Culturing and Controlling Microorganisms
Growing a bacterium in the laboratory is, at heart, a matter of feeding it and keeping watch over it. Controlling its growth, whether in an IMSS hospital, a yogurt factory, or your home refrigerator, means keeping it from thriving where it shouldn't. And understanding why some bacteria no longer respond to antibiotics is one of the most urgent health challenges of this century. You will come back to this idea at the end of the page.
Culture Media and Aseptic Technique
If you run a sterile swab over a clean table and then rub it on an empty piece of glass, you will see nothing the next day. But if you rub it on an agar plate at 25 °C, dozens of shiny spots appear that you can see with the naked eye. What does a bacterium need in order to multiply outside its natural environment?
What is a culture medium and which conditions (nutrients, temperature, pH, oxygen) are controlled when culturing bacteria in the laboratory? State the difference between…
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.
The Bacterial Growth Curve
You leave a freshly inoculated broth in the incubator. After a few hours it is cloudy, but the next day it stops getting cloudier. What happened to the growth?
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…
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.
Sterilization, Disinfection, and Food Preservation
A sealed can can last years without refrigeration, an opened quart of milk lasts only a few days in the refrigerator, and bleach cleans the counter but would damage a wound. Why isn't the same procedure suitable for preserving food, cleaning objects, and treating a patient?
Differentiate between sterilisation, disinfection, and antisepsis, and give an example of a situation (operating theater, kitchen worktop, superficial wound) in which…
Differentiate between sterilisation, disinfection, and antisepsis, and give an example of a situation (operating theater, kitchen worktop, superficial wound) in which each is applied.
Antibiotics, Resistance, and the Antibiogram
It is common to feel better on the third day of an antibiotic and stop taking it. What's wrong with that, if the symptoms are gone?
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'…
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.
Horizontal Gene Transfer
Bacteria reproduce by division, and each daughter cell is almost a clone of its mother. Yet a bacterium can become resistant to an antibiotic without having that gene in its lineage, and one species can even pass resistance to another. How?
What is horizontal gene transfer and how does it differ from 'vertical' inheritance from parents to offspring? Name its three mechanisms in bacteria.
What is horizontal gene transfer and how does it differ from 'vertical' inheritance from parents to offspring? Name its three mechanisms in bacteria.
Describe the three mechanisms of bacterial horizontal gene transfer — conjugation, transformation, and transduction — indicating which vehicle transports the DNA 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.
Hospital Outbreaks and Experimental Design
If a resistant bacterium reaches an intensive care unit, how do you trace its origin and how do you stop it? And, back in the laboratory, how do you verify that one disinfectant works better than another?
In a hospital ICU, an outbreak of carbapenem-resistant Klebsiella pneumoniae is detected, affecting four patients admitted with venous catheters and assisted mechanical…
In a hospital ICU, an outbreak of carbapenem-resistant Klebsiella pneumoniae is detected, affecting four patients admitted with venous catheters and assisted mechanical ventilation.
- 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.
- 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.
- 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…
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.
At the Boundary of Life: Acellular Agents
Everything you grow on agar has cells. But there are infectious agents that have none. Are they alive?
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…
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".
- 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.
- 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.
- 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.
Go back to the beginning: controlling microorganisms means keeping them from growing where it is not convenient (media, sterilization, disinfection, antisepsis), and resistance appears because the antibiotic selects for bacteria that were already resistant or that received the gene through horizontal transfer.