Free Microbiology multiple-choice practice questions with hints and explanations, at three levels: key terms and facts, applying the concepts, cases and analysis.
Who discovered penicillin in 1928?
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Microbiology is the study of organisms too small to see without a microscope. Most of them are harmless or useful: the bacteria in your gut help digest food and keep harmful microbes out.
A microbe that causes disease is a pathogen. The main groups differ in structure, and the structure decides how you treat them:
Answer: Influenza is a virus, which has no cell wall or ribosomes of its own. Antibiotics attack bacterial structures such as the cell wall and bacterial ribosomes. A virus has none of these and uses the host cell’s machinery. Calling influenza a bacterium is the core mistake.
Bacteria are first described by shape: cocci are round, bacilli are rods and spirilla are spirals. Cocci in clusters are typical of staphylococci; cocci in chains are typical of streptococci.
The Gram stain sorts bacteria by their cell wall:
Bacteria also differ in their need for oxygen. Obligate aerobes need it, obligate anaerobes are killed by it (for example Clostridium), and facultative anaerobes grow with or without it.
Bacteria reproduce by binary fission: one cell splits into two. Some, such as Clostridium and Bacillus, form endospores: tough resting forms that survive heat, drying and alcohol.
Many bacteria release exotoxins, proteins with a specific effect. Tetanus toxin causes muscle spasms; cholera toxin causes watery diarrhoea.
Bacteria can resist antibiotics, for example by making an enzyme that destroys the drug (beta-lactamase), pumping it out (efflux pumps) or changing its target. Resistance genes can pass between bacteria.
Answer: Purple, because its thick peptidoglycan wall keeps the crystal violet. A thick peptidoglycan wall holds the purple dye. The outer membrane and endotoxin belong to Gram-negative bacteria, which end up pink.
An infection needs a chain of infection. Breaking any link stops it:
The incubation period is the time from infection to the first symptoms. A disease that passes from animals to humans is a zoonosis. The basic reproduction number R0 is the average number of people one case infects in a fully susceptible population.
Hand hygiene is the most effective single measure against spread in hospitals. Alcohol rub does not kill spores, so after contact with Clostridioides difficile you wash with soap and water.
Sterilisation kills all microorganisms including spores, for example in an autoclave with steam under pressure at about 121 °C. Disinfection reduces microorganisms on objects but may leave spores. Antisepsis does the same on skin or tissue.
Vaccines train the adaptive immune system. They can contain live weakened microbes (MMR), killed microbes or parts of them, or mRNA that makes the cells produce one viral protein.
Answer: Sterilisation, for example in an autoclave. Only sterilisation kills spores. Disinfection and antisepsis reduce the number of microbes but can leave spores; pasteurisation kills many pathogens in food but is not sterilisation.
Most questions describe an organism or a patient. Work from structure to consequence:
Follow the reasoning. Try to predict the next step before you open it.
A hospital patient has had broad-spectrum antibiotics for a week and now has profuse, foul-smelling diarrhoea. What is the likely cause, and how is spread to other patients prevented?
Broad-spectrum antibiotics kill much of the normal gut flora. This gives room to a resistant organism that the normal flora usually keeps in check.
The classic cause is Clostridioides difficile, a Gram-positive, spore-forming, obligately anaerobic bacillus.
It releases toxins that damage the lining of the colon. This causes diarrhoea and, in severe cases, inflammation of the colon.
Spores leave the body in faeces and survive for a long time on hands, toilets and bed rails. They enter the next person through the mouth: faecal–oral spread.
Isolate the patient with contact precautions, wear gloves and an apron, and wash hands with soap and water because alcohol does not kill spores.
Clean the environment with a sporicidal agent, such as a chlorine product, and stop antibiotics that are not needed.
C. difficile infection after loss of normal gut flora; prevent spread with isolation, gloves and apron, soap-and-water hand washing and sporicidal cleaning.
Answer: Washing with soap and water. Spores are not killed by alcohol, so they must be washed off mechanically with soap and water. Repeating the alcohol rub does not solve this, and gloves never replace hand hygiene.
For hard questions, start from the organism’s structure. Wall, envelope, spores and toxins explain most symptoms, test results and treatment choices.
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Which of these is usually the smallest?
Most viruses are about 20–300 nanometres, much smaller than bacteria (around 1–5 micrometres). An electron microscope is usually needed to see them.
What is the main virulence factor of Streptococcus pneumoniae, and the target of its vaccines?
The capsule protects S. pneumoniae from phagocytosis. There are many capsular types, and pneumococcal vaccines contain capsular polysaccharides from the most common ones.
Which feature distinguishes most exotoxins from endotoxin?
Exotoxins are secreted proteins with specific actions and are often very potent; many can be inactivated into toxoids. Endotoxin is lipopolysaccharide, more heat-stable and cannot be made into a toxoid.
A microbe that causes disease is a pathogen. The main groups differ in structure, and the structure decides how you treat them:
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