Ch 10: Pathogens

Question 1

Distinguish between disease and health

Answer 1

• disease: any condition that adversely affects the functioning of an organism
• health: the state of complete physical, mental and social wellbeing and not merely the absence of disease

Question 2

Define outbreak and endemic

Answer 2

• outbreak: sudden increase in the incidence of a disease in a population in a certain time period
• endemic: the constant presence of a disease or infectious agent within a given geographic area or population group

Question 3

What is an infectious disease?

Answer 3

• caused by an agent that can be passed from one organism to another
  
  • host: a living organism that can be infected under natural conditions by an infectious agent, the infected organism
  • pathogen: an infectious agent that causes disease
  • transmission: the passing of an infectious disease from an infected host to another individual
  • contagious: diseases that are easily transmitted by close contact with an infected organism or their secretions

Question 4

What is a non-infectious disease?

Answer 4

• those that are not caused by pathogens and are non-communicable, or transmitted from one individual to another
• i.e. nutritional diseases – obesity, malnutrition, degenerative disease – diabetes, osteoporosis, Alzheimer’s disease, environmental diseases – skin cancer

Question 5

Define pathogenicity

Answer 5

• pathogenicity: the potential capacity of a pathogen to cause disease

Question 6

Define virulence

Answer 6

virulence: the degree of pathogenicity of a pathogen, the ability of a pathogen to cause severe disease within its host

Question 7

Define susceptibility and resistance

Answer 7

susceptibility: describes the level of response by an organism to a pathogen (its resistance), depends on a number of factors including age, state of health and their natural resistance to that particular pathogen
resistance: describes the extent to which an organism is or is not affected by an agent such as a pathogen or chemical toxin

Question 8

Define symptoms and incubation period

Answer 8

symptoms: symptoms are the effects a pathogen or disease has on the body of the host (e.g. high temperature, rash, cough, lethargy).

incubation period: the time between an infection and the onset of symptoms

Question 9

What are the reasons for an incubation period?

Answer 9

• need to divide multiple times to reach sufficient numbers
• need time to reach their target organs/tissues/cells
• toxins produced by the pathogen accumulate over time

Question 10

What are viruses?

Answer 10

• definition: non-cellular agents comprised of a protein coat and a nucleic acid genome (either RNA or DNA)
• examples:
  
  • DNA viruses
    
    • herpes simplex virus
    • hepatitis B
    • smallpox
    • Epstein-Barr virus
  • RNA
    
    • influenza
    • SARS
    • Hepatitis C
    • Ebola
• viruses are highly specific for the cell or organism they infect: an adenovirus infects lung epithelial cells, causing the common cold, bacteriophages are viruses that infect bacteria

Question 11

Describe the life cycle of a virus

Answer 11

1. when a virus infects an organism it injects its nucleic acid into a host cell
2. a) once inside, the viral nucleic acid takes over the host cell and directs it to make multiple copies of the viral protein coat and nucleic acid b) some viruses do not cause their host cell to produce multiple viruses immediately. Instead they enter a l__ysogenic phase
3. these then assemble into new viruses and are released when the cell undergoes lysis, or splits open. This releases many more viral particles, which can infect other cells within the host

• viruses are obligate intracellular parasites – they rely totally on host cells for their reproduction and cannot function outside the host cell. This trait poses limitations on viral research. All viruses cause some type of disease

Question 12

Describe prions

Answer 12

• definition: prions are small infectious proteins in the brain. Because they are proteins they do not contain genetic material.

Pathogenic form of prion

• the prion protein exists normally in the body. It plays a role in memory, learning and intercellular signaling. It can be converted to a disease-causing form. Two forms:
  
  • PrP^c = normal protein
  • PrP^Sc = disease-causing form
• PrP^Sc is able to convert PrP^c to the harmful PrP^Sc when they encounter each other. When there is sufficient numbers of the pathogenic form, they aggregate to form filaments. These fibres kill brain cells consequently affecting muscle coordination and brain function
• the PrP^Sc protein is very resistant to high temperatures, strong enzymes and ultraviolet radiation, making it extremely difficult for the immune system to combat. Researchers have not yet discovered a chemical that is effective in destroying the harmful protein and is also safe enough to administer to a patient
• this form of the protein can arise spontaneously as a mutation in the gene that codes for the normal protein, but it is usually transmitted by entering the body in infected food, most commonly brain tissue from an infected animal

Diseases caused by prions

• mad cow disease (BSE) in cows
• Scrapie in sheep and goats
• Creutzfeldt-Jacob disease in humans

Question 13

Describe bacteria, their adaptations and how they are identified

Answer 13

Bacterial pathogens: structures and adaptations

• 
  only a relatively small number of bacteria cause disease. Billions of commensal bacteria live on and in our bodies and are often beneficial.

adaptations

• cell wall containing peptidoglycan outside their cell membrane
• flagellum: gives them mobility
• slimy bacterial capsule: may be used to help the bacteria to stick to surfaces i.e. teeth or mucous membranes. The capsule is a large, well organized layer sitting outside the cell wall, usually increasing the virulence of a species as it makes it harder for the body’s immune system or antibiotics to attack the inner bacterium
• endospores: tough, dormant structures bacteria are capable of forming. Resistant to extreme temperatures, chemicals and drying out enabling the bacteria to resist unfavourable conditions and facilitates dispersal to new hosts
• binary fission or budding spores: forms of asexual reproduction which allow bacteria to reproduce very rapidly in favourable conditions. Some species can reproduce every 20 minutes. From one bacterium, 4.7 × 10²¹ new bacterial cells can be produced in 24 hours.

Classification and identification of bacteria

• bacteria are classified according to their shape and whether they cluster together

Shapes of bacteria

• coccus: a spherical bacterium (pl. cocci)
• bacillus: rod-shaped bacterium (pl. bacilli)
• vibrio: like a comma shape

Clusters of bacteria

• diplococci: spherical bacteria occur in pairs
• streptococci: occur in chains
• staphylococci: occur in clusters

Identification of bacteria

• different species or strains of bacteria can be difficult to distinguish under a microscope.
• one useful test for identifying bacteria is the Gram stain which can stain different strains purple or pink depending on their cell membrane/cell walls
• process: bacteria can be grown as colonies, which contain clones of the original bacterial cells. First, bacterial cells are ‘streaked’ onto an agar plate. Single colonies, originating from a single bacterium and containing identical cells, can be picked off and inoculated into a liquid medium broth. The appearance of colonies and the nutrients they require to grow vary between species. This process allows for individual strains to be isolated and grown in pure culture, allowing benign and pathogenic strains of bacteria to be distinguished according to their appearance and response to antibiotics and chemicals

Question 14

How do bacteria cause disease?

Answer 14

• bacteria can be transmitted from one host to another by direct contact, in food and water and in droplets of moisture in the air
• bacteria can have properties that make them pathogenic to other organisms, especially external components such as lipopolysaccharides and peptidoglycans. This is because they stimulate immune responses that are sometimes so strong they damage host cells and tissues
• once inside a host, bacteria divide rapidly. Some bacteria can produce toxins (often bacterial metabolic wastes) that damage host tissues, disrupt the functioning of cells or interfere with the host’s immune system - making the host susceptible to other pathogens

Question 15

Describe disease causing fungi

Answer 15

Structure

• includes large organisms such as mushrooms and toadstools, and also microscopic forms – unicellular yeasts and moulds.
• eukaryotes
• usually larger than bacteria
• reproduce using spores.
• cell wall made of chitin

Disease causing agents

• external: as disease causing agents, fungal diseases are usually external, infecting and irritating the skin. The spores produced on the skin as it grows is carried by the infected skin when it flakes off. If the spores come into contact with damaged/broken skin they may cause new fungal infections. Spores are very long lived which improves transmission rates as they can remain alive for years in a household, germinating when conditions are suitable Examples of fungal diseases are tinea and ringworm, where spore-infected skin flakes off, spreading the infection.
• Internal: fairly rare, can cause serious disease in plants (cinnamon fungus and jarrah forests) and in immunocompromised individuals (aspergillus can fill space in the lungs and limit breathing, or can spread in the bloodstream and infect major organs

Question 16

Describe protists

Answer 16

• unicellular, eukaryotic organisms that produce sexually and asexually. Very few species cause disease in humans, but those that do can cause serious illness.
• examples include: malaria, trypanosoma, giardia

Malaria

• caused by protists from the Plasmodium genus
• if left untreated can cause enlargements of the liver and spleen, or in the case of cerebral malaria, brain injury leading to death in severe cases
• one of the most infectious diseases that has evolved to become resistant to antimalarial drugs and the incidence of malaria is rising. Effective vaccines have not yet been developed making it extremely difficult to control the spread of this disease

Question 17

Describe the different types of parasites

Answer 17

Endoparasites

• definition: parasite (organism that lives on or in its host for all/part of its life, causing harm and gaining nutrition from the host) that lives inside its host
• the intermediate host is where the parasite develops but does not reach sexual maturity (can be especially dangerous to the host as cyst can form in the eyes or brain), the definitive host is where the adult form of the parasite produces gametes. Some endoparasites have a direct life cycle, completing development in a single host. Juvenile forms of the parasite can encyst in tissues, where they can be ingested by the next host.

Ectoparasites

• definition: parasites that live on the surface of another organism i.e. arthropods (fleas, ticks and lice)
• most create only minor symptoms that can be treated easily, bites may cause discomfort
• in some cases, they act as vectors for dangerous pathogens i.e. Yersinia bacterium causes the bubonic plague

Question 18

Describe transmission

Answer 18

• pathogens have a range of adaptations to enable their transmission between hosts, to persist and survive pathogens must follow a repeating cycle of transmission from current to future host (either direct transmission from one host to the next or may involve one or more steps through an intermediate host/vector). One adaptation is having a very low infective dose, that is, the number of organisms needed to establish in a new host
• transmission requires the pathogen to escape from the body of its current host. It must then gain transport to a suitable new host, enter their body, establish itself in their tissues and finally ensure it is once again passed to a new host
• understanding their infectious cycles is critical to be able to identify suitable strategies to control pathogens

Question 19

Describe forms of disease

Answer 19

­Direct contact

• definition: the immediate transfer of an infected disease causing agent (pathogen) from one organism to another through contact. In humans this may include kissing, touching or sexual contact. This also includes zoonotic diseases
• many organisms have adapted to have an infectious period before symptoms show, in this infectious period, the host may be normally active, interacting with many potential new hosts and maximising the opportunity for transmission of the pathogen

Zoonotic diseases

• definition: diseases of animal origin that can affect other vertebrates including humans, any disease/infection that is naturally transmissible from vertebrate animals to humans and vice-versa i.e. Lyssavirus, Ebola virus disease, influenza, salmonellosis
• new zoonosis can be particularly dangerous as people may have little immunity to them
• an example of transmission that does not use direct contact is when a fomite is involved. This is a non-living object such as bedding, towels and coins that can carry disease-causing organisms
• prevention:
  
  • personal health and hygiene
  • reduce exposure
  • manage animal health

­Bodily fluids

• indirect contact: the transmission of a disease causing agent from a reservoir to a susceptible host by suspended air particles, airborne droplets or by animate (vector) or inanimate (vehicle) intermediaries. There is not contact between the infected host and susceptible host
• body fluids include sweat, tears, vomit, mucus, blood, saliva and urine. Organisms that use this mode of transmission can usually survive outside the body for long periods. They include the pathogens that cause: glandular fever, cytomegalovirus, sexually transmitted diseases including HIV, Hepatitis B, human papillomavirus, gonorrhoea and chlamydia.
• a possible preventative measure is to treat every bodily fluid as if it is infectious

­Foodborne transmission

• the gastrointestinal tract is a relatively easy point of entry for a pathogen.
• examples: bacteria like Salmonella, Campylobacter, Escherichia coli and Staphylococcus aureus and viruses like norovirus, hepatitis A and rotavirus cause these food-borne illnesses
• transmission:
  
  • watery diarrhoea, nausea and vomiting help to transmit the pathogen (i.e. via hand contact in a toilet) and low infective doses (e.g. 100 to 10 000 gastroenteritis rotavirus particles) make these diseases easily transmissible
  • handling of food without washing their hands
  • between the temperature danger zone of 5-60°C, is where most disease-causing bacteria can divide by binary fission to make many copies of themselves. It is important to keep food either colder or warmer than this

­Waterborne transmission

• several important diseases are transmitted in contaminated water, including diseases transmitted by the faecal-oral route and untreated sewage is discharged into the environment/cropland
• Examples include: bacterial diseases of typhoid, cholera and Shigella, viral diseases such as Hepatitis A, protists: Giardia, Cryptosporidium, Entamoeba, endoparasites such as intestinal worms and Schistosoma
• juvenile forms live in water and secrete enzymes that break down skin, allowing them to enter the bloodstream of their new host
• prevention of transmission: improved hygiene, clean water for drinking, bathing and washing and good sewerage systems can prevent the transmission of these diseases. Water can be disinfected, by eradication or with chemicals such as chlorine, and sanitation systems can be installed

­Airborne transmission

• coughing and/or sneezing releases millions of microbes in droplets of mucus or saliva. Particles can remain suspended in air for some time and travel long distances. If the droplets lands on the mucous membranes of a person, they may catch the disease
• airborne diseases include: measles, mumps, SARS, tuberculosis, colds, Influenza
• the advent of air-conditioning aided the spread of airborne diseases, including Legionnaires Disease

­Vectors

• definition: is a living organism that transmits pathogens from one host to another, sometimes a pathogen completes part of its life cycle in the vector and is dependent on this
• vectors allow pathogens to:
  
  • come into contact with a new host
  • spread over much greater distances
  • penetrate the outer defences of the host
• biting insects such as mosquitoes, ticks, fleas, lice and flies are examples
  of vectors