4.12 - Communicable diseases Flashcards
Bacilli (bacterial pathogens)
- Rod-shaped bacteria and archaea
- Rods (bacilli) or chains (streptobacilli)
- Salmonella (S. enterica)
- Anthrax (B. anthracis)
Cocci
- spherical bacteria
- Mostly lack flagella and are non-motile
- Streptococcus pneumonia
- Staphylococcus aureus
How are bacteria classified
Basic shape:
- bacilli, cocci, vibrios, spirilla, spirochaetes
Cell wall structure:
- gram positive bacteria look purple-blue under a light microscope following gram staining e.g. MRSA
- gram negative bacteria appear red following gram staining e.g. e.coli
- type of cell wall affects how bacteria react to different antibiotics
Spirals
- Spirals (spirillum) or corkscrews (spirochaete)
- External (spirillum) / internal (spriochaete) flagella
- Campylobacter jejuni, Helicobacter pylori
Vibrios
- Comma-shaped bacteria
- Motile with flagella
- Cholera (V. cholera)
- V. vulnificus
Viruses
- 200-300nm
- non-living infectious agents
(acellular and cannot reproduce without a host) - a short section of RNA surrounded by protein.
- infect cells by inserting its own RNA into the DNA of a cell
- The infected cell will then begin to produce more viruses
- all natural viruses are pathogens
- affect all types of organisms, even bacteria, these are called bacteriophages
Examples include HIV, herpes, tobacco mosaic virus (TMV) and SARS-CoV-2
Protoctista
- 5-3000μm
- diverse range of eukaryotic organisms
- vary widely in cellular structure, locomotion, life cycles and feeding methods
- Includes unicellular organisms, and cells grouped into colonies.
- Small percentage act as pathogens, causing disease in plants and animals
- Protists which are pathogenic are parasites – using host organisms
- may require a vector e.g. malaria (mosquito) and sleeping sickness (tsetse fly)
- May enter host directly through contaminated water e.g. amoebic dysentery and Giardia
Fungi
- 5-50μm
- do not cause significant harm to animals, but can be devastating to plants
- eukaryotic organisms, which can be unicellular or multicellular without tissues
- Fungi that are human pathogens are unicellular e.g. thrush, ringworm
- Fungi cannot photosynthesise so digest food extracellularly before absorbing nutrients
- parasitic fungi are the fungal pathogens that cause disease
- can often affect leaves, preventing photosynthesis
- produce millions of spores, which enable them to spread widely and rapidly, particularly among crop plants
The two modes of action of pathogens
Damaging the host directly:
- viruses take over the cell metabolism to make new viruses and then burst out of the cell, destroying it
- some protoctista take over cells and break them open, digesting the cell contents as they reproduce
- fungi digests living cells and destroys them. This combined with the body’s response to the damage gives symptoms of disease
Producing toxins which damage host tissues:
- bacteria produce toxins (a by-product of the normal functioning of bacteria) that damage the host in some way, causing disease. Some toxins break down cell membranes, damage or inactivate enzymes or interfere with the host cell genetic material
- some fungi produce toxins which affect the host cells and cause disease
Ring rot
- a bacterial disease of potatoes, tomatoes and aubergines
- caused by gram-positive bacteria Clavibacter michiganesis
- damages leaves, tubers and fruit
- can destroy up to 80% of the crop
- there is no cure
- once bacterial rot infects a field it cannot be used to grow potatoes for 2 years
Tobacco mosaic virus (TMV)
- a virus that affects tobacco plants and around 150 other species (tomatoes, peppers etc)
- damages leaves, flowers and fruits, stunting growth and reducing yields
- can lead to an almost total crop loss
- no cure, but resistant crop strains are available
Potato blight
- caused by a fungus-like protoctist oomycete (Phytophthora infestans)
- hyphae penetrates host cells, destroying leaves, tubers and fruit
- causes millions of pounds worth of crop damage each year
- no cure, but resistant strains, careful management and chemical treatments can reduce infection risk
Black sigatoka
- banana disease caused by a fungus (Mycosphaerella fijiensis)
- attacks and destroys leaves
- hyphae penetrate and digest the cells, making the leaves black
- causes 50% reduction in yield
- resistant strains are being developed
- good husbandry and fungicide treatment can control the spread of the disease but there is no cure
Tuberculosis (TB)
- Droplet infection (inhalation)
- bacterial disease of humans, pigs, cows, pigs, badgers and deer
- commonly caused by Mycobacterium tuberculosis and M.bovis
- damages and destroys lung tissue and suppresses the immune system so the body is less able to fight off other diseases
- people affected by HIV/AIDS are more likely to develop TB infections
- curable by antibiotics
- preventable by improving living standards and vaccination
Bacterial meningitis
- Droplet infection (inhalation)
- fomites via infection routes
- bacterial infection (Streptococcus pneumoniae or Neisseria meningitidis) on the meninges of the brain (protective surfaces on the surface of the brain)
- can spread to the rest of the body, causing septicaemia (blood poisoning) and rapid death
- mainly affects very young children and teenagers
- blotchy rash that does not disappear when a glass is pressed against it
- antibiotics will cure the disease if delivered early
- vaccines can protect against some forms
HIV/AIDS
- acquired immunodeficiency syndrome (AIDS) is caused by human immunodeficiency virus (HIV)
- HIV targets T-helper cells in the immune system and gradually destroys it
- means affected people are open to other infections such as TB and some types of cancer
- can affect humans and some primates
- a retrovirus with RNA as its genetic material
- passed from person to person in bodily fluids (unprotected sex, sharing needles etc)
- no vaccine and no cure, but antiretroviral drugs slow progress of disease
- FGM increases infection rate
Influenza (flu)
- Direct contact
- droplet infection (inhalation)
- fomites
- viral infection (Orthomyxoviridae) of ciliated epithelial cells in the gas exchange system
- kills them, leaving the airways open to secondary infection
- can be fatal for young children, old people and people with chronic illnesses (usually from secondary bacterial infections such as pneumonia)
- affects mammals and birds
- three main strains; A, B and C
- A viruses are the most virulent and also classified by proteins on surface
- mutate regularly, so you cannot be totally immune from year to year
- no cure, but vulnerable groups are given a flu vaccine
Malaria
- caused by protoctista Plasmodium
- spread by the bite of infected Anopheles mosquitoes
- Reproduce inside female mosquitos and the parasite is passed to human hosts when the mosquito takes blood meals, invading red blood cells and liver
- Causes recurring fever and weakness, making hosts vulnerable to other infections
- Limited cures, no vaccine
- Vector controlled with habitat destruction and insecticides
- Prevent biting e.g nets
Ring worm
- direct contact
- fomites e.g. bedsheets, soil
- fungal disease affecting mammals
- different fungi infect different species
- causes grey-white crusty infectious areas of skin
- not damaging
- antifungal creams are an effective cure
Athlete’s foot
- direct contact
- fomites e.g. towels, floors
- form of human ringworm
- grows on and digests warm, moist skin between the toes
- antifungal creams are an effective cure
- causes cracking and scaling
- prevented by good hygiene
Forms of direct transmission
Direct contact (contagious diseases):
- contact with the body fluids of an infected person
- skin-to-skin contact
- microorganisms from faeces transmitted on the hands
Inoculation
- through a break in the skin
- from an animal bite
- through a puncture wound
Ingestion:
- taking in contaminated food or drink or transferring pathogens to the mouth from the hands
Forms of indirect transmission
Fomites:
- inanimate objects such as bedding can transfer pathogens
Droplet infection (inhalation):
- minute droplets of saliva and mucus expelled from mouth as you talk, cough or sneeze
Vectors:
- transmits communicable pathogens from one host to the other, usually animals
- different from transmission between animals and humans e.g. bird flu
Factors that increase the probability of catching a communicable disease
- overcrowded living and working conditions
- poor nutrition
- compromised immune system
- climate change (can introduce new vectors)
- culture and infrastructure (traditional medicine can increase transmission)
- socioeconomic factors such as a lack of trained health professionals
transmission of pathogens between plants
Direct transmission:
- direct contact of a healthy plant with any part of a diseased plant
Indirect transmission:
- soil contamination
- vectors (wind, water, animals, humans)
Factors affecting the transmission of communicable diseases in plants
- planting varieties of crops that are susceptible to disease
- over-crowding increases the likelihood of contact
- poor mineral nutrition reduces resistance of plants
- damp, warm conditions increase the survival and spread of pathogens and spores
- climate change - increased rainfall and wind promote the spread of diseases; changing conditions allow animal vectors to spread to new areas; drier conditions may reduce the spread of disease
The HIV cycle
- HIV virion glycoprotein binds to helper T cell receptor and fuses to cell surface membrane
- HIV RNA, reverse transcriptase, integrase and other viral proteins enter host cell (endocytosis)
- Viral DNA is formed by reverse transcriptase
- Viral DNA is transported across the nucleus and integrated into the host DNA by the enzyme integrase
- New viral RNA is used to make viral proteins
- New viral RNA and proteins move to surface and are assembled
- ‘Budding’ occurs as new virion fuses with cell membrane and is released
The malaria cycle
- Plasmodium protists in mosquito salivary glands
- Protists enter host when infected mosquito sucks blood
- Protists carried to liver by bloodstream and reproduce asexually
- Protists begin cycle of infecting RBCs, reproducing and bursting out
- Some cells develop into male and female gametes
- Gametes move into mosquito’s gut when it feeds on infected blood
- Fertilisation occurs and zygote then divides by mitosis
- New cells burst out of cells in gut and migrate to salivary glands
How do plants recognise an attack
- receptors in the cells respond to molecules from the pathogens or to chemicals produced when the plant cell wall is attacked
- this stimulates the release of signalling molecules that switch on genes in the nucleus
- defensive chemicals are produced, sending alarm signals to unaffected cells to trigger their defences
- polysaccharides (callose and lignin) are made to strengthen cell walls
Physical defences of plants - callose
- Polysaccharide callose (β-1,3 and β-1,6 linkages between glucose monomers) is synthesised
- Callose is deposited between cell walls and membranes in cells adjacent to those infected
- these callose papillae act as barriers, preventing pathogens entering cells around site of infection
- Callose continues to be deposited in large amounts
- Lignin is added, thickening and strengthening mechanical barrier
- Callose blocks sieve plates in the phloem, sealing off infected areas and preventing the spread of pathogens
- Callose is deposited in plasmodesmata between infected cells and neighbours, sealing them from healthy cells
Chemical defences of plants
- insect repellents (pine resin and citronella)
- insecticides (caffeine)
- antibacterial compounds including antibiotics (phenols = antiseptics)
- antifungal compounds (saponins interfere with fungal cell membranes - chitinases)
- anti-oomycetes (glucanases break down glucans in cell wall)
- general toxins (cyanide)
Non-specific animal defences for keeping pathogens out of the body
- the skin covers the body and also has a skin flora of healthy microorganisms that outcompete pathogens
- many body tracts are lined with mucous membranes that trap microorganisms are contain lysozymes and phagocytes
- lysozymes in tears, urine and stomach acid destroy bacteria and fungal cell walls
Blood clotting and wound repair
When platelets come into contact with collagen in the skin or the wall of a damaged blood vessel, it starts to secrete substances:
- thromboplastin, an enzyme that triggers a cascade of reactions resulting in a blood clot (thrombus)
- serotonin, which makes the smooth muscles in the walls of the blood vessels contract, so they narrow and reduce the supply of blood to the area
