L12: Infections on a surface

Question 1

What is a surface?

Answer 1

Interface between a solid and a liquid/gas

Question 2

What surfaces are there on a patient for microorganisms to colonise?

Answer 2

Skin
--> Epithelium 
--> Hairs
--> Nails 
Mucosal surface
--> Conjunctiva
--> Gastrointestinal tract
--> Respiratory tract
--> Genitourinary tract

Question 3

What microorganisms can be found on the skin?

Answer 3

Viruses –> Papilloma (warts) and Herpes simplex (coldsaw)
Bacteria
–> Gram positive–> Staph Aureus (coagulase positive–> enzyme for invading tissues) coagulase negative streptococci, corynebacterium
–> Gram negative–> Enterobacteriaceae
Fungi–> Yeast (single celled), dermatophytes (multicellular)–> Athletes foot, ringworm
Parasites–> mites

Question 4

What does the human microbiota show?

Answer 4

Range of bacteria
Varies from place to place
Varies in concentration from place to place

Question 5

Where can infections come from?

Answer 5

Self –> transferred from normal environment on patient to new abnormal environment–>
Transmitted from animal
Environment–> contaminated water, food, air, surfaces
Source–> patient or via intermediate

Question 6

What is one of the most common ways of patients getting infected?

Answer 6

Self transferred

Microbiota in one area entering a new area

Question 7

Define microbiota?

Answer 7

Micro-organisms on the surface of the skin or mucosa
Normally harmless and beneficial
Wrong area–> harmful

Question 8

How can infections be transferred from one area to another?

Answer 8

Invasion
Migration
Innoculation–> during operation etc… bacteria introduced into the body
Haematogenous–> in the blood

Question 9

What are some example of naturally occurring external surface infections?

Answer 9

Cellulitis 
Pharyngitis 
Conjuctivitis 
Gastroenteritis--> inflammation of large bowel--> diarrhoea mechanism for expelling organism but also helps distribute it
Urinary tract infection 
Pneumonia

Question 10

What are some examples of naturally occurring internal surface infections?

Answer 10

Endovascular–> endocarditis, vasculitis
Septic arthritis–> infection of joint
Osteomyelitis–> infection of bone
Empyema–> infection of pulmonary space (between external surface of wall and internal chest wall)

Question 11

What are examples of prosthetic surface infections?

Answer 11

Intravascular lines
Peritoneal dialysis catheters
Prosthetic joints 
Cardiac valves
Pacing wires
Endovascular grafts
Ventriculo-peritoneal shunts

Question 12

What is prosthetic valve endocarditis?

Answer 12

Infection of the heart valves
Native–> valves your born with which are damage
–> Normally caused by bacteria in mouth –> Staph aureus
Prosthetic valves–>replaced
–> Normally causes by skin bacteria–> coagulase negative staphylococci

Question 13

What are the organisms normally found in prosthetic joint infections? How is risk reduced? How is it treated?

Answer 13

Coagulase negative streptococci
Staphylococcus aureus
Preventative–> sterile area (filtered and high pressure room), Surgeons wear ‘space’ suits
Antibiotics aren’t very good–> poor blood supply so concentration of antibiotic to reach joint low
Removal of prosthetic–> treat with antibiotics–> redo several months later–> 2 stage replacement
1 Stage replacement possible but risk of infection to new prosthetic

Question 14

What are the causative organisms in cardiac pacing endocarditis?

Answer 14

Coagulase negative staphylococci–> pts own skin

Staphylococcus aureus

Question 15

What is the process of development of infection at the cell surface?

Answer 15

Adherence to host cell/ prosthetic surface
Biofilm formation
Invasion and multiplication of tissues
Host reponse
–> Pyogenic–> pus formation by neutrophils
–> Granulomatous–> fibroblasts, lymphocytes, macrophages–> nodular inflammatory surfaces

Question 16

What allows bacteria to stick to the surface? What happens when they bind?

Answer 16

Bacterial cell specialised structures–> pili or fimbriae (specialised proteins)
Bind to surface receptor on host cell membrane
Change from vegetative form to activated form
Start to produce biofilm

Question 17

What is a biofilm?

Answer 17

Production of 'slime' layer by multiple bacteria communicating together 
Excrete ECM (polysaccharides, nucleic acids etc)

Question 18

Why do bacteria produce a biofilm?

Answer 18

Protect bacteria from host defences

Suffocates neutrophils–> prevent antibodies and complement from attacking the bacteria

Question 19

How is a biofilm produced?

Answer 19

1. Starvation–> bacteria shrink form spores until conditions right for growth
2. Active bacteria adhere to surface
3. Attached bacteria multiply and encase the colonies within a slimey matrix
4. Nutrients diffuse into matrix
5. Molecular signs that regulate behaviour transferred between cells in matrix
6. Chemical gradients create microenvironments for different species and different activites
7. Antimicrobial damage outer cell layer but biofilm community resistant
8. Aggregates can become detached and move to a new surface but remain in biofilm

Question 20

How do bacteria communicate?

Answer 20

Quorum sensing

Enough bacteria–> communicate–> control biofilm formation, sporulation, and virulence factor secretion

Question 21

How does quorum sensing work?

Answer 21

Bacteria produce signalling molecules–> autoinducers (AI)
Cell surface receptors/ cytoplasmic receptor on other cells detect signalling molecule
Gene expression is altered –> co-operative behaviours and more AI production

Question 22

How do microorganism causes disease?

Answer 22

Exposure- adherence- invasion- multiplication and dissemination
Production of virulence factors–> exotoxins (cytolytic, AB toxins, superantigens, enzymes) and endotoxins–> host cell damage (direct, consequent to host immune response)

Question 23

How are surface infections diagnosed? What are the challenges?

Answer 23

Identify infecting organism and its antimicrobial susceptibilites
Blood culture
Tissue/ prosthetic material sonication (apply sound to remove infection) and culture
Challenges–> adherent organisms (difficult to get bacteria off)
–> Low metabolic state/ small colony variants–> more difficult to grow

Question 24

How are surface infections treated?

Answer 24

Aim–> sterilise tissue, reduce bioburden
Antibacterials
Remove prosthetic
Surgery–> resect infected material

Question 25

What are the challenges with treatment?

Answer 25

Poor antibacterial penetration into biofilm
Low metabolic activity of biofilm
Danger/difficulties of surgery

Question 26

How can surface infections be prevented?

Answer 26

Natural surfaces–> Maintain surface integrity, prevent bacterial surface colonisation, remove colonising bacteria
Prosthetic surfaces–> prevent contamination, inhibit surface colonisation and remove colonising bacteria