Re: why doesn't antibiotics harm human host cells?

     Excellent question.  There are many different types of antibiotics used to treat 
bacterial infections.  These compounds differ in their mechanism of action (how 
they kill bacteria).  Choosing which one to prescribe is an important job because 
most antibiotics work on a subset of bacteria, not all bacteria.  And, overuse of 
antibiotics can lead to bacterial populations that are resistant to the antibiotic.
     The answer to your question is related to the mechanism of action of each 
antibiotic.  For example, penicillin works by becoming incorporated into the cell wall 
of bacteria, like E. coli.  With enough penicillin replacing the normal components of 
the cell wall, the wall weakens and the bacterium breaks apart.  This antibiotic does 
not harm human cells because human cells lack the bacterial cell wall.  Thus, in 
human cells, the antibiotic has no target to attack.  However, not all antibiotics use 
this mechanism of action.  Tetracycline, another antibiotic, works by preventing an 
important step in protein synthesis.  Tetracycline binds to ribosomes.  Both bacterial 
cells and human cells contain ribosomes.  In this case, tetracycline could harm 
human ribosomes, but human cells do not accumulate tetracycline.  In order for 
tetracycline to enter human cells, the cells would have to have a way to transport it 
across the membrane.  Some other classes of antibiotics work by capitalizing on 
the differences that exist between bacterial and human ribosomes, thereby 
protecting human cells while causing the demise of bacterial cells.
    I have provided you with three different mechanisms of antibiotic action, but this 
is not an exhaustive list.  As you can see, the reason why a particular antibiotic 
does or does not harm human cells varies depending on the mechanism of action 
of the antibiotic.  If you are curious about a specific antibiotic, you must first 
determine how it works to kill bacteria.  Once you know how it works, then you can 
find out if it is harmful to human cells.  Not all antibiotics are benign (harmless to 
human cells).

Here are some sources on the internet for mechanisms of antibiotic action:
penicillin (http://www.cellsalive.com/pen.htm)

tetracycline (http://aac.asm.org/cgi/content/full/47/12/3675)

other antibiotics (http://www.elmhurst.edu/~chm/vchembook/654antibiotic.html)

Also, some basic biology textbooks and most microbiology textbooks will also have 
information on this topic.


Thank you for your question!