Detailed Notes on Enterobacteriaceae

Overview of Enterobacteriaceae

Importance: Enterobacteriaceae represents the largest family of Gram-negative bacteria, crucial in both clinical microbiology and public health. They are responsible for a variety of infections, particularly in humans, and their ability to develop antibiotic resistance poses significant challenges for treatment and control. This family includes many species that are part of the normal human gut flora, but certain pathogenic strains can lead to serious diseases.

Key characteristic: Most members are motile due to peritrichous flagella, which enhance their ability to move towards nutrients and away from harmful substances. However, notable exceptions exist among non-motile species, which can affect their ecological niche and pathogenicity.

Culturing Enterobacteriaceae

Primary Medium: MacConkey agar is the preferred medium to culture Enterobacteriaceae, differentiating them based on their ability to ferment lactose. This medium is selective for Gram-negative bacteria and allows for the identification of lactose fermenters and non-fermenters based on colony color.

Colony Appearance:

  • Lactose Fermenter: Dark pink colonies emerge after 24 hours of incubation, indicating acid production from lactose fermentation.

  • Late Lactose Fermenter: Lighter colonies may appear after 48 hours, suggesting slower fermentation rates or incomplete lactose metabolism.

Biochemical Features

Oxidase Test: The majority of Enterobacteriaceae are oxidase negative, indicating the absence of cytochrome c oxidase in their electron transport chain. An exception to this is Plesiomonas, which is oxidase positive, thus facilitating differential identification.

Antigens:

  • K antigen: This is the capsular antigen responsible for virulence; it helps the bacteria evade phagocytosis.

  • O antigen: The cell wall antigen is vital for serotyping and plays a key role in immune response.

  • H antigen: A flagellar antigen that is removed when heat is applied, necessary for isolating the O antigen and determining serological relationships.

Media Used for Differentiation

  • EMB (Eosin Methylene Blue): This medium is used for the presumptive identification of Escherichia coli. It demonstrates color differentiation in lactose fermentation, producing a greenish metallic sheen for strong fermenters.

  • XLD (Xylose Lysine Deoxycholate Agar): This medium will appear red for non-lactose fermenters and yellow for lactose fermenters such as E. coli, allowing for easy differentiation based on metabolic properties.

  • Hektoen Enteric Agar (HEA): This medium will show yellow colonies for lactose fermenters and green colonies for those that do not ferment lactose. It is selective for enteric pathogens and helps in the identification of pathogens like Salmonella and Shigella.

Salmonella Growth Characteristics

  • SSA (Salmonella Shigella Agar): On this medium, Salmonella species produce colorless colonies with a characteristic black center that indicates hydrogen sulfide (H2S) production, which is crucial for differentiation from Shigella, which does not produce H2S.

Classification of Enterobacteriaceae

  • Lactose Fermenters:

    • Rapid Lactose Fermenters (Coliforms): These show A/A plus gas in TSI reaction and can be differentiated using the methyl red test. They typically include common intestinal inhabitants.

    • Key Species:

    • Escherichia coli: Frequently associated with infections such as urinary tract infections (UTIs), neonatal meningitis, and often exists as part of normal flora, indicating the need for context in evaluating its pathogenicity.

    • Enterohemorrhagic E. coli (O157:H7): Causes severe infections, including hemolytic uremic syndrome, characterized by kidney failure following gastroenteritis.

    • Enterotoxigenic E. coli: Responsible for infantile diarrhea due to producing heat-labile toxins that stimulate fluid secretion in the intestines.

    • Enteroinvasive E. coli: Capable of invading intestinal mucosa, leading to dysentery-like symptoms.

    • Enteropathogenic E. coli: Often associated with infantile diarrhea, emphasizing the diversity of pathogenic mechanisms among E. coli types.

  • Enterohemorrhagic E. coli (EHEC)

    • also known as verotoxin-producing E. coli (VTEC), is a pathogenic strain of Escherichia coli that is associated with severe gastrointestinal illnesses.

    • It produces Shiga toxin, which can lead to serious conditions such as hemolytic uremic syndrome (HUS), characterized by kidney failure, hemolytic anemia, and thrombocytopenia.

    • The most common serotype of EHEC is O157:H7. Infection typically occurs through consumption of contaminated food or water, and symptoms can range from mild diarrhea to severe bloody diarrhea, with complications arising especially in young children and the elderly.

    • Effective management requires prompt medical attention and careful monitoring of symptoms for potential progression to HUS.

    • MUG (4-Methylumbelliferyl-β-D-glucuronide) negative and sorbitol McConkey negative results are significant for the identification of enterohemorrhagic E. coli (EHEC), particularly the strain O157:H7.

      • MUG Negative: MUG is a substrate used to differentiate E. coli from other bacteria in laboratory settings. A MUG negative result indicates that the strain does not produce the enzyme β-glucuronidase, which is common in most non-pathogenic E. coli but not in EHEC O157:H7, helping in its identification.

      • Sorbitol McConkey Negative: Sorbitol McConkey agar is a selective medium used to isolate and differentiate E. coli strains based on their ability to ferment sorbitol. EHEC O157:H7 does not ferment sorbitol, so it remains colorless on the medium, in contrast to other E. coli strains that produce pink colonies due to sorbitol fermentation.

      In summary, being MUG negative and sorbitol McConkey negative are key biochemical characteristics used to help identify EHEC O157:H7 in clinical microbiology.

  • Enteroaggregative E. coli (EAEC) is a distinct biotype of Escherichia coli that is characterized by its ability to adhere to the intestinal mucosa in a unique stacked-brick formation. EAEC is associated with acute and chronic diarrhea, particularly in children and immunocompromised individuals.

    • Virulence factor: Aggragative adhesion fimbriae(AAFs) that facilitate adherence to intestinal cells, enhancing the bacteria's ability to colonize and induce inflammation.

    Key features of EAEC include:

    • Adherence: It forms aggregates on the intestinal epithelial cells, which contributes to its pathogenicity.

    • Enterotoxins: EAEC may produce enterotoxins such as enteroaggregative heat-stable enterotoxin (EAST) and a plasmid-encoded toxin (PET), which can disrupt normal intestinal function and lead to diarrhea.

    • Clinical Manifestations: EAEC infections are often associated with prolonged diarrhea that can last for weeks and may include abdominal pain and low-grade fever.

    • Epidemiology: EAEC is commonly found in developing countries and can be transmitted through contaminated water or food.

Escherichia coli VS Shigella spp.

  • MacConkey Agar:

    • Escherichia coli: Typically produces pink colonies due to lactose fermentation, indicating acid production.

    • Shigella spp.: Colonies appear colorless or pale, as it is unable to ferment lactose.

    Acetate Utilization:

    • Escherichia coli: Generally positive for acetate utilization, indicating ability to metabolize acetate for energy.

    • Shigella spp.: Usually negative for acetate utilization, indicating it cannot utilize acetate in the same manner.

    Motility:

    • Escherichia coli: Motile; typically exhibits motility due to peritrichous flagella.

    • Shigella spp.: Non-motile; does not show motility even in semi-solid media due to lack of flagella.

Klebsiella pneumoniae

  • is a gram-negative bacterium and a member of the Enterobacteriaceae family. It is known for causing a variety of infections, including pneumonia and urinary tract infections (UTIs). This bacterium is also referred to as Friedlander’s Bacillus. It is characterized by the following properties:

  • Encapsulated: Klebsiella pneumoniae possesses a thick capsule that helps it evade the host's immune response.

  • String Test Positive: This indicates the presence of a viscous capsule.

  • Colony Appearance: It forms mucoid colonies on agar.

  • Motility: Non-Motile.

Biochemical Features

  • TSI (Triple Sugar Iron): A/A + Gas indicating acid production from fermentation of sugars with gas production.

  • LIA (Lysine Iron Agar): K/K indicating lysine decarboxylation without hydrogen sulfide (H2S) production.

  • IMVIC tests: (-)(-)(+)(+) showing results in indole, methyl red, Voges-Proskauer, and citrate utilization tests respectively.

  • LOA (Lysine Decarboxylase Test): (+)(-)(-), indicating a positive result for lysine decarboxylation and negative for ornithine and arginine.

  • Urease and Malonate: Positive for urease production and can utilize malonate as the sole carbon source.

Clinical Significance

Klebsiella pneumoniae is known to cause a range of infections:

  • Pneumonia: Significant in hospital-acquired pneumonia.

  • Wound Infections: Often occurs in patients with compromised immune systems or following surgical procedures.

  • Meningitis: Can lead to serious central nervous system infections.

  • Urinary Tract Infections (UTIs): A common pathogen in cases of nosocomial UTIs, especially in catheterized patients.

Klebsiella pneumoniae, Klebsiella oxytoca (non-motile), Klebsiella ozaenae (Malonate negative), and Klebsiella rhinoscleromatis (Malonate positive) can be differentiated based on their biochemical characteristics as follows:

  1. Lysine Decarboxylase Test (LDC):

    • K. pneumoniae: Positive

    • K. oxytoca: Positive

    • K. ozaenae: Typically positive

    • K. rhinoscleromatis: Negative

  2. Voges-Proskauer (VP):

    • K. pneumoniae: Negative

    • K. oxytoca: Positive

    • K. ozaenae: Positive

    • K. rhinoscleromatis: Negative

  3. Urease:

    • K. pneumoniae: Positive

    • K. oxytoca: Positive

    • K. ozaenae: Positive

    • K. rhinoscleromatis: Positive

  4. Indole Test:

    • K. pneumoniae: Negative

    • K. oxytoca: Positive

    • K. ozaenae: Negative

    • K. rhinoscleromatis: Negative

Enterobacter spp. represents a significant genus within the Enterobacteriaceae family, known for its clinical relevance in human health. These bacteria are characterized by their ability to ferment glucose and lactose, making them distinguishable on selective media used for diagnosis.

Biochemical Characteristics:

  • Triple Sugar Iron (TSI): Enterobacter spp. typically displays an A/A reaction with gas production, indicative of acid production from the fermentation of both glucose and lactose, along with the release of gas. This reaction is important for differentiating Enterobacter from other non-lactose fermenting species.

  • IMVIC Test Results: The results are (-)(-)(+)(+), indicating negative results for indole and methyl red tests, and positive results for Voges-Proskauer and citrate utilization tests. This pattern is crucial for differentiating between Enterobacter spp. and other Enterobacteriaceae members.

  • Urease Test: Generally negative for urease production, meaning Enterobacter spp. does not hydrolyze urea into ammonia as part of its metabolic processes, which differentiates it from urease-positive organisms such as Proteus spp.

    • EXCEPTION: Enterobacter gergoviae, which is known for being urease positive, demonstrating the need to carefully differentiate species within the genus.

Clinical Significance:

  • Urinary Tract Infections (UTIs): Enterobacter spp. can cause nosocomial infections, particularly UTIs, especially in immunocompromised individuals or those with urinary catheters. Their ability to thrive in hospital settings contributes to their prevalence in clinical infections.

  • Wound Infections: These organisms are also implicated in wound infections, particularly in diabetic patients or those undergoing surgical procedures, due to their capacity to colonize damaged tissues.

  • Septicemia: Enterobacter can lead to bloodstream infections, which pose serious risks and require prompt medical intervention. The bacteremia is often associated with underlying health issues and can be complicated by the organism's antibiotic resistance traits.

Other Important Information:

  • Resistance to Antibiotics: Enterobacter spp. are notorious for developing resistance mechanisms, particularly to beta-lactams, making infections difficult to treat. Continuous surveillance and susceptibility testing are essential in managing these infections effectively.

  • Laboratory Identification: Proper laboratory identification is crucial for the effective treatment of infections caused by Enterobacter. Testing methods include culture on MacConkey agar, where they typically produce pink colonies indicating lactose fermentation, and further confirmation using biochemical assays according to established guidelines.

To differentiate Enterobacter aerogenes, Enterobacter gergoviae, Hafnia alvei, Enterobacter cloacae, Enterobacter sakazakii, and Enterobacter pantoea agglomerans, the results of lysine, ornithine, and arginine decarboxylation tests can be utilized. Below is a summary of how these species react to these biochemical tests:

Biochemical Characteristics:

  1. Lysine Decarboxylation Test (LDC):

    • Enterobacter aerogenes: Positive

    • Enterobacter gergoviae: Positive

    • Hafnia alvei: Positive

    • Enterobacter cloacae: Variable (can be positive or negative)

    • Enterobacter sakazakii: Negative

    • Enterobacter pantoea agglomerans: Negative

  2. Ornithine Decarboxylation Test (ODC):

    • Enterobacter aerogenes: Positive

    • Enterobacter gergoviae: Negative

    • Hafnia alvei: Positive

    • Enterobacter cloacae: Positive

    • Enterobacter sakazakii: Positive

    • Enterobacter pantoea agglomerans: Negative

  3. Arginine Decarboxylation Test (ADC):

    • Enterobacter aerogenes: Positive

    • Enterobacter gergoviae: Positive

    • Hafnia alvei: Negative

    • Enterobacter cloacae: Positive

    • Enterobacter sakazakii: Negative

    • Enterobacter pantoea agglomerans: Variable (can be positive or negative)

Summary of Differentiation:

  • Enterobacter aerogenes and gergoviae: Both are lysine positive and arginine positive, but gergoviae may differ on ornithine.

  • Hafnia alvei: Characterized by being positive for lysine and ornithine but negative for arginine.

  • Enterobacter cloacae: Shows variability, can be positive for lysine and ornithine, and positive for arginine.

  • Enterobacter sakazakii: Positive for ornithine, negative for lysine, and negative for arginine.

  • Enterobacter pantoea agglomerans: Negative for lysine and ornithine; arginine test results can vary.

Arizona spp.

  • Late lactose fermenters (L.L.F)= ONPG (+)

  • TSI= A/A+ GAS+ H2S

  • LIA= K/K

  • Like Salmonella spp. there are H2S producers but they are non-lactose fermenters.

    • L.L.F = ONPG (-)

    • TSI = K/A + GAS + H2S

    • LIA = K/A

  • Escherichia coli

    • L.L.F = ONPG (+)

    • TSI = A/A + GAS + H2S (-)

    • LIA = K/A

  • Citrobacter spp.

    • L.L.F: Positive for ONPG

    • TSI: A/A with gas production and H2S production

    • LIA: K/K

  • causes:

  • C. diversus

    • Neonatal meningitis

  • C. freundii

    • UTI

    • Pneumonia

    • Endocarditis

    Similar to Salmonella spp., Citrobacter species are producers of hydrogen sulfide (H2S) but do not ferment lactose.

    • L.L.F: Negative for ONPG

    • TSI: K/A with gas production and H2S production

    • LIA: K/A

    Escherichia coli

    • L.L.F: Positive for ONPG

    • TSI: A/A with gas production and negative for H2S

    • LIA: K/A

Citrobacter freundii and Salmonella typhi using MacConkey agar and the Lysine Decarboxylase (LDC) test:

  • MacConkey Agar:

    • Citrobacter freundii: Typically produces pink colonies due to lactose fermentation.

    • Salmonella typhi: Appears colorless or pale because it does not ferment lactose.

  • Lysine Decarboxylase (LDC) Test:

    • Citrobacter freundii: Generally positive for LDC, indicating lysine decarboxylation.

    • Salmonella typhi: Usually negative for LDC, indicating it does not decarboxylate lysine.

  • Non-Lactose Fermenters:

    • Proteus Group: Known for swarm growth on agar, urease positive, and generally indole positive, except for Proteus mirabilis. This group is associated with urinary tract infections and can contribute to renal stone formation.

    • Salmonella:

    • Characterized as aerogenic; with Salmonella typhi causing typhoid fever and other species contributing to a range of gastroenteritis complications.

    • Shigella: Typically biochemically negative and known for causing dysentery; an example is Shigella flexneri, which can lead to bloody diarrhea and is highly infectious.

Other Notable Genera

  • Campylobacter: Recognized as microaerophilic and oxidase positive, this genus is significant due to its role in gastroenteritis, particularly linked to undercooked poultry.

  • Helicobacter pylori: This species is notably associated with gastric ulcers and has implications in the development of gastric cancer, highlighting its importance in gastrointestinal health.

Conclusion

Understanding the biochemical properties and the culture media of Enterobacteriaceae is essential for effective diagnosis and treatment of infections caused by this diverse family of bacteria. Their significant clinical relevance, coupled with their ability to acquire antibiotic resistance, necessitates ongoing research and monitoring to manage their impact on public health effectively.