Methods of Killing Bacteria

Heat, UV radiation, antibiotics and chemicals are all used to control bacteria, viruses and other microorganisms. How to they kill pathogens without hurting our cells?

* How Control Agents Damage Microbes *

There are many different methods used to control the growth of microorganisms. However, the modes of action (MOAs) of microbial control measures generally fall into one of four basic categories.

1. Alteration of Cell Wall: The bacterial cell wall maintains the integrity of the cell, allowing it to keep its shape even when placed in a hypotonic environment. When the cell wall is weakened or disrupted, it can no longer function to prevent the cell from bursting due to osmotic effects (water rushing into the cell).

2. Alteration of Plasma Membrane: A cell’s outer membrane contains cytoplasm and all the cells internal contents, as well as controlling the passage of chemicals into and out of cell. When damaged, the plasma membrane may allow cellular contents to leak out.

Some viruses (acellular particles) are surrounded by a membrane called the viral envelope. This envelope is responsible for attachment of the virus to target cells, so damage to the viral envelope interferes with the process of replication for enveloped viruses. Nonenveloped viruses (those that only have a capsid and nucleic acid) have a greater tolerance of harsh conditions.

* Interference with Protein Structure: Protein function depends on the 3-D shape of the molecule. Extreme heat, or certain chemicals, can denature, or change the shape of a protein. A denatured protein can no longer carry out its function within the cell.

* Interference with Nucleic Acid Structure: Nucleic acids (DNA and RNA) can be damaged or destroyed by chemicals, radiation, and heat. The result can be the production of fatal mutations to the DNA or interference with protein synthesis through action on RNA.

* Types of Microbial Control Agents *

There are three general categories of microbial control agents:

* Physical: Heat, freeze-drying, ultraviolet radiation and filtration are all physical control agents.
* Chemical: Chemical agents of control, like the disinfectants Lysol or Clorox, destroy most vegetative cells and viruses.
* Chemotherapeutic: Antimicrobics are drugs (antibiotics) used to treat patients diagnosed with an infectious disease.

* Microbial Sensitivity to Control Agents *

Different types of microbes have varying levels of sensitivity to the affects of chemical and physical control agents. Bacterial endospores and protozoan spores are very difficult to destroy. The use of an autoclave (heat combined with pressure) is the most reliable way to eliminate them.

Even some vegetative cells (active metabolizing cells as opposed to dormant endospores) are more difficult to destroy than others. For example, Mycobacterium and Nocardia bacteria have cell walls containing waxy mycolic acid. The wax makes these cells more resistant to destruction than others.

Viruses are generally easier to destroy than vegetative cells. Naked (non enveloped) viruses are more difficult to destroy than enveloped viruses.

* Selectivity of Microbial Control Agents *

Both chemical and physical agents of control are not particularly targeted agents. The affected microbe, host cell and environment are all prone to the toxic effects of these generalized agents. In contrast, chemotherapeutic agents, like antibiotics, are selectively toxic. They can harm bacteria while not harming our cells because antimicrobics target some aspect of microbial metabolism, such as protein synthesis or cell wall production. There are many different classes of antibiotics, each with and antibiotic mode of action (MOA) targeting different aspects of bacterial metabolism.

* Protocol for Testing a Microbial Control Agent *

The basic principle for testing any control agent, (whether temperature, chemical or antibiotic) is always the same:

1. Expose the organism to the agent.
2. Remove the agent.
3. Put the organisms in favorable growth media.
4. After incubation, look for reproduction of organisms.

* Sources *

Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.