give examples of one antibiotic that can work against many species of bacteria
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Answer:
The antibacterial effect of penicillin was discovered by Alexander Fleming in 1929. He noted that a fungal colony had grown as a contaminant on an agar plate streaked with the bacterium Staphylococcus aureus, and that the bacterial colonies around the fungus were transparent, because their cells were lysing. Fleming had devoted much of his career to finding methods for treating wound infections, and immediately recognised the importance of a fungal metabolite that might be used to control bacteria. The substance was named penicillin, because the fungal contaminant was identified as Penicillium notatum. Fleming found that it was effective against many Gram positive bacteria in laboratory conditions, and he even used locally applied, crude preparations of this substance, from culture filtrates, to control eye infections. However, he could not purify this compound because of its instability, and it was not until the period of the Second World War (1939-1945) that two other British scientists, Florey and Chain, working in the USA, managed to produce the antibiotic on an industrial scale for widespread use. All three scientists shared the Nobel Prize for this work, and rightly so - penicillin rapidly became the "wonder drug" which saved literally millions of lives. It is still a "front line" antibiotic, in common use for some bacterial infections although the development of penicillin-resistance in several pathogenic bacteria now limits its effectiveness (see later).
The action of penicillin is seen in Figure A. This shows an 'overlay plate', in which a central colony of the fungus Penicillium notatum was allowed to grow on agar for 5-6 days, then the plate was overlaid with a thin film of molten agar containing cells of the yellow bacterium, Micrococcus luteus. The production of penicillin by the fungus has created a zone of growth inhibition of the bacterium. This demonstration parallels what Alexander Fleming would have observed originally, although he saw inhibition and cellular lysis of the bacterium Staphylococcus aureus.
Figure B shows the typical asexual sporing structures of a species of Penicillium. The spores are produced in chains from flask-shaped cells (phialides) which are found at the tips of a brush-like aerial structure.
Penicillin has an interesting mode of action: it prevents the cross-linking of small peptide chains in peptidoglycan, the main wall polymer of bacteria. Pre-existing cells are unaffected, but all newly produced cells grow abnormally, unable to maintain their wall rigidity, and they are susceptible to osmotic lysis.
This morphogenetic effect of penicillin can be demonstrated by growing either Gram-positive or Gram-negative bacteria in the presence of sub-lethal concentrations of penicillin. The images below show Gram-stained cells of Bacillus cereus that had been cultured in the absence of penicillin (left-hand image) or in the presence of a low concentration of the penicillin derivative termed Ampicillin (right-hand image). By affecting the cross-linking of the bacterial cell wall, penicillin has caused the bacterium to grow as larger cells with less frequent cell divisions.
Penicillin is not a single compound but a group of closely related compounds, all with the same basic ring-like structure (a beta-lactam) derived from two amino acids (valine and cysteine) via a tripeptide intermediate. The third amino acid of this tripeptide is replaced by an acyl group (R in the diagram below), and the nature of this acyl group confers specific properties on different types of penicillin.
The two natural penicillins obtained from culture filtrates of Penicillium notatum or the closely related species P. chrysogenum are penicillin G (shown in the diagram) and the more acid-resistant penicillin V. They are active only against Gram-positive bacteria (which have a thick layer of peptidoglycan in the wall) and not against Gram-negative species, including many serious pathogens like Mycobacterium tuberculosis (the cause of tuberculosis). (see Gram reaction for further details). Nevertheless, the natural penicillins were extremely valuable for treating wound pathogens such as Staphylococcus in wartime Europe.
An expanded role for the penicillins came from the discovery that natural penicillins can be modified chemically by removing the acyl group to leave 6-aminopenicillanic acid (see diagram above) and then adding acyl groups that confer new properties. These modern semi-synthetic penicillins such as Ampicillin, Carbenicillin (see diagram) and Oxacillin have various specific properties such as:
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Answer:
Pencillin is the antibiotic sir