Why some antibiotic combination treatments are effective against persisters?

Researchers claim that persisters are tolerant to multiple antibiotics. In such experiments, they isolate persisters in vitro after treating with one antibiotic and then test the killing rate of persisters with other antibiotics. Based on these results, they report that persisters are multi-drug tolerant cells.

However, in many instances, persisters can be killed by combinations of antibiotics. Minardi et al. (2011) found that a combination of intraperitoneal tygecycline and stent-bonded rifampin was effective in sterilizing both urine culture and ureteral stent culture (biofilms) against Enterococcus faecalis whereas treatment with tygecycline or rifampin alone were not effective in sterilizing the culture (tygecycline alone treatment left a small subpopulation of bacteria which constituted less than 1% of the control group). Similar results were also reported with combinations of clarithromycin and amikacin against P.aeruginosa biofilms in an animal model of ureteral stent infection (Cirioni et al. 2011).

Why combination treatment was better against the persisters in biofilms in the above case? Or in other words, how did 3.8x10³ cfu/ml of bacteria in the stent culture that survived tygecycline alone treatment were killed completely when rifampin was given along with tygecycline? If the same experiment were done with tygecycline alone, one would have considered those bacteria as persisters (in the above articles, authors have not used the term 'persisters'). This raises the question whether the persisters reported in many experiments are ‘true persisters’ or simply bacteria that survived antibiotic therapy due to suboptimal conditions.

Earlier, I had argued that many of the experiments demonstrating ‘persisters’ are the result of suboptimal conditions provided in their experiments.  

Minardi et al. (2011). Efficacy of tygecycline and rifampin alone and in combination against Enterococcus faecalis biofilm infection in a rat model of ureteral stent. Journal of surgical research doi:10.1016/j.jss.2011.05.002.

Cirioni et al. (2011). Effect of the combination of clarithromycin and amikacin on Pseudomonas aeruginosa biofilm in an animal model of ureteral stent infection. J Antimicrob Chemother 66(6): 1318-1323.