MadSci Network: Microbiology

Re: What precise effect on the structure of bacteria do disinfectants have?

Date: Thu Mar 11 00:46:56 1999
Posted By: Tinsley Davis, Grad student, Microbiology, University of Wisconsin Madison
Area of science: Microbiology
ID: 920541952.Mi

Dear Emma,
Thanks for writing. Before I jump into the specific modes of action that 
different disinfectants have, I'll offer some background on bacterial 
structure to make sure we're thinking along the same track.

Bacteria are single celled organisms.  All the activities bacteria need to 
perform to live are contained within this single cell.  The cell is 
delineated from the rest of the environment by a barrier known as the lipid 
membrane.  The membrane is made up of lipid molecules that aggregate to 
exclude the fluid of the environment.  To imagine this more clearly, think 
of the leftover fat in a frying pan (perhaps not a beautiful image, but an 
effective one).  Fat is also made of lipids and is not soluble in water.   
Fat beads up and excludes the water, as you know if you've ever tried to 
clean a dirty frying pan with water alone. 

Within the lipid membrane are the life sustaining activities of the 
bacterial cell.  The agents that carry out activities such as metabolism 
and reproduction are many different kinds of proteins that the cell must 
make from scratch.  If a bacterium cannot maintain the lipid boundary or 
protein synthesis fails, the cell is doomed, and that's where disinfectants 
come in!

Disinfectants are chemicals applied to non-living surfaces that rid these 
surfaces of bacterial contamination.   All disinfectants are bactericidal; 
in other words, they kill the bacteria.  However, only some disinfectants 
are bacteriostatic meaning that they prevent the growth of new bacteria.  
The amount of time the surface stays free of bacteria depends on the 

I'll give a sampling of common disinfectants and each mode of action in 
alphabetical order:

This disinfectant category contains specific kinds of pure alcohol (in 
other words not your common table wine).  Ethanol (an ingredient in table 
wine) and Isopropanol (also known as rubbing alcohol and poisonous to 
humans) are very commonly used.  These disinfectants disrupt the structure 
of proteins in the bacteria.  If the proteins are disrupted, or denatured, 
they cannot perform their highly specific tasks, and the bacterium dies.

Bleach is also known by its formal chemical name as sodium hypochlorite.  
Now we shall delve into some basic chemistry.  In solution, the sodium 
hypochlorite dissociates, or dissolves, to produce atomic oxygen and 
chlorine.  The chlorine and oxygen are very reactive, negatively-charged 
elements that oxidize components of the bacterial cell.  This oxidation 
prevents effective operation of the cell's machinery, and the bacterium 

	I bet you've always wondered what the difference between a detergent 
and a soap is!  Detergents act like soaps though detergents are not derived 
from fats like soaps are.  Detergents are comprised of long molecules with 
very different ends. In some types of detergents, one end of the molecule 
is positively charged (insoluble in water), and the other is negatively 
charged (soluble in water).  These molecules wiggle their way in between 
the lipids of the bacterium's membrane and cause the cell to spill its 
contents into the environment, killing the cell.

Dilute solutions (as little as 1%) of iodine work in much the same way as 
the chlorine described above.  Looking at a periodic chart, you'll even 
notice that chlorine and iodine are in the same halogen family, hence their 
common mode of action.  Negatively charged iodine combines with proteins 
and disrupts their ability to function normally.

Phenolics is the generic name for a specific family of chemical structures.  
There are a variety of phenolics used as disinfectants.  Lysol is perhaps 
the most famous.  This class of disinfectants works by disrupting proteins, 
a common theme as we have seen, but also by disrupting the lipid membrane.  
A leaky or broken membrane in addition to faulty proteins are problems too 
severe for the bacterium to fix in time, and it dies.  Phenolics are also 

	  All the disinfectants described above work in an indiscriminate 
manner.  For example, the chlorine ions bind to many kinds of proteins, not 
just one.  Phenolics cause many types of bacterial species to have leaky 
membranes.  I refer to this as a "broad spectrum of action".  As a 
consequence, it is highly unlikely that every protein in a cell will evolve 
an ability to resist the effects of chlorine.   I'll call the following 
section "extra credit" because it moves beyond traditional disinfectants to 
a recent phenomenon on your grocery store shelves.

	Here is a disinfectant that does discriminate, or is picky about its 
target in a bacterial cell.  You have probably noticed quite a lot of 
"antibacterial" products on the store shelves in recent years.  The active 
ingredient in these soaps and detergents is an antibiotic known as 
Triclosan.  Triclosan, however, kills bacteria by acting on a single 
specific protein that is necessary for the bacterium to make an effective 
lipid membrane.  If this one protein evolves to resist the action of 
Triclosan, Triclosan will no longer be able to kill those bacteria with the 
new version of the protein.  Scientists are debating the effects of 
bacterial resistance to Triclosan, and the following article will give you 
more details on a hot topic in the scientific world.

Heath, R.J., Y.T. Yu, M.A. Shapiro, E. Olson, C.O. Rock. 1998.  "Broad 
spectrum antimicrobial biocides target the FabI component of fatty acid 
synthesis." Journal of Biological Chemistry.  273(46):30316-20.

For some nitty gritty information on chemical structures of disinfectants, 
check out: Grayson, M., ed. 1982. “Antibiotics, Chemotherapeutics, and 
Antibacterial Agents for Disease Control.”  New York: John Wiley & Sons, 
pp.435-473. is a  site that lists many kinds 
of biology courses available through the web, including microbiology.  This 
would be a good starting point if you wanted to know more about bacteria or 
molecular biology.

Current Queue | Current Queue for Microbiology | Microbiology archives

Try the links in the MadSci Library for more information on Microbiology.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-1999. All rights reserved.