Infection and Microorganims Flashcards
types of micro-organisms
- Commensals- organisms that are normal inhabitants of human body
- Pathogens- organisms generally absent from human microbiome in healthy individuals but causes disease in healthy individuals
- Opportunistic pathogens- commensals that can become invasive during immunosuppressive states
- Saprophytes= microorganisms that live in our environment
bacteria
- Single celled prokaryotes- no membrane bound organelles (no nucleus)
DNA found in the cytoplasm- mostly as circular chromosomes but they often have - plasmids (rings found in cytoplasm) that contain DNA
- May have cell structures like flagella (tails for swimming)
- Can live in any environment and reproduced by either binary fission or spore formation
- Genetic variation exists through horizontal gene transfer
bacterial cell wall
containing peptidoglycan, chains of which are linked by peptide bridges (which are an antibiotic target)
Gram staining bacterial cell wall
gram positive bacteria- purplish because PG layer is thick
gram negative bacteria- pink, thinner PG layer and has lipopolysaccharide layer surrounding It
lipopolysaccharide layer
in gram negative bacteria, blocks PG wall
resistant to penicillin and vancomycin
endotoxin that triggers harmful inflammation
coagulase postive gram positive staphylococcus
- S. aureus- stains golden on a blood agar plate, produces pus-forming soft tissue infections that are carried in the bloodstream, importance lies in methicillin-resistant strain (MRSA)
gram positive streptococcus (alpha haemolytic)
- S. pneumoniae causes pneumonia, meningitis and bronchitis in COPD patients
- S. oralis
gram positive streptococcus (beta haemolytic)
- Group A: S.pyogenes can cause tonsillitis, cellulitis, soft tissue necrosis and puerperal sepsis
- Group B: S. agalactiae
gram positive groups of bacteria
streptococcus, staphylococcus, enterococcus, bacilli, listeria, corynebacterium, clostridium
gram positive bacillus
B.anthacis from soil has multiple toxins/virulences and causes anthrax by inhalation or cutaneous entry
gram positive listeria
L.monocytogenes from soft cheece in pregnancy causes intrauterine or neonatal septicaemia or meningitis
gram positive corynebacterium
most pathogenic of normal skin commensals, transmitted via respiratory droplets
- inflammation of throat pseudo membrane can cause suffocation and bull neck
- exotoxins inhibit protein synthesis in host which leads to cell death in the heart and peripheral nerves
gram positive clostridium
- C. tetani- invades wounds, tetanus produces an exotoxin that stops release of the inhibitor GABA hence neurones are constantly excited. Voluntary= rigid paralysis, autonomic= hyper-sympathetic state
- C. botulinum- via food, exotoxin stops Ach release so leads to flaccid paralysis, can be used in botox
- C. difficile- alcohol resistant spores causing CD colitis (diarrhoea), often result of antibiotic use
- C. perfringens- contains damaging enzymes (a-toxin= lysis) causing soft tissue damage and food poisoning
gram negative bacteria groups
neisseria (cocci), anaerobes, bacili, moxarella
gram negative cocci
- N. gonorrhoeae
- N. meningitides
- Moraxella cattharalis (causes COPD exacerbations)
gram negative bacilli categories
fastidious- grow slowly, require enriched sugar
non-fastidious sugar fermenters- (Enterobacteriaceae commensals)
non fastidious non-fermenters
gram negative fastidious bacili
- Haemophilus influenzae- uncapsulated causes pneumonia, capsulated causes meningitis
- Legionella- species are carried by amoebae in water and causes severe pneumonia
- Helicobacter pylori- produces buffers stomach acid to survive and causes gastritis, which produces ulceration
gram negative non fastidious sugar fermenters
- Escherichia coli- has filaments that make it stick in GI and urinary tracts, also produces haemolysins and anti-phagocytic capsule, causes UTI, diarrhoea, bacteraemia
- Klebsiella- causes UTI, bacteraemia, pneumonia and liver abscesses
- Shigella- causes gastroenteritis and colitis in humans only
- Salmonella typhi- causes typhoid and traveller’s diarrhoea, while non-typhoid species cause gastroenteritis
gram negative non fastidious non fermenters
- Pseudomonas aeruginosa- huge range of virulence factors and AB resistance mechanisms, complicates cystic fibrosis and leads to ventilator pneumonia plus bacteraemia in immunocompromised
- Burholderia cepacian- poor prognosis in cystic fibrosis patients
- Vibrio cholerae- carried in water & produces rice water stools, toxin increases cyclic AMP and so reduces ion concentration in GI cells, so water is not absorbed by osmosis and leads to diarrhoea and dehydration
- Campylobacter- from contaminated animals causes most bacterial gastroenteritis- self limiting diarrhoea
bacteria that behave like viruses
smaller than other bacteria, lack a normal bacterial cell wall (can’t visualise them with a gram stain), don’t grow on agar so their diagnoses are done with serology or molecular analysis
factors that bacteria need to multiply
- Energy source eg. Glucose
- Water
- Nitrogen, sulphur, iron ect.
- Temperature, pH
- Some need complex organic compounds
- Atmosphere- these can be
→ strict aerobes (need oxygen)
→ facultative (can grow in either presence or absence of oxygen)
→ strict anaerobes (need absence of oxygen)
process of binary fission
- DNA replication
- Genome segregation
- Septum formation
- Division
spore formation
where bacteria breaks into a number of pieces which eventually form adult cells, type of bacterial replication
mechanisms of bacterial horizontal gene transfer
- Bacterial transformation- naked DNA is released from one cell and uptaken into another
- Bacterial transduction- DNA is transferred by a type of virus called bacteriophage
- Conjugation- a plasmid is passed via a cytoplasmic bridge between 2 cells
virulence
relative ability of a pathogenic organism to cause disease
virulence factors
genes, molecules and structures that either promote the survival of the microbe or increase its damaging effects
S.aureus virulence factors
→ protein A on the cell wall helps evade phagocytosis by immune cells
→ coagulase enzyme (converts fibrinogen to fibrin) produced a fibrin coat/capsule that protects against phagocytosis
→ hyaluronidase enzyme (breaks down hyaluronic acid) breaks down host tissue
→ haemolysins (exotoxin) produces pores in red blood cells
→ toxins causing toxic shock, boils and pneumonia
S. pyogenes virulence factors
→ Protein M are present in group A strep, helps organism evade phagocytosis
viruses
- Can’t survive outside a host cell
- Nucleic acid is either DNA or RNA that is enclosed in a protein coat called a nucleocapsid
- Smaller than bacteria
viral structure
- either single or double stranded DNA or RNA inside a protein coat (capsid made up of capsomere subunits), nucleic acids can either have a linear or circular structure + be in one piece or segments
- Some viruses have an outer envelope that is derived from the cell membrane in the host cell and have glycoproteins inserted into it that play role in infection
- May also contain enzymes (if not present in host) or cell membrane-derived coat from the host
- Not considered alive until they have invaded a host cell and used its functions to replicate
virus infection stages
entry into the host body → entry into host cells → nucleic acid transcription → viral replication → egress from host cell
viral transmission routes
- Orally through contaminated food or drink, saliva
- Direct skin contact
- Respiratory droplet transmission
- Sexually
- Trans-placental (vertically)
- Direct inoculation via contaminated needles, trauma or insect bites
viral replication
- Virus adsorbs to host cells via their receptor binding proteins linking to existing host receptors
- Membranes fuse or the virus enters the cell via receptor mediated endocytosis
- Capsid is shed to uncoat the nucleic acid and this replicates to syntheses more RNA and viral proteins
- These are then assembled into new nucleocapsids and released; either by egress or budding
egress
cytolysis eg. herpes destroys neurones this way
budding
glycoproteins inserted into existing cell membrane which buds off to release a viral envelope
Budding- mostly used by viruses with envelopes.
viral actions in host
- acute lytic infections (infections that have a cytopathic effect) - shorter duration and host cells burst to release the replicated viruses
- persistent infection with shedding- eg. Hepatitis B, is chronic and replication is slow
- latent viruses- eg. herpes may remain dormant in host and reactivate later (such as during a cold)
- HIV is a provirus that integrates its nucleic acid into the hosts genome in a persistent and slow infection
- Transformation mutates DNA for use in host cell growth systems, precipitating uncontrolled and increased growth, crowding, new antigens and loss of contact- cardinal signs of cancer. For example: Epstein-Barr virus associated with lymphomas, HPV is an agent in cervical carcinoma, Hep B and C are associated with liver cancer
HIV retrovirus mechanism
reverse transcriptase produces DNA from its RNA and integrase intserts it into the host genome, irreversible so HIV is impossible to cure, leads to AIDS by destroyed leukocytes at the end of latency, anti-retrovirals prolong life
herpes simplex mechanism
composed of double stranded DNA in an enveloped isocahedron, replicated in the nucleus and uses kinesin motors to ascend neuron axons, remain latent in these ganglion areas and reactivate later, so move back down same axons to epithelia and produces cold sores on skin
polio virus mechanism
infects the gut (lymphoid tissue cytoplasm) and spreads through the blood to motor neurones, 99% of infection are asymptomatic but when nerve cells are destroyed, meningitis and the paralysis associated with polio occur
antivirals
can target any stage of replication cycle and often used in combination to fight an infection, best known is acyclovir which inhibits viral DNA polymerase, it is activated (phosphorylated) by viral kinases so only exerts its actions on infected cells
fungi
- Eukaryotic organisms- can be either multicellular or single celled organisms with multiple nuclei
- Thick cell wall made of carbohydrates
- Classifications: yeasts, filamentous fungi or dimorphic fungi
- Yeasts- unicellular that reproduce by binary fission or budding, e.g. candida, cryptococcus neoformans
classifications of fungi
- Filamentous fungi- grow via long thread structures called hyphae, e.g. aspergillus
- Dimorphic fungi- can grow via two types, growth depends on environmental conditions like temperature as well as genetic factors
- Superficial fungal infections affect hair shafts and the dead layer of skin
- Cutaneous infections affect the epidermis, hair and nails
- Subcutaneous infections affect the subcutaneous tissue
- Systemic infections affect internal organs
- Opportunistic infections affect immunocompromised patients like those with AIDS or on immunosuppressants due to a transplant, e.g. candida, aspergillus, cryptococcus and pneumocystis jiroveci
examples of fungi
- Candida- causes athletes foot (cutaneous), mucosal inflammation (thrush) & candidemia (systemic in blood), has high mortality and risk of infecting heart, eyes, liver and spleen
- Cryptococcus neoformans- indicates AIDS and causes fungamia and meningitis
- Aspergillus- invasive, opportunistic, and often fatal, hypersensitivity- allergic bronchopulmonary aspergillus
- Pneumocystis jirovecci- causes pneumonia and consequently hyperoxia in immunocompromised patients
prions
- Proteinaceous infectious agents
- Abnormally folded proteins coded for by host cells which cause other molecules to become abnormally folded and can form fibrils in the brain
- Spread via contaminated food or medical tools
- Probable cause of rare, degenerative fatal brain disease Creutzfeldt Jakob disease
parasite examples
- Protozoa- single celled parasites
- Helminths- worms
- Arthropods- have exoskeletons
pathogenicity
ability of an agent to damage the host
general pathogenesis
- Disease occurs when pathogens are not controlled by host defences (because of wounds in skin barrier, antibiotics destroying natural competitive flora or foreign objects)
- Primary pathogens can instigate disease in healthy subjects, while opportunistic ones do so in immunosuppressed organisms when host defences are reduced
- Host response involves inflammation and innate and adaptive immunity, can include damaging themselves, in addition to direct pathogen damage if the response is excessive
areas without commensal bacteria
The brain, muscle, bone and bladder
ways microbes can be transmitted
- Respiratory
- Fecal-oral
- Venereal (STIs)
- Via vectors
sites of entry for pathogens
- Skin (requires injury or an insect vector) or mucosal surfaces (conjunctiva in eyes, these are thinner for function so, are more penetrable)
- Respiratory tract
- The gut
- Urogenital tract
routes of invasion for microbes
blood (most common), extracellular fluid, the lymph, the nervous system or the body cavities
pathogenicity of microbes
immune reactions, bacterial endotoxins which are released when cells are damaged and can trigger a strong damaging host response.
Coagulase Negative Gram positive Staphylococcus
- S. epidermis
- S. capitis
are coagulase negative and only pathogenic on a foreign body such as a prosthetic.
Rickettsia species
bacteria that behaves like a virus, transmitted by parasites and present as rocky mountain spotted fever or typhus (rash, fever, headache)
areas without commensals
brain, muscle, bladder, bones
