Expert: Florence M Rollwagen - 11/18/2008

Question
QUESTION: Why some bacteria appear to be more resistant to irradiation or heat stress than anther bacteria?

ANSWER: Hi Mohammed:  Thanks for your question.

Radiation resistance as well as heat stress are related to the particular genes that bacteria carry.  Some bacteria, like Deinoccocus radiodurans have unique DNA structure and specialized DNA repair enzymes so that they can recover from high radiation doses.

Other bacteria, such as Acinetobacter have extra layers in the cell wall that protect them from radiation.

Of course, one cannot leave out endospores as the ultimate radiation resistance mechanism.  Bacteria of the genera Clostridium and Bacillus form endospores which are highly resistant to most forms of disinfection (except high pressure steam sterilization - autoclaving).  The endospores later sprout into live bacteria which go on living and dividing.  This is what makes botulism so dangerous.  The organism Clostridium botulinum forms endospores which remain after incomplete sterilization.  Then the endospores grow new Clostridia which grow and make the toxin which is deadly.  The organism Bacullus anthracis (anthrax) forms endospores that are also highly resistant and can last for years in the environment.

So each specific bacterium has its own special way of surviving environmental dangers.

Hope this helps.  Write back if you have more questions.

FM Rollwagen, PhD

---------- FOLLOW-UP ----------

QUESTION: Dear Rollwagen
Thank so lot for your answer.
I have two questions please. 1- why spore bacteria more resistance to irradiation and stress than vegetative cell and what the difference between them?
2- I did experiment which is the ability of Campylobacter to survive under aerobic condition. My results showed that the OD increased during the first four hour of incubation  then decreased the the ten hour thereafter the OD was relatively stable level after 14 hour. why OD increased during the first four hour?

Regards
Mohammed

Answer
Hi again Mohammed:  Endospores are highly resistant to radiation, chemicals and other harsh environmental treatments.  This is because they are dehydrated and inert.  Loss of water eliminates the possibility of forming free radicals or toxic oxygen byproducts which are highly reactive and can damage the cell.

Here is a website:
http://www.micro.cornell.edu/cals/micro/research/labs/angert-lab/bacterialendo.c

Campylobacter is microaerophilic, which means it can grow under low oxygen conditions.  It is not, however, an anaerobe because it still needs oxygen.  

The question you asked is easily answered, and I think you have not given it careful consideration.  Think about what the OD means in a bacterial culture.  The higher the OD, the more bacteria are present.  So your culture grew until it used up all the oxygen in the system.  Then the bacteria started to die, resulting in a slight decrease.  Because the number of organisms dividing equalled the number of organisms dying, the culture stayed stable.

You need to look up the growth curve for bacteria:
http://en.wikipedia.org/wiki/Bacterial_growth

Hope this helps.

FM Rollwagen, PhD