Pathogenic Gram-Negative Cocci and Bacilli

Introduction

This section explores the medically significant Gram-negative cocci and bacilli, focusing on their taxonomy, structure, pathogenesis, and clinical relevance. These organisms are responsible for a wide range of human diseases, including sexually transmitted infections, meningitis, and various healthcare-associated infections.

Neisseria: Gram-Negative Cocci

General Characteristics

• Neisseria is the only genus of Gram-negative cocci that regularly causes disease in humans.

• These bacteria are aerobic, nonmotile, and typically arranged as diplococci (pairs of cocci).

• They are oxidase positive, which distinguishes them from many other Gram-negative pathogens.

• Pathogenic strains possess fimbriae, a polysaccharide capsule, and a cell wall antigen called lipooligosaccharide containing lipid A.

• Two species are pathogenic to humans: N. gonorrhoeae and N. meningitidis.

Neisseria gonorrhoeae (The Gonococcus)

• Causes gonorrhea, a sexually transmitted disease that only occurs in humans.

• Risk of infection increases with the number of sexual encounters; more common in females, often asymptomatic in women but symptomatic in men (painful urination, discharge).

• Can trigger pelvic inflammatory disease (PID) in women and cause infections outside the reproductive tract.

• Pathogenic mechanisms include adherence to mucosal surfaces, secretion of protease that cleaves secretory IgA, and survival within neutrophils.

Diagnosis, Treatment, and Prevention

• Diagnosis: Identification of Gram-negative diplococci in pus; nucleic acid amplification for asymptomatic cases.

• Treatment: Broad-spectrum intramuscular cephalosporins; resistance is an increasing problem.

• Prevention: Sexual abstinence, monogamy, proper condom use; newborn eye infections prevented with antimicrobials.

Neisseria meningitidis (The Meningococcus)

• Most common cause of meningitis in individuals under 20 years of age.

• Can be part of the normal microbiota of the upper respiratory tract.

• Transmitted by respiratory droplets, especially among people in close contact.

• Meningitis can cause death within hours; meningococcal septicemia is also life-threatening.

Diagnosis, Treatment, and Prevention

• Diagnosis: Rapid identification of Gram-negative diplococci in phagocytes of the CNS is critical.

• Treatment: Immediate administration of antimicrobial drugs such as ceftriaxone.

• Prevention: Vaccine available for some strains; eradication is difficult due to asymptomatic carriers.

Pathogenic, Gram-Negative Facultatively Anaerobic Bacilli

Overview

• Most common Gram-negative pathogens of humans; found in the intestinal microbiota, water, soil, and decaying vegetation.

• Two main families: Enterobacteriaceae and Pasteurellaceae.

• The oxidase test distinguishes between these families.

The Enterobacteriaceae Family

• Motile, bacilli or coccobacilli; reduce nitrate to nitrite and ferment glucose.

• Classified into three groups: Coliforms (rapidly ferment lactose), Noncoliform Opportunists (do not ferment lactose), and True Pathogens.

Pathogenesis and Virulence Factors

• Possess a variety of antigenic components (membrane, protein, and polysaccharide capsular antigens).

• Virulence factors include fimbriae, adhesins, exotoxins, and type III secretion systems.

Diagnosis, Treatment, and Prevention

• Diagnosis: Detection in urine, blood, or cerebrospinal fluid; biochemical tests for rapid identification.

• Treatment: Diarrhea is usually self-limited; internal infections require antimicrobials.

• Prevention: Good personal hygiene and proper sewage control.

Coliform Opportunistic Enterobacteriaceae

• Aerobic or facultatively anaerobic, Gram-negative rods that ferment lactose to form gas.

• Common in soil, plants, and decaying vegetation; presence in water indicates contamination.

• Genera include Escherichia, Klebsiella, Serratia, Enterobacter, Hafnia, and Citrobacter.

Escherichia coli

• Most common and important coliform; antigens used to identify strains.

• Virulent strains have plasmids for fimbriae, adhesins, and exotoxins.

• Causes gastroenteritis, pediatric diarrhea, and other diseases.

• E. coli O157:H7 produces Shiga-like toxin, causing hemorrhagic colitis and hemolytic uremic syndrome; associated with undercooked beef and contaminated foods.

Klebsiella

• Found in digestive and respiratory systems; capsule protects from phagocytosis.

• K. pneumoniae is the most common pathogenic species, causing pneumonia, bacteremia, meningitis, wound infections, and UTIs.

Serratia

• Produces a red pigment at room temperature; grows on hospital supplies and can cause life-threatening infections in immunocompromised patients.

• Frequently resistant to antimicrobial drugs.

Enterobacter, Hafnia, and Citrobacter

• Reside in digestive tracts, soil, water, and sewage; can contaminate dairy products.

• Cause healthcare-associated infections (HAIs) and are often resistant to drugs.

Noncoliform Opportunistic Enterobacteriaceae

Proteus

• Proteus mirabilis is associated with UTIs in patients with long-term catheters; can cause kidney stones and is resistant to many drugs.

Morganella, Providencia, and Edwardsiella

• Cause HAIs, primarily UTIs; Providencia may cause kidney stones.

Truly Pathogenic Enterobacteriaceae

• Include Salmonella, Shigella, and Yersinia; almost always pathogenic due to numerous virulence factors, including type III secretion systems.

Salmonella

• Motile, peritrichous bacilli; live in intestines of birds, reptiles, and mammals.

• Human infections usually from contaminated food, especially poultry and eggs.

• Can cause salmonellosis and typhoid fever (caused by Salmonella enterica serotype Typhi).

• Typhoid fever: humans are the only host; bacteria spread via contaminated food/water, pass through intestines into bloodstream, and can cause systemic disease.

Shigella

• Oxidase-negative, nonmotile; primarily a parasite of the human digestive tract.

• Produces enterotoxin causing severe dysentery (shigellosis).

• Four species: S. dysenteriae (most severe), S. flexneri (developing countries), S. boydii, S. sonnei (industrialized nations).

• Spread via contaminated hands or food; person-to-person transmission possible.

Yersinia

• Pathogens of animals; three important species: Y. enterocolitica (food/waterborne), Y. pseudotuberculosis (intestinal inflammation), Y. pestis (bubonic and pneumonic plague).

• All contain virulence plasmids for adhesins and type III secretion systems.

Sites of Infection

• Members of Enterobacteriaceae can infect various body sites, including the CNS, respiratory tract, bloodstream, gastrointestinal tract, and urinary tract.

The Pasteurellaceae Family

Overview

• Small, nonmotile, facultative anaerobes; oxidase positive and require heme or cytochromes for growth.

• Two main genera: Pasteurella and Haemophilus.

Pasteurella

• Normal microbiota in oral and nasopharyngeal cavities of animals; humans infected via animal bites or aerosols.

• Most cases (P. multocida) cause localized inflammation; can cause bacteremia in immunosuppressed individuals.

• Diagnosed by identification in specimens; treatable with antibacterial drugs.

Haemophilus

• Colonize mucous membranes; some species cause opportunistic infections.

• H. influenzae type b (Hib) was a common cause of meningitis before vaccination.

• H. ducreyi causes a sexually transmitted disease.

Summary Table: Classification of Enterobacteriaceae

Additional info: This summary integrates and expands upon the provided lecture slides, adding definitions, clinical context, and a classification table for clarity and exam preparation.