3.1 Preventing approach: combination of treatments
Combining antibiotics to prevent the formation of persister cells is a
strategy that generally uses antibiotics with radically different
targets, the membrane being one of the preferred targets. Thus, most of
the studies combine the use of the polymyxin B or colistin with
β-lactams.
Recently, it has been shown that meropenem or carbenicillin treatments
resulted in the appearance of a particular cell phenotype that survive:
round, metabolically active cells that have lost their cell wall
[126]. A modification of their envelope leads to a susceptibility to
treatment with β-lactam (ampicillin and carbenicillin) with an increase
in the killing power from 100 to 1000 fold [126]. These results
suggest that antibiotic treatment can cause a modification of the
envelope, making the bacteria more sensitive to another antibiotic
targeting the membrane. Another study focused on three clinical strains
(Acb-1, Acb-8 and Acb-2) and determined the level of persister cells
generated by polymyxin B and meropenem, tested at different
concentrations alone or in combination [130]. Mono-treatment with
these antibiotics at 5, 10 or 15 mg/L, induced between 0.0031 to 0.9743
% of persisters depending on the choice of antibiotics. However, these
antibiotics in combination did not produce persister cells. This study
highlighted and confirmed the useful effects of combined therapy.
Synergistic effects were also observed with other antibiotic
combinations: 30 μg/mL carbenicillin with 2 μg/mL colistin or 200 μg/mL
ampicillin with 2 μg/mL colistin. They increased the killing efficiency
on two clinical isolates [126]. These combinations provide a two-log
increase in bacterial eradication.
Colistin has also been tested in combination with other antibiotics,e.g . colistin plus tobramycin, rifampicin or colistin [131].
These authors show that these combinations eradicate all cells.
Sequential antibiotic combinations were also examined [132]. It was
demonstrated that treatment by colistin followed by amikacin eradicated
the entire population, when the reverse sequence did not. Even though no
high antibiotic concentration was used to select persisters, this study
clearly shows that sensitization by a membranolytic agent is highly
effective. Colistin in combination with the econazole, an imidazole
derivative, was also tested [133]. This combination showed a
synergic effect with a reduction of MIC colistin by 128 fold with 10
µg/mL econazole.
The combination of an antimicrobial agent with a phage cocktail to
control or limit biofilm formation has also been investigated [134].
Combinations of four temperate bacteriophages (SA1, Eve, Ftm and Gln)
and different antibiotics (ampicillin/sulbactam, meropenem and colistin)
enhanced biofilm inhibition and degradation on thirty extensively
drug-resistant (XDR) strains of A. baumannii . Moreover, it has
been shown that a combination of Paride and meropenem can sterilize
deep-dormant cultures in vitro and greatly reduce a resilient
bacterial infection of a tissue cage implant in mice [135]. In this
context, it would be interesting to test these phage/antibiotic
combinations on persister cells.