LOs: 4 Flashcards
Bacterial Growth Curve Phases
- Lag: initial phase after inoculation, when the bacteria are gearing up to grow (making more enzymes for growth, etc.)
- Exponential (Log): when bacteria are growing at a steady rate (“doubling time”)
- Stationary: bacteria use up nutrients and produce waste products, growth starts to slow
o Before: Total = Viable (almost all bacteria in culture are alive)
o After: Total > Viable
How pathogens can overcome host irons equestration
The body uses iron-binding proteins (lactoferrin and transferrin) to restrict the availability of iron for pathogens
Pathogens fight back:
1) By producing their own iron-binding molecules (e.g. siderophores)
2) Some bacteria (e.g. the pathogenic Neisseria spp.) can bind human transferrin and lactoferrin to their surface and then use that bound iron
Aerobes
Grow only in the presence of O2
Ex. Pseudomonas aeruginosa)
Microaerophiles
A special group of aerobes requiring O2, but at lower concentrations than found in normal air
Ex. Campylobacter spp.
Facultative anaerobes
Grow in either the presence or absence of O2 (e.g. E. coli)
Grow by fermentations in the absence of O2, but shift their metabolism to respiration in the presence of O2
More ATP is produced during respiration than fermentation, so growth is faster here
Ex. E. coli
Aerotolerants
Grow in presence or absence of O2, but metabolism always uses fermentation
Ex. most Streptococcus spp.
Strict anaerobes
Grow only in the absence of O2
Ex. Clostridium spp.
Why some bacteria are anaerobic / Why anaerobes are sensitive to oxygen
Anaerobes often lack catalase (which breaks down toxic H2O2 formed after cells are exposed to air).
Anaerobes often lack superoxide dismutase (which detoxifies free radical forms of O2 that form after cells are exposed to air).
Anaerobes may have very sensitive enzymes that require a reduced environment.
Bottom line: anaerobes must be in a low redox (reduced environment) and O2
raises the redox potential.
The redox potential of healthy tissue is too high for anaerobes to grow.
Medical conditions that can lower tissue redox potential:
- Circulatory problems
- Tight orthopedic casts
- Co-presence of facultative anaerobes (which can consume oxygen) during an infection
Body locations where anaerobes are normally found & Examples
Skin: Propionibacterium spp.
Mouth: Porphyromonas gingivalis
Vagina: Lactobacillus spp.; Prevotella bivia
Colon: Bacteroides fragilis
Gram+ Anaerobes: Sporeforming rods (1)
Clostridium spp.
Gram+ Anaerobes:
Non-sporeforming rods (1)
Actinomyces spp.
Gram+ Anaerobes:
Cocci (2)
Peptococcus spp.
Peptostreptococcus spp.
Gram- Anaerobes:
Non-sporeforming rods (2)
Bacteriods-like group
Fusobacterium spp.
Gram- Anaerobes:
Cocci (1)
Veillonella spp.
Predisposing factors / medical conditions that can lead to anaerobic infections
Low tissue redox conditions from circulatory problems
Antibiotic therapy: can lead to Clostridium difficile infection
Bite wounds
Aspiration of mouth flora into the lungs
Spillage of intestinal contents into the peritoneal cavity due to perforation of the
GI tract
Bacteriodes-like Bacteria:
Characteristics & Entry
Gram- anaerobic rods
Found as normal flora in colon, vagina, & mouth
Cause endogenous infections (breeching epithelium via surgery, wounds, ruptures, etc.)
Need low tissue redox for growth
Not contagious
Bacteriodes-like Bacteria:
Virulence Factors
LPS is not endotoxic
Makes a capsule (antiphagocytic)
Some make an enterotoxin but not exotoxins
Not all are equally pathogenic (B. fragilis is most likely to cause disease)
Bacteriodes-like Bacteria:
Diseases & Treatment
Disease near their normal flora colonization body site:
- Bacteroides fragilis: colon, abdominal infections that involve two stages: peritonitis and then (if untreated) abscess formation.
- Prevotella bivia: female genital tract, pelvic inflammatory disease and infertility.
- Prevotella melaninogenica and Porphyromonas gingivalis: mouth, respiratory tract infections (e.g., dental, chronic sinusitis, pulmonary infections).
Inflammation and abscess formation.
Can enter the bloodstream and cause bacteremia. Can be rapidly fatal.
Treatment for abscesses: surgical drainage and use of antibiotics effective against anaerobes (e.g., metronidazole).
Histotoxic Clostridia:
Species & Characteristics
C. perfringens: most important
C. septicum: infections in pts w/ cancer of GI tract
Gram+ anaerobic spore-forming rods
Some normal GI flora, some in soil, some in both
Histotoxic Clostridia:
Virulence Factors
Exotoxins: protein toxins made and secreted outside the cell.
α toxin: most important single toxin for C. perfringens, a lecithinase (phospholipase C) that disrupts mammalian plasma membranes.
Histotoxic Clostridia:
Entry
Both endogenous & exogenous infections
1) Entry of cells or spores into wounds, ruptures, tumors, etc.
2) Need low tissue redox for growth
Histotoxic Clostridia:
Diseases
- Wound infections
- Gas gangrene (clostridial myonecrosis): bacteria are present in muscle (low redox condition); toxins kill cells to form necrotic tissue (low redox), allowing progressive spread of infection. Toxins also enter circulation and damage distant organs. Rapidly fatal w/o treatment (if possible).
- Anaerobic cellulitis: somewhat less serious, involves only cutaneous and subcutaneous tissue.
- Simple wound infections: least serious, only involves cutaneous tissue. - Organ infections: e.g., uterine infections
- Septicemia/bacteremias
- C. perfringens type A food poisoning: 3rd most common food poisoning; not usually life-threatening, an enterotoxin causes diarrhea and abdominal cramps.
Histotoxic Clostridia:
Treatment
Very difficult for gas gangrene
i) Removal of affected tissue by surgery (must be performed early in infection), often involves amputation. Antibiotics are in a supportive role.
ii) Hyperbaric O2 (?)
iii) Prevention: keep wounds clean.
Bacterial Endospores:
Formed by…
Sporulation
Resistant to…
Sterilization
Why resistant
Bacillus (aerobic or facultative anaerobic) and Clostridium (anaerobic)
Survival response to a poor environment, not a reproductive strategy
- Heat
- Radiation
- Chemicals (some disinfectants)
- Drying
Aimed at killing bacterial endospores to kill all other life forms
- Low internal water
- Rich in DNA stabilizing proteins
- Lack metabolism (“suspended animation”)
Diseases that can be transmitted by bacterial endospores
Anthrax: Bacillus anthracis
Tetanus: Clostridium tetani
Infections caused by the histotoxic clostridia (C. perfringens)
Clostridium difficile
Tetanus (C. tetani):
Characteristics
Virulence Factors
Entry
Gram+ spore-forming anaerobic rod found in soils
Tetanus neurotoxin inhibits release of inhibitory NTs –> uncontrolled muscle contractions –> lethal
- Implantation of spores at a wound or during childbirth
- Spore germinates into a new cell & makes tetanus toxin
- Long incubation time while waiting for proper conditions to grow & germinate
Tetanus (C. tetani):
Disease
Treatment
Prevention
1) Toxin produced in wound and then migrates along nerves into CNS
2) Toxin blocks inhibitory neurotransmitter release; muscles are constantly stimulated.
3) Convulsive muscle contractions of voluntary muscles of the jaw (trismus, lockjaw), back, extremities.
4) Death results from respiratory failure.
Treatment: very difficult, give tetanus antitoxin, muscle relaxants and assisted ventilation.
Prevention: The disease itself does not confer immunity but immunization can be induced using tetanus toxoid.
