Pathogenic Gram-Negative Bacteria: Structure, Diseases, and Clinical Features

Pathogenic Gram-Negative Bacteria

Overview

Gram-negative bacteria are a diverse group of microorganisms characterized by their thin peptidoglycan cell wall and outer membrane containing lipopolysaccharide (LPS). Many are significant human pathogens, causing a wide range of diseases. This chapter focuses on the structure, transmission, clinical features, and prevention of important Gram-negative cocci and bacilli.

Classification of Gram-Negative Pathogens

Cocci vs. Bacilli

• Cocci: Spherical-shaped bacteria.

• Bacilli: Rod-shaped bacteria.

Bacterium	Shape	Unique Characteristics
Neisseria gonorrhoeae, N. meningitidis	Cocci (diplococci)	Kidney-shaped pairs; oxidase positive
Escherichia coli	Bacilli	Lactose fermenter; some strains produce shiga-like toxin
Klebsiella pneumoniae	Bacilli	Prominent capsule; mucoid colonies
Serratia marcescens	Bacilli	Red pigment (prodigiosin) production
Enterobacter	Bacilli	Lactose fermenter; opportunistic
Proteus mirabilis	Bacilli	Highly motile; swarming growth; urease positive
Salmonella enterica (serovars Typhi, Typhimurium)	Bacilli	Non-lactose fermenter; causes typhoid and gastroenteritis
Shigella sonnei	Bacilli	Non-motile; causes bacillary dysentery
Yersinia pestis	Bacilli (coccobacilli)	Bipolar staining; causes plague
Haemophilus influenzae	Bacilli (coccobacilli)	Requires X and V factors for growth
Bartonella henselae	Bacilli	Causes cat scratch disease
Brucella melitensis	Bacilli (coccobacilli)	Intracellular; causes brucellosis
Bordetella pertussis	Bacilli (coccobacilli)	Causes whooping cough; produces pertussis toxin
Pseudomonas aeruginosa	Bacilli	Produces blue-green pigment (pyocyanin); resistant to many antibiotics
Francisella tularensis	Bacilli (coccobacilli)	Highly infectious; causes tularemia
Legionella pneumophila	Bacilli	Requires cysteine and iron for growth; causes Legionnaires' disease
Coxiella burnetii	Bacilli (coccobacilli)	Obligate intracellular; causes Q fever

Transmission, Treatment, and Prevention

General Principles

• Transmission: Varies by species; includes respiratory droplets, fecal-oral route, direct contact, vectors, and contaminated food/water.

• Treatment: Often involves antibiotics, but resistance is common in some species (e.g., Pseudomonas aeruginosa).

• Prevention: Includes vaccination (where available), hygiene, safe food handling, vector control, and avoiding exposure to reservoirs.

Key Pathogens and Diseases

Neisseria Species

• Neisseria gonorrhoeae: Causes gonorrhea, a sexually transmitted infection. Transmitted via sexual contact. Prevention includes safe sex practices.

• Neisseria meningitidis: Causes meningococcal meningitis. Spread by respiratory droplets. Vaccines are available for prevention.

Enterobacteriaceae Family

• Escherichia coli: Causes urinary tract infections, gastroenteritis, and neonatal meningitis. Transmitted via fecal-oral route or endogenous infection.

• Klebsiella pneumoniae: Causes pneumonia, UTIs, and sepsis. Notable for its thick capsule and antibiotic resistance.

• Serratia marcescens: Opportunistic pathogen; produces red pigment; associated with hospital-acquired infections.

• Enterobacter: Causes nosocomial infections, especially in immunocompromised patients.

• Proteus mirabilis: Causes UTIs; produces urease, leading to kidney stones.

• Salmonella enterica (serovars Typhi, Typhimurium): Typhi causes typhoid fever (systemic infection); Typhimurium causes gastroenteritis. Transmitted via contaminated food/water.

• Shigella sonnei: Causes bacillary dysentery (shigellosis); transmitted via fecal-oral route.

• Yersinia pestis: Causes plague (bubonic, septicemic, pneumonic); transmitted by flea bites or respiratory droplets.

Other Gram-Negative Pathogens

• Haemophilus influenzae: Causes meningitis, epiglottitis, and pneumonia; type b (Hib) is vaccine-preventable.

• Bartonella henselae: Causes cat scratch disease; transmitted by cat scratches or bites.

• Brucella melitensis: Causes brucellosis; transmitted via unpasteurized dairy or contact with infected animals.

• Bordetella pertussis: Causes whooping cough (pertussis); produces pertussis toxin; prevented by DTaP vaccine.

• Pseudomonas aeruginosa: Opportunistic pathogen; produces blue-green pigment (pyocyanin); common in burn and cystic fibrosis patients; highly antibiotic-resistant.

• Francisella tularensis: Causes tularemia; transmitted by ticks, rabbits, or inhalation; highly infectious.

• Legionella pneumophila: Causes Legionnaires' disease; transmitted via inhalation of contaminated water aerosols (e.g., air conditioning systems).

• Coxiella burnetii: Causes Q fever; transmitted by inhalation of contaminated dust from animal products.

Unique Disease Characteristics

• Gonorrhea: Purulent urethral discharge, pelvic inflammatory disease.

• Meningococcal meningitis: Rapid onset fever, neck stiffness, petechial rash.

• Typhoid fever: Prolonged fever, abdominal pain, "rose spots" on abdomen.

• Shigellosis: Bloody diarrhea with mucus and pus.

• Plague: Buboes (swollen lymph nodes), pneumonia, septicemia.

• Pertussis (Whooping cough): Paroxysmal cough with inspiratory "whoop".

• Legionnaires' disease: Severe pneumonia with high fever and GI symptoms.

• Q fever: Flu-like illness, hepatitis, endocarditis (chronic cases).

Summary Table: Key Gram-Negative Pathogens

Bacterium	Disease(s)	Transmission	Prevention
Neisseria gonorrhoeae	Gonorrhea	Sexual contact	Safe sex
Neisseria meningitidis	Meningitis	Respiratory droplets	Vaccine
Escherichia coli	UTI, gastroenteritis	Fecal-oral, endogenous	Hygiene
Klebsiella pneumoniae	Pneumonia, UTI	Endogenous, hospital	Infection control
Serratia marcescens	Nosocomial infections	Hospital environment	Infection control
Proteus mirabilis	UTI	Endogenous	Hygiene
Salmonella enterica	Typhoid, gastroenteritis	Contaminated food/water	Safe food handling
Shigella sonnei	Dysentery	Fecal-oral	Hygiene
Yersinia pestis	Plague	Fleas, droplets	Vector control
Haemophilus influenzae	Meningitis, epiglottitis	Respiratory droplets	Vaccine (Hib)
Bartonella henselae	Cat scratch disease	Cat scratches/bites	Avoid cat bites
Brucella melitensis	Brucellosis	Unpasteurized dairy	Pasteurization
Bordetella pertussis	Pertussis	Respiratory droplets	DTaP vaccine
Pseudomonas aeruginosa	Opportunistic infections	Hospital, environment	Infection control
Francisella tularensis	Tularemia	Ticks, rabbits, inhalation	Avoid exposure
Legionella pneumophila	Legionnaires' disease	Water aerosols	Water system maintenance
Coxiella burnetii	Q fever	Inhalation of animal dust	Animal control

Key Terms

• Lactose fermenter: Bacteria that can ferment lactose, producing acid (e.g., E. coli).

• Capsule: Polysaccharide layer that protects bacteria from phagocytosis (e.g., Klebsiella pneumoniae).

• Opportunistic pathogen: Causes disease mainly in immunocompromised hosts (e.g., Pseudomonas aeruginosa).

• Obligate intracellular: Bacteria that must live inside host cells to survive (e.g., Coxiella burnetii).

Summary

• Gram-negative bacteria include both cocci and bacilli, many of which are important human pathogens.

• Understanding their unique characteristics, modes of transmission, and prevention strategies is essential for clinical microbiology.

• Vaccination, hygiene, and infection control are key to preventing many Gram-negative bacterial diseases.

Additional info: Some details (e.g., pigment production, specific disease features, and prevention strategies) were inferred based on standard microbiology knowledge to provide a complete and self-contained study guide.