When was the last time researchers isolated hipA mutants either in vitro or in vivo?

It was Moyed and Bertrand (1983) who isolated hipA7 mutants for the first time. They were screening for mutants showing high persistence and isolated ‘high persistence’ mutants that included hipA7 mutant. The hipA7 mutant generates high frequency of persisters than wild type strains when exposed to antibiotics. Similarly, it was later found that the overexpression of hipA results in high frequency of persisters. Based on these findings, it was concluded that hipA gene is very important in persister generation.

However, isolation of this mutant in vitro or in vivo has not been reported after Moyed and Bertrand (1983). In fact, some of the researchers failed to isolate hipA mutants among the persisters generated in vitro. After plating approximately 10¹⁰ bacteria on agar plates containing different concentrations of ciprofloxacin for 96 h, Marcusson et al. (2005) found that some bacteria could survive the lethal action of the antibiotic. However, the survival of this subpopulation was not associated with any mutation in the hipA or hipB genes, indicating that the persistence of bacteria in the presence of antibiotics in the above case was not due to hipA or hipB mutations.

In vivo, isolation of high persister (hip) strains has been reported among fungal and bacterial species (Lafleur et al. 2010; Mulcahy et al. 2010). However, in both cases, ‘hip’ is only a general term for high persisters and  the genetic background of hip phenotype is unknown (researchers assumed that the hip phenotype was caused by an underlying genetic change).  

Given the fact that persisters are isolated following antibiotic administration with any bacterial species tested, one would expect isolation of hipA mutants with high frequency. It is interesting to note that a mutant that has not been isolated either in vitro or in vivo for decades is considered highly important for a very common phenomenon.

Moyed and Bertrand (1983). hipA, a newly recognized gene of Escherichia coli K-12 that affects frequency of persistence after inhibition of murien synthesis. J. Bacteriol. 155:768-775

Marcusson et al. (2005). Mutant prevention concentrations of ciprofloxacin for urinary tract infection isolates of Escherichia coli. J Antimicrob Chemother 55(6), 938-43.

Lafleur et al. (2010). Patients with long-term oral carriage harbor high-persister mutants of Candida albicans. Antimicrob Agents Chemother 54(1), 39-44.

Mulcahy et al. (2010). Emergence of Pseudomonas aeroginosa strains producing high levels of persister cells in patients with cystic fibrosis. Journal of Bacteriology 192(23): 6191-6199.