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
