Blood agar is a
Answer: B
Correct answer: enriched media
Blood agar is nutrient agar with added blood, supplying the extra growth factors that fastidious organisms such as streptococci need - so it is an enriched medium. It happens to be differential as well, since the haemolysis patterns distinguish alpha, beta and gamma streptococci.
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Salt agar is a
Answer: C
Correct answer: selective media
Salt (mannitol salt) agar contains about 7.5% sodium chloride, which inhibits most bacteria while staphylococci tolerate it. Suppressing the unwanted flora so that one group can grow is exactly what makes a medium selective.
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Nutrient agar is prepared by addition of
Answer: B
Correct answer: 3% agar in nutrient broth
Agar is added at about 2-3% to set nutrient broth into a solid plate. Less than that gives a soft or semi-solid medium, used for motility testing; more would make the gel needlessly hard and hard to pour. Agar itself is not a nutrient - almost no bacterium can digest it, which is precisely why it is used as the gelling agent.
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Differential media contain all of the following EXCEPT
Answer: B
Correct answer: Substances that inhibit microorganisms
A differential medium contains a basal medium, the substrate in question (often a sugar) and an indicator to reveal what has been done to it - it lets everything grow and tells the colonies apart. Inhibitory substances belong to a selective medium, whose purpose is the opposite: to stop unwanted organisms growing at all.
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Growth factors are
Answer: D
Correct answer: Essential substances that bacteria cannot synthesise for themselves
A growth factor is an organic compound - a vitamin, amino acid, purine or pyrimidine - that an organism must have but cannot make, so it has to be supplied in the medium. Mineral salts and the bulk elements are nutrients, but they are not growth factors.
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Bacteria that cannot grow in the presence of oxygen are
Answer: A
Correct answer: Strict anaerobes
Strict (obligate) anaerobes are killed by oxygen, because they lack the enzymes that dispose of the toxic products of oxygen chemistry. Strict aerobes require it; facultative anaerobes grow with or without it; microaerophiles need it but only at low concentration.
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Strict anaerobes lack which of the following enzymes?
Answer: D
Correct answer: All the above
Oxygen metabolism produces superoxide and hydrogen peroxide, and three enzymes normally deal with them: superoxide dismutase, catalase and peroxidase. Strict anaerobes lack them, so these products accumulate and destroy the cell - which is why oxygen is lethal to such organisms.
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Lysis reaction is a
Answer: D
Correct answer: all of the above
A lysis reaction is antibody plus complement dissolving a cell, and it is named after whatever is dissolved: haemolysis of red cells, spirochaetolysis of spirochaetes, vibriolysis of vibrios. The mechanism is the same in each - the membrane attack complex of complement.
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How many systems does the complement fixation reaction include?
Answer: A
Correct answer: 2 systems
The test uses two systems. The first is antigen, antibody and complement; if they match, the complement is consumed. The second - the haemolytic indicator system of sheep red cells and haemolysin - is then added to reveal whether any complement is left.
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The haemolytic system of the complement fixation reaction consists of
Answer: D
Correct answer: 3 per cent suspension of sheep erythrocytes and hemolytic serum
The indicator system has two components: a 3% suspension of sheep red cells and the haemolytic serum (amboceptor) raised against them. Together they will lyse only if free complement remains - which is what makes them an indicator.
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