Exercises: Culture and control of microorganisms

What is a culture medium and which conditions (nutrients, temperature, pH, oxygen) are controlled when culturing bacteria in the laboratory? State the difference between a solid and a liquid medium.

Differentiate between sterilisation, disinfection, and antisepsis, and give an example of a situation (operating theater, kitchen worktop, superficial wound) in which each is applied.

What is horizontal gene transfer and how does it differ from 'vertical' inheritance from parents to offspring? Name its three mechanisms in bacteria.

Explain, using natural selection, how a bacterial population resistant to an antibiotic arises when treatment is stopped early, and why the antibiotic does not 'create' resistance.

The graph represents the growth curve of a closed bacterial culture, with its four phases labelled with numbers (1 to 4). Name each phase and explain what happens to nutrients and waste in each.

Describe the three mechanisms of bacterial horizontal gene transfer — conjugation, transformation, and transduction — indicating which vehicle transports the DNA in each case and which one is involved in the spread of resistance.

In a hospital ICU, an outbreak of carbapenem-resistant Klebsiella pneumoniae is detected, affecting four patients admitted with venous catheters and assisted mechanical ventilation.

1. a) Analyze the epidemiological chain of the outbreak: identify the probable source, the most plausible transmission mechanism (vehicle and passive vector), and justify what physiological factors make these patients especially vulnerable.
2. b) Design a contingency plan with control measures ranked in order of priority (isolation and physical barriers, active screening, and fomite disinfection), biologically justifying why hand hygiene between patients is more critical for halting transmission than environmental fumigation.
3. c) Critically evaluate the following proposal: "To eradicate the outbreak at its root, we will administer prophylactic colistin to all patients admitted to the ICU, whether or not they have symptoms." Predict the selective and evolutionary consequences of this measure in the medium term.

Design an experiment to compare the efficacy of two disinfectants against a skin bacterium: agar plates, controls, measurement of inhibition zones, and criteria for concluding which is more effective, stating the variables you would keep constant.

A patient diagnosed with the flu insists on being prescribed amoxicillin "in case the infection moves down to the chest", while a farmer continuously administers low doses of tetracycline in the drinking water of healthy pigs to "accelerate their fattening and prevent diarrhea".

1. a) Refute the patient's demand based on biochemical and cytological foundations: justify why beta-lactam antibiotics are completely harmless to viruses and predict what immediate selective impact that amoxicillin will have on the patient's own commensal microbiota.
2. b) Interpret the described farming practice from the perspective of natural selection: explain why continuous subtherapeutic concentrations represent the optimal scenario for selecting resistant bacteria compared to full lethal doses.
3. c) Explain the horizontal gene transfer mechanisms (conjugation, transformation, and transduction) through which a resistance gene generated in a commensal livestock bacterium can end up in a human pathogenic strain in a hospital setting.