Bacterial Structure and Function 1: Structure

Question 1

How small are bacteria

Answer 1

• plant 10^-4
• animal 10^-5
• bacteria 10^-6, 1 um
• need to use at least a light microscope, and then electron microscope

Question 2

Structural characteristics of bacteria

Answer 2

• genome: DNA, no introns, single circular chromosome, nucleoid- like a eukaroytic nucleus, concentrates the genome in one intracellular position, unlike a eukayotic nucleus, no membrane-bound compartment
• 70S ribosomes
• peptideoglycan cell wall
• reproduction by binary fission

Question 3

Microscopic Appearance

Answer 3

• 10-100X smaller than eukaryotes
• 500-1000X larger than viruses
• visible by light microscopy with phase contrast or staining

Question 4

Bacterial shape

Answer 4

• cocci (round): staph, strep, neisseria
• bacilli (oval): bacillus, salmonella
• spirochetes (corkscrew): Borrelia, Treponema

Some bacteria have characteristic clumping patterns:

1) Cocci in clumps: staph
2) Cocci in chains: strep
3) Diplococci: Neisseria

Question 5

Gram- stain procedure

Answer 5

1) Fix bacteria to microscope slide with heat
2) Stain with crystal violet (all bacteria turn purple)
3) Iodine treatment (Gram (+)s become permanently purple)
4) Decolorize with alcohol (leaches purple from Gram(-)s)
5) Counterstain (safrinin re-stains Gram(-)s so they’re visible)

Question 6

Basis of gram-stain

Answer 6

• Gram+ and Gram- cell walls are significantly differnet
• Gram+ has 3 layers of peptidoglycan, no exterior membrane
• Gram- has 1 layer of peptidoglycan and does have an exterior membrane
• other differences include LPS in Gram- and more frequent genetic transfer by plasmid exchange among Gram-: creates major problem with antibiotic resistance

Question 7

Acid-fast (mycobacteria)

Answer 7

• third category of cell wall type
• they do not stain Gram + but are structurally different from other Gram -
• mycloic acid layer of Acid-fast bacteria resists both CV and counterstain
• includes mycobacterium tuberculosis

Question 8

Clinical significance of gram-stain

Answer 8

• fast and cheap: THE most commonly ordered lab test
• cuts the differential in half by quickly eliminating a large category of candidate pathogens
• can occasionally make the diagnosis: gram - diplocci in cervical smear= gonorrhea

Question 9

LPS

Answer 9

• component of Gram- cell walls
• have O-antigen segment on the outside helps with lab ID
• have Lipid A near the outer membrane which is toxic
• can cause septic shock either during sepsis or after antibiotic treatment when LPS is released from dead bacteria into blood
• teichoic acids are the nearest equivalent in Gram+ much less likely to cause shock

Question 10

Glycocalyx

Answer 10

• slime layer: loose coating of polysaccharide helps bacteria attach to host cells, form biofilms (hard to clean, specially since antibiotic resistance can be easily shared)
• capsule: firm enclosure helps with attachment, resists phagocytosis, can be clinically useful for serologic testing or vaccine target, often a virulence factor

Question 11

Pili/Fimbrae

Answer 11

• used for attachment, often a virulence factor
• gram+ pili are recently-discovered covalently linked pili subunits
• gram- pili come in many types, incorporate many different molecules, some have evolved into propulsion systems or secretion systems that are virulence factors in their own right
• Type IV pili- locomotion
• Type III and IV secretion pili