Friday, May 13, 2005

Antibiotic resistance in bacteria

As soon as antibiotics are applied to a population of bacteria, only those bacteria that have the ability to survive in the presence of the antibiotic reproduce, thus resulting in strong selection for bacteria with antibiotic resistance. If the bacteria don't already have genes for antibiotic resistance, they generally have to rely on random mutations, or genetic transfers from other bacteria that already have these genes, to acquire antibiotic resistance.

However, a recent journal article (Cirz et al. 2005; my dad just sent me the synopsis), reports on a mechanism some bacteria use to increase their mutation rate in response to certain antibiotic drugs, thus causing faster evolution of resistance to those drugs. The article is in PLoS Biology, an open-access journal, so the full text of the paper is freely available. It's a good read.

Antibiotics known as quinolones (e.g., Ciprofloxacin, aka Cipro) interfere with the normal functioning of topoisomerases (proteins that help maintain DNA structure) in bacteria; this interference causes DNA damage, and eventually bacterial cell death. Bacteria can evolve resistance to these drugs, and the required mutations to obtain resistance are known:
"Resistance to ciprofloxacin requires mutations in the genes that encode the topoisomerases (gyrA and gyrB, encoding gyrase, and parC and parE, encoding topoisomerase IV) or in the genes that affect cell permeability or drug export"
By altering the function of the topoisomerases, the quinolones also stop the repression (by LexA) of a set of genes known as SOS genes. These (normally repressed) genes include three DNA polymerases (proteins that synthesize / repair DNA) that often mutate DNA as they work on it; these DNA polymerases are not essential for bacterial survival, as bacteria have other DNA polymerases they use for typical DNA replication. The end result of all this molecular biology is that bacteria treated with quinolones should begin producing DNA polymerases that cause mutations in the bacteria's own DNA; Cirz et al. set out to verify this experimentally.

Cirz et al. first infected mice with pathogenic strains of E. coli, and then administered ciprofloxacin; after 72 hours they found that approximately 3% of the E. coli in the mice had become resistant to the antibiotic (figure 1, filled circles). However, when they infected mice with a strain of E. coli that could not de-repress the SOS genes (by preventing autoproteolysis of LexA), they found that none of the bacteria in the mice evolved resistance to ciprofloxacin after 72 hours (figure 1, filled triangles).

Cirz et al. then compared the mutation rates of E. coli before and after ciprofloxacin treatment, and found that exposure to ciprofloxacin increased the mutation rate by approximately 10^4 (from 9.0 × 10−10 mutants/viable cell/d to 1.8 × 10−5 mutants/viable cell/d). However, when they examined a strain of E. coli that could not de-repress the SOS genes, they found that mutation rates after exposure to ciprofloxacin were 100-fold lower than those of standard E. coli exosed to ciprofloxacin.

In summary, Cirz et al. found that administration of quinolones to bacteria increased bacterial mutation rates, and thus promoted the evolution of resistance to the antibiotics. Or, as the paper's authors say,
"the mutations that confer resistance to ciprofloxacin and rifampicin are not simply the result of unavoidable errors accumulated during genome replication, but rather are induced via the derepression of genes whose protein products act to significantly increase mutation rates."
Then the paper's authors go on to suggest how this knowledge can be used to help prevent, or at least reduce, the evolution of antibiotic resistance:
"The traditional paradigms of DNA replication and mutation suggest that resistance-conferring mutations are the inevitable consequence of polymerase errors, and offer no obvious means for intervention. In stark contrast, the model described above suggests that bacteria play an active role in the mutation of their own genomes by inducing the production of proteins that facilitate mutation, including Pol IV and Pol V, as has been suggested with other forms of mutation. In turn, this suggests that inhibition of these proteins, or the prevention of their derepression by inhibition of LexA cleavage, with suitably designed drugs, might represent a fundamentally new approach to combating the emerging threat of antibiotic-resistant bacteria."


Cirz RT, JK Chin, DR Andes, V de Crécy-Lagard, WA Craig, FE Romesberg. 2005. Inhibition of Mutation and Combating the Evolution of Antibiotic Resistance. Public Library of Science Biology 3: e176. Full-text HTML, PDF.

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