Oxygen level molecular oxygen determines microbial

concentration inside of the cell to prevent dehydration (they counteract the effect of high external solute conc.)
- Oxygen Level
Molecular oxygen determines microbial growth
Obligate Aerobe – high levels of O
2
(environmental levels); test tube A
Microaerophile – require some O
2
, but are killed by environmental levels; test tube D
Faculative Aerobe/Anaerobe – prefer O
2
, but don’t need it to grow; test tube C
Aerotolerant – grow equally well under all conditions; test tube E
Obligate Anaerobe – are killed by the presence of O
2
; test tube B
Oxygen has many risks – ROS = reactive oxygen species
Since O
2
is highly reactive, organisms need mechanisms to prevent it from reacting with its cell components
Anaerobes often lack mechanisms to detoxify substances such as superoxide (O
2
- ), hydrogen peroxide (H
2
O
2
),
& hydroxyl radical (OH * ) that can damage DNA, RNA, proteins, & lipids
How Cells deal with Oxygen:
- Carotenoids = substances that protect against ROS and can destroy & detoxify ROS products
- Enzymes can also detoxify ROS – superoxide dismutase (SOD) & catalase (converts H
2
O
2
to H
2
O and O
2
)
Aerobes generally contain these enzymes whereas anaerobes lack the
Many anaerobic habitats exist – i.e. in dental plaque, soils, sediments, etc.
 Are microbes always growing?
-There are many natural limits to microbial growth
-Nutrients/energy sources can be limited
-Sometimes environmental factors aren’t favorable
- Cell’s Response :
Cells can sense limitations and respond by acclimating/adapting to the changes, even if they aren’t growing
Cell’s can survive by stasis or differentiation; they can often persist a long time
However, sometimes the conditions are too extreme and cells die
 Cysts = differentiated cells with increased environmental survival and decreased activity
-This is one of the ways cells can respond to limitations
-Many microorganisms can differentiate into cysts structure, which is more resilient than vegetative cells
-i.e. Giardia can form vegetative cells (trophozoites) & cysts and it can differentiate bt the two depending on environment
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 Endospores = a cell’s response to starvation; they are a microbial dormant cell that is even better at surviving extreme
conditions than cysts
-Endospores
-Only some types of bacteria can produce endospores: (Firmicutes)
Bacillus – aerobic bacteria; don’t tend to cause disease; however, pathogens include B. anthracis & B. cereus
Clostridium – anaerobic bacteria; don’t tend to cause disease; however, pathogens include C. tetani (tetanus),
C. botiulinum (botulism), & C. difficile (GI infection)
- Endospore Formation : very complex process; takes a long time – about 10 hrs; it is DIFFERENT than binary fission
Starts with a vegetative cell that is limited nutritionally
Sporulation – cell elongates & divides into two asymmetrical cells; one is the mother cell and one is the forespore; the
cells do NOT completely separate – the mother engulfs the forespore; the forespore continues to grow within the
mother cell; eventually, the forespore survives and the mother cell disintegrates; the forespore develops many