Immunity to Bacteria

Another article is up on, this one is about how the immune system deals with bacteria – Below is only the introduction, if you would like to read more then remember to check out the above ^^^


Bacteria exist naturally on many biological surfaces, for example the skin or the lining of the intestines. Bacteria like these make up the body’s natural flora and have a range of symbiotic relationships; a good example would be the flora of the rumen in cattle which degrade food materials, providing energy for both the cattle and the bacteria. The three main types of symbiotic relationship are:

•Mutualism – Both members of the symbiotic relationship benefit

•Commensalism – No apparent harm/benefit occurs to either member of the relationship

•Parasitism – One member of the relationship is living at the expense of the other resulting in disease

The pathogenicity of a certain bacteria depends on its survival inside the host – how well is it able to resist or evade host defence mechanisms and immune response. The resulting disease/damage caused to tissue is due to either the pathogenicity of the bacteria or the immune response of the host itself.

Bacterium Structure

Prokaryotes vs. Eukaryotes

Bacteria are prokaryotes, they differ from eukaryotic cells (such as those in humans) because the structures within prokaryotic cells are typically not compartmentalised. Prokaryotes also lack nuclear membranes, mitochondria, endoplasmic reticulum, a Golgi body, phagosomes and lysosomes (unlike eukaryotes). Also, prokaryotes only have a single, circular chromosome – unlike the nucleus of a eukaryotic cell.

Gram Staining

Bacteria can be very broadly categorised into two groups, gram negative and gram positive. This describes whether or not the bacterial will stain when using a gram stain. Gram-negative bacteria do not take up the gram stain; this is due to an extra outer membrane. Gram-positive bacteria do not have this extra outer membrane and so will take up the gram stain.

Bacterial Structures

•Plasmids – This is an extra-chromosomal strand of circular DNA, it is able to replicate independently from the main chromosome in the bacteria and the genes which the plasmid codes for aren’t typically essential for survival. The plasmid may be shared between bacteria which may be of concern as the plasmid often codes for pathogenesis and anti-bacterial resistance.

•Cell Envelope – This is the extra outer membrane seen in gram-negative bacteria

•Flagella – A protein organelle (consisting of flagellin) which is used for locomotion

•Pili (Fimbriae) – This is the organelle which allows adhesion to the epithelium of host cells.

•Capsules and ‘slime’ layers – These are layers outside of the cell envelope in some specialised bacteria. This extra layer allows the inhibition of ingestion by phagocytes as they are unable to detect the bacterium. A well-defined layer is known as a capsule, a lesser defined layer is known as a slime layer.

•Endospores – This is a term given to dormant forms of bacteria which are able to survive harsh conditions

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