Enterobacteriaceae are a large family of bacteria found extensively in diverse environments, including the human and animal gut. Comprising over 30 genera and more than 100 species, they are among the most studied bacterial families. While many are harmless residents within the body, others can act as significant agents of disease. Understanding Enterobacteriaceae provides insight into both human health and the broader microbial world.
Defining Characteristics
Enterobacteriaceae are primarily characterized as Gram-negative bacteria, meaning they do not retain the crystal violet stain due to their cell wall structure. They are typically rod-shaped, or bacilli, usually measuring between 1 to 5 micrometers in length. Most members of this family are motile, possessing numerous flagella that allow movement, although some genera like Klebsiella and Shigella are non-motile.
A distinguishing metabolic feature of Enterobacteriaceae is their facultative anaerobic nature, enabling them to thrive in both oxygen-rich and oxygen-deprived environments. This adaptability contributes to their widespread presence in various habitats. All species within this family are capable of fermenting glucose, a key biochemical characteristic used for their identification. Furthermore, they are generally oxidase-negative, which helps differentiate them from other bacterial families that are oxidase-positive.
Common Habitats
Enterobacteriaceae commonly inhabit the intestines of humans and animals, a distribution reflected in their name, “entero” (gut). Many species are considered normal components of the gut microbiota. Beyond biological hosts, these bacteria are also widely found in the natural environment.
Their presence extends to soil, water, and decaying organic matter. Some species can also be found in association with plants, exhibiting epiphytic, endophytic, or saprophytic relationships. This widespread environmental presence means that Enterobacteriaceae can serve as indicators of fecal contamination in water and food, which has implications for public health.
Their Varied Roles
Beneficial/Commensal Roles
Many Enterobacteriaceae are normal residents of the human gut, maintaining a healthy microbiome. They contribute to physiological processes, including digesting complex carbohydrates the human body cannot break down. Beneficial strains, such as Escherichia coli (E. coli), synthesize essential vitamins like Vitamin K, crucial for blood clotting and bone health.
These commensal bacteria also prevent colonization by harmful pathogens. They do this by competing for nutrients and space, and by producing antimicrobial substances. While E. coli is often associated with disease, most strains are harmless and contribute positively to gut health.
Pathogenic Roles
Despite beneficial roles, certain Enterobacteriaceae strains are human pathogens, causing various infections. They can be primary pathogens, directly causing disease, or opportunistic pathogens, leading to infections when they spread outside their usual habitat or in immunocompromised individuals.
Salmonella species cause food poisoning with gastrointestinal symptoms, and can lead to systemic infections like typhoid fever. Shigella species cause dysentery, a severe diarrheal illness with fever and abdominal cramps. Pathogenic E. coli strains cause traveler’s diarrhea, urinary tract infections (UTIs), and sometimes severe conditions like hemolytic-uremic syndrome. Klebsiella species, especially Klebsiella pneumoniae, are implicated in hospital-acquired infections, including pneumonia and UTIs. Enterobacter species also commonly cause healthcare-associated infections, such as UTIs, respiratory tract infections, and bloodstream infections.
Understanding Infections
Infections by pathogenic Enterobacteriaceae can occur through consuming contaminated food or water. Person-to-person transmission is also possible, especially with poor hygiene. Opportunistic infections often arise when these normal flora bacteria spread to other body sites or in immunocompromised individuals or those undergoing invasive medical procedures.
Diagnosis typically involves laboratory testing, where samples like urine, blood, or tissue are collected and cultured to identify the specific bacterial species. Gram staining provides initial information, while biochemical tests and molecular diagnostics confirm identification and determine antibiotic susceptibility. Treatment primarily relies on antibiotics.
A concern regarding Enterobacteriaceae infections is increasing antibiotic resistance. Many strains have developed resistance to multiple antibiotics, including last-resort treatments like carbapenems. This resistance complicates treatment and poses a global public health challenge, necessitating careful antibiotic stewardship and infection control.