Heterogeneous Persister Cells Formation in Acinetobacter baumannii

Abstract

Bacterial persistence is a feature that allows susceptible bacteria to survive extreme concentrations of antibiotics and it has been verified in a number of species, such as Escherichia coli, Pseudomonas aeruginosa, Staphylococcus spp., Mycobacterium spp. However, even though Acinetobacter baumannii is an important nosocomial pathogen, data regarding its persistence phenotype are still lacking. Therefore, the aim of this study was to evaluate the persistence phenotype in A. baumannii strains, as well as its variation among strains after treatment with polymyxin B and tobramycin. Stationary cultures of 37 polymyxin B-susceptible clinical strains of A. baumannii were analyzed for surviving cells after exposure to 15 µg/mL of polymyxin B for 6 h, by serial dilutions and colony counting. Among these, the 30 tobramycin-susceptible isolates also underwent tobramycin treatment at a concentration of 160 µg/mL and persister cells occurrence was evaluated equally. A high heterogeneity of persister cells formation patterns among isolates was observed. Polymyxin B-treated cultures presented persister cells corresponding from 0.0007% to 10.1% of the initial population and two isolates failed to produce detectable persister cells under this condition. A high variability could also be observed when cells were treated with tobramycin: the persister fraction corresponded to 0.0003%–11.84% of the pre-treatment population. Moreover, no correlation was found between persister subpopulations comparing both antibiotics among isolates, indicating that different mechanisms underlie the internal control of this phenotype. This is the first report of persister cells occurrence in A. baumannii. Our data suggest that distinct factors regulate the tolerance for unrelated antibiotics in this species, contrasting the multi-drug tolerance observed in other species (eg. dormancy-mediated tolerance). Supporting this observation, polymyxin B – an antibiotic that is believed to act on non-dividing cells as well – failed to eradicate persister cells in the majority of the isolates, possibly reflecting a disconnection between persistence and dormancy.

Combination of polymyxin B and meropenem eradicates persister cells from Acinetobacter baumannii strains in exponential growth

“…Furthermore, we observed that combining the highest doses tested of both antimicrobials was more efficient even after 6 h exposure, and rapidly reduced the bulk of cells in comparison with the other combinations. Thus, taking into account the present data and our previous findings that polymyxin B (15 µg ml À1 ) [8] or meropenem alone (15, 30, 100 and 200 µg ml À1 ) were not able to eradicate A. baumannii persisters [13], we may highlight that there is a synergistic bactericidal effect of this drug combination. Simultaneous treatment employing high doses of conventional antimicrobials has also been reported as being able to completely eradicate Staphylococcus aureus persister cells within a few hours of antibiotic combination exposure [14].…”

“…Nevertheless, antimicrobial therapy failure has been reported, especially in recalcitrance of chronic infections, which may be due to the tolerance mediated by persister cells [5,6] -a microbial subpopulation able to survive to normally lethal concentrations of bactericidal antimicrobials [7], as has been previously described in Acinetobacter spp. exposed to polymyxin B [8]. This tolerance appears to be a transient phenotype that may be understood as arising from a combination of epigenetic inheritance and cellular noise [9], whose regulation involves a multiplicity of pathways [10].…”

“…Simultaneous treatment employing high doses of conventional antimicrobials has also been reported as being able to completely eradicate Staphylococcus aureus persister cells within a few hours of antibiotic combination exposure [14]. Furthermore, the treatment grounded in antimicrobial combination could be an effective alternative to eradicating these cells, especially taking into account the heterogeneous pattern of persister cell levels obtained after exposure to distinct antibiotics, even those belonging to the same class [8,[15][16][17]. So, a multidrug persistence phenotype is suggested, as well as the existence of different types of persisters in the bacterial population [16,18].…”

Pyocyanin Stimulates Quorum Sensing-Mediated Tolerance to Oxidative Stress and Increases Persister Cell Populations in Acinetobacter baumannii

“…cinetobacter baumannii is an emerging opportunistic human pathogen that causes a plethora of nosocomial infections (1). The ability of this pathogen to sense and react to environmental and host stress signals allows it to persist in medical settings and in human hosts (2).…”

Acinetobacter baumannii biofilm resistance mechanisms and prevention and control of progress

“…The sensitivity of bacteria with slow growth in the membrane decreases and enters into the "hibernation" state. [21] Until the drug stops functioning, residual bacteria quickly multiplies to form a new biofilm by use of dead ones as a nutrient source, which stimulates the recurrence of infection. In addition, there is also significant different in pH value between inside and outside of biofilm, mainly due to the accumulation of acidic metabolites to reduce drug sensitivity.…”