penicillin

Penicillin was discovered by Alexander Fleming in 1929. What happened was that he noticed that, on a agar plate streaked with a bacteria, Staphylococcus Aureus, there was a contaminant mold that got onto the plate and that there was no bacterial growth directly around this mould.

The fungus was found to be Penicillum notatum and thus the substance that was found to have these anti-bacterial properties was called penicillin.

It was not until the 1940s that penicllin was purified by two other scientists, Ernst Boris Chain and Howard Walter Florey. It has since become the most widely used antibiotic to date and is still used for many Gram-positive bacterial infections.

Penicllin works by inhibiting the formation of peptidoglycan cross links in the bacterial cell wall by directly competing for the reaction site (the beta-lactam ring of penicillin is a chemical analogue for the enzyme that links the peptidoglycan molecules in bacteria) and thus preventing the bacteria from multiplying (or rather causing cell lysis or death when the bacteria tries to divide)

Antibiotic resistance to penicillin is now common amongst many hospital acquired bacteria. The resistance to penicillin has been partly (maybe mostly) due to the rise of beta-lactamase producing bacteria which secrete an enzyme that breaks down the beta-lactam ring of penicillin, rendering it harmless to the bacteria.

Symptoms of an allergic reaction to Penicillin can include the following:

Rashes
Hives
Itchy eyes
Swollen lips, tongue, or face

Rarely a Penicillin allergy can lead to an anaphylactic reaction, usually occurring within an hour of taking Penicillin. Some symptoms may be difficulty breathing, hives, wheezing, dizziness, loss of consciousness, rapid or weak pulse, drop in blood pressure, swelling of the throat or tongue, skin turning blue, diarrhea, nausea, and vomiting.

Chances of an allergic reaction are greater if you have had a positive skin test for a Penicillin allergy, had hives that showed up soon after taking Penicillin, or if you have previously had an anaphylactic reaction to Penicillin.

Usually during emergency treatment for an emergency allergic Penicillin reaction an epinephrine shot is given to the patient. Also antihistamines and corticosteroids may be given intravenously.

For less severe allergic reactions a person may want to take over the counter antihistamine(s).

A significant number of people who think they have a Penicillin allergy may not have one. Their sensitivity to Penicillin may have changed from what it was in the past. Adverse reactions to Penicillin may me mistaken for an allergic reaction. Some frequent adverse reaction symptoms may be:

Diarrhea
Hypersensitivity (an undesired reaction caused by the normal immune system)
Nausea
Rash
Neurotoxicity (exposure to natural or artificial toxic substances, neurotoxins, that alter the normal activity of the nervous system in such a way as to cause damage to nervous tissue): limb weakness/numbness, loss of memory, delusions, cognitive and behavioral problems, and headache.
Hives
Superinfection (when a cell previously infected by one virus is co-infected with a different strain of the virus or another virus, at a later date).

Less frequent adverse reactions include:

Fever
Vomiting
Erythema (redness of the skin caused by an increase in blood flow to different tissues in the body)
Dermatitis (inflammation of the skin)
Seizures
Angioedema (rapid swelling of the dermis, subcutaneous tissue, mucosa and submucosal tissues)
Pseudomembranous colitis (inflammation of the colon)

Sources:
http://www.webmd.com/allergies/guide/penicillin-allergy-topic-overview
http://www.mayoclinic.com/health/penicillin-allergy/DS00620/DSECTION=symptoms
http://en.wikipedia.org/wiki/Penicillin_allergy