Gram-Negative Rods: Enterobacteriaceae and Escherichia coli

Gram-Negative Rods

Introduction

Gram-negative rods (GNRs) are a diverse group of bacteria characterized by their rod-shaped morphology and the presence of a thin peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharide (LPS). They are clinically significant due to their association with various human diseases, particularly infections of the gastrointestinal and urinary tracts.

• Gram-negative bacilli: Rod-shaped bacteria that do not retain the crystal violet stain in Gram staining.

• Non-spore forming: These bacteria do not produce spores as part of their life cycle.

• Motile and/or capsulated: Many possess flagella for movement and capsules for protection.

Classification of Gram-Negative Rods

Enteric vs. Non-Enteric GNRs

Gram-negative rods are classified based on their primary habitat and pathogenic potential.

Enteric GNRs	Non-Enteric GNRs
Enterobacteriaceae	Haemophilus influenzae
Vibrio cholerae	Bordetella pertussis
Campylobacter jejuni	Brucella species
Helicobacter pylori	Pseudomonas aeruginosa

Enterobacteriaceae are the most clinically relevant family among enteric GNRs, responsible for a wide range of infections.

Family Enterobacteriaceae

General Features

Members of the Enterobacteriaceae family are distinguished by their ability to ferment glucose and reduce nitrate. They are commonly found in the intestinal tract and are implicated in both community-acquired and nosocomial infections.

• Lactose fermenters (LF): Produce pink colonies on MacConkey's agar (e.g., E. coli, Klebsiella pneumoniae, Citrobacter species).

• Non-lactose fermenters (NLF): Produce pale colonies on MacConkey's agar (e.g., Salmonella species, Shigella species, Proteus species).

Escherichia coli (E. coli)

Morphology

E. coli is a Gram-negative bacillus with the following characteristics:

• Non-spore forming

• Motile (possesses flagella)

• Capsulated (some strains)

Antigenic Structure

E. coli possesses several important antigens that contribute to its pathogenicity and classification:

• O (Somatic) antigen: Heat-stable polysaccharide, >180 types

• H (Flagellar) antigen: Heat-labile protein, >50 types

• K (Capsular) antigen: Heat-labile polysaccharide, >80 types

Diseases Caused by E. coli

E. coli is associated with a variety of clinical syndromes:

Type	Clinical Manifestation
Urinary tract infection (UTI)	Most common cause of uncomplicated UTI
Hospital-acquired infections (HAIs)	UTI, surgical site infection, bloodstream infection, pneumonia
Neonatal meningitis	Caused by strains with K1 capsule
Intestinal infections	Various diarrheal diseases

Intestinal Infections Caused by E. coli

Pathogenic E. coli strains are classified based on their virulence factors and clinical presentation:

Type	Abbreviation	Clinical Features
Enterotoxigenic	ETEC	Traveler's diarrhea (watery diarrhea)
Enteropathogenic	EPEC	Infantile diarrhea
Enteroaggregative	EAEC	Diarrhea in children
Enteroinvasive	EIEC	Dysentery (bloody diarrhea)
Enterohemorrhagic	EHEC	Hemolytic uremic syndrome (HUS)

Enterohemorrhagic E. coli (EHEC)

EHEC, including the O157:H7 serotype, is notable for causing hemolytic uremic syndrome (HUS), a severe complication characterized by:

• Hemolytic anemia

• Uremia (renal failure)

• Low platelet count

Source of infection: Meat, dairy products, vegetables, and fruits.

Pathogenesis: EHEC produces Shiga-like toxin, which inhibits host protein synthesis.

Laboratory Diagnosis of E. coli Infections

Sample Collection

Samples are collected according to the site of infection (e.g., urine for UTI, stool for intestinal infection).

Direct Gram-Stained Smear

• Gram-negative bacilli

• Non-spore forming

• Motile

• Capsulated

Cultural Characteristics

• Temperature: 37°C

• Oxygen: Facultative anaerobe

• CO2: Atmospheric

Culture Medium

• MacConkey's agar: Used to differentiate lactose fermenters (LF) from non-lactose fermenters (NLF).

Identification of E. coli

• Colony morphology: Rose-pink colonies on MacConkey's agar (LF)

• Gram-stained smear: Gram-negative bacilli

• Biochemical reactions: Ferments glucose, maltose, mannitol, lactose, and sucrose with acid and gas production

• IMViC test: $(++--)$ (Indole positive, Methyl Red positive, Voges-Proskauer negative, Citrate negative)

Laboratory Diagnosis of E. coli O157:H7 Infection

• Stool culture on sorbitol MacConkey's agar: E. coli O157:H7 is a non-sorbitol fermenter, producing pale colonies, while other E. coli strains produce pink colonies.

Case Study

Clinical Scenario

A 6-year-old boy presents with severe abdominal cramps, diarrhea (watery progressing to bloody), and vomiting after consuming undercooked ground beef. Stool sample is sent for microbiological testing.

• Likely causative agents: E. coli (especially EHEC), Salmonella, Shigella

• Diagnostic approach: Stool culture, Gram stain, biochemical tests, and serotyping for specific antigens.

Example: Identification of EHEC O157:H7 using sorbitol MacConkey's agar and detection of Shiga-like toxin.

Additional info: The notes include clinical, laboratory, and classification details relevant for exam preparation in medical microbiology.