What are Enterobacteria?
Enterobacteriaceae are a large family of Gram-negative, non-spore-forming bacteria that include both normal intestinal flora and important human pathogens such as Escherichia coli, Klebsiella species, Salmonella species, Shigella species, Enterobacter species, Serratia marcescens, and Proteus mirabilis. 1
Key Characteristics
- Gram-negative bacilli that do not form spores and are facultative anaerobes 1
- Normal colonizers of the gastrointestinal tract in humans and animals, though many species can cause disease 1, 2
- Coliforms represent the predominant aerobic component of normal intestinal flora 3
Major Pathogenic Species
The clinically significant members include:
- Escherichia coli - the most common cause of urinary tract infections and a frequent cause of bacteremia 4
- Klebsiella pneumoniae and other Klebsiella species - important pathogens in respiratory and urinary tract infections 4, 5
- Salmonella species (S. choleraesuis, S. typhimurium, S. enteritidis) - cause endocarditis with high mortality and have affinity for abnormal cardiac valves 3
- Enterobacter species (E. aerogenes, E. cloacae) - opportunistic multiresistant pathogens common in hospital-acquired infections 5, 6
- Serratia marcescens - typically affects injection drug users with high mortality rates (70%) 3
- Proteus mirabilis - causes urinary tract infections and occasionally endocarditis 3
- Shigella species - cause bacterial gastroenteritis 3
Clinical Significance in Infections
Intra-abdominal infections: E. coli is the predominant pathogen (71% of cases), followed by Klebsiella species (14%), Pseudomonas aeruginosa (14%), Proteus mirabilis (5%), and Enterobacter species (5%) 3
Endocarditis: Enterobacteriaceae cause rare but severe infections with mortality rates of 60-80%, particularly in injection drug users, prosthetic valve recipients, and patients with cirrhosis 3
Diabetic foot infections: Enterobacteriaceae are common in chronic or previously treated infected ulcers, often as part of polymicrobial infections 3
Nosocomial infections: Klebsiella, Enterobacter, and Serratia species are important hospital-acquired pathogens, especially in immunocompromised patients 4, 6
Antibiotic Resistance Concerns
Extended-spectrum beta-lactamase (ESBL) production is increasingly common among Enterobacteriaceae, particularly E. coli and Klebsiella species, with highest rates in Southeast Asia, Eastern Mediterranean, and Western Pacific regions 3
Risk factors for ESBL-producing strains include recent antibiotic exposure (especially third-generation cephalosporins or fluoroquinolones) within 90 days, known colonization, recent hospitalization, or travel to high-prevalence regions 3
Carbapenem resistance is an emerging threat, with plasmid-mediated carbapenemases now detected in Enterobacteriaceae 7
Fluoroquinolone resistance has expanded significantly, particularly among E. coli, Shigella, and Salmonella species, especially in Latin America and East Asia 3
Colonization vs. Infection
Intestinal colonization with Enterobacteriaceae is normal and does not require treatment 2
High colonization density (intestinal dominance) in hospitalized patients is associated with increased risk of subsequent bloodstream, urinary tract, or respiratory infections 5
Finding Enterobacteriaceae in stool typically represents colonization rather than infection unless there are systemic signs of illness or specific pathogens like Salmonella, Shigella, or Shiga toxin-producing E. coli are identified 2