Bacterial Structure and Function 2

Question 1

Flagella

Answer 1

• long polymers of flagellin that protrude from cell wall: a bacterium may have one or several polar flagella at one end or be covered in them
• corkscrew shaped
• rotate to propel bacterium forward (eukayotes do wavelike motion)
• bacterial flagella are not whiplike

Question 2

Ribosomes

Answer 2

• 70S rather than eukaryotic 80S
• “S” is Svedberg a unit of sedimentation rate in a cesium chloride density gradient under ultracentrifugation
• chemical differences between prokaryotes and eukaryotic ribosomes make them a great drug target
• > 50% of all antibiotics target bacterial ribosomes
• aminoglycosides attack small subint, interfere with initiation and accuracy
• tetracyclines also small subunit interfere with the transfer of tRNA and aa to ribosome
• macrolides attack rRNA of large subunit, elongation

Question 3

Spores

Answer 3

• spore-forming bacteria, including Bacillus and Clostridium, can survive extreme environmental conditions by assuming a very rugged dormant form
• spore formation is triggered by nutrient depletion
• the bacterial genome and a minimal entourage of macromolecules are compressed into a thick spherical coat that can survive high temperatures, dehydration, antiseptics, antibiotics
• when nutrients and water are again plentiful, spore “unpacks” into normal bacterial form, normal metabolism and reproduction resume
• thats why we autoclave

Question 4

Binary fission

Answer 4

• produces logarithmic growth under optimal environmental conditions
• bacterial cells are individual organisms and do not usually limit their growth by mechanisms like contact inhibition
• bacterial released from a low nutrient environment into a human body or laboratory culture will produce very large numbers of progeny in a very short time
• bacterial generation time is limited primarily by the available nutrients. Other factors include the bacterial species, the temperature, and any toxic factors in the environment (high pH, antibiotics)

Question 5

Growth curve of bacterial growth

Answer 5

• Lag phase: bacteria sense new environment and upregulate their gene products for metabolism and growth (liquid culture appears clear)
• Log phase: rapid cell division with an experimental growth curve (culture appears lightly cloudy)
• Stationary phase: nutrient depletion and waste accumulation force growth rate down to match death rate (culture appears very cloudy)
• death phase: waste accumulation kills off most remaining bacteria (culture looks like garbage- sediment on the bottom, pellicle on top, middle lightly cloudy with chunks)

Question 6

Obligate aerobes

Answer 6

• some bacteria, like mammalian cells, rely on oxygen for their electron transport pathway for ATP generation
• these bacteria either cannot grow or quickly die without oxygen
• example: M. tuberculosis is restricted to oxygen-rich portions of the body

Question 7

Obligate anaerobes

Answer 7

• some bacteria lack superoxide dismutase and/or catalase enzymes and also lack pathways to regenerate enzymes that have become oxidized; they use fermentation pathways rather than electron transport to generate ATP
• these bacteria cannot grow or quickly die in the present of oxygen
• example: botulism is usually contracted from eating foods that were sealed after being inadequately cooked, like home-canned vegetables, because C botulinum grew in the anaerobic environment of the Mason jar

Question 8

Facultative anaerobes

Answer 8

• some bacteria encode both fermentation pathways and the more-efficient electron-transport pathways to generate ATP, so that they can use oxygen when it’s available and fermentation when it isn’t
• these bacteria often grow faster in the presence of oxygen, but continue to grow without it
• example: E. coli can grow under a very wide range of laboratory conditions

Question 9

Sugar metabolism

Answer 9

• facultatives and anaerobes each have a repertoire of sugars that they can break down for ATP in the absence of oxygen: Fermentation
• fermentation is not as efficient as electron transport for generating energy but suffices for the growth of many bacteria
• organic acids are produced as waste products (usually lactic and/or pyruvic acid)
• in nature the acids may be further broken down by other microorganisms living nearby
• in lab culture, the acids can be detected with pH indicators in the media revealing the ability of the bacterium to ferment the provided sugar and narrowing the differential
• example: a rod-shaped Gram- enteric pathogen that ferments lactose is not shigella