Don't miss those 1.5 hours to treat persistent infections

One of the strategies proposed for the treatment of persistent infections is to kill bacterial cells with a high dose of an antibiotic and allowing the antibiotic concentration to decrease to enable the growth of persisters followed by the administration of a second dose of antibiotic shortly after persisters start to grow (Lewis 2007). Later Gefen et al. (2008) found that even the persisters are vulnerable to the action of antibiotics over a narrow time window after the exit from stationary phase. They found that, during the first 1.5 h after the transfer to fresh medium, the non-growing cells are vulnerable to the action of ampicillin. However, after this period, those cells get differentiated into the dormant state that protects them from the bactericidal action of ampicillin. Authors suggest that, by subjecting the bacterial populations to antibiotics within this time window, before the onset of full dormancy, it might be possible to prevent persistence.

However, translating this knowledge to in vivo situations will be difficult. In the above in vitro experiment, an overnight bacterial culture was diluted in fresh medium and at each time point after the dilution, an aliquot of the culture was directly exposed to ampicillin for 5 h, and the number of persisters was calculated to determine their vulnerability to the antibiotic. In vivo, the antibiotics can not be removed completely at once since the concentration of the antibiotic decreases only gradually depending on the half life of the antibiotic. The concentration of the antibiotic at which persisters start to grow is not known. The knowledge of this concentration is important because the time window during which the persisters are vulnerable to the action of antibiotics is very narrow (i.e. 1.5 h for ampicillin). How can we subject the bacterial populations to antibiotics within this time window without knowing the antibiotic concentration at which the persisters start to grow?

The fundamental problem with these type of conclusions is that the researchers try to extrapolate in vitro results as such to in vivo conditions. The effects of metabolism and hence the half-life of the antibiotic, post-antibiotic effect, plasma protein binding, tissue distribution etc. are never taken into consideration in in vitro experiments. Hence the chances of errors are high when extrapolating results from the static in vitro experiments as such to in vivo conditions.

The above article has shown that, even though persisters are intransigent to the action of antibiotics, they have a generous side also. They offer us a window of opportunity to eliminate them. Clinicians can now utilize this 1.5 h window of opportunity to treat persistent infections!!

Next- Why did Balaban et al. (2004) miss small colony variants?

Lewis, K. (2007). Persister cells, dormancy and infectious diseases. Nat Rev Microbiol. 5(1): 48-56.

Gefen et al. (2008). Single-cell protein induction dynamics reveals a period of vulnerability to antibiotics in persister bacteria. Proc Natl Acad Sci USA 105(16): 6145–6149.