General characteristics of the Enterobacteriaceae family
Enterobacteriaceae are a large family of gram-negative bacteria that inhabit primarily the large intestine of humans and animals. Many representatives are part of the normal microflora and are classified as opportunistic pathogens: when their numbers increase or immunity decreases, they are capable of causing infectious diseases.
All enterobacteria share four general biochemical features:
- they are facultative anaerobes — they grow both in the presence and absence of oxygen;
- they ferment glucose;
- they are oxidase-negative;
- they reduce nitrates to nitrites.
Enterobacteria should be distinguished from the clinically significant group of non-fermenting Gram-negative bacteria (e.g., Pseudomonas aeruginosa, a frequent cause of healthcare-associated urinary tract infections and sepsis). Pseudomonads are strict aerobes, do not ferment glucose, are oxidase-positive, and do not reduce nitrates — demonstrating the opposite set of features.
Virulence factors
All enterobacteria contain lipopolysaccharide (LPS) in their cell wall, which is characteristic of Gram-negative bacteria. Its component lipid A acts as an endotoxin.
In addition to endotoxin, enterobacteria can produce exotoxins. A classic example is the Shiga-like toxin produced by enterohemorrhagic Escherichia coli, which damages intestinal mucosal and endothelial cells.
Antigenic structure
Enterobacteria differ from one another in their set of antigens, which is used for serotyping within a single species. There are three main antigens:
- O-antigen — somatic, represented by the variable part of the lipopolysaccharide; its structure determines the serovar. It is associated with lipid A — the endotoxin.
- H-antigen — flagellar protein, present in all motile enterobacteria.
- K-antigen — capsular, present in species possessing a capsule. In Salmonella typhi, the capsular antigen is designated as the Vi-antigen (from virulence) and is associated with the pathogenicity of the agent.
Laboratory diagnostics
The main method is bacteriological. The material is inoculated onto blood agar and selective-differential media:
- MacConkey agar;
- eosin-methylene blue agar (Endo or Levine medium);
- other lactose-containing media.
Bacteria that do not ferment lactose form colorless colonies; lactose-positive species (e.g., E. coli) produce brightly colored colonies.
Final identification is carried out on triple sugar iron (TSI) agar and urea agar. Motility is determined microscopically (flagella staining by Morozov's method) or using semi-solid media.
Serological method — agglutination reaction, a universal technique for diagnosing any enterobacterium.
Differential media: TSI and urea agar
Triple sugar iron (TSI) agar contains iron sulfate, glucose, lactose, and sucrose. The medium is poured into a tube with a slant (aerobic zone) and a butt (low oxygenation zone). Inoculation is performed by stabbing the butt and streaking the slant.
- If the bacterium ferments neither glucose nor lactose, the color of the indicator does not change.
- When all sugars are fermented, the medium turns completely yellow due to a pH shift toward the acidic side; gas bubbles indicate gas production.
- If only glucose is fermented, only the butt turns yellow (there is no oxygen access and the acid is preserved), while the slant remains its original color — the acid is oxidized in the aerobic zone.
- The production of hydrogen sulfide (H₂S) is manifested by blackening of the medium due to the reaction with iron sulfate and the precipitation of iron sulfide.
Urea agar contains urea and an indicator (phenol red). Bacteria producing the enzyme urease (e.g., Helicobacter pylori, Proteus) break down urea into ammonia and CO₂. Ammonia alkalizes the medium — the indicator changes color from orange to purple. Urease is considered a virulence factor in a number of pathogens.
Principles of antibiotic therapy
Therapy for enterobacterial infections should be individualized and based on the antibiogram. Broad-spectrum drugs are prescribed at the initial stage, followed by regimen adjustment based on susceptibility test results.
In diarrhea, which is typical of most enterobacterioses, rehydration and a sparing diet are of key importance; antibiotics are not prescribed in all cases.
Groups of drugs used for infections caused by enterobacteria:
- Penicillins and cephalosporins — cell wall synthesis inhibitors;
- Aminoglycosides, chloramphenicol, tetracyclines — protein synthesis inhibitors;
- Quinolones and sulfonamides — nucleic acid synthesis inhibitors (formally classified as chemotherapeutic agents rather than antibiotics).
A serious problem is the production by enterobacteria of β-lactamases, which destroy the β-lactam ring of penicillins and a number of cephalosporins, rendering these antibiotics ineffective. This is why determining the susceptibility of a specific strain is a mandatory step.
Key points
- Enterobacteria are Gram-negative facultatively anaerobic bacilli that ferment glucose, are oxidase-negative, and reduce nitrates to nitrites.
- Endotoxin is represented by lipid A of LPS; some species produce exotoxins (Shiga toxin of E. coli).
- Serotyping is based on O-, H-, and K-antigens; in Salmonella typhi, the capsular antigen is called the Vi-antigen.
- The main diagnostic method is bacteriological using MacConkey, Endo, and EMB media; identification is performed on TSI and urea agar.
- Sugar fermentation, gas production, and H₂S production are assessed on TSI; urease activity is assessed on urea agar.
- Serologically, the agglutination reaction is used.
- Therapy is individualized, with mandatory antibiogram; the basis is β-lactams, aminoglycosides, tetracyclines, quinolones.
- Production of β-lactamases is the leading mechanism of enterobacterial resistance to penicillins.