamases.
Are Zn dependant enzymes with a different
mechanism of action.
Exhibit resistance to
penicillins, cephalosporins, carbapenems and β
Their hyrdolytic profile doesn’t include
Types include Plasmid-mediated IMP-type
carbapenemases, VIM and NDM-1 (New Delhi
NDM-1 (New Delhi metallo-β-
Originally described from New Delhi in 2009
widespread in Escherichia coli and Klebsiella
pneumoniae from India and Pakistan.
As of mid-2010, NDM-1 carrying bacteria have
been introduced to other countries (including
the United States and UK)
It’s the so called “super bug” spread due to
the large number of tourists travelling from
Class C β-lactamase
Known as the serine cephalosporinases.
Mainly Amp C plasmid induced enzymes.
Substrates include the penicillins,
cephalosporins plus cephamycins and β
lactam – β lactamase inhibitor combinations.
Cefepime is poorly hydrlolysed by it.
They are mainly inhibited by cloxacillin,
oxacillin and aztreonam.
Responsible for the resistance emerging in
Class D serine oxacillinases
Initially found to hydrolyse oxacillin at a slow
Confer resistance to
penicillins, cephalosporins, extended
spectrum cephalosporins, carbapenems
and β lactamase inhibitors.
In vitro sodium chloride and chelating
agents like EDTA inhibit some carbapenem
Source of concern because they are active
against cephalosporins and carbapenems.
Include class A, B and D β lactamases.
Plasmid mediated IMP type in enteric
Gram negative organisms, Pseudomonas and
VIM family of carbapenemases
OXA – type β lactamases
Factors influencing β
Expanded spectrum activity may decrease
intrinsic hydrolytic activity like in TEM and SHV
which can be compensated for by increased
gene dosage or a promoter with increased
Active efflux mechanism as in Pseudomonas
allows selective enzymes with limited hydrolytic
activity to inactivate the drug.
Decreased expression of OMPs required like in K.
pneumoniae allow resistance to develop to
Genetics of β lactamases
PLASMIDS: predominate in ESBLs and hospital
outbreaks if unifocal in origin like TEM but may
be multifocal too like in SHV and spread world
TRANSPOSONS: TEM – 1 and TEM – 2
CHROMOSOMES: Klebsiella strains producing
SHV – 1
INTEGRONS: Genes are incorporated into the
genome but have their origin elsewhere. So
the reservoir is large with ubiquitous spectrum.
Eg. CTX – M, Amp – C type
Risk factors Predisposing to infection by
β lactamase producing organisms
Previous antimicrobial exposure
(quinolones, third generation
Previous hospital admissions
Prolonged ICU and hospital stay.
Increased severity of illness
Use of a CVC/CAC, urinary catheter, ventilatory