Looking at A. baumannii isolates from patients around the country, they noted that all the colistin-resistant strains harbored mutations in pmrB, a regulatory gene that leads to the modification of polysaccharides on the outside of the cell in response to antibiotic exposure. Tests showed a tight correlation between the ability of individual isolates to resist high concentrations of colistin and the ability to resist attacks by LL-37 or lysozyme.
This was very convincing, write the authors, that mutations in the pmrB gene were responsible for cross-resistance to LL-37 and lysozyme, but to get closer to a causative link between treatment and cross-resistance, they studied two pairs of A. baumannii isolates taken from two different patients before and after they were treated for three or six weeks with colistin. The results helped confirm the cross-resistance link: neither strain taken before treatment was resistant to colistin, LL-37, or lysozyme, but the strains taken after treatment showed significant resistance to colistin and lysozyme. (One post-colistin isolate was no more or less resistant to LL-37 than its paired pre-colistin isolate.) Like the resistant strains tested earlier, both post-colistin isolates harbored crucial mutations in the pmrB gene that apparently bestow the ability to resist treatment.
The authors point out that the apparent link between resistance to colistin and cross-resistance to antimicrobial agents of the immune system could well extend to other pathogens that are treated with colistin, including Pseudomonas aeruginosa and Klebsiella pneumoniae. Weiss says he plans to follow up with studies to determine whether this bears out.
|Contact: Jim Sliwa|
American Society for Microbiology