Due to overuse and misuse of antibiotics and the ability of bacteria to rapidly gain resistance, we are now in an age of multidrug-resistant (MDR) and pan-drug-resistant (PDR) bacterial pathogens and face a possible return to the pre-antibiotic era. The burden of antimicrobial resistance has increased the length of hospital stays, patient mortality, and health care costs (1–5). Acinetobacter baumannii is a Gram-negative and highly antibiotic-resistant bacterial pathogen (6, 7). Due to its ability to persist on surfaces for weeks, its ability to colonize humans, and its increasing rate of antibiotic resistance, A. baumannii has become an emerging health care problem (6, 8).

Polymyxin antibiotics, including colistin, are currently used as last-line drugs to treat MDR A. baumannii infections (9). Colistin is a cationic antimicrobial peptide that disrupts both the outer and inner membranes of Gram-negative bacteria (10), a trait shared by the host cationic antimicrobials LL-37 and lysozyme (11–16). LL-37 is a human antimicrobial peptide typically found at sites of inflammation, where it is a primary defense against Gram-negative bacteria (17). Lysozyme is a host antimicrobial found within multiple immune cells as well as in secretions such as tears, breast milk, and mucus and is important for their activity against invading microbes. Importantly, the highly cationic, nonenzymatic, C-terminal portion of lysozyme has very potent antimicrobial activity (12–14).

Given the increasing prevalence of colistin resistance among A. baumannii clinical isolates (9), we set out to test whether there was a correlation with cross-resistance to host cationic antimicrobials. We assembled a panel of A. baumannii isolates (CI-1, CI-2, CI-3, 17978, CI-4, ARLC, MU134, MU215, MU181, and MU52) and first determined their colistin MICs (see Table S1 in the supplemental material) and susceptibilities to multiple other antibiotics (Table S2). Bacterial suspensions were prepared using a PROMPT (3M Company, St. Paul, MN) inoculation system, and antibiotic susceptibilities were determined using Neg Breakpoint Combo Panel type 41 on a Microscan WalkAway Plus automated system (Siemens Healthcare Diagnostics Inc., West Sacramento, CA). Additionally, the MICs for colistin were measured using Etest strips (bioMérieux, Durham, NC), following the inoculation and reading instructions of the manufacturer. The strains exhibited a range of MIC values. Using Clinical and Laboratory Standards Institute (CLSI) interpretive criteria, we found that strains CI-1, CI-2, CI-3, and 17978 were sensitive to colistin whereas CI-4, ARLC, MU134, MU215, MU181, and MU52 were resistant (Table S1).

We treated the colistin-sensitive and -resistant isolates with LL-37 or lysozyme to determine their sensitivities, as previously described (18). Briefly, overnight cultures were grown from frozen stock in lysogeny broth (LB; also known as Luria broth) (BD Biosciences, Sparks, MD) at 37°C with aeration and then diluted to a final concentration of ~10e6 CFU/ml in 25% LB. Bacteria were treated with host antimicrobials (LL-37, 6.25 µg/ml; lysosome, 2.5 mg/ml) and incubated with aeration at 37°C, and aliquots were plated at 0 h, 1 h, and 2 h for enumeration of CFU. The colistin-sensitive clinical isolates CI-1 (Fig. 1A) and CI-2 (Fig. 1B) were inhibited from replicating to wild-type levels in the presence of LL-37, while CI-3 (Fig. 1C) and 17978 (Fig. 1D) were killed. In contrast, the colistin-resistant clinical isolates CI-4 (Fig. 1E), ARLC (Fig. 1F), MU134 (Fig. 1G), MU215 (Fig. 1H), MU181 (Fig. 1I), and MU52 (Fig. 1J) each replicated roughly 100- to 200-fold after 2 h in the presence of LL-37. These data suggest that colistin resistance correlates with increased resistance to the host cationic antimicrobial peptide LL-37, which was further clearly demonstrated when results from 4 experiments performed with the aforementioned strains were pooled (Fig. 1K). We observed similar phenotypes following lysozyme treatment, with the exception of the colistin-susceptible 17978 isolate (Fig. 1D), which was able to persist in the presence of lysozyme instead of being killed, and CI-2 (Fig. 1B), which exhibited very limited replication. These results were again further illustrated when data from several experiments were pooled (Fig. 1L) and suggest that colistin resistance is highly correlated with resistance to lysozyme.

We next investigated the basis for the resistance phenotypes observed. The A. baumannii PmrAB two-component regulatory system induces phosphoethanolamine modifications to the lipid A component of lipopolysaccharide (LPS) in response to the presence of polymyxins (19, 20). Arroyo et al. showed that point mutations within the PmrB periplasmic domain, histidine kinase (HisK) dimerization/phosphoacceptor domain, or C-terminal ATP binding domain (ATPB) can confer constitutive polymyxin resistance through these lipid A modifications (20). We sequenced the pmrB genes in the colistin-resistant isolates used in this study and found that each harbored nonsynonymous mutations in the sequences encoding one or more of these three domains compared to the pmrB sequence from the colistin-sensitive 17978 strain (see Table S1 in the supplemental material). In contrast, only synonymous mutations were identified in the sequences of pmrB from the colistin-sensitive strains. Nonsynonymous point mutations conferring resistance can be selected rapidly and likely explain the ability of some of the colistin-sensitive strains to eventually acquire resistance during extended exposure to LL-37 or lysozyme (see Fig. S1A to D in the supplemental material) (20). Taken together, these data suggest that mutations within pmrB confer not only colistin resistance but also cross-resistance to host cationic antimicrobial peptides.

Since the clinical isolates used here were very diverse (isolated from different anatomical sites, from different areas of the United States, and exhibiting differing antibiotic susceptibility patterns; see Table S1 and S2 in the supplemental material), it is not surprising that there was heterogeneity in their patterns of resistance to host antimicrobials; specifically, the colistin-sensitive CI-2 isolate displayed low-level resistance to LL-37 and lysozyme (Fig. 1B) that was not observed with the other colistin-sensitive isolates (Fig. 1A and C to D). In order to overcome the complications of comparing diverse strains, and to directly determine whether colistin treatment can induce cross-resistance to host cationic antimicrobials, we obtained 2 pairs of A. baumannii clinical isolates from patients pre- and post-colistin treatment (21). Each pair of serial isolates (Ab6266 and Ab6267, and Ab3527 and Ab3941) was from a distinct patient, under 35 years of age, who had sustained severe trauma overseas (21). Ab6267 was collected after 3 weeks of patient treatment with colistin, and Ab3941 was collected after 6 weeks of patient treatment with colistin. Both pre-treatment isolates (Ab6266 and Ab3527) were colistin sensitive, while the post-treatment isolates (Ab6267 and Ab3941) displayed increased resistance to colistin (see Table S1 in the supplemental material). In addition, the transition to increased colistin resistance was not accompanied by changes in susceptibility to the other antibiotics tested (Table S2). While we cannot specify the event(s) (an induced mutational event, selection for a preexisting mutant, or reinfection) that led to the selection for the post-treatment resistant isolates, the pre- and post-treatment isolates were shown to be highly similar by pulse-field gel electrophoresis (PFGE), using a 90% similarity cutoff, compared to 1,500 A. baumannii strains collected from 24 geographically separate hospitals (21). Additionally, optical genome mapping (OGM) analysis showed that the two sets of pre- and post-treatment isolates shared >99% homology (21).

When treated with LL-37 or lysozyme, the post-colistin treatment Ab6267 isolate displayed a significant increase in resistance compared to the corresponding pre-colistin treatment isolate (Ab6266) (Fig. 2A and B). Ab3941 displayed a significant increase in resistance to killing by lysozyme compared to Ab3527 (Fig. 2D), although there was no change in its level of resistance to LL-37 (Fig. 2C). Neither pre-colistin treatment isolate demonstrated resistance to LL-37 or lysozyme over 22 h of treatment (see Fig. S1E and F in the supplemental material). As with the colistin-resistant isolates tested above, the post-colistin treatment strains both harbored nonsynonymous mutations in critical PmrB domains (Table S1). These data from sequentially isolated strains pre- and post-colistin treatment strongly support the conclusion that clinical use of colistin induces cross-resistance to host cationic antimicrobial peptides.