Pharmaceutical Microbiology

EXTENDED SPECTRUM BETA LACTAMASE

Written by MicroDok

EXTENDED – SPECTRUM BETA – LACTAMASE (ESBL)

ESBL is an “acronym” which stands for extended spectrum beta-lactamase. Though there is no consensus on the definition of ESBL, extended spectrum beta-lactamases (ESBLs) are a group of beta-lactamase enzymes with the ability to hydrolyze and cause resistance to the oxyimino 3rd-generation cephalosporins (e.g. cefotaxime, ceftazidime, cefuroxime and ceftriaxone) and monobactams (e.g. aztreonam), but not the cephamycins and carbapenems. Extended – spectrum beta – lactamases (ESBLs) are a group of enzymes that break down antibiotics belonging to the penicillin and cephalosporin groups and render them ineffective for therapy in vivo.

The name “ESBL” was originally coined to reflect the expanded substrate spectrum of enzymes derived from narrow – spectrum TEM, SHV, or OXA beta – lactamases. ESBLs are transmissible beta – lactamases that are plasmid – mediated, and can be inhibited by clavulanic acid, tazobactam or sulbactam. They are encoded by genes that can be exchanged between bacteria. ESBLs are enzymes that can hydrolyze oxyimino – cephalosporins (cefuroxime, ceftazidime, cefotaxime, ceftriaxone and cefepime) and monobactams (aztreonam) but not cephamycins (cefoxitin and cefotetan) or carbapenem. They belong to the Ambler Class A beta – lactamase enzyme according to the Bush – Jacoby – Medeiros classification scheme.

Typically, ESBLs arise or are derived from genes for TEM – 1, TEM – 2, or SHV – 1 by mutations that alter the amino acid configuration around the active site of these beta – lactamases (TEM and SHV). This leaves the enzymes with about 1-7 amino acid substitutions at the active site of the enzyme, thus enlarging the active site and allowing attack on oxyimino – cephalosporins. ESBLs carries tremendous clinical implications since most of the drugs (or antibiotics) used in therapy against bacterial infections are rendered inactive by them. The beta – lactams (penicillins and cephalosporins) are the most widely used antibiotics in clinical medicine; and if ESBLs are resistant to all of them, it then means that there is a limitation of drugs available for the treatment of bacterial infections (especially those caused by ESBL – producing bacteria). This is indeed a big threat to our therapeutic regimens.

The ESBLs emerged soon after the introduction of the extended spectrum cephalosporins. They were first isolated in clinical isolates of a patient  named “Temoneira” from Germany in 1983 and in the United States of America in 1989. The emergence of ESBL – producing bacteria has been widely reported as an important cause of nosocomial infections in most part of the world. ESBLs are found amongst strains of Enterobacteriaceae especially Escherichia coli, Klebsiella pneumoniae, and Klebsiella oxytoca. They have been associated with increased morbidity and mortality, especially amongst intensive care unit and high – dependency units in the hospitals. Accurate detection of these “evil enzymes” is imperative in order to avoid clinical failure due to inappropriate antimicrobial therapy.

TYPES OF ESBLs

The majority of the ESBLs are derivatives of the widespread beta – lactamases, TEM – 1, TEM – 2 and SHV – 1. Today, about 160 variants of ESBLs have been recognized, with new majority assigned to a TEM or SHV group in sequential numbers. There is also another type of ESBL called the CTX – M type, which has emerged and disseminated over the world causing community onset infections especially urinary tract infections (UTI’s). The CTX – M ESBLs have a high affinity to cefotaxime, which it is known to hydrolyze and cause resistance to.

TEM – TYPE ESBLs (CLASS A): TEM – 1 is the most common encountered beta – lactamase in Gram – negative bacteria. Amino acid substitution at many sites in TEM – 1 beta – lactamases can be created in the laboratory without loss of activity. Those responsible for the ESBL phenotype change the configuration of the active site of the enzyme, allowing access to oxyimino beta – lactams. This enhances the susceptibility of the enzyme (ESBL) to beta – lactamase inhibitors such as clavulanic acid. The TEM Type ESBLs are mostly found in Escherichia coli  and Klebsiella pneumoniae.

SHV – TYPE ESBLs (CLASS A): SHV is derived from its early classification as the “Sulphydryl variant”. It denotes a variable response to sulphydryl inhibitors. SHV – Type ESBLs have one or more amino acid substitutions around the active site. They are most commonly found in Klebsiella pneumoniae and they account for up to 20% of the plasmid – mediated ampicillin resistance in this species.

CTX – M – TYPES ESBLs (CLASS A): The CTX – M ESBLs was named for their greater activity against cefotaxime (CTX) than other oxyimino – cephalosporin’s (e.g. Ceftazidime). Rather than arising by mutation, they represent examples of plasmid acquisition of beta – lactamase genes normally found on the chromosomes of Kluyvera species, a group of rarely pathogenic commensal organisms. Other forms of ESBLs also exist.

PHENOTYPIC DETECTION OF ESBLs

Detection of ESBLs is less straight forward when compared to routine antimicrobial susceptibility studies carried out in most of our hospitals today. Accurate laboratory detection of ESBL – producing bacteria in our hospitals is imperative in order to avoid clinical failure due to inappropriate antimicrobial therapy as the ESBLs are known to be resistant to both the penicillins and cephalosporins. Routine antimicrobial susceptibility testing methods are not capable of detecting ESBL resistance without modification. ESBL producers are likely to be reported falsely susceptible to the cephalosporins, unless specific ESBL screening and confirmatory tests are carried out. ESBL detection should be a two step process, and it is as shown below:

A Screening Test: This is done with an indicator cephalosporin to indicate any possible ESBL production. According to the Clinical Laboratory Standard Institute, CLSI (formerly NCCLS), any isolate found to be resistant to any one of the cephalosporins (ceftazidime, ceftriaxone, cefotaxime, cefpodoxime) used in the screening test using ESBL screening breakpoints should be considered possible ESBL producer that necessitates ESBL confirmation.

A Confirmatory Test: There are Various confirmatory test available for the detection of ESBLs including genotypic test using polymerase chain reaction (PCR), E-test, double disk synergy test (DDST) amongst others. For the phenotypic test using the DDST method, the CLSI recommends the use of both ceftazidime and cefotaxime alone and in combination with clavulanic acid. A 5mm increase in zone diameter for either of the cephalosporin (ceftazidime and cefotaxime) tested in combination with clavulanic acid versus its zone when tested alone, confirms ESBL production. Both cefotaxime and ceftazidime are recommended as they are variably hydrolyzed by TEM/SHV derived ESBLs and the CTX – M ESBLs.

The ESBL screening breakpoints and the phenotypic confirmatory test interpretation are as shown in the table below:

Table 1: ESBL SCREENING BREAKPOINTS

Discs R (BP, mm) S (BP, mm) ESBL Screening (BP, mm)
Cefpodoxime 10mg 17 27 17
Ceftazidime   10mg 14 18 22
Aztreonam     30mg 15 22 27
Cefotaxime    30mg 14 23 27
Ceftriaxone    30mg 13 21 25


R – Resistant              S – Susceptible                       BP – Breakpoint

Table 2: ESBL CONFIRMATORY TEST

DISCS INTERPRETATION
Ceftazidime                                   30mg

Ceftazidime – clavulanic acid  30/10mg

AND

Cefotaxime                                    30mg

Cefotaxime – clavulanic acid   30/10mg

A 5mm increase in zone diameter for EITHER antibiotic tested in combination with clavulanic acid versus its zone when tested alone confirms ESBL production.

MEDICAL SIGNIFICANCE OF ESBL DETECTION

Extended spectrum beta-lactamases (ESBLs) are clinically important enzymes produced by pathogenic bacteria including E. coli, K. pneumoniae and P. aeruginosa to mention only but a few. They represent an important mechanism of antibiotic resistance in Enterobacteriaceae and even non-Enterobacteriaceae such as P. aeruginosa, reducing the therapeutic efficacy of available drugs against these pathogens. ESBLs hydrolyze and inactivate penicillins, cephalosporins and some non-beta lactams including quinolones, sulphamethoxazole-trimethoprim and aminoglycosides. Gram negative bacteria producing ESBLs are a clinical threat and have been associated with increased morbidity and mortality in patients with severe infections.

The detection of ESBLs in clinical isolates is associated with many health problems such as destruction of oxyimino-cephalosporins (workhorse hospital antibiotics), delayed recognition of the infection, inappropriate treatment of ESBL infections and they are multidrug resistant in nature. In summary, when ESBL is detected in clinically important bacteria pathogens such as P. aeruginosa, Klebsiella and Escherichia coli; the emergence and spread of ESBL-producing bacteria can be prevented and controlled in both the hospital and non-hospital environment. It will also go a long way to prescribe antibiotics properly, in order to achieve better prognosis after treatment – since ESBL-producing bacteria are known to be multidrug resistant in nature.

RISK FACTORS FOR ACQUIRING ESBLs

Patients at high risk for developing colonization or infection with ESBL-producing organisms are often seriously ill patients with prolonged hospital stays and in whom invasive medical devices such as catheter are present for a long period of time. Though the actual source or risk factors for acquiring an ESBL infection is often arcane, most infections due to ESBL-producing bacteria have often occurred in people with underlying medical conditions who are already very sick, and people who are normally in intensive care units (ICUs) and heavy dependency units in the hospital environment.

Another striking risk factor for the acquisition of ESBL infection is heavy antibiotic usage or prior administration of antibiotics especially the 3rd-generation cephalosporins and other expanded spectrum antibiotics. According to researchers, there is evidence to suggest that the overuse of antibiotics (especially the extended spectrum cephalosporins) has imposed a selective pressure on pathogenic bacteria to acquire genes which they mutate and amplify to confer a wide range of resistance or hydrolytic activity to these antibiotics. In conclusion, the risk factors for the acquisition of ESBL – producing bacteria are quite different from risk factors for other infections. Some of the risk factors for the acquisition of ESBLs, many of which are inter – related includes:

  • Prior exposure to antibiotic, especially extended spectrum cephalosporins.
  • Increased length of stay in the hospital, especially the ICU’s and high dependency units.
  • Increased severity of infection.
  • Use of central venous catheter.
  • Use of arterial and urinary catheter.

CONTROL OF ESBLs

It is pertinent for us to control ESBL infection as a way of preventing or eradicating them since these mutant enzymes already exist. And they have spread through the clinical population either on plasmids or clonally in resilient strains. The acquisition of ESBLs is often associated with greater sensitivity to clavulanic acid (a beta – lactamase inhibitor), and this has also led to the use of a beta – lactamase inhibitor in combination with a cephalosporins in order to control them. There is need to control the emergence an spread of ESBL – producing bacteria in both the community and in the hospitals because organisms producing these enzymes destroy beta – lactams including cephalosporins which are the workhorse antibiotic used for therapy in most of our hospitals today.

ESBLs are also associated with a number of morbidity and mortality and many of them are multidrug resistant, showing antibiotic resistance to both beta – lactams and non – beta lactams like the fluoroquinolones and aminoglycosides and trimethoprim. This is not too good as treatment options for most bacterial related infections are been narrowed down by them, thus making it hard and difficult to manage and treat these infections. The spread of ESBLs can be managed by one of the following ways:

  • Proper detection and reporting of ESBL – producing bacteria in our hospitals and community.
  • Use of antibiotics wisely.
  • Maintaining proper personal hygiene both in the community and in the hospital, especially good hand washing since hands are the chief route whereby hospital strains of resistant microbes are acquired and spread.
  • Awareness creation in both the community, hospitals and even in the educational system about the ESBLs.
  • Re – evaluation of the cephalosporins for prophylaxis.
  • Restrictions on the consumptions of cephalosporins.

TREATMENT OF ESBL INFECTIONS

Bacteria that produce ESBLs beyond all reasonable doubts should be reported resistant to all penicillins (except temocillin), cephalosporins (except cefoxitin), and to aztreonam, irrespective of the routine susceptibility test results. Treatment failures and death have occurred when cephalosporins were used against ESBL producers that appeared susceptible in vitro. The presence of ESBLs in an infection (whether community acquired or hospital acquired) complicates the whole process of selecting antibiotics for treatment, particularly in patients with severe bacteraemia. This is due to the fact that organisms producing extended spectrum beta – lactamase enzymes are multidrug resistant to a number of antibiotics.

Susceptibilities of ESBL producers to β – lactamase inhibitor combinations vary with the bacteria isolate and its amount of enzyme being produced. Carbapenems (imipenem, meropenem and ertapenem) are consistently active and are the treatment of choice in severe infections due to ESBL producers. Though combinations of a cephalosporin with a beta – lactamase inhibitor like clavulanate work in principle, their actual efficacy in vivo have not yet been formally evaluated. Treatment with imipenem or meropenem has been associated with the best outcome in terms of survival and bacteriologic clearance when used for infections caused by ESBL – producing E. coli or Klebsiella species. Data on the actual antibiotic that is most efficacious for the treatment of ESBL infections are very limited.

The history of the battle in the war against antibiotic resistance is an ongoing one which calls for us all to be on the lookout for these organisms in our hospitals. The preceding pages have very briefly and incompletely laid out some of the basic facts of antibiotics, antibiotic resistance, mechanisms of development of resistance, extended spectrum beta – lactamases and their emergence and both their clinical and economic importance in the community. All substances (drugs or antibiotics inclusive) are poisons; the difference is in their dose according to the Center for Disease Control and Prevention (CDC).

In line with this statement, it is paramount that we use antimicrobials wisely in order to limit the chances of microbes developing resistance to them and spreading in both the community and in the hospitals, causing infections. Since their emergence, antibiotic resistance (especially ESBLs) have done us no good but rather, they are gradually eroding the efficacy of our therapeutic drugs without us knowing it; thus limiting therapeutic options for treatment. This calls for a holistic action which requires all hands (physicians, scientists, nurses, pharmacists, students, and patients) to be on deck in order to alleviate the menace of antibiotic resistance. So far so good, researchers all over the world are not leaving any stone unturned in this fight against antibiotic resistance.

New drugs that microorganisms will not be easily resistant to should be developed; and people’s orientation towards the use of drugs should change as well in order to avoid unnecessary and indiscriminate use of drugs. The fight against antibiotic resistance (especially those mediated by ESBLs) should not be only scientific, as social, economic, ethical, political, and even personal will are involved and such a multidisciplinary approach is paramount in curbing it. Antibiotic resistance knows no border of any country since there is free movement of both people and trade between one continent and another, which can serve as a route for the spread of a resistant pathogen.

The high – speed and ingenuity of microbes in developing resistance to available drugs (or antibiotics) is not balanced with the slow pace of developing new antimicrobials. This gives pathogenic microbes the time to mount resistance mechanism against available drugs. Thus making it difficult in selecting drugs for treatment in cases of infections, especially those caused by extended spectrum beta – lactamase – producing bacteria. A holistic multidisciplinary action is therefore urgently needed in curbing the emergence and spread of antibiotic resistant bacteria so that we do not run into a post – antibiotic era.

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