III - Microbiology & Infection - Development of Infections

Question 1

What is the difference between symptoms and signs of an infection? Give examples.

Answer 1

Symptoms are subjective characteristics felt by patient. Signs are objective manifestations of disease observed by others.

Symptoms:
- pain, nausea, headache, chills, sore throat, fatigue, lethargy

Signs:
- Swelling, rash or redness, vomiting, diarrhoea, fever, pus formation

Question 2

What are the stages of an infection

Answer 2

Stages of infectious disease:

• Incubation period: infected with something but number of infectious organisms is low, so you don’t feel anything.
• Prodromal period: just starting to feel a bit ill. Good indicator that we are going to get sick.
• Illness: exponential growth of microorganisms. Immune system is starting to work but not fully functioning. Time from incubation to illness depends on the infection, the immune capability of the patient and infective numbers.
• Decline: immune system starts to kick in or antimicrobials are given. Immune system is working hardest in this phase.
• Convalescence: still feeling poorly because body is trying to repair itself. During any of these phases’ patient can infect others.

Question 3

What is the role of adhesion in infection?

Answer 3

Adhesion is a process by which microorganisms attach themselves to cells. They do so by using specialised structures or attachment proteins. Bacteria use ligands which are part of fimbriae. Appropriate receptors for the ligands are often glycoproteins which are part of the host cell structure.

Some bacterial pathogens do not attach to host cells that attach to each other to form biofilms that are hard to remove. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular polysaccharide substances.

Question 4

What are virulence factors of infectious agents?

Answer 4

Virulence factors of infection agents
- Pathogenicity – ability of microorganism to cause disease
- Virulence – degree of pathogenicity (how good it is at causing infection)
Neither of these however address the severity of the disease. High virulence almost always cause disease whereas less cause disease only in weekend hosts.

Other virulence factors include:

• Extracellular enzymes
• Toxins
• Endotoxins
• Antiphagocytic factors

Question 5

Give examples of extracellular enzymes

Answer 5

• Hyaluronidase (hyaluronic acid is glue that holds cells together) and collagenase (collagen is main structural protein) degrade specific molecules to enable bacteria invade deeper tissues
• Coagulase causes blood proteins to clot. Some bacteria like to live in clots (provide protection for bacterial cell from immune system)
• Kinases digest blood clots, which causes break-down of tissues, allowing subsequent invasion of damaged tissues

Question 6

Give examples of toxins

Answer 6

Exotoxins: these are compounds produced inside bacterial cells and released to the external environment:

• Cytotoxins generally directly kill host cells or affect function (could affect complement system)
• Neurotoxins specifically interfere with nerve cell function (stop information being passed down)
• Enterotoxins affect cells lining the GIT (gut wall is good barrier to prevent bacteria getting in)

Endotoxins: these are part of the outer membrane of gram-negative cells. The outer membrane will breakdown when the cell dies, the components will be released into the environment. All endotoxins of all gram-negative cells have the same activity but how severe they are is dependent on the bacterial species. Endotoxins encourage the body to over-stimulate its immune system causing severe inflammation, significant blood flow and haemorrhaging. E.g., Lipid A

Question 7

Give examples of Antiphagocytic factors

Answer 7

Capsules: discourage phagocytosis because the capsules are slippery. Also many capsules are composed of chemicals normally found in the body, so the host won’t recognise it as foreign.

Antiphagocytic chemicals: they produce chemicals preventing fusion of lysosome with phagocytic vesicles. If a macrophage encounters a foreign bacterial cell, it’ll engulf it. It’ll be in a phagocytic vesicle and the macrophage will need to fuse with enzymes (lysosome) to break down the bacterial cell. Antiphagocytic chemicals stop this fusion process.
- Leukocidins are chemicals capable of destroying phagocytic cells outright

Question 8

What is Streptococcus pneumoniae

Answer 8

Streptococcus pneumoniae are organisms that can cause pneumonia in certain people. It is a gram-positive encapsulated organism. You can see it is gram-positive because it has lipoteichoic acid which is only a component of gram-positive cells. It is a commensal organism in the naso-pharynx area. In normal, healthy people it won’t cause infection. It is an opportunistic pathogen.

One of its virulence factors is the capsule which prevents phagocytosis. It also produces hyaluronidase (glue).

• PspA (pneumococcal surface protein A) inhibits complement
• PspC associates with CbpA (choline binding protein A) which is also associated with complement and certain factors. Specifically inhibits C3b production.
• Neuraminidase A exposes binding sites of the host cells. It is an enzyme that cleaves certain components from binding sites so allows bacteria cells to attach to host
• Pneumolysin is a cytolytic toxin
• Autolysin is an antiphagocytic factor

Streptococcus pneumoniae has a lot of virulence factors but shouldn’t cause infection unless you are immunocompromised.

Therefore, care should be taken especially in hospitals because we are dealing with poorly people, and some do potentially have dangerous infections living on their skin.

Just because something has a high virulence does not correlate to the severity of the infection