🚀 go-pugleaf

There have been several postings lately on the use of erythromycin
(Maracyn) against blue-green algae in aquariums.  I'd like to 
reply to some of the unanswered questions that came up in these
postings.

The drug:  Erythromycin is a macrolide antibiotic that was originally
isolated from the bacterium Streptomyces erythreus.  Most of what is 
sold today are chemical derivatives of erythromycin that were modified 
to make it more stable.  I don't know what is in Maracyn.

Method of action:  Erythromycin binds to the bacterial ribosomes and 
prevents elongation of the peptide chain; i.e. it keeps the bacteria
from making proteins and eventually kills them.  

Spectrum:  Clinically, erythromycin is used against a variety of bacteria,
both gram-positives and gram-negatives.  Blue-green algae (cyanobacteria),
while being eubacteria like the gram-positives and gram-negatives, are
generally not put into either of these two groups.  Evolutionarily, 
they diverged from the gram-positives and gram-negatives fairly early on.
As various postings have shown, however, they are sensitive to erythromycin.
I don't know whether the bacteria in your filter would be erythromycin-
sensitive; some are and some are not.  Fungi, fish, and other higher
organisms are generally not affected directly by erythromycin.  It
is considered to be one of the "safest" antibiotics.  ..

Resistance:  Bacteria become resistant due to ribosomal mutations.  Changes
in both ribosomal proteins and in ribosomal RNA are possible.  These changes
cause the ribosomes to have reduced affinity for the drug.  Resistance 
to erythromycin, unlike resistance to some other antibiotics, tends to not 
persist in bacterial populations.  Those bacteria with ribosomal mutations
are at a competitive disadvantage in the absence of erythromycin and die off
in a few weeks.

Your friendly microbiologist, Georg.