Enterobacteriaceae is a large family of bacteria that live in the human gut, in soil, in water, and on food. The family includes some of the most common bacteria in clinical medicine, from the well-known E. coli to Salmonella and Klebsiella. Some members are harmless residents of your intestines, while others cause serious infections ranging from food poisoning to life-threatening bloodstream infections. The family has also become a major focus of public health concern because many of its members are developing resistance to antibiotics.
Basic Characteristics
All Enterobacteriaceae share a handful of traits that define the family. They are rod-shaped, Gram-negative bacteria, meaning they have a thin cell wall surrounded by an outer membrane. Most are motile, propelled by tiny whip-like structures called flagella distributed around their surface. They don’t form spores, which means they can’t enter a dormant, nearly indestructible state the way some other bacteria can.
One of their most useful traits for survival is metabolic flexibility. They are facultative anaerobes, so they can grow whether oxygen is available or not. Nearly all of them ferment glucose, often producing acid and gas in the process. They also grow in the presence of bile salts, which is part of what allows them to thrive in the human digestive tract. In the lab, they consistently test negative for an enzyme called oxidase and positive for catalase, and they reduce nitrate. These biochemical signatures are what microbiologists use to identify them.
Key Members of the Family
The family contains dozens of genera, but a handful dominate in human health. E. coli is by far the most familiar. It is the second most commonly reported infectious organism overall and lives as a normal part of your gut flora, though certain strains cause severe foodborne illness. Klebsiella pneumoniae is a leading cause of hospital-acquired pneumonia and bloodstream infections. Enterobacter species, particularly the Enterobacter cloacae complex, are increasingly important in intensive care units.
Salmonella and Shigella are the family’s most recognized foodborne pathogens. Salmonella causes millions of cases of gastroenteritis worldwide each year, while Shigella is responsible for dysentery, particularly in settings with limited sanitation. Yersinia enterocolitica, another foodborne member, causes fever and abdominal pain that can mimic appendicitis. Less commonly discussed but still clinically relevant are Proteus, Serratia, and Citrobacter, which rank among the top causes of hospital-associated infections.
Where They Live
Many Enterobacteriaceae are commensal organisms, meaning they live in your gut without causing harm. E. coli, for instance, colonizes the intestines within hours of birth and remains a permanent resident. These bacteria also exist widely in soil, freshwater, and on the surfaces of plants and animals. Their presence in water and food is actually used as a hygiene indicator: finding high levels of Enterobacteriaceae in a food production facility signals potential contamination.
This is where the distinction between “coliforms” and Enterobacteriaceae matters. Coliforms are a subset of Enterobacteriaceae defined by a single lab trait: the ability to ferment lactose and produce acid and gas within 48 hours at 35°C. All coliforms belong to the Enterobacteriaceae family, but not all Enterobacteriaceae are coliforms. Because the coliform definition is based on a lab test rather than actual taxonomy, it misses many potential contaminants. Food safety programs are increasingly switching to testing for the broader Enterobacteriaceae family, which catches a wider range of harmful bacteria.
Infections They Cause
Outside the gut, Enterobacteriaceae are opportunistic. They cause trouble when they reach parts of the body where they don’t belong, or when the immune system is weakened. Urinary tract infections are the most common example. E. coli causes the vast majority of UTIs, traveling from the intestinal tract to the urinary system.
In hospitals, these bacteria are responsible for a broad range of serious infections: bloodstream infections (bacteremia), ventilator-associated pneumonia, surgical site infections, and infections linked to catheters and other medical devices. Sepsis and septic shock are the most dangerous outcomes, carrying particularly high mortality in critically ill or immunosuppressed patients. E. coli, Klebsiella pneumoniae, and the Enterobacter cloacae complex are the species most frequently involved in these hospital-acquired infections.
The Antibiotic Resistance Problem
Enterobacteriaceae have become one of the biggest challenges in antibiotic resistance. These bacteria produce enzymes called beta-lactamases that break apart the molecular ring at the core of many common antibiotics, rendering them useless. The problem has escalated in stages, with each wave of resistance targeting a more powerful class of drugs.
Extended-spectrum beta-lactamases (ESBLs) were the first major alarm. Bacteria carrying ESBLs can break down penicillins and several classes of related antibiotics. When ESBLs spread, doctors turned to a class of powerful antibiotics called carbapenems, often considered last-resort drugs. But bacteria adapted again. Carbapenem-resistant Enterobacteriaceae, known as CRE, now produce enzymes called carbapenemases that destroy even these last-line antibiotics.
The resistance mechanisms go beyond enzymes. Some bacteria pump antibiotics back out through efflux pumps before the drugs can take effect. Others modify the tiny channels (porins) in their outer membrane so antibiotics can’t enter the cell in the first place. Many resistant strains use multiple mechanisms simultaneously, making them extraordinarily difficult to treat.
The scale of the problem is growing. In New York City alone, 7,114 CRE cases were reported between 2019 and 2024. One particularly concerning type of resistance enzyme, called NDM (New Delhi metallo-beta-lactamase), surged from 58 cases in 2019 to 388 in 2024. The rate of NDM-positive CRE infections rose from less than one per 100,000 residents to 3.8 per 100,000 in just a few years. These resistant bacteria spread readily in healthcare settings and are classified by the CDC as an urgent public health threat.
How They’re Identified in the Lab
When a clinical sample arrives at a microbiology lab, Enterobacteriaceae are typically identified using a combination of growth characteristics and biochemical tests. Commercial identification systems use panels of miniaturized biochemical reactions to generate a profile that can be matched to known species. These systems are widely used but have accuracy limitations, with some kits correctly identifying species only about half the time for certain organisms.
A newer technology called MALDI-TOF mass spectrometry has transformed identification in many labs. It works by analyzing the unique protein fingerprint of a bacterial colony and matching it against a database, delivering results in minutes rather than the hours or days required by traditional methods. It has become a cost-effective and time-efficient alternative to genetic sequencing, though species-level accuracy still varies. Genetic methods like 16S rRNA sequencing remain the gold standard when precise identification is needed, particularly for unusual or newly described species.
Taxonomic Changes
If you encounter the term “Enterobacterales” alongside Enterobacteriaceae, it reflects a recent reshuffling of the family tree. The family Enterobacteriaceae was first defined in the early 20th century and grew to include an enormous number of genera. As genetic tools improved, scientists realized the family had become too genetically diverse to remain a single group. Several genera were moved into newly created families, all grouped under the broader order Enterobacterales. In everyday clinical and public health usage, “Enterobacteriaceae” still refers to the core group containing E. coli, Klebsiella, Salmonella, Shigella, Enterobacter, and their close relatives. The practical impact of the reclassification is mostly taxonomic, but it does explain why you may see different terminology depending on the source.