7- Infectious diseases Flashcards
Basic characteristics of infectious disease
- Genus/species – Bacteroides fragilis
- Light microscope -1um diam./ 3-10um length
- They are Procaryotic so have no nucleus/ membrane bound organelles only free-floating DNA strands – divides by binary fission (Asexual reproduction = identical daughter cells)
- Doubling time – 20 min for E.coli
- Differentiating Cell wall– Blue/purple (Gram positive), red (Gram negative)
What are pathogens
Organism that causes or is capable of causing disease
What are commensals
Organism which colonises the host but causes no disease in normal circumstances, unless they are in a place they shouldn’t be
What is an opportunistic pathogen
Microbe that only causes disease if host defenses are compromised (Opportunist infections)
What is Virulence/Pathogenicity
The degree to which a given organism is pathogenic
What is asymptomatic carriage
When a pathogen is carried harmlessly at a tissue site where it causes no disease
What is the morphology of bacteria
- Shape, size and aggregation e.g. cocci, rods, clumps, chains, pairs
- Cell wall structure – Gram positive (Thick peptidoglycan layer – absorbs the dye) / Gram negative (Thin peptidoglycan layer + outer membrane with lipopolysaccharides – contribute to antibiotic resistance)
- Other structures – capsule, spore, flagella, pili
The species name can sometimes tell us…
the disease that it causes
Features of bacterial growth
- Agar medium (media), broth
- Aerobic (growth in oxygen), anaerobic (growth without oxygen), facultative (either)
- Shape of colony – diameter, edge, domes or flat
- Effect on medium – haemolysis (Breakdown of RBC) – indicated by zone clearing around colonies (Streptococcus), pigment production (Pseudomonas – Green pigment), acid from sugar – detected using a pH indicator e.g. lactose (Escherichia coli) (Catalase test – ability to produce catalase, used to separate staph a from other cocci)
Examples of microbes
most viruses
-Most viruses
-E.coli, S.aureus etc
-Mycobacterium
tuberculosis
-Fungi (Candida albicans)
-Mycobacterium
leprae
What is the situation and doubling time of most viruses
They are cells with a doubling time <1hr
What is the situation and doubling time of E coli, S aureus etc
They are broth or solid media and the doubling time is 20- 30 mins
What is the situation and doubling time of Mycobacterium
tuberculosis
They are broth or media and the doubling time is 24 hours
What is the situation and doubling time of fungi (Candida albicans)
They are broth or media and the doubling time is 30mins
What is the situation and doubling time of Mycobacterium
leprae
They are broth or media and the doubling time is 2 weeks
What do some antibiotics do
Many groups, split on chemical structure and mode of action such as penicillins (Gram +ve) and tetracyclines (Gram +ve and -ve)
When antibiotic groups split what can they be
B or B
either bactericidal (Kill bacteria) or bacteriostatic (Inhibiting bacterial synthesis)
What is the activity of antibiotics determined by
Activity determined by Minimum Inhibitory Concentration (MIC) – the lower the MIC the more active the drug
-lowest concentration of an antibiotic that can inhibit the visible growth of a particular microorganism
When does antibiotic resistance develop
soon after antibiotics used in treatment
What is the most common mechanism of resistance
Via E
Most common mechanism is via enzyme – beta-lactamase (ESBL produced by gram -ve)
More on resistance
- Widespread and often indiscriminate use has led to multiresistant organisms (selection)
- Several bacteria now very resistant (MRSA) and difficult to treat
To combat resistance there is now
greater reliance now on infection control and/or vaccines
Bacterial genetic variation can be
mutation or gene transfer
What happens during mutation
BDI
- Base substitution
- Deletion
- Insertion
What happens during gene transfer
- Transformation via plasmid (Where bacteria take up free DNA from environment and incorporate it)
- Transduction via bacteriophage (a virus) where DNA is transferred between bacterias
- Conjugation via sex pilus transfer of DNA via direct contact
What is the incidence and cost of healthcare associated infections
- 300,000 HAIs per year in UK
- Costs NHS £1 billion
What is the affect of healthcare associated infections
- Increasing problem in community settings such as nursing and residential homes
- Major impact on mortality and morbidity increasing length of stay and cost
What hygiene measures can be employed to reduce MRSA spreading
- Clean hands before and after touching patients
- Hands cleaned with soap and water, or alcohol gel or hand rub
- Wear gloves and aprons when caring for a patient with MRSA
- A patient with MRSA may be moved to a room on their own or into a separate area for people who have MRSA or other infections
Infections and spread of MRSA (Staph A that’s resistant to antibiotics)
Infections- wounds/ bacteraemia (bacteria in blood)
Spread- hands/ formatted (surfaces)
Figures and treatment of MRSA (Staph A that’s resistant to antibiotics)
Figures- 6000 cases of
bacteraemia/ yr
Treatment- Vancomycin
plus topicals if necessary
Infections and spread of C.Diffile
Infections- Hospital-acquired
diarrhoea
(80% in >65 yr age group)
Spread- Hands and/or
environmental surfaces
Figures and treatment of C diffile
Figures- 50,000
cases/yr
Treatment- Broad-spectrum antibiotics are cause – metronidazole or vancomycin plus fluids
How to avoid contracting C diff
- Wash hands thoroughly with soap and water before preparing/eating food, after handling raw food, after going to the toilet, after visiting hospitals and care homes
- Take antibiotics only when necessary
- Wash all dirty clothes, bedding, towels of infected patients in washing machine on hottest cycle. Clean toilet seats, flush handles, taps, after use with detergent and hot water
What is chlamidiya trachomatis
- Related to Chlamydophila pneumoniae and C.psittaci – Cause pneumonia
- Obligate intracellular bacterium – Requires a host cell to replicate. BUT ARE NOT VIRUSES (Has cell wall and susceptible to antibiotics that target cell wall synthesis)
- Produces Elementary (Infectious form) and Reticulate bodies (Replicative form) (EBs/RBs) can be seen by light microscope
- Unique developmental cycle – EB taken up by host, transforms to RB to replicate differentiate back to EB
Chlamidiya trachomatis growth
Grown in tissue culture – as host cell needed. Divided into 3 groups:
* Serovars A-C = Trachoma
* Serovars D-K = NGU/ Conjunctivitis
* Serovars L1-L3 = Lymphogranuloma venereum
What is chalmidiya trachomatis treated with
T A
Tetracycline, azithromycin
What are the basic characteristic of viruses
- DNA (Herpes) or RNA (HIV)
- Require electron microscope to be seen (25 – 150nm)
- Several morphological forms
- No metabolic systems – requires tissue culture
How do viruses grow
- Many different types of cells used for culture – growth determined by cytopathic effect (Damage or changes occur in host cells) (CPE)
- Growth recognised by unique appearance e.g. adenonvirus –grape-like clusters
- Also by antibody neutralisation and erythrocyte agglutination
What is the structure of viruses
- Virion: the intact virus particle
- Capsid: the protein coat
- Capsomeres: the protein structural units which make up the capsid
- Nucleic acid genome: either DNA or RNA
What is protozoa
- Eukaryote (3-2000um)
- Contains nucleus, no cell wall
- Unicellular, often in soil and water
- Life cycle, trophozoite and cyst stages
What lab tests are needed to diagnose a virus
- CPE
- Electron microscopy - morphology
- Serology – antibody tests e.g. ELISA looking for >4 fold rise in titre
- Molecular tests such as PCR and commercial DNA/RNA detection systems
Bacteria and viruses
both detected using NAATs
Although both detected by NAATs (Nucleic acid amplification tests), otherwise different diagnostic approaches for bacteria and viruses
