Involvement of pmrAB and phoPQ in Polymyxin B Adaptation and Inducible Resistance in Non-Cystic Fibrosis Clinical Isolates of Pseudomonas aeruginosa

During investigation of susceptibility testing methods for polymyxins, 24 multidrug-resistant clinical isolates of Pseudomonas aeruginosa were observed to have a distinct, reproducible phenotype in which skipped wells were observed during broth microdilution testing for polymyxin B. Possible mechanisms underlying this phenotype were investigated. The effects of various concentrations of polymyxin B on growth, the expression of resistance genes, and outer-membrane permeability were observed. Real-time PCR was performed to compare the expression, in response to selected concentrations of polymyxin B, of genes related to the PhoP-PhoQ and PmrA-PmrB two-component regulatory systems in polymyxin B-susceptible isolate PAO1, polymyxin B-resistant isolate 9BR, and two isolates (19BR and 213BR) exhibiting the skipped-well phenotype. 19BR and 213BR appeared to have similar basal levels of expression compared to that of PAO1 for phoQ, arnB, and PA4773 (from the pmrAB operon), and in contrast, 9BR had 52- and 280-fold higher expression of arnB and PA4773, respectively. The expression of arnB and PA4773 increased in response to polymyxin B in a concentration-dependent manner for 9BR but not for 19BR and 213BR. For these isolates, expression was significantly increased for arnB and PA4773, as well as phoQ, only upon exposure to 2 μg/ml polymyxin B but not at a lower concentration of 0.125 μg/ml. The sequencing of the pmrAB and phoPQ operons for all three isolates revealed a number of unique mutations compared to that for PAO1. 1-N-phenylnaphthylamine (NPN) was used to study the effect of preincubation with polymyxin B on the self-promoted uptake of polymyxin B across the outer membrane. The preincubation of cells with 2 μg/ml polymyxin B affected baseline membrane permeability in 19BR and 213BR and also resulted in a reduced rate of NPN uptake in these isolates and in PAO1 but not in 9BR. The results presented here suggest that the skipped-well isolates have the ability to adapt to specific concentrations of polymyxin B, inducing known polymyxin B resistance genes involved in generating alterations in the outer membrane.The emergence of multidrug-resistant gram-negative organisms and the simultaneous lack of new clinically available antimicrobial agents have led to a resurgence of older compounds such as the polymyxins (6). These agents, including polymyxin B and colistin, have highly potent activity against gram-negative organisms, including Pseudomonas aeruginosa, but were previously abandoned due to a reported high incidence of nephrotoxicity and neurotoxicity (12).Resistance to polymyxin B is predominantly associated with decreased uptake into the bacterial cell resulting from a reduced capacity for initial binding (23). Polymyxin B and other polycationic compounds enter the cell via a process known as self-promoted uptake (10, 11). Polymyxin B binds to outer-membrane lipopolysaccharide (LPS) displacing Mg²⁺ and disrupting the Mg²⁺ cross bridges between anionic LPS molecules in the outer membrane, thus leading to membrane destabilization. This leads to an increased permeability of the outer membrane, allowing further uptake of the antibiotic. In P. aeruginosa, the ability of polymyxin to permeabilize outer membranes reflects its ability to bind to LPS with higher affinity than the native cross-bridging cation Mg²⁺ (20, 22).A polymyxin B adaptive resistance phenotype was first reported by Gilleland and Murray in 1976 (9) when the wild-type PAO1 strain was passaged in minimal medium containing low Mg²⁺ and exposed to increasing concentrations of polymyxin B. Since this time, many studies have focused on the structural basis of adaptive resistance (7, 8). Under various environmental conditions, P. aeruginosa has been found to synthesize different forms of the lipid A component of LPS (5). In particular, under Mg²⁺-limiting conditions, P. aeruginosa exhibits lipid A modifications, including the addition of aminoarabinose and palmitate. These modifications have been associated with polymyxin B resistance (17, 18).It is well established that two distinct two-component regulators, PhoP-PhoQ and PmrA-PmrB, respond to limiting Mg²⁺ conditions, resulting in polymyxin B resistance in P. aeruginosa (14-16). Under Mg²⁺-limiting conditions, PhoP-PhoQ autoregulates the oprH-phoP-phoQ operon (15), and similarly, PmrA-PmrB autoregulates the PA4773-5-pmrAB-PA4778 operon (17). Furthermore, the arnBCADTEF-PA3559 operon (PA3552-PA3559), which is responsible for the addition of aminoarabinose to lipid A (18), is separately regulated by each of these two-component regulatory systems and is upregulated under Mg²⁺-limiting conditions (17). Both the PA4773-PA4778 and the PA3552-PA3559 operons have shown independent upregulation in response to various cationic antimicrobial peptides in laboratory mutants (18). Mutations in PmrB and the presence of aminoarabinose have been directly associated with polymyxin B resistance (18).During a comparative study of MIC methods for testing polymyxin B and colistin susceptibility, it was observed that 24 of 243 multidrug-resistant clinical isolates of P. aeruginosa demonstrated skipped wells in the broth microdilution method for either polymyxin B, colistin, or both (a “skipped well” is an isolated well showing no growth of bacteria despite the fact that a well with a higher concentration demonstrates growth). According to Clinical and Laboratory Standards Institute (CLSI) guidelines, one skipped well is acceptable and MIC readings should be taken based on the well with the highest antibiotic concentration exhibiting growth (3). The assumption is that a technical error has occurred. The multidrug-resistant isolates described here skipped one to five wells, and bacteria recovered from wells at the higher polymyxin concentrations (i.e., after the skipped wells) also exhibited a polymyxin MIC profile containing skipped wells. The phenomenon of skipped wells has previously been described and was termed cocarde growth (1, 2, 23). In this study, we set out to determine the potential causes for this type of resistance profile in the identified P. aeruginosa clinical isolates.     

Emergence of Polymyxin B Resistance Influences Pathogenicity in Pseudomonas aeruginosa Mutators

The interplay between polymyxin B pharmacodynamics and pathogenicity was examined in Pseudomonas aeruginosa PAO1 and isogenic DNA repair-deficient mutators (mutM and mutS strains). Against mutS mutators, polymyxin B initial killing was concentration dependent, with >99.9% bacterial reduction at 2 h followed by regrowth and resistance. The pre- versus postexposed strains were inoculated real time into Galleria mellonella waxworms, resulting in increased median survival times from 20 h to 23 h (P < 0.001). Emergence of resistance in mutS P. aeruginosa resulted in attenuation of virulence.     

Susceptibility of 1,500 Isolates of Pseudomonas aeruginosa to Gentamicin, Carbenicillin, Colistin, and Polymyxin B

A series of 1,500 strains of Pseudomonas aeruginosa was collected from a variety of sources to provide a group of strains which would truly represent the species. All of them were pyocine typed, and a wide range of types was included among the isolates from each source. The gentamicin, carbenicillin, colistin, and polymyxin minimal inhibitory concentration of each strain was measured by the agar dilution method employing the Steers inocula replicator. Over 99% of strains were inhibited by 8 mug of gentamicin per ml, by 256 mug of carbenicillin per ml, and by 4 mug of colistin per ml. The small number of strains more resistant to each antibiotic comprised a variety of different pyocine types. Few strains were found to be susceptible to tetracycline, chloramphenicol, kanamycin, or streptomycin at the single concentration tested.     

Characterization of the Polymyxin B Resistome of Pseudomonas aeruginosa

Multidrug resistance in Pseudomonas aeruginosa is increasingly becoming a threat for human health. Indeed, some strains are resistant to almost all currently available antibiotics, leaving very limited choices for antimicrobial therapy. In many such cases, polymyxins are the only available option, although as their utilization increases so does the isolation of resistant strains. In this study, we screened a comprehensive PA14 mutant library to identify genes involved in changes of susceptibility to polymyxin B in P. aeruginosa. Surprisingly, our screening revealed that the polymyxin B resistome of this microorganism is fairly small. Thus, only one resistant mutant and 17 different susceptibility/intrinsic resistance determinants were identified. Among the susceptible mutants, a significant number carried transposon insertions in lipopolysaccharide (LPS)-related genes. LPS analysis revealed that four of these mutants (galU, lptC, wapR, and ssg) had an altered banding profile in SDS-polyacrylamide gels and Western blots, with three of them exhibiting LPS core truncation and lack of O-antigen decoration. Further characterization of these four mutants showed that their increased susceptibility to polymyxin B was partly due to increased basal outer membrane permeability. Additionally, these mutants also lacked the aminoarabinose-substituted lipid A species observed in the wild type upon growth in low magnesium. Overall, our results emphasize the importance of LPS integrity and lipid A modification in resistance to polymyxins in P. aeruginosa, highlighting the relevance of characterizing the genes that affect biosynthesis of cell surface structures in this pathogen to follow the evolution of peptide resistance in the clinic.     

Optimization of Polymyxin B in Combination with Doripenem To Combat Mutator Pseudomonas aeruginosa

Development of spontaneous mutations in Pseudomonas aeruginosa has been associated with antibiotic failure, leading to high rates of morbidity and mortality. Our objective was to evaluate the pharmacodynamics of polymyxin B combinations against rapidly evolving P. aeruginosa mutator strains and to characterize the time course of bacterial killing and resistance via mechanism-based mathematical models. Polymyxin B or doripenem alone and in combination were evaluated against six P. aeruginosa strains: wild-type PAO1, mismatch repair (MMR)-deficient (mutS and mutL) strains, and 7,8-dihydro-8-oxo-deoxyguanosine system (GO) base excision repair (BER)-deficient (mutM, mutT, and mutY) strains over 48 h. Pharmacodynamic modeling was performed using S-ADAPT and facilitated by SADAPT-TRAN. Mutator strains displayed higher mutation frequencies than the wild type (>600-fold). Exposure to monotherapy was followed by regrowth, even at high polymyxin B concentrations of up to 16 mg/liter. Polymyxin B and doripenem combinations displayed enhanced killing activity against all strains where complete eradication was achieved for polymyxin B concentrations of >4 mg/liter and doripenem concentrations of 8 mg/liter. Modeling suggested that the proportion of preexisting polymyxin B-resistant subpopulations influenced the pharmacodynamic profiles for each strain uniquely (fraction of resistance values are −8.81 log₁₀ for the wild type, −4.71 for the mutS mutant, and −7.40 log₁₀ for the mutM mutant). Our findings provide insight into the optimization of polymyxin B and doripenem combinations against P. aeruginosa mutator strains.     

多黏菌素B与其他抗菌药物相比较治疗铜绿假单胞菌所致血流感染

通过一项回顾性列队研究,比较静脉使用多黏菌素B与其他抗菌药物在治疗医院获得铜绿假单胞菌所致血流感染的疗效。评估多种潜在混杂因素。本研究分析2004年1月至2009年12月,18岁以上并接受48 h以上抗菌治疗铜绿假单胞菌血流感染患者,使用多黏菌素B或其他抗菌药物的疗效及肾毒性。     

Resistance of Spheroplasts and Whole Cells of Pseudomonas cepacia to Polymyxin B

Sucrose-lysozyme spheroplasts were prepared from two strains of Pseudomonas cepacia and tested for susceptibility to polymyxin B and benzalkonium chloride. Spheroplasts were more susceptible than whole cells to benzalkonium chloride but not to polymyxin B. Disruption of the outer membrane layer was not by itself sufficient to render P. cepacia susceptible to polymyxin B.     

Polymyxin B in Combination with Antimicrobials Lacking In Vitro Activity versus Polymyxin B in Monotherapy in Critically Ill Patients with Acinetobacter baumannii or Pseudomonas aeruginosa Infections

There is no clinical evidence supporting the use of polymyxin B in combination with another antimicrobial for infections caused by extensively drug-resistant Acinetobacter baumannii or Pseudomonas aeruginosa isolates. We developed a cohort study of patients in two intensive care units from teaching hospitals to evaluate treatment with intravenous polymyxin B for ≥48 h for severe A. baumannii or P. aeruginosa infections. Covariates potentially associated with 30-day mortality were evaluated in a Cox proportional hazards model. A total of 101 patients were included; 33 (32.7%) were treated with polymyxin B in combination with an antimicrobial lacking in vitro activity and 68 (67.3%) with polymyxin B in monotherapy. The overall 30-day mortality was 59.4% (60 patients), comprising 42.4% (14 of 33) and 67.6% (46 of 68) in combination and monotherapy groups, respectively (P = 0.03). The mortality rates were 18.5/1,000 patient days and 36.4/1,000 patient days in the combination and monotherapy groups, respectively (P = 0.02). Combination therapy was independently associated with lower 30-day mortality (hazard ratio, 0.33; 95% confidence interval, 0.17 to 0.64; P = 0.001). Creatinine clearance of ≥60 ml/min was also a protective factor, while a higher acute physiology and chronic health evaluation (APACHE II) score and polymicrobial infection were associated with increased mortality. The results did not change after adding a propensity score for prescribing combination therapy into the model. The protective effect remained when only combination with β-lactam or carbapenem was considered and in both subgroups of patients: those with A. baumannii infection and those with lower respiratory tract infections. To our knowledge, this is the first clinical study to show a benefit of combination over monotherapy with polymyxin B for severe extensively drug-resistant A. baumannii or P. aeruginosa infections.     

Elucidation of Mechanisms of Ceftazidime Resistance among Clinical Isolates of Pseudomonas aeruginosa by Using Genomic Data

Ceftazidime is one of the few cephalosporins with activity against Pseudomonas aeruginosa. Using whole-genome comparative analysis, we set out to determine the prevalent mechanism(s) of resistance to ceftazidime (CAZ) using a set of 181 clinical isolates. These isolates represented various multilocus sequence types that consisted of both ceftazidime-susceptible and -resistant populations. A presumptive resistance mechanism against ceftazidime was identified in 88% of the nonsusceptible isolates using this approach.     

Multiple Genetic Mutations Associated with Polymyxin Resistance in Acinetobacter baumannii

We studied polymyxin B resistance in 10 pairs of clinical Acinetobacter baumannii isolates, two of which had developed polymyxin B resistance in vivo. All polymyxin B-resistant isolates had lower growth rates than and substitution mutations in the lpx or pmrB gene compared to their parent isolates. There were significant differences in terms of antibiotic susceptibility and genetic determinants of resistance in A. baumannii isolates that had developed polymyxin B resistance in vivo compared to isolates that had developed polymyxin B resistance in vitro.     

The Two-Component System CprRS Senses Cationic Peptides and Triggers Adaptive Resistance in Pseudomonas aeruginosa Independently of ParRS

Cationic antimicrobial peptides pass across the outer membrane by interacting with negatively charged lipopolysaccharide (LPS), leading to outer membrane permeabilization in a process termed self-promoted uptake. Resistance can be mediated by the addition of positively charged arabinosamine through the action of the arnBCADTEF operon. We recently described a series of two-component regulators that lead to the activation of the arn operon after recognizing environmental signals, including low-Mg²⁺ (PhoPQ, PmrAB) or cationic (ParRS) peptides. However, some peptides did not activate the arn operon through ParRS. Here, we report the identification of a new two-component system, CprRS, which, upon exposure to a wide range of antimicrobial peptides, triggered the expression of the LPS modification operon. Thus, mutations in the cprRS operon blocked the induction of the arn operon in response to several antimicrobial peptides independently of ParRS but did not affect the response to low Mg²⁺. Distinct patterns of arn induction were identified. Thus, the responses to polymyxins were abrogated by either parR or cprR mutations, while responses to other peptides, including indolicidin, showed differential dependency on the CprRS and ParRS systems in a concentration-dependent manner. It was further demonstrated that, following exposure to inducing antimicrobial peptides, cprRS mutants did not become adaptively resistant to polymyxins as was observed for wild-type cells. Our microarray studies demonstrated that the CprRS system controlled a quite modest regulon, indicating that it was quite specific to adaptive peptide resistance. These findings provide greater insight into the complex regulation of LPS modification in Pseudomonas aeruginosa, which involves the participation of at least 4 two-component systems.     

Prevalence,Resistance Mechanisms,and Susceptibility of Multidrug-Resistant Bloodstream Isolates of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an important pathogen commonly implicated in nosocomial infections. The occurrence of multidrug-resistant (MDR) P. aeruginosa strains is increasing worldwide and limiting our therapeutic options. The MDR phenotype can be mediated by a variety of resistance mechanisms, and the corresponding relative biofitness is not well established. We examined the prevalence, resistance mechanisms, and susceptibility of MDR P. aeruginosa isolates (resistant to ≥3 classes of antipseudomonal agents [penicillins/cephalosporins, carbapenems, quinolones, and aminoglycosides]) obtained from a large, university-affiliated hospital. Among 235 nonrepeat bloodstream isolates screened between 2005 and 2007, 33 isolates (from 20 unique patients) were found to be MDR (crude prevalence rate, 14%). All isolates were resistant to carbapenems and quinolones, 91% were resistant to penicillins/cephalosporins, and 21% were resistant to the aminoglycosides. By using the first available isolate for each bacteremia episode (n = 18), 13 distinct clones were revealed by repetitive-element-based PCR. Western blotting revealed eight isolates (44%) to have MexB overexpression. Production of a carbapenemase (VIM-2) was found in one isolate, and mutations in gyrA (T83I) and parC (S87L) were commonly found. Growth rates of most MDR isolates were similar to that of the wild type, and two isolates (11%) were found to be hypermutable. All available isolates were susceptible to polymyxin B, and only one isolate was nonsusceptible to colistin (MIC, 3 mg/liter), but all isolates were nonsusceptible to doripenem (MIC, >2 mg/liter). Understanding and continuous monitoring of the prevalence and resistance mechanisms of MDR P. aeruginosa would enable us to formulate rational treatment strategies to combat nosocomial infections.Pseudomonas aeruginosa is an important pathogen commonly implicated in serious nosocomial infections such as pneumonia and sepsis. The occurrence of multidrug-resistant P. aeruginosa strains is increasing worldwide and limiting our therapeutic options. Resistance in P. aeruginosa may be mediated via several distinct mechanisms (3, 13). In multidrug-resistant isolates, the relative contributions of different molecular mechanisms toward phenotypic multidrug resistance are not well established. While there are multiple surveillance studies tracking the resistance of P. aeruginosa to various antimicrobial agents over the years (6, 7), studies reporting trends in concomitant resistance to multiple agents over time are scarce.A hypermutable state is often observed in multidrug-resistant pathogens causing chronic infections in cystic fibrosis patients. It is often related to defects in genes involved in the mismatch repair system (e.g., mutS and mutL). It is also commonly believed that in multidrug-resistant P. aeruginosa isolates, reduced virulence may result due to decreased biofitness. However, recent data suggest otherwise, and multidrug-resistant P. aeruginosa may remain fully pathogenic (8).Many first-line agents are ineffective if used alone against multidrug-resistant P. aeruginosa isolates. Consequently, unconventional agents or those that are less preferred (due to toxicity concerns) may have to be used. The objectives of this study were to examine (i) the prevalence of multidrug-resistant P. aeruginosa, (ii) the mechanism and biofitness cost of multidrug resistance, and (iii) the susceptibilities of multidrug-resistant P. aeruginosa isolates to polymyxin antibiotics and a newer carbapenem (doripenem). These investigations are expected to provide a rationale for effective treatment strategies to combat nosocomial infections.(This study was presented in part at the 48th Interscience Conference on Antimicrobial Agents and Chemotherapy-Infectious Diseases Society of America 46th Annual Meeting, Washington, DC, 25 to 28 October 2008 [26].)     

428株铜绿假单胞菌的临床分布与耐药性分析

目的分析临床分离铜绿假单胞菌的感染部位分布及耐药率,为减少和控制院内感染及临床合理选择抗菌剂提供依据。方法采用法国生物梅里埃公司ATB Expression微生物分析仪进行细菌鉴定,K-B纸片扩散法进行药敏试验。结果分离获得1 165株细菌,其中铜绿假单胞菌428株,检出率为36.74%;82.00%的铜绿假单胞菌分离自痰与咽拭子,41.44%分离自干部病区。铜绿假单胞菌菌株对多黏菌素B敏感率最高(100.00%),其次为美罗培南(59.80%)、哌拉西林/他唑巴坦(59.70%)和亚胺培南(57.10%)。结论铜绿假单胞菌临床检出率较高,主要分布在老干部病区。易感因素为患者病情严重、住院时间较长、免疫功能低下、大量使用抗菌剂等。铜绿假单胞菌表现为高度和多药耐药,建议临床应根据药敏试验结果进行抗感染治疗。     

Italian Survey on Comparative Levofloxacin Susceptibility in 334 Clinical Isolates of Pseudomonas aeruginosa

A national survey on susceptibility patterns of 334 Pseudomonas aeruginosa isolates from intensive care units and hematology and oncology wards from 13 Italian hospitals compared the in vitro activity of levofloxacin, an injectable oral fluoroquinolone, to those of ciprofloxacin, ofloxacin, ceftazidime, imipenem, amikacin, and gentamicin. Amikacin and imipenem had the best susceptibility profiles. The activity of levofloxacin was superior to those of the other quinolones and was comparable to that of ceftazidime. The effect of levofloxacin in vitro on P. aeruginosa clinical isolates suggests that further clinical investigations are warranted.     

Synergistic Effect of Membrane-Active Peptides Polymyxin B and Gramicidin S on Multidrug-Resistant Strains and Biofilms of Pseudomonas aeruginosa

Multidrug-resistant Pseudomonas aeruginosa is a major cause of severe hospital-acquired infections. Currently, polymyxin B (PMB) is a last-resort antibiotic for the treatment of infections caused by Gram-negative bacteria, despite its undesirable side effects. The delivery of drug combinations has been shown to reduce the required therapeutic doses of antibacterial agents and thereby their toxicity if a synergistic effect is present. In this study, we investigated the synergy between two cyclic antimicrobial peptides, PMB and gramicidin S (GS), against different P. aeruginosa isolates, using a quantitative checkerboard assay with resazurin as a growth indicator. Among the 28 strains that we studied, 20 strains showed a distinct synergistic effect, represented by a fractional inhibitory concentration index (FICI) of ≤0.5. Remarkably, several clinical P. aeruginosa isolates that grew as small-colony variants revealed a nonsynergistic effect, as indicated by FICIs between >0.5 and ≤0.70. In addition to inhibiting the growth of planktonic bacteria, the peptide combinations significantly decreased static biofilm growth compared with treatment with the individual peptides. There was also a faster and more prolonged effect when the combination of PMB and GS was used compared with single-peptide treatments on the metabolic activity of pregrown biofilms. The results of the present study define a synergistic interaction between two cyclic membrane-active peptides toward 17 multidrug-resistant P. aeruginosa and biofilms of P. aeruginosa strain PAO1. Thus, the application of PMB and GS in combination is a promising option for a topical medication and in the prevention of acute and chronic infections caused by multidrug-resistant or biofilm-forming P. aeruginosa.     

Systematic Review of Mutations in Pyrazinamidase Associated with Pyrazinamide Resistance in Mycobacterium tuberculosis Clinical Isolates

Pyrazinamide (PZA) is an important first-line drug in the treatment of tuberculosis (TB) and of significant interest to the HIV-infected community due to the prevalence of TB-HIV coinfection in some regions of the world. The mechanism of resistance to PZA is unlike that of any other anti-TB drug. The gene pncA, encoding pyrazinamidase (PZase), is associated with resistance to PZA. However, because single mutations in PZase have a low prevalence, the individual sensitivities are low. Hundreds of distinct mutations in the enzyme have been associated with resistance, while some only appear in susceptible isolates. This makes interpretation of molecular testing difficult and often leads to the simplification that any PZase mutation causes resistance. This systematic review reports a comprehensive global list of mutations observed in PZase and its promoter region in clinical strains, their phenotypic association, their global frequencies and diversity, the method of phenotypic determination, their MIC values when given, and the method of MIC determination and assesses the strength of the association between mutations and phenotypic resistance to PZA. In this systematic review, we report global statistics for 641 mutations in 171 (of 187) codons from 2,760 resistant strains and 96 mutations from 3,329 susceptible strains reported in 61 studies. For diagnostics, individual mutations (or any subset) were not sufficiently sensitive. Assuming similar error profiles of the 5 phenotyping platforms included in this study, the entire enzyme and its promoter provide a combined estimated sensitivity of 83%. This review highlights the need for identification of an alternative mechanism(s) of resistance, at least for the unexplained 17% of cases.     

Clinical Outcomes Associated with Polymyxin B Dose in Patients with Bloodstream Infections Due to Carbapenem-Resistant Gram-Negative Rods

There is significant variation in the use of polymyxin B (PMB), and optimal dosing has not been defined. The purpose of this retrospective study was to evaluate the relationship between PMB dose and clinical outcomes. We included patients with bloodstream infections (BSIs) due to carbapenem-resistant Gram-negative rods who received ≥48 h of intravenous PMB. The objective was to evaluate the association between PMB dose and 30-day mortality, clinical cure at day 7, and development of acute kidney injury (AKI). A total of 151 BSIs were included. The overall 30-day mortality was 37.8% (54 of 151), and the median PMB dosage was 1.3 mg/kg (of total body weight)/day. Receipt of PMB dosages of <1.3 mg/kg/day was significantly associated with 30-day mortality (46.5% versus 26.3%; P = 0.02), and this association persisted in multivariable analysis (odds ratio [OR] = 1.58; 95% confidence interval [CI] = 1.05 to 1.81; P = 0.04). Eighty-two percent of patients who received PMB dosages of <1.3 mg/kg/day had baseline renal impairment. Clinical cure at day 7 was not significantly different between dosing groups. AKI was more common in patients receiving PMB dosages of ≥250 mg/day (66.7% versus 32.0%; P = 0.03), and this association persisted in multivariable analysis (OR = 4.32; 95% CI = 1.15 to 16.25; P = 0.03). PMB dosages of <1.3 mg/kg/day were administered primarily to patients with renal impairment, and this dosing was independently associated with 30-day mortality. However, dosages of ≥250 mg/day were independently associated with AKI. These data support the use of PMB without dose reduction in the setting of renal impairment.