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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
Q

What is the result of the catalase test for many gram-negative bacteria?

A

Catalase positive.

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26
Q

What is coagulase?

A

An enzyme produced by Staphylococcus. aureus that converts soluble fibrinogen into insoluble fibrin.

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27
Q

The coagulase test is used to differentiate between which bacteria?

A 
Staphylococcus. aureus (coagulase positive). 
Other staphylococci (coagulase negative).
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28
Q

Describe a positive result of the coagulase test.

A

Clumping.

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29
Q

What are haemolysins?

A

Lipids and proteins that cause lysis of red blood cells by destroying their cell membrane.

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30
Q

Describe the haemolysis test.

A

A bacterial colony’s ability to induce haemolysis when grown on blood agar is used to differentiate between bacteria types.

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31
Q

State the three possible results of a haemolysis test.

A

Alpha haemolysis.
Beta haemolysis.
Gamma haemolysis.

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32
Q

Describe how alpha haemolysis presents.

A

An indistinct zone of partial destruction of red blood cells around the colony, accompanied by greenish/brownish discolouration of the medium.

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33
Q

Give examples of alpha haemolytic bacteria.

A

Streptococcus pneumoniae.
Viridans streptococcus.
Many oral streptococci.

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34
Q

Describe how beta haemolysis presents.

A

A clear, colourless zone appears around the colonies, in which the red blood cells have undergone complete lysis.

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35
Q

Give examples of beta haemolytic bacteria.

A

Streptococci pyogenes.
Streptococci agalactiae.
Staphylococcus aureus.

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36
Q

Describe how gamma haemolysis presents.

A

No haemolysis.

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37
Q

What test can be performed to differentiate between Streptococcus pneumoniae and viridans streptococci?

A

Optochin test.

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38
Q

Describe how the optochin test is performed.

A

An optochin soaked disc is placed on agar and bacterial growth is observed.

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39
Q

Describe how bacteria sensitive to optochin will present.

A

Clear zone around the optochin disc.

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40
Q

What are the results of the optochin test to differentiate Streptococcus pneumoniae and viridans streptococci?

A

S. pneumoniae is sensitive - clear zone around disc.

Viridans streptococci is resistant - growth around disc.

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41
Q

What does the oxidase test identify the presence of?

A

The oxidase test is used to identify whether cytochrome oxidase is present in a microorganism.

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42
Q

Describe the result of a positive and negative oxidase test.

A

Positive oxidase test is blue.

Negative oxidase test has no colour change.

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43
Q

What is cytochrome oxidase?

A

An enzyme of the bacterial electron transport chain.

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44
Q

Which bacteria produce a positive oxidase test?

A

Aerobic bacteria.

45
Q

Which bacteria produce a negative oxidase test?

A

Aerobic and anaerobic bacteria.

46
Q

Macconkey agar is used to differentiate which bacteria?

A

Gram-negative/-positive bacilli and lactose-fermenting/non-fermenting bacilli.

47
Q

How does Macconkey agar identify gram-negative bacilli?

A

Macconkey agar only grows gram-negative bacilli.

48
Q

How does Macconkey agar identify lactose-fermenting bacteria?

A

Agar has a pH indicator. Lactose-fermenting bacteria produce acid and make agar appear pink/red.

49
Q

How does Macconkey agar identify non-lactose-fermenting bacteria?

A

Agar will appear white/transparent.

50
Q

Give an example of a gram-negative lactose-fermenting bacteria.

A

Escherichia coli.

51
Q

Give two examples of gram-negative non-lactose-fermenting bacteria.

A

Salmonella spp.

Shigella spp.

52
Q

How are salmonella and shigella differentiated?

A

With the use of XLD agar.

53
Q

How do salmonella and shigella present on XLD agar?

A

Salmonella: red/pink colonies with black spots.
Shigella: red/pink colonies.

54
Q

Bacteria structure: what is the capsule?

A

A polymer of sugar that protects bacteria from host immune system.

55
Q

Describe the structure of DNA in bacteria.

A

Bacteria have circular DNA (not contained within a nucleus).

56
Q

Describe the difference between cocci and bacilli?

A

Cocci are round and spherical.

Bacilli are rod-like and straight.

57
Q

Describe three possible arrangements of cocci.

A

Diplococcus - bacterium appears in form of two joined cells.
Streptococci - chains of cocci.
Staphylococci - clusters of cocci.

58
Q

Describe three possible arrangements of bacilli.

A

Streptobacilli - chains of rods.
Vibrio - curved rods.
Spirochaete - spiral rods.

59
Q

How is the growth rate of bacteria measured?

A

By shining light on bacteria and measuring absorbance.

60
Q

By what method do bacteria divide?

A

Binary fission.

61
Q

What is the bacterial growth lag due to?

A

Bacteria must first take in nutrients needed to divide and grow.

62
Q

What are the four stages of bacterial growth?

A

Lag phase.
Exponential/log phase.
Stationary phase.
Death phase.

63
Q

The length of the exponential phase is dependent on what factor?

A

Amount of nutrients.

64
Q

Bacterial toxins can be classified into which two types?

A

Endotoxin.

Exotoxin.

65
Q

Endotoxin is a component of what bacterial structure?

A

The outer, lipopolysaccharide membrane of gram negative bacteria.

66
Q

What is endotoxic shock?

A

When the host immune system recognises the endotoxin it has a huge response (endotoxic shock / septic shock).

67
Q

Of the actions of endotoxin and exotoxin, which is specific?

A

Endotoxins are non-specific.

Exotoxins are specific.

68
Q

Give two examples of specific exotoxins.

A

Botulinum toxin inhibits the nervous system (causing botulism).
Tetanus toxin stimulates the nervous system (causing tetanus).

69
Q

Of endotoxin and exotoxin, which is stable on exposure to heat?

A

Endotoxin is stable on exposure to heat while exotoxin is unstable.

70
Q

Of endotoxin and exotoxin, which can be converted to a toxoid?

A

Exotoxin.

71
Q

What is a toxoid?

A

A non-active toxin.

72
Q

What is the function of a toxoid?

A

Though toxoids are non-active, immunogenicity is maintained meaning it can be used for vaccinations.

73
Q

What is exotoxin secreted from?

A

Gram positive and gram negative bacteria. Mainly gram+.

74
Q

What is a plasmid?

A

Small circular piece of DNA.

75
Q

What do plasmids carry?

A

Antibiotic resistant genes.

76
Q

Describe three methods of gene transfer.

A

Transformation.
Transduction.
Conjugation.

77
Q

Gene transfer: describe transformation.

A

The genetic alteration of a bacterial cell via the uptake of an exogenous substance (such as a plasmid).

78
Q

Gene transfer: describe transduction.

A

The process by which foreign DNA is introduced into a bacteria via vector or virus.

79
Q

Gene transfer: describe conjugation.

A

The transfer of genetic material between bacterial cells by direct cell-cell contact.

80
Q

Give an example of how conjugation occurs through direct cell-cell contact.

A

Sex pilus.

81
Q

What are the possible results of a coagulase test on staphylococcus bacteria?

A

Coagulase positive (clumping) and coagulase negative (no clumping).

82
Q

Explain why clumping occurs in a positive coagulase test.

A

Some bacteria (staphylococcus aureus) produce coagulase to protect against white blood cells. Coagulase converts soluble fibrinogen to insoluble fibrin.

83
Q

How does staphylococcus aureus typically present?

A

Shoulder pain.
Elevated temperature.
Osteomyelitis (of C6 and C7).

84
Q

How is staphylococcus aureus treated?

A

With flucloxacillin.

85
Q

What is MRSA resistant to?

A

B-lactams antibiotics.
Gentamicin.
Erythromycin.
Tetracycline.

86
Q

Give four pathogenic factors of staphylococcus aureus.

A

Pore-forming toxins.
Proteases.
Toxic shock syndrome toxin.
Protein A.

87
Q

Give two examples of pore-forming toxins and describe their effect.

A

PVL toxin causes haemorrhage pneumonia.

Alpha-haemolysin can induce apoptosis or cause widespread necrosis.

88
Q

Give an example of a protease produced by staphylococcus aureus and describe its effect.

A

Exfoliatin causes scalded skin syndrome.

89
Q

What is the effect of toxic shock syndrome toxin?

A

Stimulates cytokine release.

90
Q

What is the effect of Protein A (released by staphylococcus aureus)?

A

Surface protein which binds immunoglobulins in the wrong direction.

91
Q

Give two examples of coagulase negative staphylococci.

A

Staphylococcus epidermis.

Staphylococcus saprophyticus.

92
Q

Give three ways by which streptococci can be classified.

A

Haemolysis.
Lancefield typing.
Biochemical properties.

93
Q

How are streptococci differentiated by haemolysis?

A 
Alpha haemolysis (green/brownish) - Strep. intermedius.
Beta haemolysis (clear/colourless) - Strep. pyogenes.
Gamma haemolysis (no lysis) - Strep. mutants.
94
Q

Lancefield typing is used to group which types of bacteria?

A

Catalase negative, coagulase negative.

95
Q

Lancefield grouping: name a Group A bacteria.

A

Strep. pyogenes.

96
Q

Lancefield grouping: name a Group B bacteria.

A

Strep. agalactiae.

97
Q

Infections of streptococci pyogenes commonly cause what? (4)

A

Wound infections.
Tonsillitis and pharyngitis.
Otitis media.
Scarlet fever.

98
Q

Give examples of complications associated with infections of streptococci pyogenes?

A

Rheumatic fever.
Glomerulonephritis.
Scarlet fever (due to release of erythrogenic toxin).

99
Q

Give five pathogenic factors secreted by streptococci pyogenes.

A 
Hyaluronidase. 
Streptokinase. 
C5a peptidase. 
Streptolysins O & S toxin. 
Erythrogenic toxin.
100
Q

Describe a common presentation of infection by streptococci pneumoniae.

A

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
Q

What is pulmonary consolidation?

A

When a normally compressible region of a lung tissue becomes filled with liquid instead of air.

102
Q

Give four examples of infections caused by streptococci pneumoniae.

A

Pneumonia.
Otitis media.
Sinusitis.
Meningitis.

103
Q

What is otitis media?

A

A middle ear infection.

104
Q

Give examples of predisposing factors to an infection of streptococci pneumoniae.

A 
Impaired mucus trapping. 
Hypogammaglobulinemia. 
Asplenia. 
Diabetes. 
Renal disease. 
Sickle cell disease. 
Very young.
105
Q

Explain why asplenia is a predisposing factor for infection by streptococci pneumoniae?

A

The spleen produces a protein called tuftsin which acts to enhance phagocytosis. No spleen - no tuftsin.

106
Q

Give four pathogenic factors of streptococci pneumoniae.

A

Polysaccharide capsule.
Teichoic acid.
Peptidoglycan.
Pneumolysin.

107
Q

Viridans streptococci is the collective name for what?

A

Oral streptococci.

108
Q

Viridans streptococci belong to which classifications?

A

Alpha or non-haemolytic.

Phase 2a Medicine (JH) (7 decks)

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