Study Guide

Prokaryotes - Sterilization

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Some strains of bacteria are just born to be pathogens. An infectious dose of Salmonella, one cause of food poisoning, is less than 20 cells of bacteria. However, our skin, teeth, and guts are covered with trillions of bacterial cells that do no harm in these places. Occasionally, generally harmless bacteria gain access to a place where they would not normally be, for example through injury. Bacteria that become pathogenic in such situations are called opportunistic pathogens.

Since bacteria run the gamut from helpful to sometimes pathogenic to always pathogenic, humans have developed a number of techniques for controlling when and where they are allowed to grow. We have three main strategies for controlling bacterial growth:

  • Sterilization by heat or radiation
  • Chemical disinfection or antiseptic treatment
  • Antibiotic treatment

Ovens and Microwave Ovens: Sterilization by Heat or Radiation

One common method is high heat. This is especially effective in water or steam as these help transfer the energy of the heat. You can hold your hand in an oven longer than you can hold it in boiling water, right? It’s like that.

The standard machine used for sterilizing medical equipment, called an autoclave, is a sealed box in which the equipment is quickly sterilized by a very hot, high pressure steam.

Radiation including the UV rays in sunlight, as well as X-rays and microwaves, can also kill off microbes.

Death by Listerine

Disinfectants and antiseptics are chemicals that kill microbes. Disinfectants are used on inanimate things, like counter tops and sinks. Antiseptics are a class of disinfectants that do not harm living tissue. So bleach is a disinfectant while rubbing alcohol is an antiseptic.

Nowadays we use antiseptics to deal with wounds at home and in hospitals, but this wasn’t always the way. Following up on findings by Louis Pasteur that certain chemicals would eliminate microbes, physician Joseph Lister experimented with treating his surgical equipment and washing his hands with antiseptics. Yes, someone had to experiment with washing his hands. Progress can be slow.

By killing off pathogens, including opportunistic ones, before performing surgery, Lister saw a dramatic decrease in infections in his patients. While he used antiseptics, as we said above, most hospital equipment is now sterilized using autoclaves. Antiseptics are, of course, still in use, including one named for Lister, Listerine brand mouthwash.


Antibiotics are another type of antimicrobial compound that specifically target bacteria. Unlike antiseptics, which are poisonous if consumed and should only be used on surfaces like the skin, or in the mouth as mouthwash—just don’t swallow, antibiotics are safe to ingest. When we swallow an antibiotic pill, we expose our whole body to the antibiotic via the bloodstream.

Antibiotics come in two varieties:

Bactericidal antibiotics, which kill bacteria

Bacteriostatic antibiotics, which stop bacterial growth

Antibiotics target a few processes that are essential in bacterial cells. Antibiotics act, specifically, by:

  • Inhibiting cell-wall synthesis
  • Disrupting membranes
  • Blocking protein synthesis
  • Stopping nucleotide (RNA or DNA) synthesis

All of these processes are crucial for cells to grow and replicate so interfering with them can be deadly for the bacteria.

Bacteriostatic antibiotics don’t kill bacteria directly, but act by freezing all bacteria in the state they were in. This gives a huge advantage to your immune system, which typically finishes the job pretty quickly.

If the nature of an infection is known, then antibiotics can be used to target specific types of infections, but, by and large antibiotics have a widespread negative affect on the bacteria in us, both the good and the bad.

The first antibiotic identified was penicillin, which was found when Alexander Fleming saw a clearing of bacteria around mold contamination on a petri dish. He identified the mold as belonging to the genus Penicillium and so he named the compound penicillin.

Brain Snack

Since it came from a mold, Fleming first referred to penicillin as "mold juice". He actually called it "mould juice"—it's a Scottish thing. We can only imagine if antibiotics would have ever taken off if people were offered "mold juice" to cure their infections. For more about Fleming, you can check this out.

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