microbiology – alcamo lecture: bacterial structures
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Microbiology – Alcamo Lecture: Bacterial Structures
-Not all bacteria have all structures
Shapes of Bacteria• 3 Different Shapes:
Bacilli• Rod shaped• Most occur singly, but some form long
chains called Streptobacilli• Examples:
– Typhoid fever– Anthrax– Diptheria
Cocci• Usually sphere shaped but can be oval• Diplococci – cocci that remain in pairs
– Examples – Gonorrhea, Menengitis• Streptococci – cocci in chains
– Examples - Strep throat, Tooth decay• Staphylococcus – irregular grape-like
cluster of cells– Examples – Food poisoning, staph skin
infections
Spiral Bacteria• Vibrios – curved rods - look like a
comma– Example - Cholera
• Spirilla – corkscrew shape with flagella– Example – Rat Bite fever
• Spirochetes – corkscrew shape but no flagella– Example - syphilis
Flagella• Used by some bacteria to achieve
motion• Made of long rigid strands of protein
called flagellin• Protein strands are permanently coiled• Permits the flagellum to rotate and
propel the bacterium forward
Flagella• Complicated structure: Filament
attached to hook-like shaft which is inserted through cell wall and attached to cell membrane
Flagella
Monotrichous
Lophotrichous
Amphitrichous
Peritrichous
Axial Filament• Only spirochete• One flagella attached at both ends of
flexible spiral MO• Motion effected by cell spinning around
filament
Pili• Look like short flagella but have nothing
to do with motion• Very tiny protein “Hairs” that enable MO
to stick to surfaces - like “Velcro”
Pili• Pili aid in transfer of genetic material
between bacteria
• Pili anchor bacteria to surfaces like living tissue
• Can enhance MO’s disease effect
• Example - gonorrhea
Capsule• Some bacteria secrete a layer of
polysaccharides and proteins that stick to its surface
• Sticky and gelatinous• Serves as a buffer between the bacteria
and its environment– Protects bacteria against dehydration– Protects bacteria against host’s immune
system
Glycocalyx• Some bacteria produce a Slime Layer• Complex sugar, made inside cell wall secreted
as liquid, polymerizes to jelly like substance• Cavities – S. mutans attaches itself to teeth by
using the sugar a person eats – creates an acid that breaks down tooth enamel
Cell Wall• All bacteria have a cell wall except
mycoplasmas• Semi-rigid structure, protects, gives
shape• Amount of chemical “Peptidoglycan”
determines characteristics of cell wall• If a lot: Thick, G+ stain reaction, sensitive
to penicillin and lysozyme in tears, saliva, mucous
• If small: thinner, G- stain reaction, not sensitive to penicillin or lysozyme
• If MO is pathogenic, disease more difficult to cure
Cell Membrane• Boundary layer of the cell inside of cell wall• Contains Cytoplasm, controls molecular traffic
in and out of the cell• Triple layer structure, 60% proteins, 38% lipids
(phospholipid bilayer), 2% sugars• Antimicrobials (detergents, alcohol, some
antibiotics) dissolve cell membrane
Cytoplasm
Gelatinous mass of proteinscarbohydrateslipidsnucleic acidssaltsions water
Important Structures in Cytoplasm
• Ribosomes – protein synthesis• Inclusion Bodies – globules of starch
or lipids – store nutrients• Bacterial Chromosome – closed loop of
DNA without a membrane or proteins (nucleoid region)
• Plasmids – smaller, separate molecules of DNA – few genes but do give bacteria drug resistance (R genes)
Endospores• Some Gram + bacteria produce highly
resistant structures - spores– Bacteria grow, mature and reproduce as
vegetative cells– Then the bacterial chromosome replicates
and the cell membrane grows in to seal off a developing spore
– Next, thick layers of peptidoglycan form to protect the cell
– Finally, the cell wall of the vegetative cell disintegrates and the spore is released
Endospores• Very resistant to poor environmental
conditions:– Extreme temperatures – boiling water– Chemicals – 70% alcohol– Spores have even been recovered alive
from an Egyptian mummy’s intestines
• Examples of spore formers – anthrax, botulism, tetanus
• “Sporulation” is spore formation - DNA + some cytoplasm wrapped in spore case formed by cell membrane
• “Germination” occurs when good environmental conditions return – vegetative cell
Kingdom Classification
Archaeobacteria• Have existed on earth longer than any
other living organism• They are different from eubacteria:
– No peptidoglycan in cell wall– Different lipids in cell membrane– Different ribosomal RNA– Now archaeobacteria and eubacteria are
classified as different kingdoms
Archaeobacteria• 3 Types:
– Methanogens – rods that live in anaerobic conditions and produce methane gas – common in marshes and the guts of cows and humans
– Thermoacidophiles – resistance to acid and high temperatures – live in hot springs and ocean vents
– Extreme Halophiles – thrive in high salt environments (Great Salt Lake)
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