Symbiosis

Types of symbiosis 

Symbiosis is the living together of two organisms.  Commensalism - symbiotic relationship in which one organism benefits and the other is neither helped nor harmed. Mutualism - " both organisms benefit   Pathogenesis - " one organism is harmed (may or may not grow inside the host) Parasitism - " one organism is harmed (parasite can not grow outside the host)

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E.g. of mutualism and commensalism 

MutualismBovine (cow) rumen - harboring bacteria that digest cellulose and other plant materials. The cow benefits from the bacteria through aided digestion and the bacteria benefit fromt the cow through an habitat for growth and continual food supply.CommensalismNon-pathonenistic E. coli in human intestine. Consumes remaining oxygen in the intestine, allowing for growth of obligate anaerobes - the E. coli does not benefit from these bacteria either. 

Role of commensal/mutualistic bacteria

Those inhabiting the intestinal flora aid the synthesis of vitamins and digestion of polysaccharides. In addition, the presence of bacteria aids the physical development of these tissues. perturbation (change to system) of these C/M bacteria can contribute to disease - inflammatory bowel, obesity ect...Areas colonised are as follows:  Skin - staphylococcus  Oral cavity/nasopharynx - streptococcus  Stomach - helicobacter pylori  GI tract - escherichia coli

Bacteria in the oral cavity

Bacteria live on teeth and in the crevices of gums, colonies are called plaque. Early colonisers e.g. strep, work commensally to enable later species of bacteria to inhabit the cavity. It is important to note, within plaque, there is a different, low pH and anaerobic environment and the bacteria inhabiting have adapted to inhabit it.   The early colonizers of teeth (strep) bind secrete polysaccharides and bind to the resulting sugar coated surface. Later colonisers (fusobacteria) bind to surface proteins on existing bacteria, this the early colonizers 'recruit' the later ones. Adaptation of oral bacteria to challenging environments is a factor. The early colonizers have to endure pH3-4, thus they have developed acid tolerance mechanisms. Late colonisation of bacteria are present in an anoxic environment, both early (facultative) and late are anaerobes, but this two step mechanism allows for late colonisation by construction of an anaerobic environment.Acid tolerance - low membrane permeability to protons; export of protons via proton pump; export of lactic acid. 

Bacteria in the GI tract

Bacteria like helicobacter adapt to colonise the stomach, despite its low (pH2) environment. The large intestine is anoxic (without oxygen) and so bacteria like bacteroides, bifidobacterium and clostridium have adapted to respire anaerobically.  Helicobacter acid tolerance mechanism: protons do enter the bacterial cytoplasm, and thus to avoid acidification, they must be neutralised. The bacterial enzyme urease converts urea (present in stomach) to carbon dioxide and ammonia, the ammonia reacts to neutralize H+, forming ammonium. This is a buffer mechanism. 

Identifying symbiotic bacteria 

There are two approaches to this:CulturingHowever, this method is limited, as many (most) bacteria in the body can not be grown in lab conditions. DNA sequencing - modern approachSequencing of the 16s ribosomal RNA from a colony isolated from a body site. This approach IDs bacteria at the genus level. These sequences are conserved. Metagenomics is the study of genetic material recovered from natural bacterial communities; the approach is whole genome sequencing of DNA isolated from a microbial community. 

Bacteria in the bowel

Firmicutes  Bacteroides - digest plant polysaccharides  Proteobacteria - synthesise vit K Actinobacteria - digest milk oligosaccharides, synthesise vit K and B There is also one known archaebacteria present, this is a methanogen