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Phase 2a Medicine (JH) > Microbiology > Flashcards

Microbiology Flashcards

1
Q

Define pathogen.

A

Organism that causes or is capable of causing disease.

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2
Q

Define commensal.

A

Organism which colonises the host but causes no disease in normal circumstances.

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3
Q

Define ‘opportunist pathogen’.

A

Microbe that only causes disease if the host defences are compromised.

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4
Q

Define virulence/pathogenicity.

A

The degree to which a given organism is pathogenic.

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5
Q

Define ‘asymptomatic carriage’.

A

When a pathogen is carried harmlessly at a tissue site where it causes no disease.

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6
Q

How does gram stain differentiate bacteria?

A

Gram positive bacteria have a single membrane while gram negative bacteria have a double membrane.

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7
Q

What colour do gram positive bacteria stain?

A

Purple.

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8
Q

What colour do gram negative bacteria stain?

A

Pink.

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9
Q

Describe the process by which a gram stain is performed.

A

Apply primary strain (crystal violet) to a heat fixed bacteria culture. Add iodine which binds to crystal violet. Add ethanol or acetone. Add safranin.

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10
Q

In gram staining: what is the purpose of adding iodine?

A

Iodine binds to crystal violet and helps fix it to the cell wall.

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11
Q

In gram staining: what is the purpose of adding ethanol/acetone?

A

Act as a decolouriser.

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12
Q

What colour is crystal violet?

A

Purple.

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13
Q

What colour is safranin?

A

Pink.

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14
Q

Explain why gram-negative bacteria stain pink.

A

The decolouriser interacts with the lipids and bacteria lose their outer lipopolysaccharide membrane - along with the crystal violet-iodide complexes (lose purple colour). Therefore appear pink.

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15
Q

Explain why gram-positive bacteria stain purple.

A

The decolouriser dehydrates the cell wall, trapping crystal violet-iodide complexes in the multi-layered peptidoglycan. Therefore appears purple.

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16
Q

What are the two main structural differences between gram-positive and gram-negative bacteria?

A

Gram-negative have a double membrane (outer lipopolysaccharide and inner peptidoglycan) while gram-positive have one.
Gram-positive bacteria have a thicker peptidoglycan membrane than gram-negative bacteria.

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17
Q

Give an example of a type of bacteria that cannot be stained by gram stain.

A

Acid-fast bacilli such as mycobacteria (includes tuberculosis).

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18
Q

Mycobacteria can be stained by what?

A

Ziehl-Neelsen stain.

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19
Q

What is the result of Ziehl-Neelsen stain on acid-fast bacilli?

A

Red stain.

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20
Q

What is the result of Ziehl-Neelsen stain on non acid-fast bacilli?

A

Blue stain.

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21
Q

The catalase test is used to differentiate between which two types of bacteria?

A

Staphylococci and streptococci.

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22
Q

How is the catalase test carried out?

A

Add hydrogen peroxide to bacteria.

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23
Q

What is the result of the catalase test for staphylococci?

A

Positive - bubbling reaction.

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24
Q

What is the result of the catalase test for streptococci?

A

Negative - no bubbling reaction.

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25
What is the result of the catalase test for many gram-negative bacteria?
Catalase positive.
26
What is coagulase?
An enzyme produced by Staphylococcus. aureus that converts soluble fibrinogen into insoluble fibrin.
27
The coagulase test is used to differentiate between which bacteria?
``` Staphylococcus. aureus (coagulase positive). Other staphylococci (coagulase negative). ```
28
Describe a positive result of the coagulase test.
Clumping.
29
What are haemolysins?
Lipids and proteins that cause lysis of red blood cells by destroying their cell membrane.
30
Describe the haemolysis test.
A bacterial colony's ability to induce haemolysis when grown on blood agar is used to differentiate between bacteria types.
31
State the three possible results of a haemolysis test.
Alpha haemolysis. Beta haemolysis. Gamma haemolysis.
32
Describe how alpha haemolysis presents.
An indistinct zone of partial destruction of red blood cells around the colony, accompanied by greenish/brownish discolouration of the medium.
33
Give examples of alpha haemolytic bacteria.
Streptococcus pneumoniae. Viridans streptococcus. Many oral streptococci.
34
Describe how beta haemolysis presents.
A clear, colourless zone appears around the colonies, in which the red blood cells have undergone complete lysis.
35
Give examples of beta haemolytic bacteria.
Streptococci pyogenes. Streptococci agalactiae. Staphylococcus aureus.
36
Describe how gamma haemolysis presents.
No haemolysis.
37
What test can be performed to differentiate between Streptococcus pneumoniae and viridans streptococci?
Optochin test.
38
Describe how the optochin test is performed.
An optochin soaked disc is placed on agar and bacterial growth is observed.
39
Describe how bacteria sensitive to optochin will present.
Clear zone around the optochin disc.
40
What are the results of the optochin test to differentiate Streptococcus pneumoniae and viridans streptococci?
S. pneumoniae is sensitive - clear zone around disc. | Viridans streptococci is resistant - growth around disc.
41
What does the oxidase test identify the presence of?
The oxidase test is used to identify whether cytochrome oxidase is present in a microorganism.
42
Describe the result of a positive and negative oxidase test.
Positive oxidase test is blue. | Negative oxidase test has no colour change.
43
What is cytochrome oxidase?
An enzyme of the bacterial electron transport chain.
44
Which bacteria produce a positive oxidase test?
Aerobic bacteria.
45
Which bacteria produce a negative oxidase test?
Aerobic and anaerobic bacteria.
46
Macconkey agar is used to differentiate which bacteria?
Gram-negative/-positive bacilli and lactose-fermenting/non-fermenting bacilli.
47
How does Macconkey agar identify gram-negative bacilli?
Macconkey agar only grows gram-negative bacilli.
48
How does Macconkey agar identify lactose-fermenting bacteria?
Agar has a pH indicator. Lactose-fermenting bacteria produce acid and make agar appear pink/red.
49
How does Macconkey agar identify non-lactose-fermenting bacteria?
Agar will appear white/transparent.
50
Give an example of a gram-negative lactose-fermenting bacteria.
Escherichia coli.
51
Give two examples of gram-negative non-lactose-fermenting bacteria.
Salmonella spp. | Shigella spp.
52
How are salmonella and shigella differentiated?
With the use of XLD agar.
53
How do salmonella and shigella present on XLD agar?
Salmonella: red/pink colonies with black spots. Shigella: red/pink colonies.
54
Bacteria structure: what is the capsule?
A polymer of sugar that protects bacteria from host immune system.
55
Describe the structure of DNA in bacteria.
Bacteria have circular DNA (not contained within a nucleus).
56
Describe the difference between cocci and bacilli?
Cocci are round and spherical. | Bacilli are rod-like and straight.
57
Describe three possible arrangements of cocci.
Diplococcus - bacterium appears in form of two joined cells. Streptococci - chains of cocci. Staphylococci - clusters of cocci.
58
Describe three possible arrangements of bacilli.
Streptobacilli - chains of rods. Vibrio - curved rods. Spirochaete - spiral rods.
59
How is the growth rate of bacteria measured?
By shining light on bacteria and measuring absorbance.
60
By what method do bacteria divide?
Binary fission.
61
What is the bacterial growth lag due to?
Bacteria must first take in nutrients needed to divide and grow.
62
What are the four stages of bacterial growth?
Lag phase. Exponential/log phase. Stationary phase. Death phase.
63
The length of the exponential phase is dependent on what factor?
Amount of nutrients.
64
Bacterial toxins can be classified into which two types?
Endotoxin. | Exotoxin.
65
Endotoxin is a component of what bacterial structure?
The outer, lipopolysaccharide membrane of gram negative bacteria.
66
What is endotoxic shock?
When the host immune system recognises the endotoxin it has a huge response (endotoxic shock / septic shock).
67
Of the actions of endotoxin and exotoxin, which is specific?
Endotoxins are non-specific. | Exotoxins are specific.
68
Give two examples of specific exotoxins.
Botulinum toxin inhibits the nervous system (causing botulism). Tetanus toxin stimulates the nervous system (causing tetanus).
69
Of endotoxin and exotoxin, which is stable on exposure to heat?
Endotoxin is stable on exposure to heat while exotoxin is unstable.
70
Of endotoxin and exotoxin, which can be converted to a toxoid?
Exotoxin.
71
What is a toxoid?
A non-active toxin.
72
What is the function of a toxoid?
Though toxoids are non-active, immunogenicity is maintained meaning it can be used for vaccinations.
73
What is exotoxin secreted from?
Gram positive and gram negative bacteria. Mainly gram+.
74
What is a plasmid?
Small circular piece of DNA.
75
What do plasmids carry?
Antibiotic resistant genes.
76
Describe three methods of gene transfer.
Transformation. Transduction. Conjugation.
77
Gene transfer: describe transformation.
The genetic alteration of a bacterial cell via the uptake of an exogenous substance (such as a plasmid).
78
Gene transfer: describe transduction.
The process by which foreign DNA is introduced into a bacteria via vector or virus.
79
Gene transfer: describe conjugation.
The transfer of genetic material between bacterial cells by direct cell-cell contact.
80
Give an example of how conjugation occurs through direct cell-cell contact.
Sex pilus.
81
What are the possible results of a coagulase test on staphylococcus bacteria?
Coagulase positive (clumping) and coagulase negative (no clumping).
82
Explain why clumping occurs in a positive coagulase test.
Some bacteria (staphylococcus aureus) produce coagulase to protect against white blood cells. Coagulase converts soluble fibrinogen to insoluble fibrin.
83
How does staphylococcus aureus typically present?
Shoulder pain. Elevated temperature. Osteomyelitis (of C6 and C7).
84
How is staphylococcus aureus treated?
With flucloxacillin.
85
What is MRSA resistant to?
B-lactams antibiotics. Gentamicin. Erythromycin. Tetracycline.
86
Give four pathogenic factors of staphylococcus aureus.
Pore-forming toxins. Proteases. Toxic shock syndrome toxin. Protein A.
87
Give two examples of pore-forming toxins and describe their effect.
PVL toxin causes haemorrhage pneumonia. | Alpha-haemolysin can induce apoptosis or cause widespread necrosis.
88
Give an example of a protease produced by staphylococcus aureus and describe its effect.
Exfoliatin causes scalded skin syndrome.
89
What is the effect of toxic shock syndrome toxin?
Stimulates cytokine release.
90
What is the effect of Protein A (released by staphylococcus aureus)?
Surface protein which binds immunoglobulins in the wrong direction.
91
Give two examples of coagulase negative staphylococci.
Staphylococcus epidermis. | Staphylococcus saprophyticus.
92
Give three ways by which streptococci can be classified.
Haemolysis. Lancefield typing. Biochemical properties.
93
How are streptococci differentiated by haemolysis?
``` Alpha haemolysis (green/brownish) - Strep. intermedius. Beta haemolysis (clear/colourless) - Strep. pyogenes. Gamma haemolysis (no lysis) - Strep. mutants. ```
94
Lancefield typing is used to group which types of bacteria?
Catalase negative, coagulase negative.
95
Lancefield grouping: name a Group A bacteria.
Strep. pyogenes.
96
Lancefield grouping: name a Group B bacteria.
Strep. agalactiae.
97
Infections of streptococci pyogenes commonly cause what? (4)
Wound infections. Tonsillitis and pharyngitis. Otitis media. Scarlet fever.
98
Give examples of complications associated with infections of streptococci pyogenes?
Rheumatic fever. Glomerulonephritis. Scarlet fever (due to release of erythrogenic toxin).
99
Give five pathogenic factors secreted by streptococci pyogenes.
``` Hyaluronidase. Streptokinase. C5a peptidase. Streptolysins O & S toxin. Erythrogenic toxin. ```
100
Describe a common presentation of infection by streptococci pneumoniae.
Heavy smoker with nasal congestion and fever. 2 days later gets cough and sever chest pain. Rust-coloured sputum. Chest X-ray shows consolidation.
101
What is pulmonary consolidation?
When a normally compressible region of a lung tissue becomes filled with liquid instead of air.
102
Give four examples of infections caused by streptococci pneumoniae.
Pneumonia. Otitis media. Sinusitis. Meningitis.
103
What is otitis media?
A middle ear infection.
104
Give examples of predisposing factors to an infection of streptococci pneumoniae.
``` Impaired mucus trapping. Hypogammaglobulinemia. Asplenia. Diabetes. Renal disease. Sickle cell disease. Very young. ```
105
Explain why asplenia is a predisposing factor for infection by streptococci pneumoniae?
The spleen produces a protein called tuftsin which acts to enhance phagocytosis. No spleen - no tuftsin.
106
Give four pathogenic factors of streptococci pneumoniae.
Polysaccharide capsule. Teichoic acid. Peptidoglycan. Pneumolysin.
107
Viridans streptococci is the collective name for what?
Oral streptococci.
108
Viridans streptococci belong to which classifications?
Alpha or non-haemolytic.

Phase 2a Medicine (JH) (7 decks)

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