Ignoring a low rate of mutation as a source of heterogeneity, bacterial division results in clonal 
expansion, with the daughter cells considered to be similar if not identical to the parent cells. 
Collectively, these cells are called a population. However, bacteria rarely exist as a single species 
within any one habitat but, instead, are usually found as collections of different species called 
communities, where each particular species will exist in a particular niche but may well contribute to 

the maintenance of the entire community (e.g. syntrophism). 
Interactions between microbial communities may have a negative (e.g. competition) or positive (cooperation) outcome. The interactions will be between both a single population (i.e. between members of 
the same species) or between members of different populations. Typically, co-operation will occur at 
low population densities, whereas competition will dominate at high population density. The net effect 
will regulate the population size to an optimum, depending on the selection pressures. 
It is of interest that bacteria that infect humans proliferate in a clonal manner seemingly independent of 
other organisms. The invading organism is thus growing as a population and nota community. Whilst 
there are examples of synergistic infections, where more than one species of organism is needed to 
initiate the infection, this appears to be the exception rather than the rule. As we shall see later, rather 
than depending on the microbial community in the host, an invading pathogen is often inhibited from 

proliferating by the presence of microbial populations residing normally in the host.

NEUTRALISM: Neither organism benefits, but neither is harmed. It is suspected that such a state is rare in nature.
COMMENSALISM:  Literally ‘eating at the same table’, cohabitation is the association of two different organisms where  neither is disadvantaged but, unlike symbiosis, the mutual dependence is minimal. Whilst neither organism has any strict dependency on the other (e.g. no nutritional needs), commensalism usually benefits one of the partners whilst having no impact on the other. 

MUTUALISM (SYMBIOSIS): Mutually-dependent association of two different organisms, conferring benefit on both. The interaction is obligatory for both parties. The association may be temporary or permanent but usually implies the latter as the interaction confers very specific benefits to each population that are not likely to be seen with other combinations of organisms.
SYNERGISM:Both populations benefit from the interaction. Unlike mutualism, however, synergistic interactions are not obligatory for both populations. Synergistic interactions permit activities that could not be carried out by either organism alone. Syntrophism is an example of a synergistic interaction.
COMPETITION: The antithesis of symbiotic interactions, competitive interactions result in both populations incurring detrimental effects. Both populations will be competing for the same resource (typically a nutrient), 
therefore by definition both cannot occupy the same niche (competitive exclusion). Competition between two bacterial strains represents an good example of competition.
PARASITISM: Parasitism represents the interaction between two different organisms, with the parasite exploiting the host to its own advantage. The state of parasitism is not necessarily harmful to the host but usually the interactions are specific and the parasite is smaller than the host. Both of these conditions are fulfilled when humans represent the host. Some definitions include the requirement for metabolic dependency of  the parasite on the host. 

The term ‘symbiosis’ has also been used to simply describe the association of two different 
organisms without any inference of benefit or harm to either organism. Symbiosis is just an 
association that is subdivided into the three categories as follows: 

As with all attempts at definitions, the above descriptions tend to divide the associations between 
organism into very distinct groups. This in turn ends up highlighting the types of association that do not
fit neatly into any one category; for example, the microbial flora that colonise the human body.It is therefore more appropriate to consider the above terms as points along a spectrum of associations ranging from mutually beneficial (symbiotic) through to an entirely one-sided relationship where the parasite derives all the benefits at the expense of the host (parasitism). Bacteria are found in most sites on the planet from hot springs and thermal sea vents to the human stomach at pH 1 and dust in the floor of our houses. What is important to consider is the sites at which bacteria can replicate. Bacteria found on floors may simply be there because they have fallen from our skin and are unable to replicate there. The Helicobacter species that live in the stomach are able to multiply under such extreme hydrogen ion concentrations. Likewise viruses can be found in inanimate situations (kitchen surfaces, bed sheets, etc.) but they are unable to multiply in such conditions. Those organisms that are able to replicate outside of a host cell are termed free-living in contrast to those that are obligate intracellular parasites such as all viruses and certain genera of bacteria such as Rickettsia and Chlamydia. Not surprisingly there are bacteria that are able to do both, i.e. replicate inside and outside host cells and these are termed ‘facultative intracellular parasites’. Salmonella typhi is an 
example: it replicates within human epithelial cells and macrophages during the course of human 
infection but is also able to multiply in the environment if nutrients are available. 
It is easy to visualise how birds that eat the insects that attack the skin of the elephant are mutualistic. 
There are also examples of bacteria that exist in mutualistic associations with higher hosts. The bacterium Buchnera sp.exists as an intracellular parasite in the aphid (Greenfly 
and Blackfly). The bacteria have an obligate intracellular existence but in return synthesise essential 
amino acids that are not present in the sap of the plants that the aphids colonise. Bacteria that exist in a 
strict intracellular mutualistic existence with their host are called endosymbionts. 
In being useful to the host cell endosymbionts can be viewed as intermediates between free-living 
organisms and the intracellular organelles that are thought to derive from bacteria: mitochondria in 
animals and plastids in plants. Free-living organisms need to utilise sufficient nutrients from an 
environment that may not provide regular nutrient supplies. Choosing to exploit the intracellular 
environment of a host guarantees nutrients and, what’s more, there are not hundreds of other organisms 
to compete for their share of the spoils. The host has evolved numerous defence mechanisms in order to 
prevent this bacterial pillaging. Thus, it is not surprising that intracellular parasites (either obligate or 
facultative), in their attempts to avoid or resist the host defences, have become pathogenic. 
In adapting to an intracellular existence, the microbe has two options: simply become increasingly 
dependent on the host for certain nutrients or provide a function that benefits the host. In other words, 
become mutualistic orsimply exploit the host as a parasite. 
What are the consequences of becoming an intracellular parasite or an endosymbiont and how do they 
differ from the intracellular organelles (likely to have once been bacteria)? 


Cited By Kamal Singh Khadka 
Msc Microbiology, TU.
Assistant Professor In Pokhara University, Pokhara Bigyan Tatha Prabidhi Campus, PNC, LA, NA.
Pokhara, Nepal.


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