This is a brief overview of some strategies that microorganisms use to gain an edge against competing microbes.

1. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
Page | 1
Competitive Strategies of Microorganisms
Microbial competition is a natural phenomenon in which populations of
microorganisms inhabiting a common environment complete for nutrients and other
resources of the environment. When two or more species use the same nutrients or
niches for growth, some of the populations will be compromised. Competition between
microbial species may be attributed to the availability of nitrogen source, carbon
source, electron donors, electron acceptors, vitamins, light, and water.
Competition may either result in the exclusion of other species or lead to the
establishment of a steady-state where multiple species coexist. Competition is seen
in aquatic environments where extensive phototrophic activity results in blooms of
single species of diatoms or cyanobacteria. In extremely hot springs where
thermophilic organisms are selected, the filaments that are present are predominantly
of a single bacterial species. When a succession of populations occurs, the final
species could be considered to result from competition exclusion.
Other examples of bacterial exclusion through competition are observed in lactic acid
fermentation of foods (e.g., sauerkraut, pickles) and in acid mine wastes where the
environment is highly acidic. Interaction between microorganisms for nutrients and
space is attributed to both inter- and intra-population competition. Competition
between species occurs in various environments such as the large intestine of
animals, where a single species does not dominate but a mixed population occurs as
a result of competition for nutrients.
The result of microbial competition is determined by multiple stress factors acting
together. The outcome of competition between two or more microbes in a given
environment may be determined by whether the benefit gained by one organism by its
response to factor A (i.e. efficiency of nutrient utilization) is greater than the benefit
gained by the other organism in response to factor B (i.e. low pH value). Microbes may

2. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
Page | 2
excel in competition by either having multiple sets of key enzymes or stress proteins
to help them combat stressful conditions.
Competitive Exclusion
The competitive exclusion principle states that two species competing for the same
limiting resource cannot coexist at constant population values. When one species has
even the slightest advantage over another, the one with the advantage will dominate
in the long term. This leads either to the extinction of the weaker competitor or to an
evolutionary or behavioral shift toward a different ecological niche.
A Biologist named Georgy Gause formulated the law of competitive exclusion based
on laboratory competition experiments using two species of Paramecium, P. aurelia
and P. caudatum. The conditions were to add fresh water every day and input a
constant flow of food. Although P. caudatum initially dominated, P. aurelia recovered
and subsequently drove P. caudatum extinct via exploitative resource competition.
Figure: Competitive exclusion of Paramecium caudatum by Paramecium aurelia.
However, Gause was able to let the P. caudatum survive by differing the
environmental parameters (food, water, etc.). Thus, Gause's law is valid only if the
ecological factors are constant. Gause also studied competition between two species

3. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
Page | 3
of yeast, finding that Saccharomyces cerevisiae consistently outcompeted
Schizosaccharomyces kefir by producing a higher concentration of ethyl alcohol.
Some Microbial Competitive Strategies
There are three major strategies used by microorganisms to tip the scales in their
favor, in a competition.
(1) Direct Competition – it involves the ability of the microorganism to excel other
competitors by being more versatile at nutrient exploitation or by producing substances
that inhibit other microbes.
(2) Ruderal Competition – the domination of some microbes in a community as soon
as favorable conditions return.
(3) Exploitation of Extremely Unfavorable Conditions – this is not a competitive
strategy but rather a result of physiochemical adaptations that enable some microbes
to live in extreme conditions where others cannot. Microbes rarely use a single
strategy in competition, most of the time they employ a combination of these
strategies. These strategies have been discussed in detail as follows:
Direct Competition
In direct competition, microorganisms can simply excel by being more versatile in their
ability to exploit nutrients (i.e. having the ability to consume a wide variety of nutrients,
etc.) or via antagonism (both specific antagonism such as production of inhibitory
substances and non-specific antagonism such as the production of metabolites). The
details are as follows:
1. Nutritional Versatility
Microbes that can use a wide variety of nutrients are obviously at an advantage over
those that do not have this ability. For example, the bacteria that have the genes for
arabinose operon can use the pentose sugar – arabinose, in case the preferred sugar
– glucose, is deficient in the environment. Other microbes, which do not have this

4. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
Page | 4
operon are unable to use arabinose and hence will not be able to compete with the
former in a glucose deprived environment, given that arabinose is available.
2. Specific Antagonism
Competition between species resulting from detrimental products or activities is
commonly known as antagonism. In specific antagonism, the inhibitory substances
target specific competitors of the microbe that produced those substances.
Antagonistic behavior commonly focuses on the exclusion of an organism from
growing on a specific site not because space is required for the dominant bacteria but
to exclude the other bacterium from utilizing limiting nutrients. Several different
processes account for successful antagonism, and many of these require close
contact between bacteria. It involves:
Production of Antibiotics: Several species of bacteria and fungi produce chemicals that
inhibit the growth of other microorganisms. As a result of this production of antibiotics
in nature, susceptible organisms are prevented from becoming dominant in the
population and the producers of the antibiotics are given the competitive advantage
for growth. The producers of the antibiotic are not affected because the chemical may
inhibit a metabolic step not found in the producer.
An example of this is the production of penicillin by fungi; this antibiotic inhibits a type
of cell wall (composed of peptidoglycan) found only in bacteria. Penicillin and other β-
Lactam antibiotics inhibit the formation of peptidoglycan cross-links in the bacterial cell
wall. The four-membered β-lactam ring of penicillin binds to the enzyme DD-
transpeptidase. As a consequence, DD-transpeptidase cannot catalyze the formation
of these cross-links. But the enzymes that hydrolyze the peptidoglycan cross-links
continue to function (because bacterial cells consistently remodel their cell walls). This
weakens the cell wall of the bacterium, eventually causing cell death (cytolysis) due to
osmotic pressure imbalance. Since the cell wall of fungi does not contain
peptidoglycan (it is mainly composed of chitin), it is unaffected by the penicillin is
produced.

5. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
Page | 5
Another possibility is resistance to the antibiotic produced; and this occurs with
Streptomyces spp. producing streptomycin. The impact of antibiotic production in
nature is to greatly reduce the sensitive population but not to eradicate it.
Figure: Penicillium spp. growing on the same nutrient medium as Staphylococcus spp.
can inhibit its growth due to the production of penicillin.
As a means of defense against the action of antibiotics, bacteria, and to some extent
fungi have developed several mechanisms of antibiotic resistance. Resistance to the
action of antibiotics by bacteria has been attributed to several mechanisms. The
principal ones are: (1) production of enzymes that degrade the antibiotic, (2) excluded
entry of the antibiotic by the production of binding proteins in the cell wall, and (3)
transporter systems that rapidly export antibiotics from the cell.
Production of Bacteriocins: Many different taxonomic groups of bacteria produce
bacteriocins – proteinaceous or peptidic toxins produced by bacteria to inhibit the
growth of similar or closely related bacterial strain(s). There is high host specificity for
the bacteriocins in that they will kill only strains of bacteria that are closely related to
the producer but do not harm the cell that produces them. Most frequently the

6. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
Page | 6
bacteriocins generate holes in the plasma membrane of susceptible bacteria, and this,
of course, results in the killing of the bacterial cell.
By targeting bacteria of similar physiological and nutritional activities, the bacteriocin
producer will exclude susceptible bacteria from the habitat. Bacteriocins act more or
less like a highly specialized antibiotic. Currently, there is some interest in using
bacteriocins to control spoilage of sliced lunch meat. Bacteriocins have been proposed
as a replacement for antibiotics to which pathogenic bacteria have become resistant.
Potentially, the bacteriocins could be produced by bacteria intentionally introduced into
the patient to combat infection. Some bacteriocins had been studied in in-vitro studies
to see if they can stop viruses from replicating.
3. Non-specific Antagonism
It involves the end products of microbial metabolism which may inhibit the growth of
other microorganisms. As a result of fermentation or modification of chemicals in the
environment, bacteria produce chemicals that can serve to inhibit other organisms.
This impact is not limited to cell-cell contact but may become dispersed throughout the
environment. For example, ethanol produced by Saccharomyces cerevisiae (yeast)
during fermentation is inhibitory for many of its competitors. Extracellular enzymes
may also be considered to have antagonistic activity because once the hydrolytic
enzymes are released from the microbial cell, the enzymes will digest a suitable
substrate even if the polymeric molecule is part of another microbial cell.
Ruderal Competition
This type of competition relies upon the ability of a minority subset of the population to
respond more rapidly to improved environmental conditions than the current majority
subset and subsequently attain dominance. These favorable conditions may be
optimal nutrient concentrations, tolerable temperature, ideal pH, dilution of NaCl due
to rainfall, etc.
The microbes, which are good at ruderal competition, rapidly uptake easily assimilable
substrates and they have a fast growth rate. The effectiveness of such microbes is

7. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
Page | 7
also affected by the duration of this improved state, i.e. the longer it lasts, the better
for these microbes. But these microbes must have a strategy to survive during less
favorable times. For example, the heat and desiccation resistant endospores of
Bacillus spp. and Clostridium spp. serve this purpose well.
The same task is also achieved by the exospores produced by fungi and
actinomycetes, which are not only desiccation resistant but also get dispersed over
long distances and hence colonize new areas. Some ruderal microbes do not have
any strategy to survive such unfavorable states. Their population declines immediately
if such conditions prevail, i.e. nutrient depletion due to microbial growth. In such cases,
microbes maintain a minimal population, reduce their metabolic rate and employ
several survival strategies such as downsizing of cells by reduction divisions, switching
from low substrate affinity to high affinity, etc.
Exploitation of Extreme Conditions
In extreme environments such as hydrothermal vents (having boiling water), extremely
cold environments or hypersaline water bodies only allow extremophilic microbes to
thrive in them. The competition here is almost non-existent because average microbes
cannot survive in these severe conditions. The extremophiles have made certain
physiochemical adaptations to dominate these inhospitable environments, i.e.
halophiles tend to have high amounts of KCl in their intracellular environments to be
able to tolerate the high concentrations of NaCl in their extracellular environments.
Although long-term colonization is possible in this case but in a very restricted number
of niches (as most of the environments are moderate).