Microbiological Identification and Clinical Management of Intestinal Pathogenic Enterobacteria

Identification of Intestinal Pathogenic Enterobacteria

• Definition and Taxonomic Context: The order Enterobacterales includes species that cause infectious diarrheal syndromes. These comprise primary pathogenic species within the genera Salmonella, Shigella, and Yersinia, alongside diarrheagenic pathotypes of Escherichia coli (a conditionally pathogenic species).

• Microscopic Characteristics:

  • Enterobacteria are Gram-negative bacilli.

  • They typically show no particular cellular arrangement (as seen in Gram-stained smears at $1000\times$ magnification).

• Cultural Characteristics:

  • Nutritional Status: Non-fastidious.

  • Respiration: Facultatively anaerobic.

  • Colony Morphology: They form S-type (Smooth) colonies on simple media; most species produce morphologically similar colonies.

• Selective and Differential Media:

  • MacConkey agar: A low-selectivity medium. It uses bile salts to inhibit the growth of Gram-positive bacteria. It differentiates bacteria based on their ability to ferment lactose (pathogens are typically lactose-negative).

  • Hektoen agar: A medium with moderate selectivity. It inhibits Gram-positive bacteria and a portion of Gram-negative coliform bacilli (lactose-positive). It differentiates based on lactose fermentation and the production of hydrogen sulfide ($H_2S$). Strains producing $H_2S$ appear with black centers.

Diagnostic Identification of Intestinal Pathogens

• Biochemical Identification Flow:

  • At least three lactose-negative (suspect) colonies must be tested from a stool culture.

  • Initial Biochemical Screening: Uses two multitest identification media:

    • TSI (Triple Sugar Iron).

    • MIU (Motility-Indole-Urease).

  • Advanced Identification: If screening suggests a pathogen, genus or species-level identification is achieved via:

    • Biochemical identification panels.

    • MALDI-TOF mass spectrometry.

    • Molecular techniques (16S rRNA sequencing or Whole-genome sequencing).

• Antigenic Identification:

  • Performed using slide agglutination reactions with immune sera.

  • Surface Antigens:

    • $O$: Somatic antigen.

    • $H$: Flagellar antigen.

    • $Vi$: Capsular antigen (primarily found in Salmonella Typhi, S. Paratyphi C, and rarely S. Dublin).

  • Serotyping: Combinations of these antigens define serotypes (serovars).

  • Taxonomic Schemes: Salmonella antigenic structures are classified according to the Kauffmann–White scheme using anti-$O$ and anti-$H$ sera (anti-$Vi$ is used less frequently).

  • Shigella Specifics: Species identification (S. sonnei, S. flexneri, S. boydii, S. dysenteriae) is determined using specific anti-$O$ sera.

Acute Diarrheal Disease (ADD) and Microbiota

• Microbiological Condition of the Colon:

  • Indigenous Microbiota: Predominantly anaerobic bacteria ($95$%\u2013$99$%), including Bacteroides spp., Clostridium spp., and Bifidobacterium spp.

  • Concentration: Reaches up to $10^{11}$ bacteria per gram.

  • Functions of Colon Microbiota: Degrading indigestible dietary fibers, stimulating local immunity, preventing pathogenic colonization, and maintaining intestinal mucosal integrity.

  • Opportunistic Flora: Aerobic and facultatively anaerobic bacteria such as Escherichia coli, Enterococcus spp., Proteus spp., and Lactobacillus spp. are present in lower proportions.

• Definition of ADD: The passage of at least $3$ stools per day of decreased consistency (not retaining shape). This leads to water and electrolyte loss, potentially causing hydroelectrolytic imbalance and cardiovascular collapse.

• Clinical Syndromes:

  • Dysenteric Syndrome: Caused by an enteroinvasive mechanism leading to inflammation.

    • Symptoms: Small-volume stools containing mucus and blood, fever, abdominal pain, rectal tenesmus, and fecal leukocytes.

    • Etiology: Shigella spp., Salmonella spp., Yersinia enterocolitica, Campylobacter spp., EIEC, EHEC, and Entamoeba histolytica.

  • Cholera-like Syndrome: Caused by an enterotoxigenic mechanism.

    • Symptoms: Profuse, watery stools without blood or mucus. Absence of fever, abdominal pain, rectal tenesmus, and fecal leukocytes.

    • Etiology: Vibrio cholerae, non-O1 agglutinating vibrios, ETEC, Aeromonas hydrophila, and Plesiomonas shigelloides.

Microbiological Examination of Fecal Samples

• Clinical Indications: Severe diarrhea with persistent high fever, blood/mucus in stools, frequent watery stools, high-risk status (children, elderly, pregnant, immunocompromised), or suspected outbreaks. Mandatory for food handlers (to detect Salmonella, Campylobacter, and Shigella).

• Sample Collection Protocols:

  • Timing: Collected before initiating antibiotic therapy.

  • Quantity: Single sample of $1$\u2013$2\,g$ up to a maximum of $10\,g$.

  • Procedure: Use the scoop of the collection container. Sample from at least $3$ different areas, specifically targeting mucus or blood. Avoid urine contamination.

  • Contraindications: Swabs should be avoided as they provide insufficient material.

• Transport:

  • Timeframe: Maximum of $2$ hours after collection.

  • Medium: Cary-Blair transport medium (specimen must be mixed with the medium).

  • Documentation: Form must include clinical info, date, and time of collection.

Laboratory Diagnostic Methodologies

• Macroscopic Examination: Assesses consistency (liquid/soft vs. formed) and presence of mucus, pus, or blood (suggestive of invasive mechanisms).

• Rapid Etiological Methods:

  • Antigen Detection: Tests for microbial antigens (viral, bacterial, parasitic) or toxins like verotoxins via antigen-antibody reactions.

  • Nucleic Acid Amplification Tests (NAAT): Multiplex PCR (syndromic testing) targets a broad range including Salmonella, Shigella, Campylobacter, diarrheagenic E. coli, rotavirus, norovirus, adenovirus, Giardia, Cryptosporidium, and E. histolytica.

• Microscopic Examination: Wet mounts (saline or Lugol\u2019s) identify leukocytes, indicating inflammatory diarrhea (Salmonella, Shigella, Yersinia, Campylobacter).

• Stool Culture (Target Bacteria): Minimum targets are Salmonella, Shigella, Yersinia, and Campylobacter. Cholera-like syndrome investigation requires explicit clinician requests.

Laboratory Processing Timeline (4-Day Procedure)

• Day 1: Inoculation:

  • Selective Media: MacConkey (Gram-negative differentiation by lactose) and Hektoen (Lactose and $H_2S$ differentiation).

  • Enrichment Media: Selenite broth. This allows Salmonella to multiply faster than commensals due to inhibitory sodium selenite. Incubation at $37^{\circ}C$ for $6$\u2013$12$ hours, followed by subculture onto Hektoen agar.

  • Chromogenic Media: Complementary method for Salmonella. Specific enzymes degrade substrates, turning colonies pink for preliminary identification.

• Day 2: Screening:

  • Selection of at least $3$ lactose-negative colonies ($H_2S$ positive or negative).

  • Biochemical screening on TSI and MIU.

  • Subculture on lactose differential medium for purity check.

• Day 3: Identification:

  • Read biochemical tests.

  • Antigenic identification of pure cultures (using anti-$O$ and anti-$H$ sera for species/serovar determination).

  • Confirmation via biochemical panels or mass spectrometry.

  • Initiation of antimicrobial susceptibility testing (AST).

• Day 4: Reporting:

  • Final report including presence/absence of pathogens and AST result.

  • Reporting Restrictions: AST is reported for Shigella and Yersinia. For immunocompetent hosts, Salmonella AST is not reported because antibiotics do not shorten clinical symptoms but may prolong the carrier state.

Treatment and Antimicrobial Guidelines

• First-line Treatment: Fluid and electrolyte rehydration is mandatory to prevent cardiovascular collapse.

• Antibiotic Guidelines: Recommended only for severe/moderate disease or immunocompromised patients, guided by antibiogram.

• Exclusions: Antibiotics are generally not recommended for immunocompetent patients with gastroenteritis from non-typhoidal Salmonella or STEC (EHEC).

• Active Antibiotics by Genus:

  • Salmonella* and *Shigella: Ampicillin, co-trimoxazole, ciprofloxacin. Severe cases utilize third-generation cephalosporins (ceftriaxone, ceftazidime, cefotaxime).

  • Yersinia: Co-trimoxazole, ciprofloxacin, ceftriaxone, and tetracyclines.