The family Enterobacteriaceae is a large and diverse group of Gram-negative bacteria commonly referred to as “enterics,” a name derived from their typical habitat in the intestinal tract. This expansive group encompasses over 30 genera and more than 110 species. While many members are harmless and even beneficial to their hosts, others cause a wide range of human diseases. Their ubiquitous presence means they play a dualistic role in the health and illness of humans and animals worldwide.
Defining the Family
All members are classified as Gram-negative, which means their cell wall structure includes a thin layer of peptidoglycan sandwiched between two membranes, including a distinctive outer membrane. This structure prevents the retention of the crystal violet stain used in the Gram staining procedure, causing them to appear pink or red under a microscope after counterstaining with safranin.
These bacteria are uniformly rod-shaped (bacilli) and are non-spore-forming, meaning they do not produce dormant, highly resistant structures. Metabolically, they are classified as facultative anaerobes, meaning they can generate energy through respiration or fermentation. A common biochemical property is their ability to ferment glucose. The ability to ferment lactose is used to differentiate species in the clinical laboratory, dividing the family into coliforms (lactose fermenters) and non-lactose fermenters.
Natural Habitat and Role
Enterobacteriaceae are ubiquitous, thriving in environments including soil, water, and vegetation. Their most significant habitat, however, is the intestinal tract of humans and warm-blooded animals, where they are part of the normal, resident flora. In the gut, many species exist commensally, benefiting without harming the host, and some provide distinct advantages.
For example, non-pathogenic strains of Escherichia coli contribute to host health by producing Vitamin K2, a nutrient necessary for blood clotting. These resident bacteria also occupy space and consume nutrients, preventing harmful pathogens from establishing a foothold. They transition to opportunistic pathogens when they leave the intestine and colonize other body sites, such as the urinary tract, bloodstream, or lungs, particularly in individuals with compromised immune systems.
Major Pathogenic Members
Several genera within the Enterobacteriaceae family are responsible for a substantial portion of human bacterial infections, ranging from gastroenteritis to sepsis. Escherichia coli is the most well-known member; while many strains are harmless gut residents, pathogenic strains are the primary cause of urinary tract infections (UTIs). Specific virulence factors allow some E. coli strains, such as Enterotoxigenic E. coli (ETEC) and Enteropathogenic E. coli (EPEC), to cause diarrheal diseases and foodborne illness.
The genus Salmonella contains non-typhoidal strains that cause salmonellosis (acute gastroenteritis) and typhoidal strains, such as Salmonella Typhi, which cause typhoid fever. Typhoid fever is a serious systemic infection that can lead to high fever, abdominal pain, and a rash, and it has a high mortality rate if left untreated. Shigella species are highly invasive and cause shigellosis (bacillary dysentery), characterized by bloody diarrhea and inflammation of the colon lining.
Klebsiella, particularly Klebsiella pneumoniae, is frequently implicated in hospital-acquired infections, primarily causing pneumonia and infections in the urinary tract or bloodstream. These organisms often possess a large polysaccharide capsule that helps them evade the host’s immune response, making infections difficult to clear.
The Challenge of Antibiotic Resistance
Enterobacteriaceae are at the forefront of antibiotic resistance challenges. They rapidly acquire and spread resistance genes primarily by exchanging plasmids. Plasmids are circular pieces of DNA transferred between bacteria, carrying genes that encode resistance mechanisms against multiple antibiotic classes.
A threat comes from organisms that produce Extended-Spectrum Beta-Lactamase (ESBL) enzymes, which inactivate many penicillin and cephalosporin antibiotics. ESBL-producing E. coli and Klebsiella pneumoniae are increasingly common in hospital and community settings, limiting treatment options for infections like UTIs.
The emergence of Carbapenem-Resistant Enterobacteriaceae (CRE) is also a concern. CRE are resistant to carbapenems, which are often considered last-resort antibiotics for severe infections. CRE frequently produce carbapenemases that break down these drugs, leaving few effective antibiotics. The transmissibility of resistance genes, such as blaKPC or blaNDM, means a CRE strain can transfer its resistance to other bacteria present in the gut flora.