Persister cells and antibiotic tolerance

[Fanghur]

I'm currently writing a paper on persister cells and their role in biofilm tolerance to all known antibiotics for one of my classes, and I've read several articles about persisters, but there is something I don't understand. The accepted explanation (or at least one of them) of how persisters are unaffected by antibiotics is that they are completely dormant, that is, nothing inside them is active; no cell wall synthesis, protein translation, enzymes, etc, all turned off. This means that even though the antibiotics do successfully bind their targets, it has no effect since their targets are effectively inanimate, and thus cannot be corrupted by the antibiotic. This I understand.

 

But what I don't understand is that persisters eventually do become active again, as they have to in order to regrow the biofilm, and indeed this is the case. But wouldn't the antibiotics still be bound to their targets when the persisters 'awaken'? And if so, why don't they carry out their intended function once the persisters activate?

 

An analogy; let's say that shoelaces are the antibiotic, and your friend jokingly ties your laces together while you are sleeping so that you'll trip when you wake up. While you are asleep, you are analogous to a persister cell in this situation; the 'antibiotic' (ie tied laces) is on you, but since you aren't moving, it doesn't affect you - you can't trip if you aren't walking. But once you wake up and try to get out of bed you'll still trip, because your laces are still knotted together.

 

Can someone explain to me why this isn't the case for actual persister/antibiotic relations?

[ecoli]

I'm not all that familiar with the intimate details of biofilms and antibiotics (haven't taken my bacteriology course yet) but antimicrobials aren't completely stable, so it's possible that they're being degraded or washed out before the persister cells re-activate?

[CharonY]

Indeed. Persisters usually rise when AB treatment is discontinued. IIRC the presence of ABs and other stresses actually promotes the formation of persisters.

Edited March 18, 2011 by CharonY

[Fanghur]

Indeed. Persisters usually rise when AB treatment is discontinued. IIRC the presence of ABs and other stresses actually promotes the formation of persisters.

 

I understand that part, CharonY. What I don't understand is why the antibiotics which had previously bound to the cells while they were in the persister state don't kill the persisters when they (the cells) become active again. Do antibiotics degrade rapidly once they enter the cells?

 

In other words, how does the 'sleeper' (persister cell) untie (unbind/destroy) his shoelaces (the bound antibiotic) while he is sleeping (dormant) so that he doesn't trip (get killed) when he tries to walk (protein synthesis, DNA replication, or whatever the antibiotic targets).

 

I understand that persister 'awaken' once the danger has past, so to speak, but what about the antibiotics/radicals that have already gotten inside them? What happens to them?

[SMF]

Fanghur, I am not sure about this specific example, but antibiotics have to be kept at a certain concentration over some amount of time to kill a bloom of a species of bacteria. If the bacteria ducks and hides in a biofilm, in a dormant state, until the antibiotic agent goes away it is like a hibernating bear escaping winter starvation. Antibiotics sneak into cells, via a variety of routes, and they have mechanisms for removing them (e.g. the ATPase multi drug resistance, or MDR, pump). This can result in an antibiotic concentration war against time and the biofilm persisters can bypass the period of high concentration. Stated another way, the accumulated intracellular concentration of antibiotic when a bacterium hides in a biofilm can be easily pumped out when it reawakens, but only if the extracellular concentration is reduced. SM

Edited March 19, 2011 by SMF

[Fanghur]

OK; I don't think you guys are understanding my question. Let's use quinolones as an example; quinolones kill by covalently binding to DNA Gyrase and Topoisomerase IV, and inhibiting their ability to re-ligate the DNA strands after they nick them. Now, let's say a bunch of quinolones get inside of a persister cell and bind to gyrase and topo IV. While the cell is dormant, the antibiotic is useless. But when the cell becomes active again, wouldn't those quinolones still be bound to DNA gyrase and topo IV? And if not, why not?

[jimmydasaint]

I think SMF has given you a pretty good answer.

 

The transient phenotype expressed by persisters allows escape from antibiotic effects but the exact mechanism seems to be unclear.

 

The relationship between S. aureus SCVs and biofilm phenotype is unclear, but their shared characteristics suggest that they may have a similar underlying physiology (Higashi & Sullam, 2006). Both are slow-growing and more resistant to antimicrobials, and the diseases with which they are associated overlap considerably (Higashi & Sullam, 2006). Once adhered, SCVs are almost completely resistant to antibiotics (Chuard et al., 1997). Williams et al. (1997) reported isolation of SCVs from adherent S. aureus cultures even in the absence of antibiotics, suggesting that biofilm formation may correlate with this mode of growth. However, following antibiotic treatment, these variants appeared with equal frequency in both biofilms and planktonic cells (Williams et al., 1997). The specific role of SCVs in conferring antibiotic resistance to S. aureus biofilms thus remains unclear.

Link to journal

 

 

As a speculation, is it possible that:

 

a) the antibiotic targets in the cells (ribosomes, membranes etc...) are inactive so no binding occurs and the antibiotics diffuse out?

 

b) Additionally, is it possible that the dose of the antibiotic and the half life/ persistence of the antibiotic is also important?

 

Animals are commonly used in preclinical infectious disease research. To demonstrate the potential clinical significance of animal pharmacokinetic studies, the terminal halflives of renally excreted cephalosporins and monobactams

in various animal species were collected from the literature.

These data were fitted by the allometric equation t1,2 = aWb.

(Half-life data were chosen for analysis because drug halflife is customarily reported in mammals, and the hybrid

nature of drug half-life contributes to its successful use in

allometry [2].) Human antibiotic half-life values were predicted with each equation and compared with reported

half-life values to evaluate this method of pharmacokinetic

forecasting

Refer also to Table 1 in this article.

Half Life of Antibiotics