Gram Negative Pathogens

Gram-Negative Pathogens

Examples of Gram-Negative Species

  • Serratia marcescens
    • Shower curtain bacteria (pink at the bottom).
    • Forms pink colonies on agar plates.
    • Opportunistic pathogens, especially in immunocompromised individuals with catheters or intubation in hospital settings.
  • Neisseria
    • Diplococci (pairs of cocci).
    • Neisseria gonorrhoeae is the causative agent of gonorrhea.
  • Proteus mirabilis
    • Opportunistic pathogen causing nosocomial infections.
  • Salmonella
    • Associated with food poisoning, requiring hand washing and careful handling of chicken.
  • Yersinia pestis
    • Causative agent of the bubonic plague or Black Death.

Gram-Negative Bacteria as Pathogens

  • Gram-negative bacteria constitute the largest group of human bacterial pathogens.
  • Despite the vast diversity of bacteria (over 6,000 described genera), fewer than 30 genera of gram-negative species are pathogenic.
  • Microbes are ubiquitous, but only a small fraction are pathogenic, often acting as opportunistic pathogens in immunocompromised individuals.

Lipid A and Pathogenicity

  • Gram-negative cells have an inner membrane, a thin peptidoglycan layer, and an outer membrane.
  • The outer membrane contains lipopolysaccharide (LPS), with lipid A as its lipid component.
  • Lipid A is antigenic, provoking an immune response.
  • Consequences of lipid A-induced immune response:
    • Fever
    • Inflammation
    • Shock
    • Vasodilation
    • Disseminated Intravascular Coagulation (DIC)
      • Intravascular: In the blood vessels.
      • Coagulation: Clotting.

Requirements for Pathogenicity

  • Ability to grow at 37 degrees Celsius (mesophiles).
  • Ability to evade the immune system.

Neisseria

  • Named after Neisser, who discovered this organism and other STIs.
  • The only genus of gram-negative cocci that regularly causes disease in humans; other pathogenic genera are typically bacilli.
  • Aerobic, non-motile diplococci.
  • Oxidase positive (possesses oxidase enzyme to detoxify reactive oxygen species like superoxide, hydrogen peroxide).
  • Pathogenic strains possess fimbriae for attachment to epithelial cells of mucous membranes.
  • Two species pathogenic to humans; focus on Neisseria gonorrhoeae.
    • Neisseria meningitidis is one cause of bacterial meningitis.
Neisseria gonorrhoeae
  • Causes gonorrhea, exclusively in humans; a sexually transmitted infection (STI).
  • Most US cases occur in adolescents with high-risk behavior.
  • More common in females due to higher diagnosis rates through regular gynecological exams (Pap smears).
  • Females often asymptomatic, acting as carriers.
  • Epidemiology shows varying incidence across the US, with the highest rates in the South.
  • Cases have decreased since 1975 due to increased education and detection.
  • Pathogenesis: Gonococci adhere to the genitals, urinary, and digestive tracts.
  • Evade the immune system by:
    • Possessing protease that cleaves IgA (immunoglobulin A) antibodies.
    • Surviving within neutrophils.
  • Symptoms:
    • In men: Painful urination due to inflammation and pus-filled discharge.
    • In women: Often asymptomatic but can trigger pelvic inflammatory disease.
  • Prevention: Safer sex practices, such as using condoms.
  • Historical Perspective: Sexist public service campaigns in the 1940s during World War II blamed women for spreading STIs.
  • Infections can occur outside the reproductive tract, causing proctitis, pharyngitis, and gingivitis, necessitating safer sex practices for oral sex.
  • Infection of the cornea or respiratory tract of newborns can occur during childbirth.
  • Diagnosis:
    • Genetic probes (PCR, ELISA) for asymptomatic cases.
    • Sampling cells from pus in inflamed areas.
  • Treatment:
    • Complicated by antibiotic-resistant strains.
    • Condom usage is crucial.

Gram-Negative Facultatively Anaerobic Bacilli

  • Two families contain the most human pathogens:
    • Enterobacteriaceae (infect the gastrointestinal tract).
    • Pasteurellaceae.
  • Focus on Enterobacteriaceae (studied in unknown strain tests).
  • An oxidase test differentiates between these families.
    • Oxidase-negative: Enterobacteriaceae.
    • Oxidase-positive: Pasteurellaceae.
  • Pasteurellaceae are important nosocomial pathogens (acquired in a hospital setting).
  • Gram-negative bacteria cause more than half of nosocomial infections.
  • Enterobacteriaceae are categorized as:
    • Coliforms:
      • lactose fermenting
      • Escherichia (25% nosocomial infections).
      • Klebsiella (KPC).
      • Serratia marcescens.
      • Enterobacter.
      • Citrobacter.
    • Non-coliforms:
      • Non lactose fermenting
      • Proteus.
      • Salmonella.
    • Pseudomonas:
      • Not Enterobacteroaceae.
Overview of Enterobacteriaceae
  • Part of the intestinal microbiota of animals and humans.
  • Ubiquitous in water, soil, and decaying vegetation.
  • Most common gram-negative pathogens of humans.
  • Mostly opportunistic pathogens, taking advantage of immunocompromised individuals.
  • Beneficial microbes keep their numbers low in healthy individuals.
Structure and Physiology
  • Most are bacilli and coccobacilli.
  • Reduce nitrate to nitrite.
  • All ferment glucose (facultative anaerobes).
  • Grow best in aerobic conditions but can switch to fermentation.
  • Species distinguished by biochemical properties, motility, and colony characteristics.
Dichotomous Key for Intestinal Bacteria
  • Rapid lactose fermentation (coliform vs. non-coliform).
  • Motility (CIM medium).
  • Citrate utilization.
  • Gelatin utilization.
Antigens and Virulence Factors of Enteric Bacteria
  • Antigens (surface structures):
    • O antigen: Lipopolysaccharide and lipid A.
    • K antigen: Capsule.
    • H antigen: Flagellar proteins.
  • Virulence Factors:
    • Type III secretion system (injects toxins into host cells).
    • Fimbriae (attachment).
    • Exotoxins (toxic proteins secreted by the cell).
    • Adhesins (attachment).
    • Plasmids (encode F factor, R genes, and virulence factors).
    • Hemolysin (lyses red blood cells).
    • Iron-binding proteins (sequester iron from lysed red blood cells).
Diagnosis and Treatment
  • Diagnosis from urine, blood, and cerebrospinal fluid (CSF).
  • Biochemical tests for rapid identification (e.g., ELISA).
  • Treatment:
    • Hydration (IV fluids and electrolytes).
    • Antibiotics as a last resort (to avoid antibiotic resistance and release of lipid A).
  • Prevention: Good personal hygiene and proper sewage control.
Groups of Pathogenic Enterobacteriaceae
  • Coliforms (lactose fermenters): Normal microbiota, but opportunistic pathogens.
  • Non-coliforms (non-lactose fermenters): Opportunistic pathogens.
  • True pathogens: Always pathogenic (e.g., Salmonella).
  • MacConkey agar distinguishes between lactose and non-lactose fermenters.
Coliforms
  • Aerobic or facultatively anaerobic, rod-shaped bacteria that ferment lactose to form gas in lactose broth.
  • Found in soil, on plants, and decaying vegetation.
  • Coliforms in water indicate impure water and poor sewage treatment.
Escherichia coli
  • Most common of the coliforms.
  • Possess virulence plasmids (encoding fimbriae, adhesins, and exotoxins).
  • Gastroenteritis is the most common disease.
  • Major cause of pediatric diarrhea in developing countries.
E. coli O157:H7
  • Most prevalent pathogenic E. coli in developed countries.
    • Causes diarrhea, hemorrhagic colitis, and uremic syndrome.
    • Associated with undercooked ground beef, contaminated milk or fruit juice.
    • Produces a type III secretion system and Shiga-like toxin.
    • Shiga like toxin: N-glycosylase, that cleaves the glycosidic bond between the ribose sugar and the atomine of a ribonucleotide. ribonucleotides→RNAribonucleotides \rightarrow RNA N-glycosylase \cleaves \Rightarrow adenine \& ribose
      • Stops Translation.
      • O157 - lipopolysaccharide. H7 - Flagellin Protein. K antigens for capsule, fimbriae are the F, and toxins.
Serratia marcescens
  • Produces a red pigment when grown at room temperature.
  • Common nosocomial infection.
  • Can cause life-threatening infections in immunocompromised patients with catheters, IVs, or intubation.
  • Frequently resistant to antimicrobial drugs.
Proteus mirabilis
  • Associated with UTIs in patients with long-term urinary catheters.
  • Grows in concentric waves on plates due to swarmer cells moving out in unison.

Truly Pathogenic Enterobacteriaceae

  • Always pathogenic: Salmonella, Shigella, and Yersinia.
  • Possess virulence factors, including type III secretion systems.
  • Type III Secretion System:
    • delivers toxins
    • Injection of toxins causes apoptosis of macrophages and neutrophils.
    • Electron micrograph of protein structures embedded in cell walls of gram-negative cells.
Salmonella
  • Motile, peritrichous bacilli (flagella all around the cell).
  • Live in the intestines of birds, reptiles, and mammals.
  • Human infections from consuming food contaminated with animal feces (poultry and eggs).
  • Causes salmonellosis and typhoid fever.
  • Salmonellosis proceeds by:
    • Ingesting Salmonella, which evades stomach acid and enters the small intestine.
    • Attaching to epithelial cells lining the small intestine using fimbriae.
    • Triggering endocytosis but avoiding digestion.
    • Using type III secretion systems to inject toxins, causing apoptosis.
  • Host cell death induces fever, cramps, and diarrhea.
    • If it enters the bloodstream it causes Bacteremia.
Typhoid Mary

  • Typhoid Mary was an asymptomatic carrier by having salmonella in her gall bladder that worked as a cook for many families. She infected multiple families with salmonellosis and was eventually forcibly detained and had her gallbladder removed.

  • Salmonella infection:

    • Salmonella through ingestion enterica cells in the small intestine that adhere in the lumen, epithelial cells and goes through many cell cycles. When its severe it causes bacteremia and symptoms occur within twelve to seventy two hours. If mild, then the symptoms last four to seven days without treatments and if severe, you may have it for teo weeks. Treatment involves antibiotic injections for two weeks.
Yersinia pestis
  • Causative agent of the bubonic plague or Black Death.
  • Normal pathogens of animals.
  • Contain virulence plasmids (encoding adhesins and type III secretion systems for injecting Shiga toxin).
  • Injection of toxins causes apoptosis of macrophages and neutrophils.
  • Bubonic plague symptoms:
    • Buboes: Pus-filled lumps in the groin or armpits (lymph nodes).
    • Blackening of the extremities.
  • Life cycle involved rodents, fleas, and humans.
    • Humans can transmit with each other.
  • Pneumonic plague: Infection of the lungs, spread via respiratory droplets.
  • Diagnosis and treatment must be rapid due to fast progression and deadliness
    • lumps near lymph nodes.
    • blackening of the extremities can cause clostridium.
    • Treatable with antibacterial drugs.
  • Harry Potter Connection: J.K. Rowling based 'Bubotubers' concept on buboes.
Sites of Infection for Common Enterobacteriaceae
  • Escherichia, including E. coli: Central nervous system.
  • Klebsiella pneumoniae: Damaged lung tissues.
  • Gastrointestinal tract.
  • Urinary tract (catheter-associated infections).