Infections of Surfaces

Question 1

What are the common microrganisms found on the skin?

Answer 1

• Viruses – papilloma (warts and verrucas) and Herpes simplex
• Bacteria Gram positive – Staph Aureus, Coagulase negative staphylococci (large group of bacteria that do not poses coagulase enzyme) and Corynebacterium
• Bacteria Gram Negative – Enterobacteriaceae (usually found in gut)
• Fungi – yeasts and dermatophytes
• Parasites – mites, scabies, body lice and head lice

Question 2

How is coagulase also a virulent factor?

Answer 2

Coagulase also a virulence factor allowing the bacteria to wall itself off from immune cells.

Question 3

Where/when does normal flora change?

Answer 3

Normal flora changes between different parts of the body but also between individuals.

Question 4

How can we gain an infection from one surface to another?

Answer 4

• Invasion – strep pyogenes pharyngitis
• Migration – E.Coli urinary tract infection
• Innoculation – coagulase negative staphylococcus prosthetic joint infection
• Haematogenous – viridans strep endocarditis

Question 5

Discuss some external examples of surface infections

Answer 5

• Cellulitis – severe inflammation of dermal and subcutaneous layers of the skin
• Pharyngitis – pharynx
• Conjunctivitis – eyes
• Gastroenteritis – food poisoning
• UTI – Urinary tract infections
• Pneumonia – infection of the lungs

Question 6

Discuss some internal examples of surface infections

Answer 6

• Endovascular such as Endocarditis and Vasculitis
• Septic arthritis – infection of the joint from the blood stream or inoculation
• Osteomyelitis – infection of bone
• Empyema – pus build up in the pleural space

Question 7

What is the common cause of prosthetic joint infections and cardiac pacing wire endocarditis?

Answer 7

Prosthetic Joint infections and cardiac pacing wire endocarditis are usually caused by coagulase Negative Staphylococci and sometimes Staph Aureus.

Question 8

What are the common causes of infective endocarditis after valve surgery?

Answer 8

valve endocarditis after 1 year since operation are likely to be caused by: Viridans Streptococci, Enterococcus faecalis, Staph Aureus, HACEK group or Candida.

Prosthetic valve endocarditis after less than a year since operation is usually caused by coagulase Negative Staphylococci.

Question 9

Describe the pathogenesis of infections at a surface

Answer 9

1. Adherence to host cells or prosthetic surfaces
2. Biofilm formation – complex structures, bacteria embed in matrix that they excrete and bind to water creating a jel like structure which the bacteria sit within (not necessarily just one organism). Bacteria aren’t multiplying so antibiotics aren’t effective – also antibiotics only reach outside structure of the biofilm
3. Invasion and multiplication
4. Host response – extremely important as you can wipe out the pathogen but this doesn’t stop the pathological process that has been stimulated by the pathogen.
   • Pyogenic (neutrophils - pus)
   • Granulomatous (fibroblasts, lymphocytes, macrophages - nodular inflammatory lesions)

Question 10

Describe how biofilm forms

Answer 10

1. Starvation stimulates spore like state of bacteria
2. Active bacteria can attach to almost any surface swapping from swimmers to stickers in minutes due to changes in gene expression
3. Attached bacteria multiply and encase themselves within slimy matrix
4. Nutrients diffuse and cellular signals are exchanged easily due to proximity
5. Chemical gradient create microenvironment for different species
6. Some cells may become detached and roll along within they protective biofilm state

Question 11

What is Quorum sensing?

Answer 11

This a communication mechanism between bacteria that controls sporulation, biofilm formation and virulence factor secretion. Only relevant to build biofilm if lots of other bacteria are present. This signalling takes place by the release of molecules that are detected by other bacteria. Potential here to turn off the signalling molecules to prevent biofilm formation.