Structure and Function of Bacterial Cells Flashcards
Bacterium/Animal cell comparison
➢ Eukaryotic cells are, generally, 10,000 to 100,000 times the size of their Bacterial and Archaeal counterparts
(compare boxed cell to animal cell on right).
➢ There are a number of differences between the cell types
Bacteria/Eukaryote comparison
Bacteria
-diameter < 2 μm
Eukaryotes
-diameter 10 to > 100 μm
Nuclear structure and function
Bacteria
Nucleoid region, lacks nuclear membrane
and nucleoli
Eukaryote
Nucleus, Nucleolus
Nuclear membrane
DNA
Bacteria
Usually single circular double stranded
chromosome, complexed with histone-like
proteins
Eukaryotes
One or more linear
chromosomes, complexed with histones
Cell division
Bacteria
Binary fission and budding, no mitosis Mitosis
Sexual Reproduction Irregular, fragmentary process, no meiosis Regular, meiosis, whole
chromosome
Location of genes Functionally related genes usually
clustered
Functionally related genes not
clustered
Archaea characteristics are very similar to bacteria. However…..
➢ Archaea are never photosynthetic,
➢ Do not have peptidoglycan,
➢ Do not form endospores
Classification and study of Bacteria
➢ How are the relationships between bacteria studied?
By comparing 16S rRNA sequences
➢ Useful characteristics include:
➢ Nature of the cell wall and membranes.
➢ Cell morphology.
➢ Cell motility.
➢ Growth characteristics.
➢ Cell structures.
➢ However, traditional methods examining phenotypic characteristics are
extremely important in understanding bacterial form and function.
Importance of bacterial cell walls and membranes of Life Forms
➢Obviously, a barrier is required between the internal and external environment.
➢The nature of this barrier has important functions and consequences. Such as…
➢Transport of nutrients, wastes etc,
➢Regulation of interactions with the environment.
➢As a target for antibiotic therapy.
➢Classification of bacteria.
➢The external “face” is antigenic and is recognised by the immune system.
(What is an antigen? An antigen is a molecule capable of inducing an immune
response in the host organism
The GRAM stain
➢ Named after its inventor, the Danish scientist
Hans Christian Gram (1853–1938).
➢ Developed while working in the morgue of
Berlin City Hospital in 1884 to make bacteria
more visible in stained sections of lung tissue.
➢ First report demonstrated that the typhus
bacillus, Rickettsia prowazekii, did not retain the
stain.
➢ Has become a key step in classification of
Bacteria.
Gram stain method outline
Step 1: Fixation
Cells fixed to
microscope slide using
mild heat
Step 2: Flood with Crystal violet stain
Penetrates cell wall and cell membrane.
Cells stained a dark blue colour (excess washed off)
Step 3: Flood with Iodine
Iodine complexes with crystal violet
Washed off the excess iodine
Step 4:
Decolourisation
with alcohol
* Alcohol dehydrates
the peptidoglycan
layer, shrinking and
tightening it
Gram positive: The large crystal violet-iodine complex is not able to penetrate this tightened thick peptidoglycan layer and is thus trapped in the cell.
The outer membrane of Gram-negative bacteria is degraded, and
the thinner peptidoglycan layer of Gram-negative cells is unable to retain the crystal violet-iodine
complex and the colour is lost.
Step 5:
Counterstain with
Safranin
Stains clear cells pink
(wash off the excess dye)
Gram positive: will stay
purple
Gram negative: will turn
pink.
Peptidoglycan structural unit
➢ Peptidoglycan (murein) is made of units
of sugars and amino acids.
➢ The two sugar types are:
* N-acetylmuramic acid (MurNAc or
NAM)
* N-acetylglucosamine (GlcNAc or
NAG)
➢ NAM and NAG are bound to each other
with a -(1,4) glycosidic bond.
➢ Each unit is made of a NAM, NAG and a
tetra/pentapeptide
he presence of D-amino acids.
The amino acid at position 4 is always D-Ala.
DAP = diaminopimelic acid
Variations in tetra/penta-peptide and bridge
provides a high variation in structure.
More than 100 types of peptidoglycan are
known.
➢ DAP is replaced by L-lysine in Gram positive bacteria.
Peptidoglycan structure
➢ Peptidoglycan subunits form a
mesh like polymer, composed of
many subunits.
➢ To form this polymer structure,
subunits are linked through ‘cross- linking’ between the peptides
Peptidoglycan cross-linking
➢ Formation of interpeptide
bridges (between
tetra/pentapeptides) is
catalysed by a transpeptidase.
➢ This enzyme is irreversibly
inhibited by penicillin
antibiotics (e.g. amoxicillin)
Penicillin structure
➢ Penicillin and related antibiotics are called ‘beta-lactams’. They all contain a 4-
member β-lactam ring.
➢ This ring mimics the shape of the terminal D-Ala that serves as the substrate for
cell wall transpeptidases that form covalent bonds between different
peptidoglycan chains during cell wall synthesis.
➢ Binding of penicillin to the enzyme inhibits its activity. Transpeptidase catalyses
ring opening and the formation of an inactive complex → no further
peptidoglycan cross-linking.
Lipopolysaccharide
➢ Abundant in Gram-negative bacteria.
➢ Very rare in Gram positive.
➢ Composed of variety of sugars.
Lipopolysaccharide structure
O antigen – terminal sugars are called ‘O-antigen’
➢ This a repetitive glycan polymer (includes unusual sugars, often branched).
➢ O antigen is exposed on the very outer surface of the bacterial cell.
➢ It acts as a target for recognition by host antibodies.
➢ In E. coli there are over 170 different O antigen structures.
➢ Full-length O antigens give colonies a smooth appearance, reduced O chains make
colonies appear rough.
Core
➢ Oligosaccharide component that attaches directly to lipid A.
➢ Contains unusual sugars which may also be linked to non-carbohydrate
components.
Lipid A (endotoxin)
➢ A phosphorylated glucosamine disaccharide attached to multiple fatty acids.
➢ Lipid A domain is responsible for much of the toxicity of Gram-negative bacteria.
➢ Cells lysed by the immune system release fragments containing lipid A into
circulation.
➢ Causes fever, diarrhea, and possible fatal endotoxic shock
Porins
➢ Porins in the outer membrane are
channels allowing entrance and exit
of low molecular weight
substances.
➢ Include both specific and
nonspecific classes
The Periplasm
➢ Periplasm is the space between the
cytoplasmic membrane and outer
membrane.
➢ Stops proteins, enzymes and other
large molecules from diffusing away.
➢ Contents include:
* Hydrolytic enzymes for initial degradation of
large food molecules.
- Binding proteins involved in substrate
transport. - Chemoreceptor proteins involved in
chemotactic response
S-layers – Bacterial Armour
Ø Found in Bacteria and Archaea.
Ø Composed of crystalline arrays of
glycosylated proteins.
Ø Functions not entirely understood but
include:
* Protection: acts as a selective sieve
* Adhesion
* Maintenance of cell integrity
* Interactions with host and immune
system in pathogens
* Enzyme display
Glycocalyx (sugar coat)
Ø It is a polysaccharide-rich envelope that surrounds a cell -
outermost layer.
Ø In bacteria it exists as a capsule or a slime layer.
Ø Also present in Archaea.
Ø Capsule: arranged as a thin, rigid layer and is firmly attached to the cell wall.
Ø Slime: arranged as a thick, loose layer and is more loosely attached.
Ø Not all bacteria have capsules.
Ø A capsule can be found in Gram-positive and Gram-negative bacteria
Ø Functions include:
Attachment to solid surfaces, including to host tissue.
Formation of biofilms.
Protection from desiccation, attack by viruses and phagocytic
organisms (including body defences).
Ø Antigenicity: Contains the K antigen.
Ø Not essential for growth.
Antigens of cell surface
IMPORTANT
LPS contains: O antigen
Capsule contains: K antigen
Fimbriae contain: F antigen
Flagella contain: H antigen
Teichoic Acids
Ø Present only in the cell wall of Gram-positive bacteria.
Ø Can be found in capsular layers of Gram-negative bacteria.
Ø Composed of polyglycerol- or polyribitol- phosphate.
Ø It is either attached to the peptidoglycan or
to lipids (lipoteichoic acid) in the
membrane.
Ø It is antigenic.
Ø It is negatively charged
Cocci
spherical- shaped
Bacillus
rod-shaped
spirillum and
spirochete
Spiral-shaped cells,
two types
Fimbriae
Ø Fimbriae is a filamentous structure composed of
protein.
Ø Fimbriae enable organisms to stick to surfaces,
including those of animal tissues.
Ø Fimbral antigens are referred to as F
antigens.
Ø Many classes of fimbriae/pili are known:
* Type IV fimbriae are involved in colonisation,
genetic exchange with host and twitching motility.
* Archaeal flagella share similarity with type IV
fimbriae, bacterial flagella are structurally and
evolutionarily distinct.
Pili
Ø Pili are structurally similar
to fimbriae - are
filamentous structures
composed of protein but
are typically longer.
Ø Pili are involved in the
process of conjugation
(bacterial sex)
