334
MAURICE WELSCH
media. Finally, it should be stressed that the production of a diffusible
antibiotic by a given microorganism would confer some advantage not
only to the producer itself but equally so, or even more so, under given
circumstances to the resistant individuals living in his vicinity.
In fact, a few examples of microbiological competition occurring in
nature can be attributed, beyond any reasonable doubt, to antibiotic
production (15, 328, 329a,b). At any rate, the production of any kind of
metabolite by one member of a complex microbial population is likely
to modify its composition, if not on account of its direct and specific
toxicity, then because it will make the surroundings more favorable to
some microorganisms than to others, either by modifications of the
physicochemical conditions, such as pH, rH, surface tension, or by being
more or less readily utilized as nutrient or growth-enhancing factor. The
dynamic equilibrium of mixed microbial populations (parabiosis) and
the shifting of populations successively inhabiting a given, but chemically changing, microenvironment (metabiosis), are the result of an
infinitely complex series of metabolic reactions, continuously leading to
probiotic and antibiotic effects, in which the actual participation of
antibiotics as such, although possible, is far from being the rule (330).
If antibiotics are not to be necessarily considered as the chemical
agents of a struggle for life among microorganisms, what other role can
they conceivably play?
They have sometimes been regarded as reserve materials, together
with their biologically inactive close relatives which have also to be accounted for. But such an interpretation certainly has no general significance since it can hardly apply to external metabolites.
Products of "secondary metabolism," among which antibiotics may
be included, were described as "shunt metabolites" (331), since they are
formed from "normal" intermediates thanks to special mechanisms different from, but related to, those operating in general metabolism.
These products, in many instances, are known to be mainly synthesized
when cultural conditions have become such that growth has either
completely stopped or been greatly reduced. In some cases, a relation
between antibiotic production and formation of specialized resting cells,
endospores or conidia, has even been demonstrated. Linking these considerations with the observation that enzymes are often stabilized by
their specific substrate, Bu'Lock (28) proposed that secondary metabolites would "maintain mechanisms essential to cell multiplication in
operative order when that cell multiplication is no longer possible."
Such a function, by preventing the progressive breakdown of biosynthetic mechanisms when the reactions of general metabolism are stopped
or depressed, would ensure a rapid resumption of normal cell activities
MAURICE WELSCH
media. Finally, it should be stressed that the production of a diffusible
antibiotic by a given microorganism would confer some advantage not
only to the producer itself but equally so, or even more so, under given
circumstances to the resistant individuals living in his vicinity.
In fact, a few examples of microbiological competition occurring in
nature can be attributed, beyond any reasonable doubt, to antibiotic
production (15, 328, 329a,b). At any rate, the production of any kind of
metabolite by one member of a complex microbial population is likely
to modify its composition, if not on account of its direct and specific
toxicity, then because it will make the surroundings more favorable to
some microorganisms than to others, either by modifications of the
physicochemical conditions, such as pH, rH, surface tension, or by being
more or less readily utilized as nutrient or growth-enhancing factor. The
dynamic equilibrium of mixed microbial populations (parabiosis) and
the shifting of populations successively inhabiting a given, but chemically changing, microenvironment (metabiosis), are the result of an
infinitely complex series of metabolic reactions, continuously leading to
probiotic and antibiotic effects, in which the actual participation of
antibiotics as such, although possible, is far from being the rule (330).
If antibiotics are not to be necessarily considered as the chemical
agents of a struggle for life among microorganisms, what other role can
they conceivably play?
They have sometimes been regarded as reserve materials, together
with their biologically inactive close relatives which have also to be accounted for. But such an interpretation certainly has no general significance since it can hardly apply to external metabolites.
Products of "secondary metabolism," among which antibiotics may
be included, were described as "shunt metabolites" (331), since they are
formed from "normal" intermediates thanks to special mechanisms different from, but related to, those operating in general metabolism.
These products, in many instances, are known to be mainly synthesized
when cultural conditions have become such that growth has either
completely stopped or been greatly reduced. In some cases, a relation
between antibiotic production and formation of specialized resting cells,
endospores or conidia, has even been demonstrated. Linking these considerations with the observation that enzymes are often stabilized by
their specific substrate, Bu'Lock (28) proposed that secondary metabolites would "maintain mechanisms essential to cell multiplication in
operative order when that cell multiplication is no longer possible."
Such a function, by preventing the progressive breakdown of biosynthetic mechanisms when the reactions of general metabolism are stopped
or depressed, would ensure a rapid resumption of normal cell activities