Since the early pioneering days of antibiotic research, many new antibiotics have been discovered. Advances in the fields of microbiology have allowed researchers to not only
discover new antibiotics, but also to assess the mechanisms by which these new antibiotics target bacteria, they’re mode/mechanism of action (MOA).19-20
Although certainly not an exhaustive list, eight different current antibiotics will be illustrated here. These include Gentamicin, Ampicillin, Oxacillin, Ciprofloxacin, Vancomycin, Erythromycin, Tetracycline, and Rifampin. These eight drugs are all used in later chapters as controls for compounds synthesized in this work.
1. Gentamicin.
Figure 1-1: Structure of Gentamicin.
Gentamicin was discovered in 1963 as a fermentation product of the gram-positive bacteria Micromonospora purpurea.21 Due to poor oral bioavailability, it is generally used either
as a topical agent or intravenously.22-23 Gentamicin is a type of aminoglycoside which works by stopping bacterial protein synthesis, this typically kills the bacterium and is known as a
bactericidal antibiotic.24 It is also listed by the World Health Organization (WHO) as an essential medicine, one of the most important medications needed in a basic health system.
Gentamicin is generally used for gram-negative infections, especially blood-related infections.25 Although certain gram-positive strains are susceptible to Gentamicin, most are not, so in general it is not used to treat gram-positive infections. Unfortunately, Gentamicin has several severe side-effects including nephrotoxicity (kidney damage) and ototoxicity (cochlea toxicity) which limit its use as a treatment.22, 26 Recently due to the emergence of resistance in many gram-negative strains, Gentamicin has found renewed use since it is still effective against a number of these resistant strains.27 This lack of resistance is directly tied to its infrequent use in the past rather than an inability for Gentamicin to confer resistance, as resistant strains are now becoming more prevalent due to this resurgence.28
2. Ampicillin.
Figure 1-2: Structure of Ampicillin.
Ampicillin is a drug related to penicillin and is in the ß-lactam family of antibiotics.29 These antibiotics inhibit transpeptidase, a protein necessary for cell wall synthesis.30 This often
leads to cell lysis and death of the cell, so ß-lactam antibiotics are bactericidal.30-31 Ampicillin was the first ‘broad-spectrum’ penicillin with activity against both positive and gram-negative bacteria. It differs from penicillin G only with the inclusion of an amino group, which helps Ampicillin penetrate gram-negative bacterial cell walls.32 It is also on the WHO’s list of essential medicines for basic health systems.
Like other penicillins, Ampicillin is not effective against methicillin-resistant S.
aureus (MRSA). The effectiveness of penicillin analogs relies on their ability to both reach
penicillin-binding proteins (PBPs) and bind to the PBPs. Some bacteria are able to produce enzymes that cleave the ß-lactam rings, known as ß-lactamase or penicillinase.33 Cleavage of the ß-lactam rings leaves the resulting compound ineffective. Another method bacteria have
employed to gain resistance to ß-lactams is altering the PBPs. This is most notable in MRSA strains in which the PBP is altered to such a degree that the ß-lactam ring is no longer able to bind and the antibiotics are thus ineffective.33-34
In the case of ß-lactamases, it has been shown that ß-lactamase inhibitors such as clavulanic acid can increase the potency of penicillin’s when co-administered.35 However, treatment in this fashion can potentially lead to bacterial strains that possess higher levels of ß-lactamase expression which can make future treatments with ß-lactams even less effective.36 Still, treatments with ampicillin or the closely related amoxicillin are among the most prescribed treatments for bacterial infections.37
3. Oxacillin.
Similarly to Ampicillin, Oxacillin is also a ß-lactam antibiotic. They differ greatly in that Oxacillin is resistant to penicillinase38, however, this has led to extensive use against
penicillin-resistant S. aureus. Unfortunately, this led to the previously mentioned penicillin-resistant strains in which the PBPs active sites are altered in such a way that these ß-lactams can no longer bind, leaving drugs like Oxacillin and the related Methicillin ineffective for MRSA strains.39
Figure 1-3: Structure of Oxacillin.
Despite these issues, Oxacillin remains a popular choice for treating bacterial infections that are penicillin resistant.40 It is increasingly important, however, that these drugs are not over prescribed and that patients adhere to the treatment protocols to slow the emergence of new strains of MRSA.41
4. Ciprofloxacin.
Ciprofloxacin, commonly known as simply Cipro, is a relatively modern antibiotic released in 1987. It is also on the list of WHO’s essential medications for a basic health system.
It is used for both gram-positive and gram-negative infections and works by inhibiting DNA gyrase and topoisomerase IV.42 This prevents bacterial DNA from separating and thus inhibits cell division. Cipro has been shown to be bactericidal when used in higher concentrations, however at its MIC it is bacteriostatic.43 It is thought this bactericidal nature is due to the release
of DNA gyrase complexes leading to chromosomal DNA fragmentation, while at lower concentrations the inhibitory effects just stop cell division.44
Figure 1-4: Structure of Ciprofloxacin.
Cipro is a second generation fluoroquinolone and is derived from the original antibiotic quinolones discovered in the early 1960’s. In general, these fluoroquinolones are known to quickly develop resistance, sometimes even within one course of treatment.45 This problem was compounded with prescriptions for conditions not approved by the FDA and overuse in
veterinary medicine.46-47 Some bacteria developed efflux pumps that decrease intracellular quinolone concentration and some others developed mutations to DNA gyrase and
topoisomerase IV that decreased binding affinity for quinolones.45
One literature source published in the late 1980’s reports that clinical oral dosage of Cipro to 37 MRSA patients resulted in Cipro-resistant mutants which developed in 6 out of the 37 cases. While the article claims 91% of patients were clinically cured or their conditions improved, this rate of resistance was alarming.48
5. Vancomycin.
Figure 1-5: Structure of Vancomycin.
Vancomycin is a gram-positive antibiotic biosynthesized by the bacteria Amycolatopsis orientalis. It was first isolated in 1953 from a soil sample collected from the jungles of
Borneo.49 The mode of action is the inhibition of proper cell-wall synthesis leading to death of the cell. Due to the difference in cell wall configurations, Vancomycin is ineffective in treating bacterial infections from gram-negative bacteria.
Originally Vancomycin was developed as a solution to the penicillin-resistant S. aureus infections mentioned earlier, however as Methicillin and Oxacillin were developed and were both more efficacious and easier to dose, Vancomycin fell behind as a drug of last-resort. This was due in part to its low oral bioavailability, which necessitated IV administration.50 Recently, however, Methicillin and Oxacillin resistant S. aureus (MRSA) have necessitated Vancomycin as a first-line treatment for a number of these infections.51 While IV injections are necessary for
most infections, it can be orally administered for bowel or stomach infections since it has rather poor oral absorption.52 It is now also on the WHO’s list of essential medications.
Vancomycin resistance, however, is becoming more prevalent, especially in hospital settings.51, 53 If these infections are not identified as Vancomycin resistant early enough, patients are at extreme risk. As these strains of Vancomycin-intermediate and Vancomycin-resistant S.
aureus (VISA and VRSA respectively) become more prevalent, a new antibiotic will necessarily become the new first-line treatment for bacterial infections, with resistance likely to follow.
6. Erythromycin.
Figure 1-6: Structure of Erythromycin.
Erythromycin is yet another antibiotic that was originally separated from a natural source, in this case the bacteria Saccharopolyspora erythraea.54 It is also another antibiotic listed by the WHO as an essential medication for a basic health system. It was first isolated in 1952 and was found to be active against gram-positive bacterial strains. Erythromycin is in the antibiotic class of macrolides, compounds that inhibit protein synthesis by binding to the 50S subunit of
bacterial ribosomes, humans do not have 50S ribosomal units so these compounds act selectively on the bacteria.55
Resistance to macrolides and Erythromycin is seen in many bacteria with mutations in the 50S subunit which inhibits binding. MRSA strains, for example, are notoriously resistant to Erythromycin and other macrolides.56-58 Growing resistance in other species is also an alarming concern.
7. Tetracycline.
Figure 1-7: Structure of Tetracycline.
Tetracycline was also originally isolated from bacteria, in this case strains of Streptomyces produced the compound.59 It was first isolated in 1945 and was surprisingly already prescribed as a drug as early as 1948.60 It is yet another antibiotic listed by the WHO as an essential medicine.
Tetracycline acts by inhibiting protein synthesis byblocking the attachment of charged aminoacyl-tRNA to the A site on the ribosome.61 Tetracycline binds to the 30S ribosomal subunit of microbial ribosomes and also binds to the 40S subunit of mammalian ribosomes.
However, while bacteria actively pump Tetracycline into the cytoplasm, mammalian cells do not.
The relatively small off-site effects in humans by Tetracycline can be explained by this 40S binding.62 The inhibition of protein synthesis is reversible and thus bacteriostatic.59
Bacteria actively form resistance to Tetracycline by either encoding efflux pumps that actively pump Tetracycline from the cytoplasm, or by ribosomal protection proteins that dislodge Tetracycline from the ribosome.59, 63 Tetracyclines are sometimes prescribed for MRSA related infections, however identification of the strain and its susceptibility to Tetracycline are important factors considered before treatment.64-66
8. Rifampin.
Figure 1-8: Structure of Rifampin.
As has been the case for most antibiotics discussed above, Rifampin was also first isolated from another species of bacteria, in this case Amycolatopsis rifamycinica. It was first discovered in 1957 and first sold as a medication in 1971.67 Along with most of the
aforementioned antibiotics, it is also on the WHO list of essential medications, particularly for Tuberculosis (TB).68
Rifampin inhibits RNA polymerase halting bacterial RNA synthesis. It is extremely potent in wild-type strains of M. Tuberculosis and many strains of MRSA; however, resistance is
easily conferred.69 Bacteria confer resistance through altered residues in the Rifampin binding site on RNA polymerase.69-71 Rifampin resistant TB is one of the most dangerous microbes in developing countries, with TB infecting nearly 10 million people in 2014 alone. It is estimated that ~20% of all cases involve strains resistant to at least one medication.72-74 This often leads to a ‘cocktail’ drug composed of, in many cases, three separate drugs (isoniazid, pyrazinamide, and rifampin).75
Due to side-effects and the conference of resistance, Rifampin is generally not used to treat MRSA infections, but when it is used, it is generally used in combination therapy to reduce the likelihood of forming resistance.76