Clonal variation in maximum specific growth rate and susceptibility towards antimicrobials

Abstract

Aims: To examine associations between growth rate within bacterial populations and survival patterns following treatment with antimicrobial agents. Methods and Results: Time survival data were generated for the inactivation of Escherichia coli populations, grown as batch and continuous cultures, exposed to ciprofloxacin, benzalkonium chloride and tetracycline. Timesurvivor plots were biphasic. Surviving cells were collected and immediately re-exposed to agent or were regrown and then re-exposed. Survivors were resistant to immediate challenge with any of the treatment agents. This resistance was lost on regrowth suggesting that survival reflects an expressed phenotype within a subset of the culture (persisters) rather than individual resistant clones or nonspecific quenching of the test agent. The fraction of persisters increased with decreasing growth rate when cultures were prepared in continuous culture. Conclusions: Clonal growth rates within populations were determined by culture of individual cells within microtitre plate wells. The fraction of clones, in batch cultures, growing maximally at rates below the apparent threshold for susceptibility to the test agents was sufficient to explain the results of continuous culture experiments. Significance and Impact of the Study: The presence of persisters in populations of bacteria relate to small subset of cells that are growing only slowly or are metabolically quiescent.

Senescence can explain microbial persistence

“…5 (though at sufficiently high dilution rate, washout of cells exceeds the maximum specific growth rate and thus the biomass concentration, including persisters, becomes zero in steady-state). We again note the qualitative match to the experimentally reported results in Sufya et al (2003). In particular, note the rapid transition from high to low persister fraction separating the two regimes when D 21 is smaller than and larger than the persister age dd.…”

“…We remark that the model predicts that most cells become persistent in late stationary phase. Late-stationary-phase data are scarce since all cells approach quiescence; however, data from continuous flow systems at low dilution rate (Sufya et al, 2003), which may be somewhat analogous to late-stationary-phase batch cultures, suggest that the persister percentage does indeed approach 100. Fig.…”

“…4 might be considered as an approximation to a chemostat with roughly 2 h turnover. So we also consider persister numbers in the chemostat system [equations (11)- (13)] at steady-state (see Sufya et al, 2003). In particular, by setting hb/ht50 in equations (11) and suppressing t dependence, we obtain…”

“…That is, persister cells with respect to one antimicrobial agent can also be tolerant upon exposure to different antimicrobial agents. As an example, see Sufya et al (2003), where survivors (from an Escherichia coli batch culture) of a tetracycline challenge were also tolerant to ciprofloxacin and quaternary ammonium compound.…”

“…$ In continuous culture experiments, persister fractions are observed to increase with decreasing dilution rates (Sufya et al, 2003).…”