Microbiology Flashcards
1
Q
Define ‘pathogen’.
A
Organism that causes, or is capable of causing, disease
2
Q
Define ‘commensal’
A
Organism which colonises the host, but causes no disease in normal circumstances.
3
Q
What is an ‘opportunist pathogen’.
A
Microbe that only cases disease if host defences are compromised.
4
Q
Define ‘virulence / pathogenicity’.
A
The degree to which a given organism is pathogenic, and any strategy used to achieve this.
5
Q
Define ‘asymptomatic carriage’.
A
When a pathogen is carried harmlessly at a tissue site where it causes no disease.
6
Q
Name nine tests / stains used to identify bacteria.
A
- Gram stain
- Ziehl-Neesen stain
- Catalase test
- Coagulase test
- Haemolysis test
- Optochin test
- Oxidase test
- Macconkey agar
- XLD agar
REMEMBER: Gay Zoologists Can Cum Heavily Over My Orange Xylophone
7
Q
What are the two families of bacteria?
What is the difference?
A
- Gram positive - single membrane
- Gram negative - double membrane
8
Q
What does gram staining differentiate between?
A
Gram positive and gram negative bacteria
9
Q
Describe the process of gram staining.
A
- Apply primary stain (e.g. crystal violet) to heat fixed bacteria
- Add iodine, which binds to crystal violet and helps fix it to cell wall
- Decolourise with ethanol or acetone
- Counterstain with safranin
10
Q
What is the result of the gram stain test for gram negative and gram positive bacteria? Explain why.
A
Gram negative = pink
- Decolouriser INTERACTS with lipids, and cells lose their outer LPS membrane and CV-I complexes, thus appear pink
Gram positive = purple
- Decolouriser DEHYDRATES cell wall, and CV-I complex gets trapped in multi-layered peptidoglycan, resulting in purple appearance
REMEMBER: gram Negative = piNk
11
Q
What does Ziehl-Neesen differentiate between?
A
Acid-fast bacilli and non-acid fast bacilli
12
Q
What family of bacteria does Ziehl-Neesen identify? Why?
A
Mycobacteria, because they are acid-fast bacilli
13
Q
What are the results for the Ziehl-Neesen stain?
A
Acid-fast bacilli = red (e.g. mycobacteria)
Non-acid fast bacilli = blue (e.g. E.coli)
14
Q
What does the catalase test differentiate between?
A
Staphylococcus and streptococcus bacteria
15
Q
How do you carry out the catalase test?
A
Add H2O2 to bacteria and observe if there is bubbling
16
Q
What is the result if you do a catalase test on staphylococcus and streptococcus?
A
Staphylococcus are catalase POSITIVE so you see BUBBLING
Streptococcus are catalase NEGATIVE so there is NO REACTION
17
Q
What is coagulase?
What produces it?
A
An enzyme produced by Staphylococcus aureus that converts soluble fibrinogen to insoluble fibrin
Other staphylococci do NOT produce coagulase
18
Q
What are the results of the coagulase test?
A
Staphylococcus aureus is coagulase POSITIVE so you see clumping
Other staphylococci are coagulase NEGATIVE so there is no clumping
19
Q
What is haemolysis?
A
The ability of bacteria to break down red blood cells in blood agar
20
Q
What does haemolysis require?
A
Haemolysin
21
Q
What is the result of alpha haemolysis in the haemolysis test?
A
An indistinct zone of partial destruction of RBCs appears around the colony, often accompanied by an opaque green/brown discolouration of the medium
22
Q
Which bacteria are alpha haemolytic?
A
- Streptococcus pneumoniae
- Many oral streptococci (viridians streptococci)
- S.intermedius
23
Q
What are viridians streptococci and what can they cause?
A
Collective name for oral streptococci
They can cause serious pathology e.g. infective endocarditis
24
Q
What is the result of beta haemolysis in the haemolysis test?
A
A clear, colourless zone appears around the colonies, in which the RBCs have undergone complete lysis
25
Which bacteria are beta haemolytic?
- Streptococci pyogenes
- Streptococci agalactiae
- Many other streptococci
26
Can bacteria other than streptococci be beta haemolytic?
Yes, S.aureus and Listeria monocytogenes are both beta haemolytic
27
If you conduct the haemolysis test on Streptococci and Staphylococcus, but they both appear beta haemolytic;
1. Which staphylococcus bacteria is it?
2. How do you differentiate?
1. S.aureus - beta haemolytic staphylococci
| 2. Coagulase test - staphylococci are coagulase positive, streptococci are coagulase negative
28
What is the result of gamma haemolysis in the haemolysis test?
No change, bacteria are non-haemolytic
29
What does the optochin test differentiate between?
| What do you observe?
Streptococcus pneumoniae and Streptococcus mitis
A clear zone is seen around S.pneumoniae as it is susceptible to optochin, but S.mitis is uninhibited as it is resistant
30
Describe the process of the optochin test.
Place an optochin soaked disc on agar and observe bacterial growth
31
What is seen with alpha haemolytic bacteria in the optochin test?
They are resistant to optochin so there will be growth around the disc
32
What does the oxidase test test for?
Tests if a micro-organism contain cytochrome oxidase, an enzyme in the bacterial ETC
33
What does an oxidase positive test mean?
Bacteria are definitely AEROBIC
34
What does a negative oxidase test mean?
Bacteria are either aerobic or anaerobic
35
Name four oxidase positive bacteria.
- P.aeruginosa
- V.cholerae
- Campylobacter (e.g. C.jejuni)
- Helicobacter
36
Which type of bacteria grow on Macconkey agar?
Gram-negative bacilli
37
What does Macconkey agar differentiate between?
| What are the results?
Differentiated between lactose-fermenting and non-lactose fermenting G-ve bacilli
Lactose-fermenting = pink/red
Non-lactose fermenting = white/transparent
38
Why does the Macconkey agar turn pink/red with lactose-fermenting G-ve bacilli?
It contains a pH indicator, so bacteria which ferment lactose produce acid, which alters the pH
39
Why can't G+ve bacteria grow on Macconkey agar?
| What else can't grow on it?
Bile salts present inhibit G+ve bacteria, and inhibit swarming of G-ve bacteria Proteus spp.
40
Give three examples of bacteria which would turn Macconkey agar red/pink.
- E.coli
- Klebsiella pneumoniae
- Enterobacter spp.
41
Give two bacteria which would cause the Macconkey agar to appear white/transparent.
- Salmonella spp.
| - Shigella spp.
42
What does XLD agar differentiate between?
Salmonella and Shigella
43
What is observed in the XLD agar test?
```
Salmonella = red/pink colonies with black spots
Shigella = red/pink colonies
```
44
Name three genera of Gram positive cocci.
- Staphylococci
- Streptococci
- Enterococci
45
Name two genera of Gram negative cocci.
- Neisseria
| - Moraxella
46
Name four genera of Gram positive bacilli.
- Bacillus
- Clostridia
- Corynebacteria
- Listeria monocytogenes
47
Name six genera of Gram negative bacilli.
- E.coli
- Campylobacter
- Pseudomonas
- Salmonella
- Shigella
- Proteus
48
What is the bacterial capsule?
| What can it do?
A polymer of sugar that protects bacteria from host immune system
It can inhibit parts of the innate immune system
49
What is the bacterial cell wall made of?
Phospholipids
50
What makes bacteria prokaryotes?
The absence of a nuclear membrane
51
What is a plasmid?
A circular loop of DNA, often containing genes for antibiotic resistance
52
Which enzyme is present in bacterial cytoplasm?
RNA polymerase
53
What is cell envelope like in G+ve bacteria?
- Single cytoplasmic membrane
- Large amount of peptidoglycan on outer surface
- No endotoxin (lipopolysaccharide)
54
What is the cell envelope like in G-ve bacteria?
- Double membrane; inner and outer
- Less peptidoglycan; between the membranes
- Outer membrane has lipopolysaccharide (endotoxin) which the immune system reacts to
55
What do antibiotics target in G-ve bacteria?
Peptidoglycan between cell envelope membranes
56
Which mucosal surfaces are vulnerable to bacterial colonisation?
- Nasal cavity
- Larynx
- Stomach
- Colon
57
What is a spore? How do you destroy one?
A hardy capsule which bacteria can store themselves inside
They need to be autoclaved to be destroyed
58
What temperature limits can bacteria survive within?
-80ºc to 80ºc
| 120ºc for spores
59
What pH limits can bacteria survive within?
4-9
60
How long can bacteria survive in water / desiccation?
2hrs - 3m
| 50yrs for spores
61
How do you measure growth of bacteria?
Shine a light through them and measure absorption
62
How do bacteria divide?
Binary fission
63
What are the four phases of a bacterial growth curve?
| Explain them.
1. Lag phase - bacteria are taking in nutrients needed to divide and multiply
2. Log phase - bacteria grow exponentially until they run out of nutrients
3. Stationary phase - nutrients run out so bacteria can no longer divide
4. Death phase - bacteria die without nutrients
64
What is an endotoxin?
A component of the outer membrane of G-ve bacteria - LIPOPOLYSACCHARIDE
65
How does an endotoxin trigger an immune response?
Host immune system will recognise toxin and initiate immune response called ENDOTOXIC SHOCK
66
Is endotoxic shock specific or non-specific?
Non-specific
67
What happens if you subject an endotoxin to heat?
It is stable
68
Which type of bacteria mainly produce endotoxins?
Gram negative
69
Can an endotoxin be converted into a toxoid?
No
70
Which bacteria secrete exotoxins?
Gram positive and gram negative
71
What are exotoxins?
Proteins
72
What is the action of the immune system to exotoxins?
Specific
73
What effect do botulism and tetanus have on the nervous system?
Botulism - inhibits nervous system
| Tetanus - stimulates nervous system
74
What happens to exotoxins if you expose them to heat?
They denature
75
T/F: Exotoxins can be converted to toxoids.
True
76
What is transcription?
RNA polymerase acts on the bacterial chromosome to form mRNA
77
What is translation?
Occurs at 30s/50s ribosome to produce proteins
78
What sort of mutations can occur on the bacterial chromosome?
- Base substitutions
- Deletions
- Insertions
79
Why can mutations cause antibiotics to be ineffective?
Protein coded for is altered by mutation, meaning it is no longer susceptible to the antibiotic
80
Name three ways bacteria transfer genes.
1. Transformation
2. Transduction
3. Conjugation
81
What is transformation?
The genetic alteration of a bacterial cell via the uptake of an exogenous substance (e.g. via plasmid)
82
What is transduction?
Process by which foreign DNA is introduced into a bacteria via vector or virus (e.g. bacteriophage)
83
What is conjugation?
The transfer of genetic material between bacterial cells by direct cell-to-cell contact (e.g. via pilus)
84
Where will you normally find staphylococcus?
On the nose and skin
85
How many species are there of staph?
At least 40
86
Why might a patient with a S.aureus infection present with shoulder pain?
Referred pain from osteomyelitis in cervical spine vertebrae (C6 & C7)
87
What is the preferred method of imagine for osteomyelitis?
MRI scan
88
What is the standard treatment for S.aureus infection?
Flucloxacillin for 3 months
89
What does the coagulase test reveal about S.aureus?
It's coagulase positive
90
How is S.aureus spread?
Aerosol and tough (e.g. coughing and breathing)
91
Which antibiotics is MRSA resistant to?
- B-lactams
- Gentamicin
- Erythromycin
- Tetracycline
92
What are the four virulence factors S.aureus has?
- Pore-forming toxins
- Proteases
- Toxic shock syndrome toxin
- Protein A
93
How does S.aureus use pore-forming toxins to cause disease?
- PVL toxin produced caused haemorrhage pneumonia
| - Alpha-haemolysin induces apoptosis (at low levels) and necrosis (at high levels)
94
How does S.aureus use proteases to cause disease?
Produces exfoliatin, which attacks desmosomes between skin cells, which causes scalded skin syndrome
95
How does S.aureus cause disease using toxic shock syndrome toxin?
Stimulates cytokine release, triggering an inflammatory response and tissue damage
96
How does S.aureus use protein A to cause disease?
Uses protein A, a surface protein, to cause immunoglobulins to bind in the wrong orientation to antigens
97
Which conditions are associated with S.aureus infections?
- Wound infections
- Abscesses
- Osteomyelitis
- Scalded skin syndrome
- TSS
- Food poisoning
98
Give three examples of coagulase negative staphylococci.
- S.epidermis
- S.saprophyticus
- Acute cystitis
99
What sort of bacteria is S.epidermis and where does it tend to infect?
Opportunistic bacteria, causing infections in prosthetic limbs and catheters
100
What are the 3 classifications of streptococci?
- Haemolysis
- Lancefield typing
- Biochemical properties
101
Give an example of an alpha haemolytic streptococcus.
S.intermedius
102
Give an example of a beta haemolytic streptococcus.
S.pyogenes
103
Give an example of a gamma haemolytic streptococcus.
S.mutans
104
Explain the Lancefield typing classification.
A method of grouping coagulase negative and catalase negative bacteria based on the bacterial carbohydrate cell surface antigens
105
What does S.pyogenes cause?
- Wound infections (e.g. cellulitis)
- Tonsillitis & pharyngitis
- Otitis media
- Scarlet fever
106
What are the complications of a S.pyogenes infection?
- Rheumatic fever
- Glomerulonephritis
- Immunologically mediated complications
107
How does S.pyogenes stimulate an immunologically-mediated complication?
It produces erythrogenic toxin which is a super antigen, meaning it gives rise to an exaggerated immunological response and increased circulating cytokine levels
108
How do you assess the risk of a S.pyogenes infection:
Anti-streptolysin O titre
109
What are the pathogenic factors associated with S.pyogenes?
Secretion of...
- Hyaluronidase - spreading
- Streptokinase - clot degradation
- C5a peptidase - reduces chemotaxis
- Streptolysin O & S toxin - binds cholesterol
- Erythrogenic toxin - exaggerated response
Surface factors...
- Hyaluronic acid capsule - protection
- M protein - encourages complement degradation
110
What might the clinical presentation look like in a patient with a S.pneumoniae infection?
- Heavy smoker with nasal congestion and fever
- Cough and severe chest pain 2 days later
- Rust-coloured sputum
- Chest X-ray showing consolidation
111
Where might you find normal commensal of S.pneumoniae? How much of the population have this?
30% of population have commensal in the oro-pharynx
112
What can S.pyogenes cause?
- Pneumonia
- Otisis media
- Sinusitis
- Meningitis
113
What factors can pre-dispose you to a S.pyogenes infection?
- Impaired mucus trapping (e.g. viral infection)
- Hypogammagloninaemia
- Asplenia
- Diabetes
- Renal disease
- Sickle cell disease
- <2yrs
114
Why can asplenia make you pre-disposed to S.pyogenes?
Spleen produces tuftsin, which enhances phagocytosis. Therefore without a spleen, this is impaired and the host is susceptible to infection.
115
What are the pathogenic factors of S.pyogenes?
- Polysaccharide capsule - anti-phagocytic and there are 84 types, 61 of which are unvaccinated against
- Teichoic acid - binds to choline receptors
- Peptidoglycan - protects bacteria
- Pneumolysin cytotoxin - pore-forming
116
What are viridans streptococci?
Collective name for oral streptococci
117
What type of haemolysis do viridans streptococci show?
Alpha and gamma haemolysis
118
Which type of viridans streptococci are responsible for infective endocarditis?
- S.sanguinis
| - S.oralis
119
Which group of viridans streptococci are the most pathogenic?
Which bacteria are in this group?
Milleri group;
- S.intermedius
- S.anginosis
- S.constellatus
120
What do the milleri group cause?
Deep organ abscesses, e.g. in brain and liver
121
What is a common presentation for a corynebacterium diptheriae infection?
- Child with severe sore throat
- Fever & malaise for 2 days
- Lymphadenophathy in neck
- Rapid breathing
- Thick greyish membrane on tonsils
- Swab shows G+ve bacilli
- Treat with antitoxin and erythromycin
122
How does corynebacterium diptheriae spread?
| Explain the spread
Droplet spread
Caused by production of a toxin which inhibits protein synthesis
123
What is the prevention for corynebacterium diptheriae infection?
Vaccination with toxoid
124
Describe the structure of a lipopolysaccharide.
Lipid A - toxic portion that is anchored in the outer leaflet of the outer membrane
Core (R) antigen - short chain of sugars
Somatic (O) antigen - highly antigenic repeating chain of oligosaccharides
125
What are coliforms?
Bacteria that are normally found in the environment and in human faeces
126
What can you use to differentiate between lactose fermenting and non-lactose fermenting bacteria?
Macconkey agar
127
E.coli are commensal enterobacteria. What does this mean?
They are present in the host, but don't cause disease in normal circumstances
128
What does 'facultative' mean?
What does 'facultative anaerobe' mean?
Facultative = is able to
Facultative anaerobe = able to respire aerobically and anaerobically
129
How do E.coli move about?
Flagella
130
What infections are caused by pathogenic strains of E.coli?
- Wound infections
- Cystitis
- Gastroenteritis
- Traveller's diarrhoea
- Bacteraemia
- Meningitis (in infants)
131
What do pathogenic and commensal serotypes / strains of E.coli have in common?
They all share a common 'core' genome
132
What is the difference between pathogenic E.coli and commensal E.coli?
Pathogenic E.coli contain blocks of genes not found in commensal E.coli
133
How do E.coli become pathogenic?
They acquire their pathogenicity by mating with other bacteria and acquiring their blocks of genes
134
What are the pathogenic factors for E.coli causing traveller's diarrhoea?
- Strain is enterotoxigenic E.coli (ETEC)
- Have pili, which enable it to adhere to tissue of SI
- When it adheres to SI tissue, it releases a labile toxin that alters the role of the Gs protein on the GI cell surface
- This means it can no longer stimulate adenyl cyclase, resulting in more Cl- being released into GI lumen
- Causes watery diarrhoea
135
Which other enterobacteria is closely related to E.coli?
Shigella
136
What are the four main species shigella?
- S.dysenteriae
- S.flexneri
- S.boydii
- S.sonnei
137
Shigella damages the intestinal mucosa, which results in...
- Acute infection of LI
| - Painful diarrhoea, often with blood and mucus within it
138
Describe the pathogenesis of shigella infections
- Acid-tolerant, so survive the stomach acid
- Infective dose = 100 (very low)
- Spread from person-to-person or via contaminated water / food
- Target microfold (M) cells for transport across the GI epithelium
- Cross epithelial cell layer, and are engulfed by macrophages
- Induce apoptosis in macrophages, causing release of free radicals, resulting in inflammatory response and cell damage
- Release shiga toxin which disrupts protein synthesis, causing necrosis
139
Where are microfold (M) cells found?
- In gut-associated lymphoid tissue (GALT) in Peyer's patches
- In mucosa-associated lymphoid tissue (MALT)
140
Briefly describe the action of the shiga toxin
- Disrupts protein synthesis, which can cause necrosis
| - Targets the kidney, resulting in haemolytic uraemia syndrome
141
What are the two main species of salmonella?
- S.enterica
| - S.bongori
142
Which species of salmonella is responsible for salmonellosis?
S.enterica
143
What is salmonellosis?
Any infections caused with salmonella
144
How do you catch S.bongori?
Contact with reptiles
145
Which three infections are caused by salmonella?
- Gastroenteritis
- Enteric fever (typhoid)
- Bacteraemia
146
Describe the clinical presentation of gastroenteritis.
- Frequent cause of food poisoning from milk and poultry
- 6-36 hour incubation
- Resolves in a week
- Localised infection
147
Describe the clinical presentation of enteric fever.
- Systemic disease
- Caused by Salmonella type and Salmonella paratyphi
- Spread: faecal-oral
- Results in fever, headache, diarrhoea & dry cough
- Splenomegaly and hepatomegaly
148
Describe the pathogenesis of (general) salmonellosis.
- Ingestion of contaminated water / food
- High infective dose
- Salmonella mediates its endocytosis across gut lumen
149
Describe the pathogenesis of salmonellosis in gastroenteritis.
- Bacteria presence results in chemokine release and neutrophil recruitment
- Results in neutrophil-induced tissue injury due to inflammatory response
- Fluid and electrolyte loss due to cell damage, resulting in diarrhoea
- Eventual inflammation / necrosis of gut mucosa
150
Describe the pathogenesis of salmonellosis in enteric fever.
- Migrates to the basolateral membrane of cells in the intestinal lumen (Peyer's patch)
- Causes inflammation, ulceration and diarrhoea
- Little damage to gut mucosa initially
- Bacteria is engulfed and survives, then spreads systemically via lymph nodes
- Enters bloodstream via thoracic duct, the multiplies in macrophages of liver, spleen, and bone marrow
- Results in septicaemia and massive fever
- Spreads to gall bladders from liver, where person can be a carrier for 1yr-lifetime
151
Is Klebsiella pneumoniae opportunistic or not?
Opportunistic
152
Which nosocomial infections does Klebsiella pneumoniae cause?
- Pneumonia
- Bloodstream infections
- Wound / surgical site infections
- Meningitis
153
How do Vibrio cholerae respire?
They are facultative anaerobes
154
Describe the structure of vibrio cholerae?
Curved bacilli with single polar flagellum
155
What does vibrio cholerae cause?
Cholera
156
How is vibrio cholerae transmitted?
Faecal-oral route
157
Does vibrio cholerae have a high or low ID?
High, it is sensitive to acid so requires a lot to pass through the stomach
158
What is the incubation time for V.cholerae?
5 hours - it multiplies in the SI
159
What is the main symptom of a V.cholerae infection?
Voluminous "rice-water" stools
160
Why can cholera cause death?
Causes patients to lose 20L fluid a day plus electrolytes, causing dehydration and death
161
How are the majority of cholera patients treated?
80% treated with oral rehydration
162
What are the virulence determinants of V.cholerae?
- Pilli, for colonissation
| - Cholera toxin (acts in the same way as E.coli labile toxin)
163
How does the cholera toxin cause its harmful effects?
- Results in uncontrolled cAMP production
- Which causes activation of protein kinases
- Which causes modification of ion transporter
- Therefore there is loss of Cl- and Na+
= H₂O loss
164
What localised acute infections can pseudomonas aeruginosa cause?
- Burns / surgical wound infections
- UTIs (catheters)
- Keratitis
165
Who tends to get acute systemic infections caused by P.aeruginosa?
Neutropenic patients (e.g. leukaemia, chemotherapy, AIDs, etc)
166
What is the leading cause of nosocomial pneumonia in ICU patients?
Pseudomonas aeruginosa
167
Who tends to get chronic infections caused by P.aeruginosa? Why?
Cystic fibrosis patients
They have poor functioning Cl- transporters meaning they have dehydrated lung mucus, which is the perfect environment for bacteria to grow
168
Why is P.aeruginosa hard to eradicate in cystic fibrosis patients?
It's antibiotic resistant
169
Where is commensal haemophilus influenzae found?
Nasopharyngeal carriage in 25-80% of the population
170
What can haemophilus influenzae cause in under 5 year olds? How?
If haemophilus influenzae crosses the BBB, it causes meningitis
171
What can H.influenzae cause?
- Meningitis (<5yrs)
- Bronchopneumonia
- Epiglottitis, sinusitis, otitis media
- Bacteraemia
- Lung infections (CF/COPD patients)
172
What does 'fastidious' mean?
Specific requirements for survival
173
What is the only medium H.influenzae can grow on?
| Why?
Chocolate agar - blood agar heated to 80ºc to allow release of haem by RBCs
174
What does H.influenzae need to grow?
Haem
175
How does H.influenzae move around?
It doesn't - it's non-motile
176
What are the virulence determinants for H.influenzae
- Pilli - adherence to epithelial cells and mucin
- Capsule (invasive strains only)
- LPS - causes inflammation
177
Name two beta-proteobacteria.
- Bordetella pertussis
| - Neisseria
178
What bacteria causes whooping cough?
Bordetella pertussis
179
How is B.pertussis transmitted?
Via aerosol
180
What is the clinical presentation of whooping cough?
- Non-specific flu-like symptoms
| - Paroxysmal coughing - cough followed by inhalation resulting in whooping sound
181
What can whooping cough lead to?
Sub-conjunctival haemorrhage
182
Describe the structure of Neisseria.
Non-flagellated diplococci
183
What are the two important species of neisseria?
- N.meningitidis
| - N.gonorrhoeae
184
Where can you find neisseria bacteria during an infection?
- Polymorphonuclear lymphocytes of CSF
| - Urethral discharge
185
Where can you find asymptomatic N.meningitidis in 5-10% of the population?
Nasopharynx
186
How is N.meningitidis transmitted?
Via aerosol transmission
187
Describe the pathogenesis of N.meningitidis.
Crosses nasopharyngeal epithelium and enters bloodstream
188
What happens if low or high numbers of N.meningitidis enter the blood stream?
Low numbers: asymptomatic bacteraemia
High numbers: septicaemia
189
How do you get meningitis from N.meningitidis?
If it crosses the BBB and colonises in the subarachnoid space
190
What is the mortality risk if you are septicaemic with N.meningitidis?
Very high risk of mortality
191
What are the virulence determinants of N.mengitidis?
- Capsule (non-capsulated in nasopharynx, capsulated is anti-phagocytic)
- Pilli - promote colonisation and cell invasion
- LPS - cytokine cascade and inflammatory response which leads to sepsis
192
How is N.gonorrhoeae transmitted?
Person to person
193
What does gonorrhoea cause?
- Urethritis
| - Infection of fallopian tubes (if bacteria ascends)
194
Is N.gonorrhoeae capsulated or non-capsulated?
Non-capsulated
195
Give two examples of campylobacter.
- C.jejuni
| - C.coli
196
Does campylobacter have a high or low infectious dose?
Low
197
How do you get campylobacter infections?
- Undercooked poultry
| - Unpasteurised milk
198
What symptoms do you get with a campylobacter infection?
Mild-severe diarrhoea with blood
- Self-limiting within a week
199
What are the virulence factors of campylobacter?
- Invasins
| - Cytolethal distending toxin
200
How do campylobacter invasins work?
Invade ill and colonic epithelial cells, causing local acute inflammatory response
201
How does cytolethal distending toxin (CDT) cause disease?
Arrests cell cycle, meaning target cells swell and lyse
202
Give two examples of e-proteobacteria.
- Campylobacter
| - Helicobacter pylori
203
Describe the shape of H.pylori.
Spiral shaped
204
What percent of the population is H.pylori present in?
50% - only a fraction will develop the disease
205
What can H.pylori lead to?
- Gastritis
| - Peptic ulcers
206
What is the virulence factor of H.pylori?
Urease - hydrolyses urea to generate ammonia to act as a buffer to gastric acid
207
Describe the two developmental stages in the growth cycle of chlamydia.
1. Elementary bodies (EBs) - DORMANT
- infectious
- enter cell through endocytosis
- prevent phagosome fusion
2. Reticulate bodies (RBs) - ACTIVE and FRAGILE
- replicative
- non-infectious
208
What happens if Chlamydia trachomatis spreads to the uterus / ovaries?
Pelvic inflammatory disease
209
What infections can chlamydia trachomatis cause?
- Conjunctivitis
| - Trachoma (blindness)
210
What are the most common G-ve bacteria responsible for respiratory tract infections?
- Bordetella pertussis
| - Haemophilus influenzae
211
What are the most common G-ve bacteria responsible for urinary tract infections?
- Some E.coli strains
| - Klebsiella pneumoniae
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What are the most common G-ve bacteria responsible for GI tract infections?
- Vibrio cholerae
- Shigella dysenteriae
- Some E.coli strains
- Campylobacter jejune
- Helicobacter pylori
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What are the most common G-ve bacteria responsible for meningitis?
- Neisseria meningitidis
| - Haemophilus influenzae
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What are the most common G-ve bacteria responsible for STIs?
- Klebsiella pneumoniae
| - Chlamydia trachomatis
215
What are the most common G-ve bacteria responsible for wound infections?
- Pseudomonas aeruginosa
| - Some E.coli strains
216
Are fungi eukaryotic or prokaryotic? Why?
Eukaryotic - they have a nuclear membrane
217
What is the cell wall of fungi made of?
Chitin
218
What does 'heterotrophic' mean?
Get nutrients from what they are living on
219
Which type of pathogen is heterotrophic?
Fungi
220
How to fungi travel?
Growth or spore release
221
What are yeast?
Small, single cell organisms that divide by budding
222
What do moulds form?
Multicellular hyphae and spores
223
What are dimorphic fungi?
Fungi which exist as yeasts and moulds, switching between the two when conditions suit
224
Give an example of a dimorphic fungus and the conditions required for each form.
Coccidioides immitis
- Mould at ambient temperatures
- Yeast at body temperature
225
Why do only a few fungi have an effect on humans?
- Inability to grow at body temperature
| - Cannot evade adaptive / innate immune response
226
What is the burden of fungal disease on healthcare?
Enormous, since most will have at least one during their life
227
Which three conditions can lead to fungal infections becoming life-threatening / invasive?
- Immunocompromised patients
- Post-surgical patients
- Healthy hosts (only certain types of fungi)
228
Which fungal diseases are life-threatening / invasive to immunocompromised patients?
- Candida line infections
- Pneumocystis
- Invasive aspergillosis
229
Which fungal diseases are life-threatening / invasive to post-surgical patients?
Intra-abdominal fungal infections
230
Which fungal diseases are life-threatening / invasive to healthy hosts?
- Fungal asthma
| - Travel associated fungal infections
231
What is the aim of antimicrobial drug therapy?
To achieve inhibitory levels of agent at site of infection, without host cell toxicity
232
What does antimicrobial drug therapy need to be effective?
A molecule with selective toxicity for organism targets
233
What does an antimicrobial drug molecule rely on to work?
- Target does not exist in humans
- Target is significantly different to human analogue
- Drug is concentrated in organism cell with respect to humans
- Organism has an increased permeability to the compound
- Human cells are 'rescued' from toxicity by alternative metabolic pathways
234
Why is selective toxicity easier to achieve with fungi than bacteria?
Fungi are eukaryotic cells, and are therefore more similar, thus harder to differentiate from human cells
235
Describe fungal nucleus.
- DNA/RNA synthesis and protein synthesis
| - Similar to mammalian
236
Which drug targets fungal nuclei?
Flucytosine
237
Name four molecules within the cell wall of fungi.
- Mannoproteins
- B1,3-glucan
- B1,6-glucan
- Chitin
238
Which drugs target cell walls in fungi?
Echinocandins
239
What is the difference between human cell membranes and fungal cell membranes?
Humans' contain cholesterol, fungi's contain ergosterol
240
Name six families of drugs used to fight fungal infections.
- Polyenes
- Allylamines
- Azoles
- Echinocandins
- Pyrimidine
- Grisan
241
What is an example of a polyene drug?
Amphotericin B Nystatin
242
What is the mechanism of action of polyenes?
Bind to sterols and destabilises cell membranes - pore forming compound
243
What would you use polyenes for?
Systemic and topical fungal disease
244
Why can polyenes be toxic to humans?
Although they have a 10x higher affinity for ergosterol than cholesterol, it can still be toxic in high doses
245
What toxicity do polyenes cause in humans?
- Nephrotoxicity (dose dependent, reversible)
- Distal renal tubular acidosis (hypokalaemia)
- Hyperkalaemia (if infused rapidly)
- Infusion related chills, riggers, hypotension, and acute anaphylactoid reactions
246
What is an example of an allylamine?
Terbinafine
247
What is the mechanism of action for allylamines?
Inhibit ergosterol biosynthesis
| Reversible inhibition of enzyme required for growth of fungus
248
What can you use allylamines for?
Superficial infections, e.g. onychomycocis
249
Are allylamines well absorbed or not?
They are well absorbed, but undergo extensive first phase metabolism, resulting in bioavailability of only 45%
250
What are the main fungi allylamines are used against?
- Candida
| - Aspergillus
251
What are the side effects of allylamines?
- Taste disturbance
| - Deranged LFTs
252
Give some examples of azoles.
- Clotrimazole (active against Candida)
- Miconazole
- Ketoconazole (active against Candida)
- Fluconazole (active against Cryptococcus)
- Itraconazole (active against Aspergillus and dimorphics)
- Voriconazole
- Posaconazole
253
What is the mechanism of action for azoles?
Inhibit ergosterol biosynthesis
254
What can you use azoles for?
Topical or systemic fungal diseases
255
What are the side effects of azoles?
- Transaminitis (elevated transaminases in liver)
- Hepatitis
- Alopecia (long term fluconazole)
- GI: Nausea, abdominal pain, diarrhoea (common with itraconazole)
256
What are the drug interactions for fluconazole?
- Warfarin
- Calcineurin inhibitors
- Anxiolytics
257
What are the drug interactions for itraconzaole?
- Same as fluconazole
- Steroids
- Statins
258
Give three examples of echinocandins.
- Caspofungin
- Anidulafungin
- Micafungin
259
What is the mechanism of action of echinocandins?
Inhibit the formation of cell wall glucan
260
What can you use echinocandins for?
Systemic diseases
261
Why can't you use echinocandins for certain types of fungal infections?
Fungi genera that have little glucan in cell wall are relatively unaffected by echinocandins
262
How are echinocandins best administered?
IV; they have poor oral bioavailability
263
What is easier to culture from blood; fungi or bacteria?
Bacteria; blood cultures are only half as sensitive for fungi as for bacteria
264
What is the best way to culture candida?
From tissue / fluids
But only if cultured properly and if these samples can be obtained
265
Which drugs can you use on Candida?
Echinocandins and amphotericin B
266
Which element of fungi can you identify from serum samples?
1,3 B-D glucan
267
What is onychomycosis?
Very common fungal infection of the nail
268
What is the most common cause of onychomycosis?
Trichophtyon rubrum
269
What are the differential diagnoses for onychomycosis?
- Psoriasis
- Lichen planus
- Trauma
- Eczema
- Malignant melanoma
270
What are the treatment options of onychomycosis?
- Topical amorolfine
| - Systemic itraconazole or terbinafine
271
What are the down sides to treatment of onychomycosis?
- Takes ages
| - High failure rate
272
What is pneumocystis?
A form of pneumonia caused by a yeast-like fungus
273
Who is vulnerable to develop pneumocystis?
Moderate-severe immunocompromised patients (esp. HIV, transplant, and steroids)
274
What clinical presentation would make you think 'pneumocystis'?
When a patient has hypoxia that is more severe than the CXR would suggest
ESPECIALLY with gradual onset or risk factors
275
How do you treat pneumocystis?
- Co-trimoxazole
- Clindamycin
- Pentamidine
- Trimetrexate
276
When is a person likely to become colonised by the fungus responsible for pneumocystis?
During early life
277
What are the five mycobacteria of medical importance?
| What do each of them cause?
- M.tuberculosis - tuberculosis
- M.leprae - leprosy
- M.avium complex - disseminated infections in AIDs, and patients with chronic lung disease
- M.kansasii - chronic lung infections
- M.marinum - fish tank granuloma
- M.ulcerans - Buruli ulcer
278
Are mycobacteria aerobic or anaerobic?
Aerobic
279
What can you not use to destain mycobacteria?
- Acid
| - Alcohol
280
Tuberculosis can cause meningitis. True or false?
True
281
Are mycobacteria resistance to phagolysosomal killing?
Yes
282
Describe the cell wall of mycobacteria.
- High content of high molecular weight lipids
- Weakly gram-positive or colourless
- Contains lipoarabinomannan and mycolic acids - make up strong waxy cell wall which is hard for immune system to damage
- Survives inside macrophages and low pH environments
283
Why is it difficult to stain mycobacteria?
High lipid content with mycolic acids in cell wall makes Mycobacteria resistant to gram stain
284
How long does it take to generate one new generation of mycobacteria?
15-20 hours (compared to 1 hour for common bacteria)
285
Why does slow growth of mycobacteria affect antibiotic efficacy?
It is more difficult for antibiotics to target division phase of mycobacteria
286
Who initially identified tuberculosis?
Robert Koch
287
What are the four Koch postulates?
1. Bacteria should be found in all people with disease
2. Bacteria should be isolated from the infected lesions in people with disease
3. A pure culture inoculated into a susceptible person should produce symptoms of the disease
4. The same bacteria should be isolated from the intentionally infected individual
288
What stain should you use to identify mycobacteria?
| What is the result of a positive test?
Ziehl-Neesen stain - pink/red stain for acid-fast mycobacteria
289
What method of identification is used to quickly diagnose TB in TB endemic countries?
Nucleic acid detection
- PCR amplifies TB DNA
290
What happens when a macrophage phagocytoses mycobacteria?
- Mycobacteria are engulfed into phagosome, which forms a phagolysosome
- Bacteria has adapted to withstand phagolysosomal killing and escape into the cytosol
291
Describe the immune response to mycobacteria.
1. Macophages phagocytose mycobacteria
2. Acidification and degradation of mycobacteria by proteases results in generation of antigens for presentation to T cells
292
Effective immunity to mycobacteria requires...
CD4 T-cells which generate interferon gamma. This helps activate intracellular killing by macrophages
293
What do macrophages release when they phagocytose mycobacteria?
IL-12
294
How does IL-12 aid the immune response to mycobacteria?
It stimulates the generation of T helper cells and interferon gamma release
295
Name two genetic defects that increase susceptibility to mycobacterial infection.
Defects in...
- interferon gamma / IL-12 receptors
- elements of their signalling pathways
296
What are granuloma?
Lesions that arise in response to trying to contain mycobacteria
297
How do granuloma affect macrophages?
They stimulate macrophages to become epithelioid cells
298
What is a 'langhans giant cell'?
Multinucleate cells formed of fused macrophages
299
Which cells are able to infiltrate granuloma?
Cytotoxic CD8 T-cells
300
What can happen to central tissue in a granuloma?
It can necrotise and form a caveating granuloma
- In the lung, this can cause a cavity
301
How does the formation of a granuloma affect the mycobacteria within it?
Granuloma prevents nutrients from entering, thereby starving the mycobacteria. TB goes into dormant state, but can reactivate when conditions are optimal
302
The highly immunogenic nature of mycobacterial lipids stimulate T-cell responses ___ weeks after exposure to M.tuberculosis
3-9 weeks
303
What are the positive effects of mycobacterial lipids stimulating an immune response?
- Macrophage killing of mycobacteria
- Containment of infection
- Formation of tissue granuloma
304
What are the negative effects of mycobacterial lipids stimulating an immune response?
- Hypersensitivity reaction (type 4), with skin lesions, eye lesions, and joint swelling
305
How can you measure the reactivity of T-cells to mycobacterial lipids?
Tuberculin skin test or interferon gamma release assays
306
What is the tuberculin skin test?
Intradermal injection of purified protein derivatives, inducing swelling and redness
307
What are IGRAs? Describe them.
Interferon gamma release assay
- Use antigens specific to M.tuberculosis to distinguish between this and BCG vaccine / environmental mycobacteria
- IGRAs demonstrate exposure to M.tuberculosis but NOT active infection
308
What are the four principles of mycobacteria treatment?
1. Prolonged treatment (6 months) - slowly replicating bacteria
2. Different populations of bacteria, in different locations (intracellular and extracellular), with different pH
3. Resistance may emerge so use multi-drug combinations to target all populations and mutants
4. Compliance is essential - directly observe treatment for many patient groups
309
What is the standard therapy for tuberculosis?
- Isoniazid (INH), Rifampicin (RIF), Pyrazinamide (PZA), and Ethambutol (ETH) for two months
- Isoniazid and rifampicin for a further four months
310
What is the treatment for tuberculosis if resistance develops?
- Fluroquinolones
- Injectable agents (e.g. streptomycin, cyclosporine, and capreomycin)
- Prothionamide
311
What are the side effects of anti-tuberculous drugs?
- Hepatotoxicity (isoniazid, rifampicin, pyrazinamide)
- Peripheral neuropathy (isoniazid) - give vitamin B6 to prevent
- Optic neuritis (ethambutol)
312
Describe primary tuberculosis.
- Bacilli settle in lung apex and form granulomas
| - Bacilli taken in by lymphatics to hilar lymph nodes
313
What is a 'primary complex' in primary tuberculosis?
Granuloma + lymphatics + lymph nodes = primary complex
314
Why do tuberculous bacilli settle in apex of lungs?
More air and less blood supply, thus fewer defending white cells
315
Describe latent tuberculosis
- Cell mediated immune (CMI) response from T-cells
- Primary infection is maintained by CMI persists
- No clinical disease (normal CXR)
- Detectable CMI to TB on tuberculin skin test / IGRA
316
Describe pulmonary tuberculosis
- Could occur immediately following primary disease (post-primary) or after latent reactivation
- CMI response from T-cells
- Necrosis in lesion
- Caseous material coughed up leaving cavity
- TB may spread
- CMI and caseation in lesion results in cavity
317
How does TB spread from the lungs to other parts of the body?
Bacilli spreads from lung apex to hilar lymph nodes, and then to anywhere else
318
What can a spread of TB cause?
- TB meningitis
- Miliary TB (widespread dissemination and tiny spotted lesions all over lungs and elsewhere)
- Pleural TB
- Bone and joint TB
- Genitourinary TB
