Fed-Batch Bioproduction of Spectinomycin
5
The resistance mechanisms developed by the antibiotic-producing microorganism against its own antibiotic are similar to those of clinically resistant
bacteria. Some of the more common mechanisms found in resisting antibiotics
are [1]:
a) Permeability modifications: modifications of the cell wall structure during
the antibiotic production phase reduce permeability and protect the microorganism against the high extracellular concentration of its antibiotic.
b) Compartmentation: special compartments developed during the production
phase, allow the microorganism to hold the antibiotic before excretion into
the environment. This protects the cell from internal antibiotic build-up.
c) Enzymatic deactivation: synthesis of special enzymes which convert the
antibiotic into inactive or weaker derivatives helps in resisting the biological
activity of the molecules.
d) Modification of target sites: modification of the target site of the antibiotic
within its own cellular machinery, e.g., in the ribosomes, negates its effect on
the microorganism.
e) Feedback inhibition and repression: shutting off the biosynthetic pathway
for the antibiotic prevents the concentration from becoming lethal. Feedback
inhibition and repression almost always accompanies other modes of resistance.
The beginning of secondary metabolism is strictly regulated due to the bioactive nature of antibiotics. The precise nature of the molecular mechanisms of
the various regulatory factors are still not known. However, activation of
transcription of many different operons having the same physiological functions
regulates the start of secondary metabolism for most producing microorganisms. These functions may involve synthesis of certain proteins or control
cellular activities not directly related to growth. If such functions affect antibiotic synthesis directly or indirectly, their control mechanisms become essential
regulatory phenomena. Among the various regulatory factors, carbon, nitrogen
and phosphate play key roles in antibiotic synthesis.
The nutrients present in the environment exert a strong influence on the
production of antibiotics. General observations indicate that the onset of
antibiotic production occurs when the nutritional conditions limit growth.
Often the specific growth rate reflects regulatory phenomena because it depends
on the type and quantity of nutrients present in the environment. For example,
in the case of the steady-state production of ~-l,3-glucanase, an exoenzyme
secondary metabolite, by an actinomycete in a glycerol-limited chemostat culture occurs only at growth rates below 0.2 h-1 [2]. Tylosin production by
steady-state chemostat cultures of Streptomyces fradiae also depends on the
growth rate [3]. Likewise, there is a direct correlation between growth rate and
gramicidin-S production by Bacillus brevis in chemostat cultures [4].
Nutritional limitations of carbon, nitrogen and phosphorus exert the strongest influence on regulatory patterns [5-9]. There is sufficient evidence indicating the absence of activity of enzymes, specifically involved in the biosynthetic
5
The resistance mechanisms developed by the antibiotic-producing microorganism against its own antibiotic are similar to those of clinically resistant
bacteria. Some of the more common mechanisms found in resisting antibiotics
are [1]:
a) Permeability modifications: modifications of the cell wall structure during
the antibiotic production phase reduce permeability and protect the microorganism against the high extracellular concentration of its antibiotic.
b) Compartmentation: special compartments developed during the production
phase, allow the microorganism to hold the antibiotic before excretion into
the environment. This protects the cell from internal antibiotic build-up.
c) Enzymatic deactivation: synthesis of special enzymes which convert the
antibiotic into inactive or weaker derivatives helps in resisting the biological
activity of the molecules.
d) Modification of target sites: modification of the target site of the antibiotic
within its own cellular machinery, e.g., in the ribosomes, negates its effect on
the microorganism.
e) Feedback inhibition and repression: shutting off the biosynthetic pathway
for the antibiotic prevents the concentration from becoming lethal. Feedback
inhibition and repression almost always accompanies other modes of resistance.
The beginning of secondary metabolism is strictly regulated due to the bioactive nature of antibiotics. The precise nature of the molecular mechanisms of
the various regulatory factors are still not known. However, activation of
transcription of many different operons having the same physiological functions
regulates the start of secondary metabolism for most producing microorganisms. These functions may involve synthesis of certain proteins or control
cellular activities not directly related to growth. If such functions affect antibiotic synthesis directly or indirectly, their control mechanisms become essential
regulatory phenomena. Among the various regulatory factors, carbon, nitrogen
and phosphate play key roles in antibiotic synthesis.
The nutrients present in the environment exert a strong influence on the
production of antibiotics. General observations indicate that the onset of
antibiotic production occurs when the nutritional conditions limit growth.
Often the specific growth rate reflects regulatory phenomena because it depends
on the type and quantity of nutrients present in the environment. For example,
in the case of the steady-state production of ~-l,3-glucanase, an exoenzyme
secondary metabolite, by an actinomycete in a glycerol-limited chemostat culture occurs only at growth rates below 0.2 h-1 [2]. Tylosin production by
steady-state chemostat cultures of Streptomyces fradiae also depends on the
growth rate [3]. Likewise, there is a direct correlation between growth rate and
gramicidin-S production by Bacillus brevis in chemostat cultures [4].
Nutritional limitations of carbon, nitrogen and phosphorus exert the strongest influence on regulatory patterns [5-9]. There is sufficient evidence indicating the absence of activity of enzymes, specifically involved in the biosynthetic