Structures of the Bacterial Cell Wall

Bacteria

Bacteria grow under all types of conditions ranging from the cold of the arctic (where they produce anti-freeze liquids) to the warmth of the human gut

bacteria compose most of the biomass on earth

During the first 9 months the baby is sterile. As soon as the head emerges from the birth canal the baby gets smeared with bacteria. From this point on the immune system primes itself for a life long battle.

Amounts: (bacteria=bug)

dry skin has ~10³ bugs/cm²

wet skin (armpits, groin) has ~10⁷ bugs/cm²

Saliva has ~10⁹ bugs/mL

Dental Plaque has ~10¹¹ bugs/gram

Stomach has ~100 bugs/mL

Upper Small Intestine has ~10³ bugs/cm²

Lower Small Intestine has ~10⁸ bugs/cm²

Lower Large Intestine has ~10¹² bugs/cm²

Total bacteria in a human = ~10¹⁴

Total cells in a human (non-bugs) = ~10¹³ ß They are winning

Bacteria vs. Viruses

Bacteria	Viruses
Have degrees of self-sufficiency	Are host cell dependent
Contain both DNA and RNA.	Infectious particle contains either DNA or RNA.
Enclosed by a cytoplasmic membrane and walls of varying complexity.	Enclosed in a protein coat (capsid) of identical subunits or a membrane layer obtained from the host cell.
Many proteins for biosynthesis and metabolism.	Few proteins, no metabolism
Usually can generate ATP.	Cannot generate ATP.
Maintain integrity of cell during division.	Integrity of infectious particle lost during replication.

Differences between Prokaryotes and Eukaryotes

PROKARYOTES (Bacteria and Blue-green algae)	EUKARYOTES (Protozoa, Fungi, Algae, Higher organisms) (Higher?)
No nuclear membrane, no meiosis, one chromosome.	True nuclear membrane >1 chromosome, meiosis and mitosis occur.
No mitochondria, respiration and electron transport in cytoplasmic membrane.	Mitochondria present.
Ribosomes 70s.	Ribosomes 80s (70s in mitochondria)
No apparent organelles, no golgi, no ER, no lysosomes, no chloroplasts, no glyoxosomes, no microtubules, no nucleolus	Lots of organelles, chloroplasts only in plants, glyoxosomes not always present
Cell wall complex peptidoglycan except mycoplasma and archaebacter	Cell walls, if present are homopolymers, chitin, mannan cellulose etc.
No phagocytosis	Some are phagocytic
Genetic exchange by conjugation is plasmid mediated, fragmentary, and unidirectional.	Genetic exchange by gamete fusion, meiosis occurs.
Flagella (if present) are single chain of homologous protein molecules.	Flagella are 20 fibril sheathed, organized structures.
No ameboid motion, no cytoplasmic streaming.	Some have ameboid motion, some stream.

Characteristics of Bacteria

(1) Size and Shape

Bugs have a huge surface to volume ratio, and must therefore maintain a high rate of metabolism which produces heat. Though some bugs grow aerobically, most prefer to grow anaerobically.

Major Shapes:

Spherical (Cocci)
Rods (Bacilli)
Short Rods (Coccobacilli)
Rods with tapered ends (Fusiform)
Short rods that are curved/bent (Vibrios)
Spiral-shaped bacteria (Spirilla if hard, Spirochete if flexible)
Comma shaped (Vibrio)

Major Arrangements:

Diplococci – pairs

Streptococci – chains

Sarcinae – cubical arrays

Staphylococci – irregular clusters

(2) Nucleus

don’t have one, but chromatin material is kept in one area of a cell
supercoiled with histone-like proteins
most bacteria have one circular chromosome, though others have multiple or straight strands.

The chromosome is haploid.
Bugs sequester divalent cations (Mg²⁺, Ca²⁺, etc.) to protect against radiation.

(3) Cytoplasm

no golgi, no mitochondria, no vacuoles. No visible internal structure, though there may be some responsible for division.
ribosomes – 70s in bugs, 80s in humans.
Bugs stash energy as crystals in the cytoplasm.

(4) Cytoplasmic Membrane

The bacterial membrane does similar functions to that of the mitochondria. It forces protons out using the energy to make ATP. It also does transport.
The membrane invaginates inwards in order to increase the surface area of the bug. Such an invagination is called a mesosome.
The membrane also maintains pressure, pH, and osmolarity.
It can sense temperature changes and external nutrient concentrations.
The bug can use the membrane for transport purposes (i.e. the bug can move around).
The membrane has such powerful active transport pumps that it can concentrate a solute up to 10,000 times its extracellular concentration.
bimolecular leaflet (50:50 lipid:protein); no cholesterol (except mycoplasma)
site of electron transport and all nutrient transport (group translocation, membrane integral transport processes, periplasmic binding protein-dependent transport processes)

The Cell Wall

prevents osmotic lysis, determines shape, protects against toxins.

Gram-positive vs. Gram-negative bugs:

Put bacteria on slide, warm bottom to attach bugs to surface, put crystal violet stain, rinse, put potassium iodide stain, rinse, decolorize

Gram-positive bugs: blue to purple
Gram-negative bugs: clear to pink
Why? because the gram-positive bugs will absorb the blue dye into their murein (peptidoglycan) layer, while the gram-negative bugs will not due to their 2^nd outer membrane.

Gram Positive vs. Gram Negative Cell Walls

Peptidoglycans

3D mesh of NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid)

A set of 2 enzymes sets up this 3D mesh around the bug
Antibiotics are aimed at disrupting this process
If you do this gently (i.e. break the wall in only one spot) you will get ghosts, not guts just wall
Stationary phase cultures (i.e. non-growing bugs) are not affected by the above mentioned wall antibiotics because they only work on replicating bacteria.

Teichoic Acid – found in the wall of Gram-positive bugs

polymers of either glycerol phosphate or ribitol phosphate, with various sugars, amino sugars, and amino acids as substituents
purpose is to create a negative ‘cloud’ which attracts divalent cations
recall that these cations protect the DNA against radiation damage

Lipopolysaccharide – found in the outer wall of Gram-negative bugs

Lipopolysaccharide (LPS) is extremely toxic to humans and other animals and is called endotoxin; even in minute amounts, such as the amount released to the circulation during the course of a gram-negative infection, this substance can produce a fever and shock syndrome called Gram-negative shock, or endotoxic shock

LPS consists of a toxic Lipid A (a phospholipid containing glucosamine rather than glycerol), a core polysaccharide, and O antigen polysaccharide side chains. Though LPS provides outstanding protection for the bug, it blocks the normal passage for nutrient uptake. The provisions the bug makes for this consist of active transport mechanisms and porins on the outer membrane.

lipopolysacchraide Þ permeability barrier Þ released when bug dies Þ induces host immune response

S Layer – Crystalline layer on top of either gram-positive or gram-negative bugs. The S Layer may increase virulence, and it may also help in adhesion. This is not seen on every bacteria, and it is still being researched. Gram negative = above outer membrane; gram positive = above murein layer

(1) Capsules (polysaccharides or polypeptides)

Beyond the S Layer the bug secretes a hydrophobic gel which:

(1) protect it from drying
(2) protect it from phagocytosis (macrophages think it’s snot)
(3) adhesion - aid in colonization by assisting bug to attach to surfaces

(2) Flagella (only rods, antigenic, movement – chemotaxis)

Bugs can walk!!!
Flagella can be one or several hair like strands. They use the energy from the proton pumps to spin around and move the cell (run). When the bug wants to stop the flagella spread out in a star-like formation (twiddle).
Salmonella can change antigenic features in their tail. This change occurs once in ~10,000 generations and protects the bug from being discovered by the host system. The mechanism for this involves the bug flipping a segment of its DNA so transcription effectively reverses direction and the antigen product is different.

(3) Pilli (fimbria)

Tiny hair-like strands sticking out of the bug membrane. Functions: adhesion and conjugation.