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-1Prokaryotic structure and function 1
Morphology, size and other features to distinguish
Size
Bacteria – 1-6!m length, 0.2-1.5!m wide/thick = prokaryote – no nuclear membrane,
mitochondria, golgi bodies, ER.
Eukaryotes – approx 20!m
Shape – Spherical (coccus), rod (bacillus, cylindrical), curved (spiral = spirillum, helical
= spirochete), square (halophile)
Spatial – single cells, chains, clusters
Bacteria –
Pleomorphism – existence of an organism in different morphological forms –
environmental factors influence size and shape.
Fungi ! Yeasts (unicellular) + Molds (filamentous form). Have no basic shape.
Eukaryotes – well defined nucleus, mitochondria, ER and golgi bodies.
Protozoa – variation due to flexibility of plasma membrane
Algae – various: plankton – photosynthesiser.
Composition of bacterial cells
Water – 70%
Dry weight – 30% ! DNA (3%), RNA (12%), Protein (70% = ribosomes, enzymes and
surface structures - = polysaccharides, lipid, phospholipids).
! All bacterial cells (except mycoplasma – suffix of plasma = no cell wall) have
Cell wall – Extracellular polymeric substance (EPS), pilus, mesosome.
Is outside cytoplasmic membrane. Almost all bacteria have a rigid layer of material that
surrounds the plasma membrane = CW. All bacteria (except mycoplasma – no cell wall,
and Archaea – unusual polysaccharide proteinaceous walls) havee a wall made up of 2
alternating sugars
! N-Acetylglucosamine and N-Acetylmuramic acid – joined together by Glycosidic
bonds – an attack site for many antibacteria’s
! Forms polysaccharides, disaccharides – joined together by short peptides (tetrapeptide
– attach to each N-acetylmuramic acid) into 3D framework called Peptidoglycan.
Peptide cross bridges – consisting of as many as 5 AA’s (pentapeptide), crosslink
tetrapeptides and hold the polysaccharides together.
Functions of cell wall – bacteria often grow in aqueous environments where
concentration of salts and sugars is much lower than inside cell – therefore water tends to
diffuse into the cell. CW prevents this by resisting excessive influx of water, water flows
into cell until osmotic pressure balanced by resistance of CW. CW – pretective and
structural function, allows passage of nutrients.
Cytoplasmic (plasma) membrane – Thin pliable lipid and protein envelope that defines
the cell – controls movement of small molecules in + out of cell – 7nm thick. Consists of
2 layers of phospholipids (lipid bilayer) in which proteins and glycoproteins embedded.
Proteins are free to move in the phospholipid. PL’s form the matrix of cytoplasmic
membrane, but proteins that carry out the functions. Some bacteria – respiratory enzymes
associated with the membrane. Has role in distribution of genetic information to daughter
cells. Genome binds to membrane – after genome replicates both copies attached to
-2membrane – membrane grows, a copy of genome foes to each end of cell. Daughter cells
have complete copy of genetic material after cell division.
Functions – barrier, nutrient transport, energy metabolism.
- Releases toxins, bacteriocins (affects other bacterium), enzymes produced in cytoplasm.
- Bacterial PM is bound to interior of cell wall but sometimes membrane appears to be
invaginated into cytoplasm forming a mesosome (used for DNA attachment during
replication or cell wall synthesis).
Mesosome – A coiled cytoplasmic membrane acts as an anchor to bind and pull apart
daughter chromosomes during cell division.
DNA – genome – hereditary material
Ribosomes – protein synthesis
Other structures – for selective advantage etc – only associated with particular bacteria
Endospores Capsules – around for protection
Thylakoids – membrane-enclosed compartment
Gas vesicles –
Reserve materials –
Purple bacteria and cyanobacteria – Photosynthetic membrane – thylakoids – continuous
with plasma membrane
Green bacteria – photosynthetic bacteria – chlorosomes
Bacterial flagella anchored in plasma membrane and powered by energy from energygenerating systems found in membrane – allow movement.
Most bacteria divided into 2 groups based on chemical components outside
peptidoglycan (PG) layer – a rigid mesh made up of repetitive linear polysaccharide
chains cross-linked by peptides. PG can be degraded with lyzozyme. Effect of
decolourising solution of alcohol on crystal violet-iodine complex – based on the ability
of cell wall to absorb crystal violet-iodine complex.
Gram positive – e.g. Staphylococcus. Multilayered PG 30-60nm thick surrounding
cytoplasmic membrane. Almost exclusively PG and attached teichoic acid polymers (for
cell viability) + lipoteichoic acids (surface antigens). No outer membrane. Stain purple.
Gram negative – e.g. E. Coli. Thin PG (2-3nm thick), no teichoic acid. Additional outer
membrane 7nm thick surrounding PG later – it is part of cell wall similar to plasma
membrane (e.g. lipid bilayer). Many have polysaccharide attached (e.g.
lipopolysaccharide – LPS). The outer membrane is a selectively permeable barrier but
small molecules pass through. When LPS fratction of outer membrane is toxic to animas
= endotoxin (powerful stimulator of innate + immune responses).
- Stain pink/red. The 2 membranes are separated by periplasmic space – containing a
variety of hydrolytic enzymes.
- Outer membrane functions – molecular sieve (barrier to certain molecules that pass
through Gram +ve bacteria – eg crystal violet, antibiotics, bile salts), endotoxin (lipid A
portion of LPS – acts as antigen), antigen (varying sugars), attachment for bacterial
viruses.
! relative impermeability to hydrophobic compounds (bile salts) may be reason why
many Gram –ve bacteria in gut, where bile salts are present, e.g. selective advantage.
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Structures outside the cell wall – Extracellular polymeric substances (EPS). Adjacent to
CW – these polymers provide certain advantages to cells that posses them
S-layers – Sub-unit surface layers – represent outermost structure of the cell. Protein or
glycoprotein linked to each other and CW – probably protective. E.g. Archaea and
Bacillus (shield against degradation).
Capsules and slime layers – Polymers surrounding CW (many bacteria).
Capsules – Loose polysaccharide or protein layers. Firmly adherent to CW with
structural integrity – slimy or sticky layers of polysaccharide or polypeptide. Influenced
by cultural conditions (e.g. require sucrose). Protective – promote attachment – some
require to cause disease. If no capsule, are ingested by macrohages, when encapsulated
resist digestion. The presence of capsule does not mean pathogenic.
- India ink – shows boundary/boarder of capsule
Slime layer – amorphous mass released by cell
Glycocalyx – polysaccharide fibres that extend from bacterial surface, allows bacteria to
attach to objects, other bacteria, plant or animal cells. Thick glycocalyx = capsule. Many
disease by bacteria are due to ability to synthesise glycocalyx.
Sheaths- stiff polysaccharide coverings that some bacteria secrete around a chain or
group of bacteria
Flagella – built from microtubules and dynein – in many protozoa. Movement not
essential but advantageous. Bacterial flagella are thin hairlike appendages 10-20!m in
diameter. Originate in plasma membrane.
- A rope-like propeller composed of helically coiled protein subunits (Flagellin) –
anchored in the bacterial membranes through hook and basal body structures – driven by
membrane potential. Rotates, pushes against water – propels bacterium. Flagellated
bacterium moves in one direction then tumbles, before moving off in another direction.
Moving towards nutrients – long runs, tumbles less
Moving away – more tumbling.
- Not seen under light microscope – require flagella stains to coat and thicken flagella.
Flagella suitable for movement in non-viscous liquids. Suffix Trichous – has flagella.
Axial filaments – Spirochetes have modified flagella – fibrils which form axial filaments
which coil around body of cell b/w PG layer and outer membrane, rotating filaments like
a corkscrew – not hampered by viscosity.
Gliding micro-organisms – no flagella, no axial filaments. Produce slime layer at forward
end of cell – contracts b/w cell and surface to propel forward.
OR – flagella stubs that act as paddle wheels (no organelles, no wriggling or contraction).
Pili or fimbriae – Long hollow tubes protruding peritrichously (from everywhere/all over)
from some bacteria. Made of Pilin, 0.02!m diameter, 0.2!m length. 6 different groups of
pili classified on adhesive, morphological properties. Mostly found on Gram-ve bacteria.
- Pili are antigenic – immune system can make antibodies.
Sex pili – Bind to other bacteria and are a tube for transfer of large segments of bacterial
chromosomes b/w bacteria. Involved in transfer of DNA from donor (‘male’) bacteria to
recipient (‘female’) bacteria – may also draw cells together. Contribute to adhesiveness
of bacteria. Adhesiveness determines Virulence (disease-causing ability) of some
bacteria. Some bacteria need to be palliated to cause disease
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- Pili and glycocalyx allow bacteria on surface of liquid medium to form thin films =
Pellicles – allows bacteria to float on surface where O2 is greatest.
Prosthecae – bacterial extensions that increase surface area to absorb nutrients for
attachment. A few bacteria form stalks from polysaccharide they secrete – become
encrusted with inorganic materials (e.g. Fe(OH)3) for attachment.
Infection diseases epidemiology
Epidemiology – The study of the distribution and determinants of health-related states or
events in specified populations, and the application of this study to control health
problems – embracing all diseases whether infectious or not
Sporadic – Occurs infrequently at irregular intervals. Isolated – eg typhoid fever
Endemic – Regular, low-level frequency, infection maintained within a population – eg
common cold. The habitual presence of a disease within a given geographic area.
Hyperendemic – Frequency increased above endemic level but not epidemic – eg winter
colds
Epidemic – A sudden increase above the expected level. The occurrence in a community
or region of a group of illnesses of similar nature, clearly in excess of normal expectancy
Pandemic – transcontinental spread (worldwide epidemic)
Index case – the 1st case
Outbreak – sudden increase in disease usually in a limited segment of population – eg
Legionnaires disease in Philadelphia
Zoonoses – animal disease which can be transmitted to humans
- All findings must related to a defined population
Target pop ! Study pop ! study sample (may possibly not reflect accurately represent
target population)
! oriented to groups rather than individuals, conclusions are based on comparisons,
epidemiologists measure rates and compare them
Case definition – should be precise and unambiguous
Sensitive – Lots of false +ve’s with the true +ve results – will get lots of results.
Sensitivity – The proportion of people who truly have the designated disorder who are so
identified by the test. The ability of the test to identify correctly those who have disease
Specific – Will get minimised false +ve’s but many true +ve’s with false –ve’s
Specificity – The proportion of people who truly are free of a designated disorder, who
are so identified by the test. Measure of the probability of correctly identifying a nondiseased person w a screening test. Ability of the test to correctly identify those who do
not have the disease.
Measuring disease frequency – Fractions/proportions
Numerator – number of individuals experiencing event
Denominator – population at risk
! Expressed as a % but usually per 100 000 population
Prevalence – The proportion of a population that are cases at a point in time
Incidence – number of new cases of disease for a particular time period (true rate)
Specific rates – eg gender specific rate – specifying a selected denominator.
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Classification of disease – ICD 10 = International classification of disease
Attributable risk – Disease rate in exposed – disease rate in unexposed
! e.g. rate of lung cancer in smokers – rate of lung cancer in non-smokers
- Shows proportion of the disease in exposed subjects due to exposure
Relative risk – ratio of disease rates in exposed Vs unexposed. (Risk in exposed/risk in
unexposed).
Confounding – An extraneous variable that correlates to both the in/dependent variables
Measurement error and bias – selection bias (for control or case), information bias,
measurement error (regression towards mean), analysing validity, sensitivity Vs
specificity.
Validity – ability to distinguish b/w who has disease or not – 2 components – specificity
and sensitivity.
! Causative agent, transmission, receiver, the source/reservoir, host/environmental
factors, and control
Surveillance – The systematic, active, ongoing observation of the occurrence and
distribution of disease within a population and of events or conditions that inc/dec the
risk of such disease occurrence.
Define event to be surveyed ! collecting the data in systematic way ! tabulating data
! analysing/interpretation ! using information to bring about changes.
Study design
Case control-study – to examine possible relation to an exposure for a certain disease.
Ecological studies – eg relationship b/w eating meat in certain countries + bowel cancer
Longitudinal or cohort studies – over selected time
Case-control or cross-sectional studies – observation at a single point in time/interval
Experimental studies – eg randomised clinical trial.
Prokaryotic structure and function 2
Cytoplasm: Fluid portion of cytoplasm consists of soluble enzymes and various
in/organic compounds. Cytoplasm is very granular (ribosomes). Cytoplasmic membrane
has a lipid bilayer but has no steroids.
Nucleoids: Bacterial chromosome is a single, double-stranded circle (genome – contains
all information for controlling development and metabolic cell activities), not contained
in a nucleus but a discrete area (nucleoid). Nuclear material occupies nucleoid region.
Multiple copes of mitochondrial + chloroplast DNA are contained within the matrix or
stroma + organelles are usually distributed in several clusters (= nucleoids) – attached to
inner mitochondrial membrane. Genome is supercoiled and folded into a tight mass,
complexed w protein + RNA core keeps it folded.
Plasmids: smaller, circular extra-chromosomal (additional) DNA’s, most commonly in
gram –ve. In cell, but not part of genome. Not often for cell survival but often provide a
selective advantage (e.g. offer resistance to antibiotics, heavy metals or able to synthesise
or break down unusual compounds). Are multiple copies of small self-duplicating pieces