Bacteria is the term to decribe all organisms whose cells are prokaryotic, which means they have no membrane-bound nucleus. It is now becoming more common to divide the bacteria up into two kingdoms: the archaebacteria and eubacteria. Most of the bacteria we commonly see belong in the eubacteria kingdom. This kingdom is further subdivided into the gram-positive, gram-negative, and mycoplasmas. This division is dependent on the external structure of the bacteria. The mycoplasmas are the bacteria which have no cell walls.
The gram-positive and gram-negative distinctions originally came from the resultant color of the bacteria after a process in which a smear was stained violet, rinsed in alcohol, then stained red. If the bacteria held onto the violet stain, the end product would appear violet and be considered "gram-positive". Otherwise it would look red and be "gram-negative". This difference is now believed to depend on the amount of peptidoglycan, and how accessible it is to the outside. The gram-positive bacteria contain a thick outer coating of peptidoglycan which holds the violet stain, while gram-negative bacteria have a much thinner layer, and also have structures external to that layer. The importance of this difference will be seen when we discuss how antibiotics work. Many of them attack the peptidoglycan layer, which eukaryotic cells don't have, so they are safe to humans. However, it is clear that this sort of agent will have a much more limited effect on gram-negative bacteria with their hidden peptidoglycan layer.
When people hear about bacteria, it is always in a negative light. However bacteria is everywhere, and many types are vital to our existance and play crucial roles in several parts of the various cycles required for the continuation of life. Very few of the bacteria that have been cataloged are in fact pathogenic (disease causing). However, those that are pose a serious threat to human health. The two factors that figure into whether or not a bacteria is pathogenic is it's invasiveness (abilty to multiply within the body of the host) and it's toxigenicity (ability to produce substances detremental to the host organism). While some bacteria aren't too toxic alone, they reproduce so rapidly that sheer volume of them causes infection. Similarly, some bacteria can barely penetrate the body, but they produce so much toxin that they cause infection.
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