General characteristics and morphology
The family is characterized by enormous diversity and is the largest among pathogenic and opportunistic bacteria. In terms of the GC content in DNA, the family is extremely heterogeneous: microorganisms with both low content (38–42% in Proteus and Providencia) and high content (52–60% in Klebsiella and Enterobacter) are found. The type genus Escherichia, as well as Shigella and Salmonella, occupy a central position with 50–53% GC pairs.
Enterobacteriaceae are small to medium-sized gram-negative bacilli (1–5 × 0.4–0.8 µm). They never form spores, although some genera are capable of synthesizing a protective polysaccharide capsule. Many species are motile due to peritrichous flagella, but non-motile species also occur.
Physiology and biochemical properties
Most enterobacteria are completely undemanding regarding environmental conditions and grow exceptionally well on simple nutrient media. In liquid media, they cause diffuse turbidity, while on solid media they form convex, shiny colonies (mucoid if a capsule is present). The optimum growth temperature is usually 37 °C, with the exception of the genus Yersinia, whose representatives exhibit greatest activity at lower temperatures.
In terms of respiration type, they are facultative anaerobes combining fermentative and respiratory metabolism. The most important differential features of the family are oxidase negativity, catalase positivity, and the ability to reduce nitrates (nitrate reductase).
Glucose is fermented with the production of acids. This process proceeds via one of two pathways:
- Mixed acid type: produces many organic acids and sometimes gas. Determined by the methyl red test.
- Butanediol type: the end products are 2,3-butanediol and ethanol. Determined using the Voges–Proskauer test.
Pathogenicity factors
The synthesis of virulence factors in enterobacteria is encoded by genes located on specialized pathogenicity islands, plasmids, or in the genome of converting bacteriophages. The main factors include:
- Endotoxin (somatic O-antigen) — present in all species and released only upon bacterial wall destruction.
- Exotoxins — thermostable and thermolabile enterotoxins, as well as various cytotoxins.
- Adhesion factors — type IV pili and filamentous structures of the type III secretion system (T3SS), necessary for attachment to the epithelium.
- Aggressive enzymes — DNase, phosphatase, neuraminidase.
- Capsule — reliably protects the microbe from phagocytosis and the lethal action of the complement system.
Types of interaction with the intestinal epithelium
The mechanism of infection development depends on how deeply the pathogen invades the tissues and whether it damages them:
- Superficial without damage: bacteria (e.g., enterotoxigenic Escherichia coli) multiply directly on the cell surface without disrupting the intestinal architecture.
- Superficial with damage: microbes destroy microvilli, causing a cytopathic effect on the apical surface (enteropathogenic and enterohemorrhagic Escherichia).
- Invasive with cytotoxicity: bacteria invade deep into the cells of the large intestine, multiply there, and cause enterocyte death (genus Shigella, enteroinvasive Escherichia coli).
- Transcistosis and systemic spread: pathogens penetrate through M cells into Peyer's patches, after which they multiply inside macrophages (genera Salmonella, Yersinia).
Briefly: answers to topical questions
Which differential diagnostic nutrient media are used for stool inoculation in intestinal infections?
Differential lactose-containing and selective media are used for stool inoculation in intestinal infections.
- Endo agar and Levine agar — differential lactose-containing media (Levine has low selectivity).
- Ploskirev agar — combined medium of low selectivity.
- MacConkey agar and deoxycholate agar — low selectivity media.
- Salmonella-Shigella agar and xylose lysine deoxycholate agar — moderately selective media.
- Bismuth sulfite agar — highly selective combined medium.
Which antigens are part of the antigenic structure of enterobacteria and what is their chemical nature?
The antigenic structure of enterobacteria includes somatic, flagellar, capsular, and additional antigens.
- O-antigen — somatic, present in all enterobacteria.
- H-antigen — flagellar, found in motile species.
- K-antigen — superficial, capsular; has a polysaccharide nature.
- Additional antigens — type IV pili and species-specific antigens.
What specific infectious diseases are caused by members of the Enterobacteriaceae family?
Members of the Enterobacteriaceae family cause a wide range of diseases.
- Acute intestinal infections — salmonellosis, escherichiosis, and shigellosis; salmonellae also cause typhoid fever.
- Yersiniosis — plague (Yersinia pestis), pseudotuberculosis (Yersinia pseudotuberculosis), and intestinal yersiniosis (Yersinia enterocolitica).
- Diseases caused by Klebsiella pneumoniae subspecies — classical pneumonia, sepsis, ozaena, and rhinoscleroma.
- Urinary tract infections — in infections caused by Proteus, urease contributes to kidney stone formation.
- Nosocomial infections — caused by opportunistic genera Klebsiella, Proteus, Providencia, and Serratia; clinical forms include foodborne toxic infections, respiratory tract infections, and urinary tract infections.
To memorize the basic enterobacterial antigens, use a simple rule: O is Base (somatic), H is Legs (flagellar, from Russian "ножки"), K is Hood/Cocoon (capsular, from Russian "капюшон/кокон").
Class and exam questions
How is microbiological diagnostics of enterobacterial infections performed?
The main method is bacteriological. During the febrile period (first week), blood is inoculated into bile broth to obtain a hemoculture. From the second week onward, stool, urine, or bile serve as the material.
What is the purpose of epidemiological typing of cultures?
To trace the source of infection, phage typing is used — treatment of the isolated culture with specific bacteriophages (e.g., Vi-phages for salmonellae).
What is the basis of the intraspecific and genera classification of enterobacteria?
The main criterion is biochemical activity. The ability to ferment carbohydrates (lactose, sucrose), split urea, produce indole and hydrogen sulfide, and utilize citrate is assessed.
What else to study on this topic
- Features of the genus Klebsiella: temperature-dependent motility and bipolar staining
- Principles of the type III secretion system (T3SS) in enterobacteria
- Endo, Ploskirev, and bismuth sulfite agars: mechanisms of differentiation
- The role of opportunistic enterobacteria (Proteus, Providencia, Serratia) in the development of nosocomial infections
- Cultivation features of Klebsiella granulomatis (use of the yolk sac)
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