Summary BHCS 2006 - Infection, Immunity and Disease

BHCS2006 Summary Notes
Microbiology

Bacteria structure
• Classic structure but probably Gram neg
due to lots of pili and flagella
• Flagella used for movement, chemotaxis
and ATP production – mainly composed of
protein flagellin
• Types of bacteria determined via cell wall
structure
o Gram positive
o Gram negative
o Acid fast
• Gram staining
o Crystal violet iodine ethanol Safranin
o Ethanol washes out crystal violet not taken up (usually in Gram neg)
o Safranin then stains Gram neg pink
o Gram pos stains purple with crystal violet
o Gram positive bacteria
90% of cell wall is peptidoglycan – very rigid
High muramic acid content
Low lipid content
Teichoic acid
Can form spores
Less pathogenic and less resistant to antibiotics than Gram neg
o Gram negative bacteria
10% of cell wall is peptidoglycan – elastic
Contains liposaccharide (LPS) – endotoxin responsible for severe pathogenic effect
of Gram neg
No teichoic acid
High lipid content
Bacteria that is antibiotic-resistant and/or difficult to tackle/eradicate is usually
Gram neg
• Bacterial cell wall
o Structure
All have proteins NAG and NAM linked by AAs
Gram pos have lots of rigid peptidoglycan between NAG and NAM – makes walls
rigid and takes up crystal violet
Gram neg has very little peptidoglycan so crystal violet cannot stick to anything
(washes away with ethanol)
o Unique to prokaryotes
o Rigid structure surrounding cell membrane
o Maintains cell’s shape – counters osmotic pressure
o Protects from external stresses – host IR, pH etc
o Provides attachment sites for bacteriophage
o Anchorage of fimbriae, flagella and pili
o Essential role in cell division – binary fission
o Sites of major antigenic determinants of cell surface
o Natural intrinsic resistance to some bacterias
o Acquire resistance from gene transfer
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,BHCS2006 Summary Notes
o Communication with other bacteria
o Potential target of antimicrobials
• Cytoplasmic membrane
o Site of biosynthesis of DNA, cell wall polymers and membrane lipids
o Selectively permeability and transport of solutes
o Electron transport and oxidative phosphorylation
o Efflux pump – pump antibiotic straight out = resistance
• The capsule
o Presence of capsule makes bacteria very pathogenic
o Contributes to virulence by preventing phagocytosis by WBCs
o Masks presence of bacteria to host IR – bacteria could live in macrophage undetected
• Not all bacteria have flagella – associated with Gram neg
o Many genes and parts involved in flagella activity
• Fimbriae
o Filamentous
o Adherence factors at tips
o Short and thin (opposed to pili)
o Gram pos and Gram neg
o Highly immunogenic – strong host IR
• Pili
o Long hair-like projections
o Virus receptors
o Produce protein pilin
o Long and thick (opposed to fimbriae)
o Gram neg – e.g., N. gonorrhoea
• Common pili/fimbriae – fine, rigid, numerous, bacterial adhesion
• Sex pili – long, coarse, bacterial conjugation
• Endospores
o Only 2 species produce spores – both Gram pos
Bacillus Clostridia
o Produced when bacteria are under stress or in wrong
environment – protects bacteria until
environment suitable
o Calcium layer makes spore hard to penetrate
o Bacteria breaks down and forms spores, which is
dormant (metabolically inactive), to be
transferred to suitable environment where
bacteria sprout and is active again
o Spore is resistant
o Contains calcium dipicolinic acid
• Plasmids
o Small extra DNA molecules, separate from genomic DNA (gDNA)
o Circular or linear
o Extrachromosomal
o Double-stranded DNA
o Self-replicating
o Carry genes – virulence factors, toxins etc
o Used in gene transfer/sharing
o Not essential of bacterial survival – just contribute to pathogenicity

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,BHCS2006 Summary Notes
Classification methods
• Gram stain • Infection patterns
• Morphology • Obligate intracellular
• Metabolic behaviour • Antigenic composition
• Growth temperature • DNA sequence
• Bio typing • Mole % G+C in genome
• Nutritional requirement

Bacterial size
• Micron/micrometre (m)
• 1m = 10-3m
• Size varies with bacteria type, age and external environment
• Cocci = sphere, 1m
• Bacilli = rod, 0.5-1m width, 2-8m length
• Spiral = 1-3m length, 0.3-0.6m width

Bacterial morphology
• Very important in identification – often determine where bacteria infect without knowing exact
species, just on shape
• Key shapes
o Coccus o Neisseria
o Rod/bacillus o Spirillum/spirochete
o Diplococci – mysterious pathogens
• Gram pos cocci – pyogenic (pus producing)
o Streptococcus pyogenes
o Staphylococcus aureus
• Gram neg cocci – pyogenic
o Neisseria gonorrhoea
o Neisseria meningitidis
• Spirals – chronic infections
o Treponema pallidum
o Borrelia burgdorferi
o Leptospira

Factors affecting bacterial growth
• Energy metabolism
• Nutrients
o Autotrophs
o Heterotrophs – saprophytes, parasites
o Water
o Carbon source
o Nitrogen source
o Minerals
o Growth factors – AAs, purine
• Temperature
o Psychrophilic forms (15-20°C)
o Mesophilic forms (30-37°C)
o Thermophilic forms (50-60°C)
• Hydrogen ion concentration

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, BHCS2006 Summary Notes
o Neutrophiles (pH 5-8)
o Acidophiles (pH below 5.5)
o Alkaliphiles (pH above 8.5)
• Oxygen requirements
• Osmotic pressure
• Generation time – population to double, usually 20-60 minutes for common bacteria

Viruses are genetic elements that contain either RNA or DNA, they are acellular and need a host to be
active and be able to replicate, but can survive dormant extracellularly

Origin of viruses
• Regressive theory – degenerated from cellular forms of intracellular parasites
• Progressive theory – evolved from cellular nucleic acids that gained ability to replicated
autonomously such as plasmids or transposable elements (most popular theory)
• Coevolution theory – viruses coevolved with cellular life forms

Viral size
• Use electron microscope to visualise
• Filterable agents – can pass through filters that catch bacteria
• Largest – poxvirus (300nm)
• Smallest – parvovirus (20nm)

Viral morphology
• Basic structure – nucleic acid + capsid with or without envelope
o DNA/RNA + capsid protein nucleocapsid
o Nucleocapsid + lipid membrane, glycoprotein enveloped virus
• Icosahedral
• Filamentous
• Head-tail
• Capsomer – repeating protein subunits to form impenetrable shell (capsid)
• Protomers – polypeptide chains which make up the capsomer
• Viral capsid
o Capsomers symmetrically arranged to form impenetrable shell (capsid) around nucleic acid
core
o Symmetry has 2 types – icosahedral, helical
• Viral envelope
o Some species have an envelope
o Derived from host cell membrane
o Protein subunits seen as projecting spikes on surface of envelope may be present
o May have more than one type of spike – e.g., influenza virus
o Confers chemical, antigenic and biological properties
o Enveloped virus susceptible to lipid solvents

Viral life cycle
• Every virus has 2 stages
o Metabolically inactive – particulate, transmissible virion stage consisting of nucleic acid,
capsid and envelope; entirely dependent on external physical factors for chance movement
and spread to infect other susceptible cells/hosts


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