Chapter 4

Prokaryotic Profiles

 

Prokaryotic cells (Prokaryotes)

Simplest cells

Lack a true nucleus and membrane-bound organelles

Usually have a rigid cell wall and all are single-celled

Include all members of Domains Archaea & Bacteria

 

Flagella

Functions in movement of the bacterium

Spins like a propeller

Made up of 3 components:

Filament

  Whip-like, helical structure made up of flagellin

Hook

  Curved structure that connects the filament to the cell surface

Basal body

  A stack of rings within the cell wall and membrane

  Anchors the hook in place but allows it to swivel

4 types of flagellar arrangements:

Monotrichous - a single flagellum

Lophotrichous - multiple flagella at one end

Amphitrichous - flagella located at each end

Peritrichous - flagella found randomly over the cell surface

Chemotaxis - movement of the cell in response to chemical signals

Positive chemotaxis – towards

Negative chemotaxis – away from

Run - movement in a linear direction

Tumble - when the cell stops and reverses direction

 

Pilus

Elongated, tubular structure involved in conjugation

The partial transfer of DNA from one cell to another

 

Fimbriae

Small, hair-like fibers that stick to each other and to other surfaces

 

Glycocalyx

Gel-like outer covering of some bacteria

Slime layer - diffuse & irregular structure

Capsule - distinct & gelatinous structure

Protects against host immune cells

Encapsulated bacteria have a greater pathogenicity because of this

Help adhere bacteria to its environment or other bacteria

Allow bacteria to stick to a large number of substances including tooth enamel and hospital equipment

 

Cell Wall

Provides the bacteria with structure and protection from lysis

Lies immediately below the glycocalyx

Composed primarily of peptidoglycan

composed of 2 repeating subunits bonded together to form a glycan chain:

  N-acetylmuramic acid (NAM)

  N-acetylglucosamine (NAG)

adjacent glycan chains are held together by tetrapeptide chains attached to each NAM

Penicillin prevents crosslinking of glycan chains

Lysozyme breaks covalent bonds between NAG and NAM

Gram staining categorizes bacteria based on the staining of their cell walls

 

 

Gram-positive Bacteria

Have a thick layer of peptidoglycan in cell wall

less susceptible to lysis

2 molecules (besides peptidoglycan) are commonly found:

teichoic acid – binds together layers of peptidoglycan

lipoteichoic acid – link the peptidoglycan layers to the cell membrane

both bind to NAM and extend above the cell wall to give the cell a negative charge

 

Gram-negative Bacteria

Have a thin layer of peptidoglycan in cell wall

More susceptible to lysis

Surrounded by an outer membrane:

lipid bilayer structure similar to cytoplasmic membrane

outer layer of lipopolysaccharides (LPS)

LPS has 2 important components:

Lipid A (endotoxin) – anchors LPS to lipid bilayer; recognized by our immune systems

O-specific polysaccharide – found externally; used to identify certain strains of bacteria (E. coli O157:H7)

protein porins span LPS and regulate which substances move through the membrane

 

Cytoplasmic Membrane

Composed of phospholipid bilayer embedded with proteins

Important site for metabolic activities

Functions in the transport of substances into and out of the cell and in secretion of metabolic products

 

Chromosome

A circular, double-stranded DNA molecule that is tightly coiled

Contains all genetic information for cells

Aggregated in a dense area of the cell known as the nucleoid

 

Plasmids

Extra nonessential pieces of circular, double-stranded DNA

Are reproduced and passed on to offspring or exchanged during conjugation

Often contain protective traits (antibiotic resistance and toxins)

 

Ribosomes (70S)

Composed of rRNA and protein

Small subunit (30S)

Large subunit (50S)

The site of protein production within the cell

Facilitate the joining of amino acids

 

Storage Granules

Aggregations of nutrients and other substances needed by the cell

Allows the cell to go for long periods in the absence of essential nutrients

 

Endospores (Spores)

Dormant bodies produced primarily by 3 groups of Gram-positive bacteria:

o    Bacillus, Clostridium & Sporosarcina

Function in the survival of the bacteria in hostile conditions

Can survive extreme heat, drying, freezing, and radiation

Vegetative cell - a metabolically active and growing cell

Endospore - dormant body capable of becoming a vegetative cell

Sporulation – process of spore formation

Depletion of nutrients causes the vegetative cell to convert to a spore-forming cell (sporangium)

The chromosome is duplicated and the cell separates into a sporangium and forespore which has minimum machinery to maintain life

The sporangium engulfs the forespore and a thick coat forms around the forespore making it resistant to extreme conditions; it is now an endospore

The endospore is released as the sporangium deteriorates and remains dormant until conditions improve around it

Germination of endospores requires water and an environmental stimulus

 

Prokaryotic Shapes

Coccus – round

Bacillus – rod

Coccobacillus - short rod

Vibrio - curved rod

Spirillum – spiral

Pleomorphic - vary in shape and size depending on environment

 

Classification of Prokaryotes:

A Brief look at each Domain

 

Bacteria

Most prokaryotes fall in this category

Have a rigid cell wall composed mainly of peptidoglycan

Reproduce via binary fission

Many move via flagella

 

Archaea

Similar in size, shape, and appearance to Bacteria

Also carry out binary fission and move via flagella

Main difference is that cell walls lack peptidoglycan

Many live in extreme environmental conditions

 

Eukarya

Include every other living organism except prokaryotes

In Microbiology includes algae, fungi, and protozoa

 

 

Classification of Medically Important Bacteria

 

Gram-negative Bacilli

Brucella

Bordetella

Francisella

Pseudomonas

resistant to many disinfectants and anti-microbials

Legionella

commonly found within protozoa in water

Haemophilus

Bacteroides

Fusobacterium

Obligate intracellular parasites - unable to reproduce outside a host cell

Chlamydia

Rickettsia

Coxiella

Bartonella

Family Enterobacteriaceae - reside in the intestinal tract (coliforms)

Escherichia

Salmonella

Shigella

Yersinia

Klebsiella

Proteus

 

Gram-negative curved bacilli (vibrio)

Campylobacter

Helicobacter

Vibrio

 

Gram-Negative Cocci

Neisseria

 

Gram-Positive Bacilli

Spore-forming

Bacillus

  Common inhabitant of soil

  Potential bioterrism agent

Clostridium

  obligate anaerobe

  common inhabitant of soil

Non-spore forming

Bifidobacterium

Propionibacterium

Nocardia

Actinomyces

Corynebacterium

Lactobacillus

 

Gram-positive Cocci

Staphylococcus

Thrive in salty environments

Catalase positive      

Streptococcus

catalase negative

 

Acid-fast Bacilli

Mycobacterium 

Gram stain poorly because of waxy lipid in their cell wall (mycolic acid)

resistant to disinfectants

 

Cell Wall-less

Mycoplasma

cell membrane is stabilized by sterols and resistant to lysis

 

Spirochetes

Gram-negative and move by means of an axial filament

Treponema

Borrelia

Leptospira