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Control of Microbial Growth

Initially, when bacteria are placed into a new environment there will be no increase in numbers in the poplulation.

Lag Phase

After the bacteria, have made the necessary enzymes to break down new food sources, they begin to grow at rapid rate and the population doubles at regular intervals. Phase where bacteria are most metabolically active.

Exponetial or Log Phase

As nutrients begin to run out, wastes begin to accumulate, growth slows, the nmber of new cells begins to equal the number of dying cells.

Stationary Phase

Growth slows even further, the number of dying cells becomes greater than the number of new cells.

Death Phase

Who first instituted hand washing as a control of microbial growth?

Dr. Ignaz Semmelweis


Kills everything, including endospores


Human tissue disinfectant


Kills cells but not endospores


Physically removing germs


Heat, disinfection, reduces the number of organisms


Free from disease causing microorganisms

Physical Methods of microbial control

Exposure to extreme heat (Slows), or cold (Kills)
Desiccation (Bacteriostaic - preserve, not kill)
Filtration (must make sure pore size is smaller than bacteria, or use several filters-useful for drugs)
Osomotic Pressure (Canning)
Radiation (breaks up chromosomes)

Which chemical groups inhibit/kill microbes by destroying the plasma membrane?

Phenols, Alchols and surface active agents


Alde-hydes - can sterilize when used at high concentrations. Denature proteins.
Ethy-lene oxide - disrupts proteins and can achieve complete sterilization in 4 to 18 hrs.
Ionizing Radiation (gamma, x, electron beams)

Radiation that sterilizes?

X-ray, Gamma ray

denaturing proteins?

Phenols - denature proteins and "disrupt" plasma membranes pg 449
I2 (Iodine) - antiseptic, used to denature proteins
Iodophores - they do not stain or cause allergic reactions, slow release of I2
Alchohols - denature proteins and have a solvent action on lipids such as those in the plasma membrane

Non-ionizing radiation

UV Light -- when it hits DNA it causes thymines diamers next to each other to break H-bonds and form covalent bonds. Our cells now can't make use of that gene. Repair enzyme will cut out and fix thymine diame and H-bond. Because of the repair ability, it is only a disinfectant. Also does not penetrate well.

Selective toxicity

Kill/control the microbe without harming the host
Takes advantage of the differences between microbes and humans. Toxic to microbe. Non-toxic to the human.

Who discovered antimicrobial drugs?

Paul Ehrlich. Discovered this by seeing that some dyes would stain microbes but not animal cells. That led to the concept called Selective Toxicity where the microbial cells are attacked and the host cells are left unaffected.

Who discovered antibiotics? How are drugs and antibiotics different?

Alexander Fleming.The first antibiotic, Penicillin. Discovered this by an unusual clear area around the mold in which no bacteria grew there. Realized that the fungus was inhibiting the growth of the bacteria.

Antibiotics vs. drugs?

Drugs are synthetically made while antibiotics are naturally occurring.

What are the targets of antimicrobial drugs? How does this relate to selective toxicity?

The targets of antimicrobial drugs are: inhibition of RNA synthesis, inhibition of DNA replication, inhibition of cell wall synthesis, inhibition of protein synthesis, inhibition of enzyme activity, and disruption of plasma membrane. Each of the drugs affects a different part of the cell, and only affects that one part. The drug will inhibit the bacterial cell, but not the host cells because they will perform the same cell functions using a different method or getting energy/food using a different source. This is done in order to stop a microbe from growing and replicating. The drug must be chosen based on the microorganism that needs to be fought. If the wrong one is chosen, it will most likely not affect the bacteria as intended. Also the wrong drug could kill/harm our cells or our normal flora.

Selective toxicity. Be able to define selective toxicity and apply it, by giving specific examples of how it is used to control growth. How do antiviral drugs work?

Able to poison the microbe without harming host. Must be like one of Ehr-lich's magic bullets. Bacterial cells have many differences from humans, which can be used as a target for drugs. Because humans do not have a peptidoglycan wall any drug that affects that structure would have a high degree of toxicity.


Antibiotics that have been chemically altered. Used to make them more powerful, or to fight resistance.

Which microbes are the most resistant?

Prions, Endospores, mycobacteria, protozoan cysts, protozoa, gram-neg bacteria, fungi, un-enveloped viruses, gram + bacteria, enveloped viruses


Synthetic antibiotic

Why are some microbes resistance?

Certain microbes have structures or abilities that make it very difficult to destroy. These include: waxy walls, ability to break down a wide variety of carbon-containing compounds (some can even break down disinfectants), unenveloped viruses (can't be controlled by any substance that disrupts plasma membrane), Prions (High Temperature resistant, resistant to inactivation, resistant to autoclaving)


category implies that isolates are not inhibited by the usually achieveable concentrations of the agent with normal dosage schedules, and/or that demonstrate zone diameters that fall in the range where specific microbial resistance mechanisms (e.g. beta-lactamases) are likely, and clinical efficacy of the agent against the isolate has not been reliably shown in treatment studies.'


category implies that isolates are inhibited by the usually achievable concentrations of antimicrobial agent when the recommended dosage is used for the site of infection.'

Which microbes are the most resistant?

Enveloped Viruses, Gram + Bacteria, Un-enveloped Viruses, Fungi, Gram - bacteria, Protozoa, Protozoa Cysts, Mycobacteria, Endospores, Prions

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