What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Extended Spectrum Beta Lactamase (ESBL) Explained

Name: Extended Spectrum Beta Lactamase (ESBL) Explained
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

Extended spectrum beta lactamase (ESBL) enzymes are bacterial proteins that destroy penicillins, most cephalosporins and aztreonam, but they do not destroy carbapenems or cephamycins, and they are blocked by clavulanate. When a bacterium such as Escherichia coli (E. coli) produces an ESBL, ordinary beta-lactam antibiotics stop working, and the infection becomes much harder to treat.

This article explains how ESBL enzymes work, which enzyme families matter most, how laboratories detect them, and why ESBL-producing E. coli in dogs, cattle and poultry is a growing concern for both animal and human health.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

What an ESBL Actually Is

Beta-lactam antibiotics share a chemical ring called the beta-lactam ring. Penicillins, cephalosporins, monobactams (aztreonam) and carbapenems all carry this ring, and the ring is what kills bacteria by disrupting cell wall construction. Beta-lactamase enzymes are bacterial weapons that cut the ring open, and a broken ring means a dead antibiotic.

An ESBL is a beta-lactamase with an unusually wide reach. The classic narrow-spectrum beta-lactamases, such as TEM-1, hydrolyze penicillins well but leave later-generation cephalosporins alone. ESBL variants acquire amino acid substitutions that widen the active site, so the enzyme also hydrolyzes extended-spectrum cephalosporins such as cefotaxime, ceftriaxone, ceftazidime and cefepime, plus the monobactam aztreonam.

Three features define the ESBL group:

Hydrolysis of penicillins, extended-spectrum cephalosporins and aztreonam. This is the "extended spectrum" part of the name.
No meaningful activity against carbapenems. Meropenem and imipenem remain intact, which is why carbapenems are often the treatment of choice.
No meaningful activity against cephamycins. Cefoxitin and cefotetan are usually spared.
Inhibition by clavulanate. Clavulanic acid, tazobactam and sulbactam bind the enzyme and protect the antibiotic. This is the basis of the combination-disk test used in diagnostic laboratories.

That last point matters clinically. A bacterium that is resistant to ceftazidime but becomes susceptible when clavulanate is added is behaving like an ESBL producer. A bacterium that stays resistant even with clavulanate is producing something else, most often an AmpC beta-lactamase or a carbapenemase.

Why "Plasmid-Encoded" Changes Everything

Most ESBL genes sit on plasmids, which are small circular DNA molecules that bacteria can copy and pass to neighboring bacteria. Plasmid transfer happens by conjugation, a direct cell-to-cell connection that does not require the recipient to be the same species. A plasmid carrying *bla*CTX-M can move from one E. coli to another, and in some cases from E. coli into Klebsiella pneumoniae or Salmonella.

Chromosome-encoded resistance is inherited vertically, from parent cell to daughter cell. Plasmid-encoded resistance spreads horizontally, across species and across hosts, sometimes within hours. That is why ESBL genes travel so efficiently through farms, households, waterways and hospitals.

Plasmid-borne resistance also travels in packages. A single plasmid often carries ESBL genes alongside genes for fluoroquinolone resistance, aminoglycoside resistance, sulfonamide resistance and tetracycline resistance. In a study of unweaned dairy calves in Germany, 41% of ESBL-producing E. coli isolates carried antibiotic resistance genes exclusively on plasmids, and conjugative plasmids accounted for 94.6% of plasmid-borne resistance gene occurrences [1]. Conjugative plasmids carried significantly more resistance genes than mobilizable plasmids, and class 1 integrons marked the most heavily loaded multidrug plasmids [1].

The Three Main ESBL Families

The ESBL family tree is large, but three groups dominate in veterinary and human isolates.

TEM

TEM enzymes take their name from Temoniera, the patient from whom the first variant was isolated. TEM-1 is one of the most common beta-lactamases in Gram-negative bacteria and confers resistance to ampicillin and early cephalosporins. Point mutations convert TEM-1 into an ESBL. TEM ESBLs are inhibited by clavulanate and hydrolyze aztreonam well.

SHV

SHV stands for sulfhydryl variable, a reference to the enzyme's behavior with inhibitors. Like TEM, SHV-1 is a narrow-spectrum penicillinase that can mutate into an ESBL. SHV ESBLs are common in Klebsiella species and appear in E. coli as well.

CTX-M

CTX-M enzymes are named for their strong preference for cefotaxime. They originated in environmental Kluyvera species and jumped into E. coli and other Enterobacterales on mobile elements. CTX-M is now the most common ESBL family worldwide, and it is the family most often found in animals.

The dominance of CTX-M shows up repeatedly in surveillance. In ESBL-positive E. coli from hospitals in Tabuk, Saudi Arabia, the CTX-M-1 gene was the most prevalent determinant, found in 67% of ESBL-positive samples, with *bla*SHV in 46% and *bla*TEM in 50% [2]. In broiler chickens in Jinan City, China, whole-genome sequencing identified *bla*CTX-M-55 as the predominant genotype at 29.58%, followed by *bla*CTX-M-64 at 23.94%, *bla*CTX-M-15 at 19.71%, *bla*CTX-M-14 at 12.68% and *bla*CTX-M-65 at 9.86% [3]. In ESBL-producing E. coli from camels with subclinical mastitis, *bla*CTX-M was the gene detected in the ESBL-positive isolates [4].

The practical takeaway is that CTX-M is the enzyme family a veterinarian or microbiologist should expect first when an animal isolate tests ESBL-positive.

Resistance Profile Table

The table below compares the major beta-lactamase classes that matter in veterinary practice. Substrate profiles describe what each enzyme class typically hydrolyzes. Inhibitor susceptibility describes whether clavulanate class inhibitors block the enzyme.

Beta-lactamase class	Representative enzyme	Substrate profile	Inhibited by clavulanate?
Narrow-spectrum penicillinase	TEM-1, SHV-1	Penicillins, early cephalosporins	Yes
ESBL	TEM-52, SHV-12, CTX-M-15	Penicillins, extended-spectrum cephalosporins, aztreonam. Not carbapenems, not cephamycins	Yes
AmpC (chromosomal or plasmid)	CMY-2, chromosomal AmpC of Enterobacter	Penicillins, cephalosporins, cephamycins, aztreonam. Not carbapenems	No (weakly inhibited at best)
Carbapenemase (serine)	KPC, OXA-48	Nearly all beta-lactams including carbapenems	Variable, often poorly
Carbapenemase (metallo)	NDM, VIM, IMP	Nearly all beta-lactams including carbapenems	No

Two clinical rules follow from this table. First, an isolate that is resistant to cefoxitin is probably not a pure ESBL producer, because ESBLs spare cephamycins. Second, an isolate that is resistant to meropenem is producing a carbapenemase, and that is a different and more serious problem.

How Laboratories Detect ESBL Production

Phenotypic detection relies on the clavulanate effect. The Clinical and Laboratory Standards Institute (CLSI) publishes the disk diffusion and broth microdilution breakpoints that laboratories use, and the combination-disk synergy test is the workhorse method.

The logic is straightforward. A disk containing cefotaxime alone produces a small zone of inhibition if the bacterium is an ESBL producer. A disk containing cefotaxime plus clavulanate produces a larger zone, because clavulanate disables the enzyme. A difference of 5 mm or more between the two zones is a positive result under standard methods. The double-disk synergy test places a clavulanate disk near cephalosporin disks and looks for a characteristic keyhole-shaped enhancement of the inhibition zone.

Molecular confirmation uses polymerase chain reaction (PCR) to detect specific genes. Multiplex PCR panels typically target *bla*TEM, *bla*SHV, *bla*CTX-M group 1, *bla*CTX-M group 9 and *bla*CTX-M group 25 [2]. Whole-genome sequencing goes further and identifies the exact variant, the plasmid context and any co-carried resistance genes. In a study of ESBL-producing E. coli from broiler chickens, WGS was what allowed investigators to distinguish *bla*CTX-M-55 from *bla*CTX-M-64 from *bla*CTX-M-15 in the same population [3].

Phenotypic and genotypic results do not always agree perfectly. In one survey of 31 phenotypically ESBL-positive isolates from bovine carcasses in Türkiye, 29 carried at least one ESBL-associated gene, and the predominant gene was *bla*CTX-M at 79.3% [5]. The small gap between phenotype and genotype is a normal feature of ESBL testing and reflects the limits of each method.

ESBL in E. coli: The Veterinary Picture

E. coli is the single most important ESBL-producing organism in veterinary medicine. It is a normal gut inhabitant of mammals and birds, it causes urinary, enteric, respiratory and systemic infections, and it readily accepts plasmids from other bacteria. When ESBL genes enter E. coli, the bacterium becomes both a pathogen and a resistance reservoir.

Dogs

Companion animals sit close to people, so ESBL-producing E. coli in dogs has direct household relevance. A survey of rectal swabs from 113 dogs at a quarantine station in Surabaya, Indonesia, recovered E. coli from 75.22% of animals, and 3.53% of those isolates carried ESBL genes [6]. Two of the three positive isolates carried a combination of *bla*CTX-M and *bla*TEM, and one carried *bla*CTX-M alone. No *bla*SHV was detected [6]. The prevalence was low, but the finding confirms that dogs can carry and shed ESBL-producing E. coli.

Free-roaming cats show the same pattern at higher prevalence. In a study of 214 free-roaming cats in Israel, ESBL-producing Enterobacterales were recovered from 51 cats (23.8%), yielding 61 isolates, of which E. coli was the most common species at 62.3% [7]. Multidrug resistance, defined as non-susceptibility to at least one agent in three or more antimicrobial categories, was present in 72.1% of isolates, and four isolates were carbapenemase-producing [7].

Cattle

Cattle carry ESBL-producing E. coli at variable rates depending on age, management and antimicrobial exposure. In Swiss dairy cattle sampled between 2021 and 2023, ESBL-producing E. coli was detected in 3 of 475 rectal swabs, a prevalence of 0.6% [8]. In the same study, wildlife samples showed a higher prevalence of 5%, with birds at 6.1% and mammals at 4.5% [8]. Whole-genome sequencing of 54 isolates identified 28 sequence types with no overlap between livestock and wildlife, which suggests distinct evolutionary trajectories rather than simple livestock-to-wildlife spillover [8].

Veal calves are a different story. In French veal calves, 12.5% of animals were extended-spectrum cephalosporin-resistant E. coli carriers at their farm of origin, and an additional 6.2% became carriers during transport to the sorting center [9]. Among 176 sequenced isolates, 55.1% had an ESBL phenotype conferred by a *bla*CTX-M gene, while 44.9% had an AmpC phenotype from chromosomal mutations [9]. Calves from farms that fed waste milk had a higher proportion of resistant isolates (30.3%) than calves from farms that discarded it (13.1%) [9].

Poultry

Broiler chickens can carry ESBL-producing E. coli at substantial rates. In a study of 600 fecal samples from broiler farms in Jinan City, China, 537 E. coli isolates were recovered at an 89.5% isolation rate, and 71 of those (13.2%) were ESBL producers [3]. Resistance among ESBL-producing isolates was severe. All were resistant to ampicillin, 94.37% to sulfamethoxazole/trimethoprim, 87.32% to streptomycin, 78.87% to chloramphenicol and 70.42% to tetracycline [3]. Resistance to cefuroxime and ceftriaxone reached 43.66% and 36.62% respectively, and susceptibility to amoxicillin/clavulanic acid fell to 61.97% [3]. The plasmid-mediated colistin resistance gene mcr-1 was found in 25.35% of ESBL-producing isolates [3].

A comparison of intensive and free-range chickens in Zambia found ESBL-producing E. coli in 13 of 112 pooled fecal samples, an overall prevalence of 11.6%, with no significant difference between the two production systems [10]. All 13 isolates were multidrug-resistant, with complete resistance to tetracycline and cefotaxime [10]. Production system alone did not explain the difference.

Co-Resistance: The Hidden Complication

An ESBL-positive isolate is rarely resistant to beta-lactams alone. Plasmids accumulate resistance genes, and the result is multidrug resistance that narrows treatment options sharply.

Fluoroquinolone and aminoglycoside co-resistance is the rule rather than the exception. In multidrug-resistant E. coli from diseased food-producing animals in Taiwan, fluoroquinolone resistance was driven by both chromosomal mutations in the quinolone resistance-determining regions and plasmid-mediated quinolone resistance genes, including qnr variants and aac(6')-Ib-cr [11]. The same study identified plasmid-mediated colistin resistance genes mcr-1.1, mcr-3.1 and mcr-3.5 alongside chromosomal pmrB substitutions, showing that colistin resistance can arise through multiple pathways [11].

Co-resistance shapes empirical treatment. When a veterinarian suspects an ESBL infection, the safe assumption is that fluoroquinolones, aminoglycosides, sulfonamides and tetracyclines may also fail. Susceptibility testing is not optional in these cases.

Treatment Options

Carbapenems are often the treatment of choice for serious ESBL infections because ESBL enzymes do not hydrolyze them. In the Swiss livestock and wildlife survey, all ESBL-producing E. coli isolates remained susceptible to meropenem and tigecycline [8]. That susceptibility pattern is the reason carbapenems hold their position as reliable agents when an ESBL infection is confirmed.

Carbapenem use in animals is restricted in many jurisdictions because of the need to preserve these drugs for human medicine. When a carbapenem is not appropriate or not available, the options depend entirely on the susceptibility report.

Fosfomycin has renewed interest as an alternative for urinary isolates. A long-term surveillance study of urinary pathogens in Pakistan tracked fosfomycin susceptibility across ESBL-producing and non-ESBL E. coli categories from 2013 to 2025, reflecting the clinical need for oral options when multidrug resistance limits the formulary [12]. Fosfomycin susceptibility is not universal, and it must be confirmed for each isolate.

Beta-lactam/beta-lactamase inhibitor combinations are a middle path. Clavulanate inhibits the ESBL enzyme, so amoxicillin/clavulanate can retain activity against some ESBL producers. The Jinan broiler study found that susceptibility to amoxicillin/clavulanic acid had declined to 61.97% among ESBL-producing isolates, meaning roughly four in ten were already resistant to the combination [3]. This is not a reliable empirical choice.

Adjunctive strategies are under investigation. A study of baicalin, a flavonoid from Scutellaria baicalensis, found that it potentiated cefquinome against animal-origin ESBL E. coli by suppressing *bla*CTX-M gene expression, reducing ESBL-mediated cefquinome hydrolysis and limiting the emergence of reduced susceptibility during serial passage [13]. In a mouse intestinal infection model, the combination improved survival and reduced intestinal pathology [13]. These findings are early and do not yet translate into clinical protocols.

The practical rule for treatment is simple. Do not guess. Culture and susceptibility testing determine which agent will work, and carbapenems remain the anchor for serious infections.

Zoonotic Transfer Risk and the One Health View

ESBL-producing E. coli moves between animals and people, and the movement is not one-directional. A cross-sectional study in Catalonia, Spain, sampled livestock and farmers and used whole-genome sequencing to evaluate genomic overlap between animal and human isolates [14]. ESBL-producing Enterobacterales were predominantly detected in swine, with 63.5% of sampled animals testing positive, while no ESBL, AmpC or carbapenemase producers were identified in cattle or poultry in that particular sampling frame [14]. The genomic comparison was designed to explore shared origins and interspecies transmission.

The wildlife-livestock interface adds another layer. A review of ESBL-producing E. coli at this interface concluded that livestock, wildlife and environmental matrices can be interconnected reservoirs, and that transmission should be understood as a bidirectional ecological phenomenon shaped by antimicrobial use, farm management, biosecurity, wildlife ecology, environmental contamination and mobile genetic elements [15]. Wildlife may act as a sentinel, a reservoir or a disperser of resistant bacteria, although detecting a resistant organism in wildlife does not by itself prove direct transmission [15].

Environmental routes matter too. In surface waters of the Shkumbini river in Albania, ESBL-producing E. coli was detected in 80% of samples, and 94.2% of isolates were multidrug-resistant [16]. The *bla*CTX-M-1 genotype was most prevalent at 57.7%, and almost half of the isolates carried multiple ESBL genes [16]. Untreated municipal, agricultural and industrial waste were identified as likely contamination sources [16].

For pet owners, the practical implications are modest but real. Hand hygiene after handling animals, cleaning litter boxes or picking up feces reduces exposure. Owners should not share antibiotics prescribed for one animal with another, and leftover antibiotics should never be used without veterinary direction. Animals with recurrent infections that fail to respond to standard antibiotics deserve culture and susceptibility testing rather than repeated empirical courses.

Clinical Relevance, Limitations and Common Mistakes

The most common mistake is treating an ESBL infection with a cephalosporin because the laboratory report shows "resistant" without the clinician registering what that means. A bacterium that produces an ESBL will not respond to cefovecin, cefpodoxime, ceftiofur or cefquinome, and using one of these drugs delays effective treatment and adds selection pressure.

A second mistake is assuming that a positive culture means active infection. E. coli is a normal gut inhabitant, and ESBL-producing E. coli can be carried without causing disease. Detection in a fecal sample does not automatically mean the animal needs treatment. Clinical signs, the site of culture and the overall picture determine whether treatment is warranted.

A third mistake is skipping susceptibility testing because the animal "always responds to the usual antibiotic." ESBL prevalence is rising, and the usual antibiotic may already be failing.

A fourth mistake is confusing ESBL with carbapenemase production. These are different resistance mechanisms with different clinical implications. ESBL producers usually remain susceptible to carbapenems. Carbapenemase producers do not. The distinction changes the treatment plan and the public health response.

Individual cases vary, and every animal with a suspected ESBL infection needs a veterinarian who can interpret the culture results in the context of that specific patient.

Frequently Asked Questions

What does ESBL stand for?

ESBL stands for extended spectrum beta lactamase. It is an enzyme that bacteria produce to destroy common beta-lactam antibiotics.

Are ESBL infections contagious to people?

Yes, ESBL-producing bacteria can transfer between animals and people, particularly through direct contact and environmental contamination. Good hand hygiene reduces the risk.

Can ESBL infections be treated?

Yes, but treatment requires susceptibility testing. Carbapenems are often the treatment of choice, and other options depend on what the laboratory reports as effective.

Why are carbapenems used for ESBL infections?

Carbapenems resist hydrolysis by ESBL enzymes, so they remain active when penicillins and cephalosporins fail.

Does clavulanate work against ESBL enzymes?

Yes. Clavulanate inhibits ESBL enzymes, which is why it is combined with amoxicillin and why it is used in laboratory detection tests.

Can my dog pass ESBL bacteria to my other pets?

Yes. ESBL genes sit on plasmids that transfer easily between bacteria, and animals in the same household can share resistant strains.

Is ESBL the same as MRSA?

No. ESBL refers to beta-lactamase enzymes in Gram-negative bacteria such as E. coli. MRSA is methicillin resistance in Staphylococcus aureus, a Gram-positive organism.

Should I stop giving antibiotics if my pet has an ESBL infection?

No. Do not stop or change antibiotics without veterinary direction. Stopping partway through a course can worsen the infection and increase resistance.