What Is ESBL? Causes, Diagnosis, and Treatment

ESBL stands for extended-spectrum beta-lactamase, a type of enzyme that certain bacteria produce to destroy a wide range of common antibiotics, including penicillins and most cephalosporins. When bacteria carry the genes for ESBL, many of the go-to drugs doctors reach for simply stop working. ESBL-producing bacteria have spread worldwide and are now a significant public health concern, particularly because they narrow the treatment options for infections that would otherwise be routine to manage.

How ESBL Actually Works

Beta-lactam antibiotics, which include penicillin, amoxicillin, and the cephalosporin family, all share a chemical structure called the beta-lactam ring. That ring is what makes them effective at killing bacteria. Beta-lactamase enzymes attack and break open this ring, rendering the antibiotic inactive before it can do its job.¹ Ordinary beta-lactamases have been around for decades. What makes ESBLs different is that mutations in the genes encoding these enzymes expanded their reach. Standard beta-lactamases could only break down older, narrower-spectrum antibiotics like basic penicillins. ESBLs can destroy newer, more powerful antibiotics too, including third-generation cephalosporins like ceftazidime, cefotaxime, and ceftriaxone, as well as aztreonam.²

There are some important limits to what ESBLs can break down. They generally cannot defeat cephamycins (like cefoxitin) or carbapenems (like meropenem and imipenem), which is why carbapenems have historically served as the last-resort treatment for ESBL infections.² That said, bacteria evolve, and the emergence of carbapenem resistance in some strains is a growing worry.

Which Bacteria Carry ESBL

ESBL genes show up most often in two species: Escherichia coli (E. coli) and Klebsiella pneumoniae. These are members of a larger bacterial family called Enterobacterales, which live naturally in the human gut and in the environment. Most of the time they are harmless, but when they cause infections, particularly in the urinary tract or the bloodstream, the presence of ESBL makes treatment far more complicated. In a study examining ESBL-producing bacteria in raw food samples, about 86% of the ESBL-producing isolates were E. coli and 12% were K. pneumoniae.³ Other species like Pseudomonas aeruginosa and Morganella morganii occasionally carry ESBL genes too, but they are far less common culprits.

Among the ESBL gene families, the CTX-M type has become dominant globally. This matters because different ESBL types can have slightly different resistance profiles and respond differently to diagnostic tests. The CTX-M genotype, particularly in E. coli and K. pneumoniae, is now the major ESBL type circulating in both clinical and community settings.⁴

How ESBL Genes Spread Between Bacteria

One of the most alarming features of ESBL is how efficiently resistance spreads. ESBL genes frequently sit on plasmids, which are small loops of DNA that bacteria can pass to one another through a process called horizontal gene transfer. Unlike chromosomal DNA, which a bacterium only passes to its offspring when it divides, plasmids can be transferred sideways between entirely different species of bacteria.⁵ In practical terms, an E. coli carrying ESBL can hand its resistance genes to a nearby K. pneumoniae or even a Pseudomonas in the same environment.

Research on clinical isolates from hospitals in India demonstrated this vividly: out of 20 ESBL-carrying isolates tested, 12 successfully transferred their plasmid-borne resistance genes to other bacteria through conjugation, the bacterial equivalent of sharing a USB drive.⁶ This plasmid-mediated transfer is a major reason why ESBL has spread so rapidly around the world. It does not require the original resistant bacterium to survive and multiply; it just needs to share its genetic instructions once.

Who Is Most at Risk

ESBL infections are not distributed evenly across the population. Some people are substantially more likely to pick up an ESBL-producing organism, and the risk factors fall into a few consistent patterns across multiple studies.

Prior antibiotic use is the single most consistently identified risk factor. A systematic review of community-acquired urinary tract infections caused by ESBL-producing E. coli found that previous antibiotic use raised the odds anywhere from about two to twenty-fold depending on the study.⁷ Fluoroquinolones deserve special mention here. An 11-year population study in Denmark found that fluoroquinolone treatment more than tripled the odds of ESBL E. coli or K. pneumoniae bloodstream infection compared to controls.⁸ A separate study found that previous fluoroquinolone use carried more than seven times the odds of having an ESBL-producing E. coli infection compared to a non-ESBL infection.⁹

Beyond antibiotics, recent hospitalization and a history of urinary tract infections are strong risk factors. The same systematic review identified previous hospitalization (roughly two to four times the odds) and UTI history (roughly one and a half to four times the odds) as consistent predictors.⁷ Two environmental factors also emerged: travel abroad and swimming in freshwater, both of which reflect exposure to bacteria in settings where ESBL organisms circulate freely.

Neonates in intensive care units face particular danger. A study in a neonatal ICU found that low birth weight, mechanical ventilation, prolonged hospitalization, total parenteral nutrition, and prior use of certain antibiotics were all independent risk factors for ESBL K. pneumoniae infection. ESBL infections in those infants were associated with roughly three times the mortality risk compared to non-ESBL infections.¹⁰

How ESBL Is Diagnosed

Identifying whether a bacterial infection involves ESBL production is critical because it changes the treatment plan entirely. Diagnosis typically happens in two stages: first, the lab identifies the bacterial species causing the infection, then it determines whether that organism produces ESBL.

The most widely available methods are phenotypic tests, which observe how bacteria behave when exposed to specific antibiotic combinations on agar plates. The double disc synergy test places antibiotic discs at specific distances from a disc containing a beta-lactamase inhibitor (like clavulanic acid). If the bacteria produce ESBL, the zone where bacteria cannot grow will be visibly distorted or enlarged near the inhibitor disc.¹¹ However, these methods are not equally reliable. One study of urinary E. coli isolates found that the standard double disc synergy test detected ESBL in only about 19% of confirmed ESBL-producing strains, while alternative disc diffusion methods and the E-strip test achieved 100% sensitivity.¹² The choice of testing method matters, especially in resource-limited settings where labs may rely on less sensitive techniques.

Molecular methods offer a more precise alternative. These use DNA-based techniques to detect the actual ESBL genes (such as TEM, SHV, and CTX-M) rather than observing the bacterial behavior. A line probe assay evaluated against hundreds of clinical Enterobacterales strains showed 100% sensitivity and specificity in detecting and differentiating TEM, SHV, and CTX-M genes.¹³ Molecular diagnostics are faster and more accurate, but they remain more expensive and less available in many healthcare settings. Because ESBL-producing bacteria are rapidly evolving, there is an acknowledged lack of one standardized examination method that covers all variants reliably.⁴

Treatment for Serious ESBL Infections

Carbapenems have long been considered the drugs of choice for serious ESBL infections. These antibiotics belong to the same beta-lactam family but have a structure that ESBLs typically cannot break down. For severe infections like bloodstream infections or pneumonia caused by ESBL-producing bacteria, carbapenems remain the standard first-line treatment.¹⁴¹⁵

The problem with relying on carbapenems is that overuse accelerates the emergence of carbapenem-resistant bacteria, which are even harder to treat. This has driven significant interest in carbapenem-sparing strategies. Piperacillin-tazobactam, a combination of a penicillin with a beta-lactamase inhibitor, has been a leading candidate. A systematic review and meta-analysis found no significant difference in mortality between piperacillin-tazobactam and carbapenems for ESBL-producing bloodstream infections overall.¹⁶ A large retrospective multicenter study reached a similar conclusion, finding piperacillin-tazobactam non-inferior to carbapenems for 30-day mortality.¹⁷

However, the picture is not uniformly reassuring. The meta-analysis found that mortality was significantly higher with piperacillin-tazobactam when the source of infection was something other than the urinary or biliary tract. For infections originating from the lungs, abdomen, or other sources, carbapenems appear to retain a meaningful advantage.¹⁶ When the infection is clearly originating from the urinary tract and is not causing severe sepsis, ertapenem (a narrower-spectrum carbapenem) can be a reasonable middle ground.¹⁸

Treating Uncomplicated Urinary Tract Infections

Not every ESBL infection is a life-threatening bloodstream crisis. Urinary tract infections are the most common clinical presentation, and uncomplicated UTIs caused by ESBL-producing bacteria can often be treated with oral antibiotics, avoiding hospitalization and intravenous carbapenems altogether.

Three oral antibiotics show consistently high activity against ESBL producers: pivmecillinam, fosfomycin, and nitrofurantoin. One study found that more than 95% of all ESBL-producing Enterobacterales were susceptible to these three drugs and recommended their use for uncomplicated UTIs.¹⁹ Fosfomycin and nitrofurantoin have also been found to be effective options for ESBL-positive urinary infections in children.²⁰ These drugs work for bladder infections because they concentrate in the urine, where they reach levels high enough to kill the bacteria even if their overall blood levels would be inadequate for a deeper infection. They are not appropriate for kidney infections, bloodstream infections, or any infection that has spread beyond the urinary tract.

What Happens When the Wrong Antibiotic Is Given First

Because ESBL infections resist many commonly prescribed antibiotics, there is a real risk that a patient receives an antibiotic that does not work before lab results come back confirming ESBL production. The clinical consequences of this delay depend heavily on where the infection is.

For bloodstream infections that originate from the urinary or biliary tract, the impact of initially receiving the wrong antibiotic appears to be more forgiving. One study found no statistically significant difference in 30-day mortality between ESBL-producing and non-ESBL E. coli bloodstream infections, even though the ESBL group experienced a delay in receiving an effective antibiotic.²¹ For bloodstream infections originating from other sites, the stakes are much higher. Inadequate initial antibiotic therapy was found to be an independent risk factor for death in non-urinary ESBL infections, with a roughly ten-fold increase in the odds of mortality.²² Other factors linked to worse outcomes in ESBL E. coli bloodstream infections include mechanical ventilation, presence of a central venous catheter, and higher organ failure scores at the time of infection.²³

ESBL in the Food Supply and the Environment

ESBL-producing bacteria are not confined to hospitals and clinics. They circulate in the wider environment, particularly in food-producing animals and the food chain. Research has found genetic similarities between ESBL-producing E. coli isolated from animals and animal-derived foods and those found in human infections, suggesting a possible transmission pathway between the two.²⁴ In Thailand, nearly 18% of raw food samples tested positive for ESBL-producing organisms, with ESBL-producing E. coli accounting for the vast majority.³

The use of antibiotics in livestock farming contributes to the problem. When animals receive antibiotics (often the same classes used in human medicine), resistant bacteria can flourish in their guts and be passed along through meat, eggs, dairy, or environmental contamination of water and soil. This is the basis of the “One Health” framework, which treats human health, animal health, and environmental health as interconnected pieces of the same puzzle. Addressing ESBL effectively requires reducing inappropriate antibiotic use not just in clinics but on farms as well.

The Financial and Hospital Burden

ESBL infections are not just medically complicated. They are expensive. A study from Korea found that patients with ESBL-producing Enterobacterales bloodstream infections had hospital stays about 1.5 times longer than those with susceptible infections, and their total costs were significantly higher, averaging about $11,150 compared to $8,700 for susceptible infections and $6,060 for uninfected controls.²⁵ A matched analysis of ESBL-producing E. coli and K. pneumoniae bloodstream infections found that ESBL production reduced the effectiveness of initial empirical antibiotics by about 20%, extended hospitalization by an average of three days, and increased patient costs by roughly $2,000.²⁶ Even in lower-income settings, the cost difference is meaningful. A study in Laos found that per-patient costs for ESBL-producing E. coli infections averaged about $689 compared to $489 for non-ESBL E. coli infections, with the out-of-pocket difference remaining statistically significant even after statistical adjustment.²⁷

Antibiotic Stewardship and Reducing ESBL

The most effective strategy for slowing the rise of ESBL is reducing unnecessary antibiotic use, both in hospitals and in the community. Traditionally, stewardship programs have focused on hospital prescribing, but modeling research suggests that a given reduction in community antibiotic use could have an even greater impact on overall ESBL prevalence than an equivalent reduction in hospital use.²⁸ On a per-prescription basis, hospital antibiotic changes still matter more, but the sheer volume of antibiotics prescribed in outpatient clinics means community-level changes move the needle at a population scale.

Within hospitals, restricting specific antibiotic classes has been shown to reduce resistance. Some institutions have seen declines in ESBL cases after tightening prescribing rules, though the relationship is not always straightforward. Resistance can persist even after antibiotic consumption drops because resistant bacteria do not necessarily die off quickly once established.²⁹ Coordinated efforts between stewardship programs and infection control teams are considered essential for making real progress.³⁰

One interesting development in hospital infection control has been the re-evaluation of contact precautions for ESBL. Gowning and gloving for every room entry adds cost and can reduce the frequency of healthcare worker visits. One hospital that discontinued contact precautions for ESBL-producing organisms found no increase in the prevalence of ESBL E. coli or Klebsiella infections afterward, while hand hygiene compliance actually improved (from 84% to 89%), and the institution saved over $120,000 in gown costs in the first year.³¹ This suggests that good hand hygiene alone may be sufficient to prevent hospital transmission of ESBL, though whether this applies in all settings remains debated.

Newer Drugs and Experimental Approaches

The pharmaceutical pipeline includes several newer antibiotic combinations designed to overcome ESBL and related resistance mechanisms. Ceftazidime-avibactam pairs a cephalosporin with a next-generation beta-lactamase inhibitor, and it has shown effectiveness against ESBL-producing bacteria as well as some carbapenem-resistant strains.³² In laboratory testing against ESBL-producing Enterobacterales that were not carbapenem-resistant, the newer combinations showed high susceptibility rates: cefepime-zidebactam at about 97%, aztreonam-avibactam at about 93%, ceftazidime-avibactam at about 91%, and meropenem-vaborbactam at about 91%.³³ These numbers are encouraging, though in-vitro susceptibility does not always translate perfectly to clinical outcomes.

An entirely different approach involves trying to eliminate ESBL-producing bacteria from the gut before they cause infection, a strategy called decolonization. Fecal microbiota transplantation (FMT), already used for recurrent Clostridioides difficile infection, has been explored for this purpose. Early studies have shown mixed results. In one set of cases, decolonization of antibiotic-resistant organisms was achieved in about 60% of patients at one month and 93% at six months after FMT, with E. coli responding better than K. pneumoniae. A randomized trial was less dramatic: about 50% of patients in the FMT group were decolonized compared to roughly 23% of controls, per protocol.³⁴ FMT for ESBL decolonization remains experimental and is not part of standard clinical practice, but the concept of using a healthy microbial community to outcompete resistant bacteria is being actively investigated.