Bacterial Structure and Function 1: Structure Flashcards Preview

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Flashcards in Bacterial Structure and Function 1: Structure Deck (11)

How small are bacteria

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


Structural characteristics of bacteria

-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


Microscopic Appearance

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


Bacterial shape

-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


Gram- stain procedure

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)


Basis of gram-stain

-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


Acid-fast (mycobacteria)

-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


Clinical significance of gram-stain

-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



-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



-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



-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