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Tag words: antibiotic, antimicrobial, antimicrobial agent, antibiotic resistance, penicillin, methicillin, vancomycin.











Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

Bacillus cereus bacteria.Print this Page

Antimicrobial Agents in the Treatment of Infectious Disease
(page 5)


(This chapter has 6 pages)

© Kenneth Todar, PhD

Effects on Nucleic Acids

Some antibiotics and chemotherapeutic agents affect the synthesis of DNA or RNA, or can bind to DNA or RNA so that their messages cannot be read. Either case, of course, can block the growth of cells. The majority of these drugs are unselective, however, and affect animal cells and bacterial cells alike and therefore have no therapeutic application. Two nucleic acid synthesis inhibitors which have selective activity against procaryotes and some medical utility are the quinolones and rifamycins.

Nalidixic acid is a synthetic chemotherapeutic agent that has activity mainly against Gram-negative bacteria. Nalidixic acid belongs to a group of compounds called quinolones. Nalidixic acid is a bactericidal agent that binds to the DNA gyrase enzyme (topoisomerase) which is essential for DNA replication and allows supercoils to be relaxed and reformed. Binding of the drug inhibits DNA gyrase activity.

Some quinolones penetrate macrophages and neutrophils better than most antibiotics and are thus useful in treatment of infections caused by intracellular parasites. However, the main use of nalidixic acid is in treatment of lower urinary tract infections (UTI). The compound is unusual in that it is effective against several types of Gram-negative bacteria such as E. coli, Enterobacter aerogenes, K. pneumoniae and Proteus species which are common causes of UTIs. It is not usually effective against Pseudomonas aeruginosa, and Gram-positive bacteria may be resistant. Some quinolones have a broadened spectrum against Gram-positive bacteria. The fluoroquinolone, Cipro. (ciprofloxacin) was recently touted as the drug of choice for treatment and prophylaxis of anthrax, which is caused by a Gram-positive bacillus, Bacillus anthracis.


Ciprofloxacin (cipro), a fluoroquinolone is a broad-spectrum antimicrobial agent that is active against both Gram-positive and Gram-negative bacteria. It functions by inhibiting DNA gyrase, a type II topoisomerase, which is an enzyme necessary to separate replicated DNA, and thereby inhibits cell division.

The rifamycins are a comparatively new group of antibiotics, also the products of Streptomyces species. Rifampicin  is a semisynthetic derivative of rifamycin that is active against Gram-positive bacteria (including Mycobacterium tuberculosis) and some Gram-negative bacteria. Rifampicin acts quite specifically on the bacterial RNA polymerase and is inactive towards DNA polymerase or RNA polymerase from animal cells. The antibiotic binds to the beta subunit of the polymerase and apparently blocks the entry of the first nucleotide which is necessary to activate the polymerase, thereby blocking mRNA synthesis. It has been found to have greater bactericidal effect against M. tuberculosis than other anti-tuberculosis drugs, and it has largely replaced isoniazid as one of the front-line drugs used to treat the disease, especially when isoniazid resistance is indicated. It is effective orally and penetrates the cerebrospinal fluid so it is useful for treatment of bacterial meningitis.



Rifampicin (or rifampin) is a bactericidal antibiotic from the rifamycin group. It is a semisynthetic compound derived from Amycolatopsis rifamycinica (formerly known as Amycolatopsis mediterranei and Streptomyces mediterranei). Rifampicin is typically used to treat Mycobacterium infections, including tuberculosis and leprosy; and also has a role in the treatment of methicillin-resistant Staphylococcus aureus (MRSA) in combination with fusidic acid. It is used in prophylactic therapy against Neisseria meningitidis (meningococcal) infection. It is also used to treat infection by Listeria monocytogenes, Neisseria gonorrhoeae, Haemophilus influenzae and Legionella pneumophila.



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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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