16. Infectious Diseases Flashcards
Infection Transmission
Any communicable disease, transmitted from:
• Human to human.
• Animal to human.
Transmitted by:
• Direct contact, e.g. hand-to-hand touching.
• Indirect contact, e.g. hand to a surface to a person.
Infectious Agents
Micro-organisms (microbes): • Bacteria • Viruses • Fungi • Protozoa Multi-cellular organisms
Microorganisms
- Non-pathogenic:
• Not disease-causing.
• Many microbes are ‘symbiotic’ (live together). However, these can become pathogenic.
• Commensal bacteria live on our skin and mutualistic bacteria in our intestines.
• There are around 10 times as many bacterial cells as human cells in the body. - Pathogenic:
• Disease-causing microbes (i.e. ‘pathogens’).
Commensal
A symbiotic relationship between organisms whereby one benefits and the other is unaffected; e.g. microbes on your skin.
Mutualistic
symbiotic relationship between organisms, whereby both benefit; e.g. E. coli make vitamin K for humans.
Parasitic
A symbiotic relationship between organisms whereby one benefits at the other’s expense; e.g. head lice.
Opportunistic
A symbiotic relationship between organisms which is initially commensal / mutualistic then becomes parasitic; e.g. candida
Anton van Leeuwenhoek (1670s)
• Invented the microscope — documented microbes.
Dr Ignaz Philipp Semmelweis (1847)
• Washing hands reduces disease spread in hospitals.
Louis Pasteur v. Antoine Béchamp (1860s):
- Pasteur: ‘Germ theory of disease’ — he said “germs are all bad and cause disease”. Developed pasteurisation.
- Bechamp: “Germs are ‘opportunistic’ in nature and live with us symbiotically”. He proposed the ‘terrain theory’. To prevent illness, he advocated not to kill germs but to promote health (i.e. through diet, exercise).
Alexander Fleming (1900s)
• Founder of antibiotics incidentally in his lab — penicillin
Identifying Microbes
Microbes are identified and characterised so the causative agent of a disease can be identified. This allows for appropriate treatment and applying appropriate infection controls to inhibit disease spread.
• Some organisms are cultured (grown) in laboratories so that they can be examined under microscope.
• Most whole micro-organisms can be seen with a light microscope. However, some organisms are so small (i.e. viruses) that they can only be seen with more advanced laboratory equipment such as electron microscopes.
Bacteria and Their Classification
Bacteria are prokaryotic cells. This means the cell structure is simple and has no nucleus or membrane-bound organelles.
•The ‘control centre’ of bacteria is a single loop of DNA.
Bacteria Cell Wall: Gram-positive
thick, mesh-like cell wall made of peptidoglycan (stains purple with Gram’s method).
Bacteria Cell Wall: Gram-negative
• Gram-negative — thinner cell wall layer (stains pink with Gram’s method) with an additional outer lipid-rich membrane.
Bacilli
Rod-shaped e.g. E. coli
Cocci
Spherical e.g. streptococci, staphylococci
Spirochetes
Spiral shaped
Bacterial Growth
Reproduction is asexual by ‘binary fission’.
• This type of cell division is rapid. For example, Escherichia coli (E, coli) can divide in as little as 20 minutes. Binary fission enables bacteria to reproduce in high numbers at a rapid rate. This enhances the ability of the bacteria to cause disease.
• ‘Spore’ formation — a structure that is extremely resistant to hostile physical and chemical conditions such as heat and disinfectants.
• Spores also form part of the lifecycle of many fungi and protozoans. Spores are a dormant life form.
Exotoxins
- Gram-positive and negative bacteria.
- Released by a living microbe.
- Very toxic, protein toxins.
- Have a variety of effects on bodily functions.
- Example: Diphtheria (inhibits protein synthesis), E. coli, C. tetani.
Endotoxins
- Gram-negative bacteria only.
- Released after organism death from cell wall.
- Often cause fever / malaise.
- Endotoxins stimulate inflammatory cascade.
- Usually less toxic.
- Example: Salmonella.
Invasive Enzymes
Some bacteria produce enzymes to help them invade a host; e.g. haemolytic streptococci produce enzymes that break down RBCs
Resident and Transient ‘Microflora’
The ‘microflora’ encompasses naturally occurring micro-organisms inhabiting the healthy human body — mostly bacteria:
- Present: Nasal cavity, skin, mouth, small and large intestine, vagina and perineum.
- Absent: Blood, cerebrospinal fluid (csf), lungs, stomach, uterus, fallopian tubes, ovaries, bladder and kidneys.
• Organisms within the microflora can become pathogenic if the local environment changes or if the immune system becomes compromised
Antibiotics
Antibiotics are conventional drugs that destroy bacteria.
• Antibiotics can be broadly categorised as being either broad or narrow spectrum. Broad spectrum antibiotics possess no specificity for the bacteria they kill and instead can have devastating effects on the body’s microflora.
• Antibiotics are often overprescribed. This, accompanied
by the presence of antibiotics in some foods, has led to
more adverse effects and antibiotic resistance.
• Antibiotic use is also clearly linked to a higher risk of diseases including asthma and inflammatory bowel diseases.
• Antibiotics do not work against viruses; e.g. colds, flu, cold sores.
• Other adverse effects of antibiotics include: impaired immunity, candida (yeast) overgrowth (due to loss of friendly flora), diarrhoea.
Viruses
Viruses are obligate intracellular parasites — needing a living host for survival and replication.
• They are not cells but rather consist of either a strand of RNA or DNA in a hard protein coat — ‘capsid’.
• Protein coat is unique for each virus.
• Viruses are the smallest microbes and are too small to be seen with light microscopy. They are said to be so small that 500 million rhinoviruses (associated with the common cold) could fit on the head of a pin.
Virus Types
- DNA viruses (mostly double stranded) include varicella zoster, herpes, smallpox.
- RNA viruses include measles, mumps, HIV.
- They reproduce by injecting their RNA or DNA strand into a living host cell and use the host cell’s apparatus for reproduction.
- Viruses only infect certain host cells — they express specificity due to surface viral proteins.
- When a virus binds with a cell, it only allows entry of genetic material and the capsid remains outside.
- A virus will generally burst its host cell as it leaves it.
Virus challenges
Viruses are more difficult both for the body and for anti-viral agents and drugs to identify and destroy. This is because:
- They hide inside the host cells (can be latent).
- They do not have a metabolism of their own — hence anti-microbial agents cannot be targeted towards enzymes.
- They do not have many structures of their own.
- Able to mutate — this allows them to change their surface antigens and avoid host immune response.
Viral Replication
Viral lysis
Viral budding
Latent stage
Viral lysis
Virus particles burst out of the host cell into the extracellular space, resulting in the death of the host cell.
Viral budding
Process by which a virus exits a cell and acquires an envelope (outer membrane) of its own from the host cell membrane and usually leads to cell death. Used in viruses which need an envelope; e.g. HIV.
Latent Stage
Stage of infection where the disease is present but hidden and inactive. Viruses can remain in a latent stage for years; e.g. herpes simplex.
Fungi
Fungi are eukaryotic organisms.
• Fungi can be single-celled or very complex multicellular organisms.
• Fungi have a cell wall that contains melanin.
• Fungi are found everywhere; e.g. soil, mouldy bread, medicine, food.
• Filaments of fungi are called ‘hyphae’, whilst a mass of fungi is a ‘mycelium’.
• We are generally resistant to fungal infections. Fungal infections are often opportunistic and occur in immune compromised patients (e.g. HIV, diabetes mellitus).
Fungi Forms
Yeasts
Moulds
Dermatophytes
Yeasts
Single-celled fungus that reproduces by budding; e.g. Candida albicans
Moulds
Multi-cellular fungi.
•Some moulds cause disease, others are involved in production of various foods and antibiotics; e.g. aspergillus and penicillium.
Dermatophytes
Fungi causing skin disease.
•They obtain nutrients from keratin in skin (no living tissue is invaded) and colonise in the stratum corneum.
•It’s the only fungus dependent on humans; e.g. ringworm or tinea.
Fungi Growth
Growth results in formation of:
• Hyphae: Fungal filaments.
• Mycelium: A ‘mesh’ of intertwined filaments.
• Growth is promoted by warmth, moisture, rich nutrition, acidity
Fungi Reproduction
- Asexual reproduction occurs via budding (extension of the hyphae) containing chromatin that eventually detach and develop into an independent organism.
- Sexual reproduction is less common and occurs by forming fungal spores.
Protozoa
Protozoa are eukaryotic organisms.
• Exist as single cells and have no cell wall, just a membrane.
• They live in moist environments such as fresh water, soils and in the ocean.
• Can move in various ways; e.g. flagella, cilia or vector.
Protozoa Reproduction
- Binary fission and budding.
- Sexual reproduction (only in unfavourable conditions).
- Can form cysts — can survive harsh conditions.
Helminths
Helminths are parasitic worms.
• Eukaryotic and macroscopic organisms.
• Helminths are multi-cellular.
• Lifecycle: egg, larva, adult.
• Can live in intestines, blood or tissue; i.e. intestinal epithelium.
• Include roundworm, tapeworm, pinworms.
• Many helminths are intestinal parasites.
Infection
Infection refers to invasion of a host by a pathogen causing disease. Infections may occur in: • Single individuals. • Larger groups (‘epidemic’). • Worldwide (‘pandemic’). • Restricted to an area (‘endemic’).
Reservoir
Location where the pathogen exists, reproduces and spreads to new hosts.
Carrier
infected individuals who are asymptomatic.
Vector
intermediate carrier, transporting pathogens from reservoir to host. (mosquito)
Host
an infected person or animal.
Infection Transmission
- Droplet transmission
- Direct contact,
- Indirect contact,
- Vectors:
- Nosocomial:
Transmission: Droplet transmission,
e.g. coughing with flu, scarlet fever.
Transmission: Direct Contact
e.g. via faeces.
Transmission: Indirect Contact
e.g. via food, transport.
Transmission: Vectors
carried by an insect or animal.
Transmission: Nosocomial
acquired from a medical setting (hospital or care facility); e.g. surgical site, UTIs, pneumonia, ulcers.
Infection Acquisition
- Ingestion ingest microbes / faeces, larva eggs. etc.
- Direct to blood stream injection, open wounds, bites.
- Sexual intercourse.
- Inhalation.
- Touch.
- Placental.
Infection Stages
Incubation Period
Prodromal Period
Acute Period
Chronic Period
Incubation Period
Time interval between the initial exposure to the infecting organism and the appearance of the first signs or symptoms it produces.
Prodromal Period
Interval from non-specific symptoms (malaise, fever, fatigue) to more specific acute symptoms.
Acute Period
- The pathogen peaks in population.
* Associated with very pronounced symptoms specific to the disease.
Chronic Period
•Infection with insidious or slow onset of long duration.
Host Resistance
Intact skin and mucous membranes Body secretions — stomach acid, tears, etc. Phagocytosis Interferon production Effective inflammatory response Effective immune system
Microbial Virulence
Production of invasive / destructive enzymes.
Production of endo- or exo-toxins.
Spore formation.
Entry of a large number of organisms: form colonies (CFUs).
Presence of bacterial capsule and pili.
Ability to mutate, e.g. MRSA, HIV.
Infection Control
It is important to prevent the spread of infections, especially for healthcare professionals.
• Ways to reduce the spread of infection include:
• Reduce the reservoir.
• Stop droplet transmission — covering the mouth (not with the hand).
• Block the method of transmission (hand washing, gloves, condoms).
• Kill the organism.
• Sterilisation.
Sterilisation
- ALL micro-organisms & their spores are destroyed:
- Hot air: 30 min at 180oC
- Autoclaving (steam): 20 min at 120oC.
Disinfectants
destroy pathogenic microbes, but usually not their spores.
Antiseptics
reduce the number of organisms on the skin, but not their spores e.g. alcohol.
Pasteurisation
71.7°C for 15 to 25 seconds: will kill most pathogenic microbes but not their spores
