He confirmed this observation with a new experiment on 28 September 1928. He published his experiment in 1929, and called the antibacterial substance (the fungal extract) penicillin.C. J. La Touche identified the fungus as Penicillium rubrum (later reclassified by Charles Thom as P. notatum and P. chrysogenum, but later corrected as P. rubens). Fleming expressed initial optimism that penicillin would be a useful antiseptic, because of its high potency and minimal toxicity in comparison to other antiseptics of the day, and noted its laboratory value in the isolation of Bacillus influenzae (now called Haemophilus influenzae).Fleming did not convince anyone that his discovery was important. This was largely because penicillin was so difficult to isolate that its development as a drug seemed impossible. It is speculated that had Fleming been more successful at making other scientists interested in his work, penicillin would possibly have been developed years earlier.The importance of his work has been recognized by the placement of an International Historic Chemical Landmark at the Alexander Fleming Laboratory Museum in London on 19 November 1999.
For example, resistance to vancomycin in S. aureus is due to additional peptidoglycan synthesis that makes the cell wall much thicker preventing effective penicillin entry. Resistance in Gram-negative bacteria is due to mutational variations in the structure and number of porins. In bacteria like Pseudomonas aeruginosa, there is reduced number of porins; whereas in bacteria like Enterobacter species, Escherichia coli and Klebsiella pneumoniae, there are modified porins such as non-specific porins (such as OmpC and OmpF groups) that cannot transport penicillin.Resistance due to PBP alterations is highly varied. A common case is found in Streptococcus pneumoniae where there is mutation in the gene for PBP, and the mutant PBPs have decreased binding affinity for penicillins. There are six mutant PBPs in S. pneumoniae, of which PBP1a, PBP2b, PBP2x and sometimes PBP2a are responsible for reduced binding affinity. S. aureus can activate a hidden gene that produces a different PBP, PBD2, which has low binding affinity for penicillins. There is a different strain of S. aureus named methicillin-resistant S. aureus (MRSA) which is resistant not only to penicllin and other β-lactams, but also to most antibiotics. The bacterial strain developed after introduction of methicillin in 1959. In MRSA, mutations in the genes (mec system) for PBP produce a variant protein called PBP2a (also termed PBP2), while making four normal PBPs. PBP2a has poor binding affinity for penicillin and also lacks glycosyltransferase activity required for complete peptidoglycan synthesis (which is carried out by the four normal PBPs).