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 13
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
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- Summer '19