Clinical use of colistin induces cross-resistance to host antimicrobials in Acinetobacter baumannii

Abstract

The alarming rise in antibiotic resistance has led to an increase in patient mortality and health care costs. This problem is compounded by the absence of new antibiotics close to regulatory approval. Acinetobacter baumannii is a human pathogen that causes infections primarily in patients in intensive care units (ICUs) and is highly antibiotic resistant. Colistin is one of the last-line antibiotics for treating A. baumannii infections; however, colistin-resistant strains are becoming increasingly common. This cationic antibiotic attacks negatively charged bacterial membranes in a manner similar to that seen with cationic antimicrobials of the innate immune system. We therefore set out to determine if the increasing use of colistin, and emergence of colistin-resistant strains, is concomitant with the generation of cross-resistance to host cationic antimicrobials. We found that there is indeed a positive correlation between resistance to colistin and resistance to the host antimicrobials LL-37 and lysozyme among clinical isolates. Importantly, isolates obtained before and after treatment of individual patients demonstrated that colistin use correlated with increased resistance to cationic host antimicrobials. These data reveal the overlooked risk of inducing cross-resistance to host antimicrobials when treating patients with colistin as a last-line antibiotic. IMPORTANCE Increased use of the cationic antibiotic colistin to treat multidrug-resistant Acinetobacter baumannii has led to the development of colistin-resistant strains. Here we report that treatment of patients with colistin can induce not only increased resistance to colistin but also resistance to host cationic antimicrobials. This worrisome finding likely represents an example of a broader trend observed in other bacteria against which colistin is used therapeutically such as Pseudomonas aeruginosa and Klebsiella pneumoniae. Furthermore, these data suggest that the possible future use of an array of cationic antimicrobial peptides in development as therapeutics may have unintended negative consequences, eventually leading to the generation of hypervirulent strains that are resistant to innate host defenses. The potential for the induction of cross-resistance to innate immune antimicrobials should be considered during the development of new therapeutics.

FIG 1

Colistin resistance correlates with resistance to host cationic antimicrobials in A. baumannii clinical isolates. (A to J) Colistin-sensitive (A to D) and colistin-resistant (E to J) A. baumannii clinical isolates were treated with 6.25 µg/ml of LL-37 (green) or 2.5 mg/ml of lysozyme (blue) for the indicated times and plated for enumeration of CFU. Time zero CFU is indicated by dashed black lines. (K to L) Percent changes of CFU from time zero to 2 h after treatment with LL-37 (K) or lysozyme (L) from 4 individual experiments with 3 to 4 samples of each isolate represented in panels A to J. Data were analyzed for significance using the unpaired Student’s t test. **, P < 0.001; ***, P < 0.0001. Error bars represent the standard deviations of the results determined for triplicate samples.

FIG 2

Clinical treatment with colistin can induce increased resistance to host cationic antimicrobials. Sequential A. baumannii isolates were collected from 2 patients (Ab6266 and Ab6267, patient 1; Ab3527 and Ab3941, patient 2) pre-colistin treatment (Ab6266 and Ab3527) and post-colistin treatment (Ab6267 and Ab3941). Isolates were treated with 6.25 µg/ml of LL-37 (A and C) or 2.5 mg/ml of lysozyme (B and D). Percent changes from time zero to 2 h after treatment with host antimicrobials from 3 experiments with 3 to 4 samples of each isolate are represented. Data were analyzed for significance using the unpaired Student’s t test. *, P < 0.05; **, P < 0.001. Error bars represent the standard deviations of the results determined for triplicate samples.