Pathogenic Microbiology: Gram-Negative Bacteria and Symbiotic Relationships

Introduction to Rickettsial Pathogens and Pathogenesis

• Definition of Pathogen: A group of obligate intracellular pathogens that cause disease.
• Symbiotic Relationships: These pathogens exist in a symbiotic relationship with other organisms, specifically classified as parasitism.
  • Parasitism: A relationship where one organism benefits while the other is harmed.
  • Types of Parasites: Commonly thought of as multicellular organisms (ectoparasites like ticks and fleas, or endoparasites like worms), but germs and pathogens are technically parasites.
• Obligate Pathogens: These organisms have no other way to survive and must inhabit a host cell (intracellular).

Specific Rickettsial Diseases and Vectors

-Rickettsia typhi: - Common Names: Causes endemic typhus, also known as murine typhus or bush typhus. - Transmission: Transmitted by fleas. The flea breaks the skin to consume blood, allowing the bacterium in the flea's digestive system to enter the human bloodstream. - Key Symptoms: Skin rash, dangerously high fever, and flu-like symptoms (typical of all rickettsial diseases). - Mortality: Untreated typhus has approximately a $4\%$ mortality rate. Exact numbers are difficult to calculate due to its rarity in the US.

• Epidemic Typhus:
  • Transmission: Spread by body lice (distinct from hair lice).
  • Prevention: Body lice are contracted when clothing cannot be changed regularly. Simple hygiene, like changing clothes every 2-3 days, prevents infestation.
  • Risk Factors: In the US, this is primarily seen in the unhoused community where access to personal hygiene is limited.

-Rickettsia rickettsii: - Disease: Causes Rocky Mountain spotted fever. - Transmission: Transmitted by dog ticks (large ticks roughly the size of a pencil eraser). - Nomenclature: Named after the Rocky Mountain National Laboratory where the pathogen was identified, though the ticks are actually native to the Eastern US. - Mortality: Approximately $4-5\%$ mortality rate if untreated. - Prevention: Check for ticks after hiking or spending time outdoors.

Nitrogen Fixation and Diazotrophs

• Key Genera: Rhizobium, Bradyrhizobium, Nitrobacter, and Nitrosomonas.
• Diazotrophs: Soil bacteria capable of nitrogen fixation.
• Nitrogen in Biology: Nitrogen is one of the four essential elements in biological macromolecules.
  • The atmosphere is mostly elemental nitrogen ($N_2$), which is diatomic and held by a triple covalent bond ($N \equiv N$).
  • Most living things, including humans, cannot break this triple bond and cannot access elemental nitrogen.
• Biological Availability: Nitrogen must be converted into "biologically available" forms, such as ammonia ($NH_3$) or nitrate ($NO_3^-$).
• Mutualistic Symbiosis: Diazotrophs live in a mutualistic relationship with legumes (e.g., soybeans).
  • Nodules: Bacteria colonize plants, invading roots to form lumps called nodules.
  • Resource Exchange: Diazotrophs provide available nitrogen; plants provide a carbon source (trading).
• Environmental Impact: Excess nitrogen leaches into the surrounding soil, benefiting other plants. Life on Earth depends on this process for nitrogen source acquisition.

Agrobacterium tumefaciens and Genetic Engineering

• Characteristics: A soil bacterium related to diazotrophs but is a plant pathogen.
• Disease: Causes Crown Gall disease, which manifests as large growths or tumors (plant cancer).
• Mechanism of Infection:
  • Detects fluids leaking from a wounded plant and swims toward it.
  • Contains the Ti Plasmid (Tumor Inducing plasmid).
  • Uses a T-pilus to attach to the plant cell and performs a process similar to conjugation.
• Genetic Modification: Only a fragment of the Ti plasmid (T-DNA) is inserted into the plant's nucleus and chromosome.
  • Auxins: Genes on the plasmid induce the production of plant growth hormones, causing uncontrolled cell division (the tumor).
  • Opines: Genes cause the plant to produce unique amino acids that most organisms cannot use, but Agrobacterium uses them as a nitrogen and energy source.
• Biotechnology Applications: Scientists use modified Ti plasmids to insert beneficial genes into plants.
  • Niacin: Modified to produce more vitamins.
  • BT Toxin: An insecticide gene inserted into crops (corn, rice, cotton, tobacco). It is toxic to pests but not humans, reducing the need for chemical sprays.

Bordetella pertussis (Whooping Cough)

• Disease: Causes pertussis, or "whooping cough," sometimes called the "100-day cough."
• Infection Site: Upper respiratory system (mouth, throat).
• Stages of Disease:
  1. Catarrhal Stage: Lasts about a week; resembles a common cold (drippy/bloody nose, coughing).
  2. Proximal (Paroxysmal) Stage: Lasts 8-12 weeks. Large numbers of bacteria produce pertussis toxin.
  3. Convalescent Stage: The recovery period.
• Mechanism of Toxin: Paralyzes the cilia in the trachea and bronchi. Normally, cilia push mucus (trapping particles like pollen, dust, and smoke) up to the esophagus to be swallowed. When cilia stop, mucus builds up, necessitating violent coughing fits (paroxysms).
• "Whooping" Sound: The sound made during the desperate intake of breath after a coughing fit; more pronounced in children with smaller airways. Can lead to oxygen deprivation, brain damage, and death.
• Vaccinations: Tdap, DTP, and DTaP. The "P" stands for pertussis. Formulations vary by age (1 year, 7 years, and decennial boosters).

Genus Neisseria and Symbiotic Categories

• Arrangement: Gram-negative diplococcus (pairs).
• Commensalism: Most Neisseria are human commensals.
  • Commensalism: One organism benefits, while the other is unaffected.
• Pathogenic Species:
  • Neisseria gonorrhoeae: Causes the STI gonorrhea (also called "the drip" or "the clap").
    • Symptoms: Pain and discharge at the site of infection (penis, testicles, abdomen, or anus).
    • Congenital Gonorrhea: Acquired during birth. Causes ocular gonorrhea (eye infection). This is why all newborns regardless of screening receive antibiotic eye drops.
    • Note: Does not have a capsule.
  • Neisseria meningitidis:
    • Structure: Possesses a capsule.
    • Disease: Invades the bloodstream and migrates to the central nervous system, infecting cerebrospinal fluid and inflaming the meninges (meningitis).
    • Symptoms: Severe headache, high fever, stiff neck, confusion, disorientation. Often lethal and progresses rapidly.
    • Transmission: Common in close living quarters (dorms, barracks, prisons).

Vibrio cholerae and AB Toxins

• Disease: Causes cholera.
• Cholera Toxin (Choleragen): An AB toxin (a type of heat-labile protein exotoxin).
  • B Subunit: Binding subunit; attaches to specific molecules on the cell surface and facilitates entry.
  • A Subunit: Active subunit; enters the cell to perform toxic activity.
• Mechanism in the Intestine:
  • Toxin enters epithelial cells of the small intestine.
  • Activates adenylate cyclase, which produces cyclic AMP ($cAMP$).
  • The toxin "breaks the switch," keeping adenylate cyclase permanently ON.
  • High concentrations of $cAMP$ ($50-100 \times$ normal) cause cells to secrete salt ($NaCl$) into the intestine.
• Symptoms: Water follows the salt, leading to extreme watery diarrhea known as rice water stool (containing clumps of dead tissue).
• Treatment: Dehydration is the primary killer. It is a self-limiting disease; if the patient stays hydrated (via IV or electrolyte drinks), they can survive until the pathogen is washed out.

Pseudomonas aeruginosa

• Classification: Gram-negative, soil bacterium, obligate aerobe (must have oxygen).
• Opportunistic Pathogen: Only causes disease if given the opportunity.
• Clinical Significance:
  • Burn Wards: Infects open surface wounds where oxygen is available.
  • Pigment: Produces pyocyanin, a blue-green protein pigment that turns pus that color.
  • Cystic Fibrosis (CF): CF patients have water in their lungs due to a genetic mutation. Pseudomonas grows in this moist, oxygenated environment, causing pneumonia.
• Antibiotic Resistance:
  • High metabolic diversity allows it to break down organic molecules, including some antibiotics.
  • Multidrug Resistance Pumps: Active transport proteins that use energy to pump antibiotics out of the cell.

Escherichia coli (E. coli)

• General Facts: The most well-studied bacterium. Comprises roughly $10\%$ of the bacteria in the human large intestine.
• Mutualism: Helps with digestion, produces vitamin K, and competes with incoming pathogens for nutrients.
• Pathogenesis: Can cause UTIs if it leaves the gut.
• Enterohemorrhagic E. coli (EHEC):
  • Produces Shiga-like toxin (an AB toxin acquired via horizontal gene transfer).
  • Toxin destroys ribosomes, killing human cells.
  • Causes hemorrhaging (bleeding) in the intestine, which can lead to organ failure if the toxin enters the bloodstream.
• Cerevar Identification: Identified using the Kauffman-White system based on serology (antibodies).
  • O Antigen: Part of the lipopolysaccharide ($LPS$) chain (e.g., $O157$).
  • H Antigen: Flagellum antigen (e.g., $H7$).
  • K Antigen: Capsule antigen.

Questions & Discussion

• Question: Are all Neisseria species commensal?
• Answer: Most are commensals, except for a few "troublemakers" that act as pathogens, though N. meningitidis can occasionally hang out as a commensal in the mouth before invading.
• Question: How do CF patients contract Pseudomonas?
• Answer: Likely from the dirt; potentially through inhaled dust or being brought inside by pets, and it can possibly be transmitted person-to-person via coughing.