Survey of Common Antimicrobial Drugs
About 260 different antimicrobial drugs; classified into 20 drug families.
The number may appear larger because it is common for one generic drug to be marketed under multiple
different trade names.
Most antimicrobial drugs are effective against bacteria. There are fewer antiviral and antifungal drugs.
I. Antibacterial Drugs
1. Penicillins
Cell wall inhibitors – relatively non-toxic to human cells
All have an active unit, the Beta-lactam ring
Natural Penicillins ( extracted from a mold, Penicillium sp.)
 narrow spectrum - good for gram(+) organisms, not good against gram negatives
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Penicillinases - enzymes produced by some bacteria that will inactivate penicillins by
destroying the B-lactam ring; (also called Beta-lactamases).
An important way that bacteria can develop resistance to penicillins is to acquire the
ability to produce these enzymes
Semi-synthetic Penicillins
 natural penicillin which is chemically modified - more resistant to penicillinase
 Methicillin is the usual drug of choice to treat beta-lactamase producing gram positive
organism.
 Methicillin-resistant Staphylococcus aureus ( MRSA) is a growing problem

Some are Broad spectrum - effective against gm(+) & gm(-)’s,
o e.g. ampicillin, amoxicillin,
Problems
 Allergic reactions
 Resistant organisms develop ( due to the production of Beta-lactamase enzymes)
2.
Cephalosporins
 cell wall inhibitors
 Source: a mold, Cephalosporium sp.
 the Beta-lactam ring is slightly different – but can still be inactivated by some types of Betalactamases
 more effective against gram negatives than the penicillins
 The majority of all antibiotics administered belong to this group – over 70 versions –
 1st, 2nd, 3rd, 4th generations- each generation is broader spectrum and more resistant to
bacterial B-lactamase,
 1st generation -- Cephalothin,
 3rd generation – Keflex, ceftriaxone (Rocephin) – commonly used
3. Aminoglycosides
 Inhibit protein synthesis
 broad spectrum
 toxic ( can cause hearing or kidney damage, diarrhea)
 Streptomycin ( for M. tuberculosis , bubonic plague)
 Gentamycin
 Neomycin ( topical use only)
4. Tetracyclines
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blocks protein synthesis
Very broad spectrum
Can be natural (tetracycline) or semisynthetic (e.g.,doxycycline)
can wipe out normal flora in gut  gastrointestinal upset
children - discolors teeth
pregnant women  liver damage
animal feeds  produces weight gain, but contributes to development of resistant strains
5. Chloramphenicol – synthetic
 blocks protein synthesis
 very broad spectrum
 small - can diffuse through tissues, even into CSF
 serious side effects include irreversible bone marrow suppression, aplastic anemia and
fatality; used cautiously
 reserved for serious infections - typhoid fever, brain abscesses, rickettsia & chlamydia
6. Erythromycin
o Broad-spectrum; low toxicity
o Inhibits protein synthesis
o Prophylaxis before intestinal surgery
o Good for penicillin-resistant infections
o Azithromycin is a newer version
7. Vancomycin
o very toxic to patients - only used when necessary; almost always given as IV.
o narrow spectrum;
o used for (MRSA), and resistant strains of Enterococcus
o some Staphylococcus aureus and Enterococcus faecalis (VRE)strains have recently become
resistant - major problem
8. Bacitracin - from Bacillus sp.
 narrow spectrum -- only for gm positives like staph or strep
 usually in topical ointments because it is so toxic.
9. Synthetics - Not derived from bacteria or fungus
Sulfonamides (Sulfa drugs) - 1st modern antimicrobial drugs
 compete with PABA for the enzyme that usually converts PABA to folic acid
 used mostly for urinary tract infections
 Trimethoprim - Sulfamethoxizol (TMP-SMZ) - good example of synergy
Quinolones
 Inhibit DNA synthesis
 very broad spectrum; widely used; very potent
 can affect cartilage development - problem for children or pregnant women
 Ciprofloxacin was one of the first antibiotics used to treat patients exposed to B.anthracis during
the anthrax exposures in 2001. Most patients were eventually switched to narrow spectrum
alternatives ( e.g., doxycycline)
II. Antifungal Drugs
Fungi are eukaryotes and are very similar to animal cells. This makes it more difficult to find a selective target.
Fungal infections are increasing in number and there is pressure to develop more antifungal drugs
1. Polyenes – Bind to ergosterol in the fungus membrane
 Amphotericin-B – one of the oldest antifungal drugs
 highly toxic ( esp. to kidney); reserved for systemic (body-wide) fungal infections
 Nystatin – topical or oral treatment for Candidiasis (yeast infection)
2. Azoles - damage the membrane by interfering with sterol synthesis
 topical - for athlete’s foot, yeast infections ( e.g., Miconazole, Clotrimazole)
 oral or IV - widely used as an alternative to amphotericin-B because they are less toxic
III. Antiprotozoan and Antihelminthic Drugs - for parasitic infections
Antiprotozoans drugs
Protozoan parasites are one-celled members of the Kingdom Protista
Most antimalarial drugs are quinine based
o Plasmodium sp. -- causes malaria
o Quinine - from bark of the cinchona tree - used since 1600’s as an antimalarial drug
o Synthetic Quinine derivatives are now used
o Chloroquine - most widely used antimalarial - Plasmodium strains are becoming resistant
Other Antiprotozoan Drugs
Metronidazole ( Flagyl) –
 Widely used to treat intestinal parasites such as Giardia
 Used to treat Trichomonas vaginalis, a sexually transmitted parasite
 also used to treat anaerobic bacterial infections
Antihelminthic drugs
 Helminths are multi-celled parasites (fukes, tapeworms and roundworms)belonging to the Animal
Kingdom
 Niclosamide , and Ivermectin are common antihelminthics
Antiviral Drugs
In U.S. - viral infections are about 60% of all infectious diseases. Antibiotics are NOT effective against
viruses. Viruses are intracellular pathogens. It is hard to find a drug that will penetrate the host cell, and target
the virus without harming the host cell. There are very few effective antivirals available right now , but many are
in the research and development stage. Currently the most effective strategy in dealing with viruses is to use
vaccines to prevent infection.
Three Major Modes of Action –
1. Block viral from attaching to cell or entering cell
2. Block transcription and translation
3. prevent the maturation and assembly of viral particles
These methods can protect uninfected cells, but cannot destroy extracellular viruses or viruses that are in the
latent state.
Amantadine
 one of the 1st antivirals - prevents the virus from entering the cell
 limited use in preventing influenza A infections
 Tamiflu and Relenza are newer agents with similar action
Nucleotide analogs
 synthetic nucleotides – mimic the structure of normal nucleotides, therefore will substitute for
normal bases, A,G,C, or T & inhibit DNA or RNA synthesis
 Acyclovir (Zovirax) - for herpes virus
 AZT - a Thymine analog - blocks reverse transcription of RNA into DNA ( used for retroviruses such
as HIV)
Viral Enzyme inhibitors
 will inhibit some viral enzyme that is necessary for the virus to complete its life cycle
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Reverse-transcriptase inhibitors - Retroviruses have RNA as their genetic material. To replicate,
they must copy their viral RNA into DNA. This process is called reverse transcription and requires
an enzyme called reverse-transcriptase. Drugs that inhibit reverse transcriptase will prevent the
virus from replicating. HIV ( Human Immunodefficiency Virus), the virus that causes AIDS is a
retrovirus.
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Protease-inhibitors – Proteases are viral enzymes that are necessary for viruses to assemble their
proteins and build mature viral particles.. If you inhibit these proteases, the virus cannot mature.
Interferons
 Proteins produced by fibroblasts and WBC’s in response to immune stimulation
 Have ant-cancer and anti-viral properties
 When a virus infects a cell, the infected cell produces interferons.
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Interferons can activate neighboring cells to produce antiviral proteins (AVPs) which prevent the
virus from infecting those cells .
 Interferons can also activate cells of the immune system to begin a protective immune response
 Alpha-interferon is a standard treatment for hepatitis infections. So far, this has not been very
successful for treating HIV, most cancers, or most other viral infections