Fluoroquinolones: Mechanisms of Action and Resistance

in this animation we demonstrate the biology of DNA replication leading to bacterial cell division in a gr positive bacterium such as s pneumonia the DNA is shown as a circular double strand within the bacterial cell like the DNA of all living organisms it contains the unique genetic code for all of the proteins required for bacterial survival bacteria replicate by a process known as binary fision whereby one bacterium separates into two new daughter cells however before this can occur the bacterium must make an identical copy of its complete circular DNA DNA replication requires that the two

strands of DNA separate so that the genetic Cod of the bacterium can be read and a new complimentary strand can be created for each of the original strands to accomplish this various enzymes known as helicases break the hydrogen bonds between the bases in the two DNA strands unwind the strands from each other and stabilize the exposed single strands preventing them from joining back together the points at which the two strands of DNA separate to allow replication of DNA are known as replication forks the enzymes DNA polymerase then moves along each strand of DNA behind each

replication fork synthesizing new DNA strands in red complimentary to the original ones as the replication forks move forward positive superhelical twists in the DNA begin to accumulate ahead of them in order for DNA replication to continue these superhelical twists must be removed the bacterial enzyme DNA gyas which is also known as topoisomerase 2 is responsible for removing the positive super helical twists so that DNA replications can proceed DNA gyas is an essential bacterial enzyme composed of 2 a and 2B subunits which are products of the G and gyrb genes this enzyme has other important functions

which affect the initiation of DNA replication and transcription of many genes with the combined involvement of these enzymes an entire duplicate copy of the bacterial genome is produced as the two replication forks move in opposite directions around the circular DNA genome eventually as the two replication forks meet two new complete chromosomes have been made each consisting of one old and one new strand of DNA this is referred to as semiconservative replication in order to allow the two new Interlink chromosomes to come apart another bacterial enzyme is needed which is known as topoisomerase 4 this enzyme

is structurally related to DNA gyes and is coded for by the PC and P genes topoisomerase 4 allows for the two new interl chromosomes to separate so that they can be segregated into two New Daughter bacterial cells this animation will demonstrate two mechanisms of fluroquinolone action fluroquinolone antibiotics shown here are synthetic molecules that are bacteriocidal the potency of these drugs is greatly improved by the addition of a fluorine molecule at position six and thus the term fluroquinolones fluroquinolones rapidly inhibit bacterial DNA synthesis resulting in bacterial cell death although much is known about the molecular events

that underly the action of quinolone antibiotics much remains to be clarified fluroquinolones act by inhibiting the activity of both the DNA gyas and the topoisomerase 4 enzymes for most gram negative bacteria DNA gy is the primary fluroquinolone target fluroquinolones have been shown to bind specifically to the complex of DNA gyes and DNA rather than the DNA gyes alone as a result of this binding quinolones appear to stabilize the enzyme DNA complexes which in turn results in breaks in the DNA that are fatal to the bacterium a second mechanism of fluroquinolone action is shown here with

some exceptions topor isomerase 4 is the primary target of fluoroquinolone action in most grand positive bacteria such as stafilococ ey and strepto Coy with DNA gyate being a secondary Target the separation of two New interl Daughter strands of circular DNA is disrupted the final result on the bacteria however is the same bacterial replication is disrupted and the bacterium breaks apart the relative potency of different fluroquinolone antibiotics and thus their spectrum of activity is dependent in part on their affinity for either DNA gyes or topoisomerase 4 or both one of the most common mechanisms by which

bacteria acquire resistance to fluoroquinolones is by spontaneously occurring mutations in chromosomal genes that alter the target enzymes DNA gyas and topoisomerase 4 or both the frequency with which these spontaneous mutations occurs may be in the range of 10 ^ of -6 the effective mutations on the activity of an individual fluroquinolone will vary depending on the number of mutations the location of the mutations and which Target enzyme is affected if a mutation occurs either in the G or gyrb Gene that alters DNA gyes and results in a reduced Affinity of the fluroquinolone antibiotic for this enzyme

the organism will become resistant similarly a mutation may occur that alters Topo is 4 either in the PC or P Gene and results in a reduced Affinity of the fluroquinolone antibiotic for this enzyme and the bacterial organism will become resistant it is important to note that for some fluroquinolones that have similar affinity and potency against both Target enzymes mutations in both DNA gyres and Topo isomerase 4 will be needed for resistance to occur in resistant organism semiconservative replication continues