In vivo activity of levofloxacin alone or in combination with imipenem or amikacin in a mouse model of Acinetobacter baumannii pneumonia

Critical role of growth medium for detecting drug interactions in Gram-negative bacteria that model in vivo responses

The rise in infections caused by multidrug-resistant (MDR) bacteria has necessitated a variety of clinical approaches, including the use of antibiotic combinations. Here, we tested the hypothesis that drug-drug interactions vary in different media, and determined which in vitro models best predict drug interactions in the lungs. We systematically studied pair-wise antibiotic interactions in three different media, CAMHB, (a rich lab medium standard for antibiotic susceptibility testing), a urine mimetic medium (UMM), and a minimal medium of M9 salts supplemented with glucose and iron (M9Glu) with three Gram-negative ESKAPE pathogens, Acinetobacter baumannii (Ab), Klebsiella pneumoniae (Kp), and Pseudomonas aeruginosa (Pa). There were pronounced differences in responses to antibiotic combinations between the three bacterial species grown in the same medium. However, within species, PaO1 responded to drug combinations similarly when grown in all three different media, whereas Ab17978 and other Ab clinical isolates responded similarly when grown in CAMHB and M9Glu medium. By contrast, drug interactions in Kp43816, and other Kp clinical isolates poorly correlated across different media. To assess whether any of these media were predictive of antibiotic interactions against Kp in the lungs of mice, we tested three antibiotic combination pairs. In vitro measurements in M9Glu, but not rich medium or UMM, predicted in vivo outcomes. This work demonstrates that antibiotic interactions are highly variable across three Gram-negative pathogens and highlights the importance of growth medium by showing a superior correlation between in vitro interactions in a minimal growth medium and in vivo outcomes.

IMPORTANCE

Drug-resistant bacterial infections are a growing concern and have only continued to increase during the SARS-CoV-2 pandemic. Though not routinely used for Gram-negative bacteria, drug combinations are sometimes used for serious infections and may become more widely used as the prevalence of extremely drug-resistant organisms increases. To date, reliable methods are not available for identifying beneficial drug combinations for a particular infection. Our study shows variability across strains in how drug interactions are impacted by growth conditions. It also demonstrates that testing drug combinations in tissue-relevant growth conditions for some strains better models what happens during infection and may better inform combination therapy selection.

Population pharmacokinetics and humanized dosage regimens matching the peak, area, trough, and range of amikacin plasma concentrations in immune-competent murine bloodstream and lung infection models

Amikacin is an FDA-approved aminoglycoside antibiotic that is commonly used. However, validated dosage regimens that achieve clinically relevant exposure profiles in mice are lacking. We aimed to design and validate humanized dosage regimens for amikacin in immune-competent murine bloodstream and lung infection models of Acinetobacter baumannii. Plasma and lung epithelial lining fluid (ELF) concentrations after single subcutaneous doses of 1.37, 13.7, and 137 mg/kg of body weight were simultaneously modeled via population pharmacokinetics. Then, humanized amikacin dosage regimens in mice were designed and prospectively validated to match the peak, area, trough, and range of plasma concentration profiles in critically ill patients (clinical dose: 25-30 mg/kg of body weight). The pharmacokinetics of amikacin were linear, with a clearance of 9.93 mL/h in both infection models after a single dose. However, the volume of distribution differed between models, resulting in an elimination half-life of 48 min for the bloodstream and 36 min for the lung model. The drug exposure in ELF was 72.7% compared to that in plasma. After multiple q6h dosing, clearance decreased by ~80% from the first (7.35 mL/h) to the last two dosing intervals (~1.50 mL/h) in the bloodstream model. Likewise, clearance decreased by 41% from 7.44 to 4.39 mL/h in the lung model. The humanized dosage regimens were 117 mg/kg of body weight/day in mice [administered in four fractions 6 h apart (q6h): 61.9%, 18.6%, 11.3%, and 8.21% of total dose] for the bloodstream and 96.7 mg/kg of body weight/day (given q6h as 65.1%, 16.9%, 10.5%, and 7.41%) for the lung model. These validated humanized dosage regimens and population pharmacokinetic models support translational studies with clinically relevant amikacin exposure profiles.

Exploring Roles of the Polysaccharide Capsule in Pathogenesis of Hypervirulent Acinetobacter baumannii Clinical Isolate Lac-4

The frequently multidrug-resistant bacterial pathogen Acinetobacter baumannii is a leading cause of nosocomial infections, including ventilator-associated pneumonia, such that the World Health Organization and US Centers for Disease Control and Prevention have declared it a top priority candidate for novel drug development. Nearly all clinical A. baumannii strains express a thick surface polysaccharide capsule that protects against desiccation, host defenses, and disinfectants. In this study, we investigated the contribution of the polysaccharide capsule to virulence caused by the A. baumannii clinical isolate Ab Lac-4, which is rare in its ability to cause pneumonia and disseminated sepsis in healthy mice. We assessed the role of the capsule in wildtype Lac-4 (WT) by generating a premature stop codon in wza, which codes for the polysaccharide export protein. The wza# mutant was hypersensitive to killing by complement, whole blood, and healthy human neutrophils compared to WT and a revertant mutant (wza-Rev). Furthermore, the wza# mutant was highly attenuated in murine sepsis and unable to disseminate from the lungs during pneumonia. This study reinforces the capsule as a key contributor to Ab Lac-4 hypervirulence.

How to treat severe Acinetobacter baumannii infections

PURPOSE OF REVIEW

To update the management of severe Acinetobacter baumannii infections (ABI), particularly those caused by multi-resistant isolates.

RECENT FINDINGS

The in vitro activity of the various antimicrobial agents potentially helpful in treating ABI is highly variable and has progressively decreased for many of them, limiting current therapeutic options. The combination of more than one drug is still advisable in most circumstances. Ideally, two active first-line drugs should be used. Alternatively, a first-line and a second-line drug and, if this is not possible, two or more second-line drugs in combination. The emergence of new agents such as Cefiderocol, the combination of Sulbactam and Durlobactam, and the new Tetracyclines offer therapeutic options that need to be supported by clinical evidence.

SUMMARY

The apparent limitations in treating infections caused by this bacterium, the rapid development of resistance, and the serious underlying situation in most cases invite the search for alternatives to antibiotic treatment, the most promising of which seems to be bacteriophage therapy.

Antibacterial Activity of the Green Synthesized Plasmonic Silver Nanoparticles with Crystalline Structure against Gram-Positive and Gram-Negative Bacteria

Nanoparticles (NPs) have attracted considerable interest in numerous fields, including agriculture, medicine, the environment, and engineering. The use of green synthesis techniques that employ natural reducing agents to reduce metal ions and form NPs is of particular interest. This study investigates the use of green tea (GT) extract as a reducing agent for the synthesis of silver NPs (Ag NPs) with crystalline structure. Several analytical techniques, including UV-visible spectrophotometry, Fourier transform infrared (FTIR) spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and X-ray diffraction (XRD), were used to characterize the synthesized Ag NPs. The results of UV-vis revealed that the biosynthesized Ag NPs exhibited an absorbance plasmonic resonance peak at 470 nm. According to FTIR analyses, the attachment of Ag NPs to polyphenolic compounds resulted in a decrease in intensity and band shifting. In addition, the XRD analysis confirmed the presence of sharp crystalline peaks associated with face-centered cubic Ag NPs. Moreover, HR-TEM revealed that the synthesized particles were spherical and 50 nm in size on average. The Ag NPs demonstrated promising antimicrobial activity against Gram-positive (GP) bacteria, Brevibacterium luteolum and Staphylococcus aureus, and Gram-negative (GN) bacteria, Pseudomonas aeruginosa and Escherichia coli, with a minimal inhibitory concentration (MIC) of 6.4 mg/mL for GN and 12.8 mg/mL for GP. Overall, these findings suggest that Ag NPs can be utilized as effective antimicrobial agents.

The Protective Role of Interleukin 17A in Acinetobacter baumannii Pneumonia Is Associated with Candida albicans in the Airway

Recent studies have found that the coexistence of fungi and bacteria in the airway may increase the risk of infection, contribute to the development of pneumonia, and increase the severity of disease. Interleukin 17A (IL-17A) plays important roles in host resistance to bacterial and fungal infections. The objective of this study was to determine the effects of IL-17A on Acinetobacter baumannii-infected rats with a previous Candida albicans airway inoculation. The incidence of A. baumannii pneumonia was higher in rats with C. albicans in the airway than in noninoculated rats, and it decreased when amphotericin B was used to clear C. albicans, which influenced IL-17A levels. IL-17A had a protective effect in A. baumannii pneumonia associated with C. albicans in the airway. Compared with A. baumannii-infected rats with C. albicans in the airway that did not receive IL-17A, recombinant IL-17A (rIL-17A) supplementation decreased the incidence of A. baumannii pneumonia (10/15 versus 5/17; P = 0.013) and the proportion of neutrophils in the lung (84 ± 3.5 versus 74 ± 4.3%; P = 0.033), reduced tissue destruction and inflammation, and decreased levels of myeloperoxidase (MPO) (1.267 ± 0.15 versus 0.233 ± 0.06 U/g; P = 0.0004), reactive oxygen species (ROS) (132,333 ± 7,505 versus 64,667 ± 10,115 AU; P = 0.0007) and lactate dehydrogenase (LDH) (2.736 ± 0.05 versus 2.1816 ± 0.29 U/g; P = 0.0313). In vitro experiments revealed that IL-17A had no significant effect on the direct migration ability and bactericidal capability of neutrophils. However, IL-17A restrained lysis cell death and increased apoptosis of neutrophils (2.9 ± 1.14 versus 7 ± 0.5%; P = 0.0048). Taken together, our results suggest that C. albicans can depress IL-17A levels, which when supplemented may have a regulatory function that limits the accumulation of neutrophils in inflammatory areas, providing inflammatory response homeostasis.

Histopathological Analysis of Acinetobacter baumannii Lung Infection in a Mouse Model

Acinetobacter baumannii is the main causative pathogen of nosocomial infections that causes severe infections in the lungs. In this study, we analyzed the histopathological characteristics of lung infection with two strains of A. baumannii (ATCC 19606 and the clinical isolate TK1090) and Pseudomonas aeruginosa PAO-1 in C3H/HeN mice to evaluate the virulence of A. baumannii. Survival was evaluated over 14 days. At 1, 2, 5, or 14 days postinfection, mice of C3H/HeN were sacrificed, and histopathological analysis of lung specimens was also performed. Histopathological changes and accumulation of neutrophils and macrophages in the lungs after infection with A. baumannii and P. aeruginosa were analyzed. Following intratracheal inoculation, the lethality of ATCC 19606- and TK1090-infected mice was lower than that of PAO-1-infected mice. However, when mice were inoculated with a sub-lethal dose of A. baumannii, the lung bacterial burden remained in the mice until 14 days post-infection. Additionally, histopathological analysis revealed that macrophages infiltrated the lung foci of ATCC 19606-, TK1090-, and PAO-1-infected mice. Although neutrophils infiltrated the lung foci of ATCC 19606- and TK1090-infected mice, they poorly infiltrated the lung foci of PAO-1-infected mice. Accumulation of these cells in the lung foci of ATCC 19606- and TK1090-infected mice, but not PAO-1-infected mice, was observed for 14 days post-infection. These results suggest that A. baumannii is not completely eliminated despite the infiltration of immune cells in the lungs and that inflammation lasts for prolonged periods in the lungs. Further studies are required to understand the mechanism of A. baumannii infection, and novel drugs and vaccines should be developed to prevent A. baumannii infection.

Infectious Diseases Society of America Guidance on the Treatment of AmpC β-lactamase-Producing Enterobacterales, Carbapenem-Resistant Acinetobacter baumannii, and Stenotrophomonas maltophilia Infections

BACKGROUND

The Infectious Diseases Society of America (IDSA) is committed to providing up-to-date guidance on the treatment of antimicrobial-resistant infections. A previous guidance document focused on infections caused by extended-spectrum β-lactamase-producing Enterobacterales (ESBL-E), carbapenem-resistant Enterobacterales (CRE), and Pseudomonas aeruginosa with difficult-to-treat resistance (DTR-P. aeruginosa). Here, guidance is provided for treating AmpC β-lactamase-producing Enterobacterales (AmpC-E), carbapenem-resistant Acinetobacter baumannii (CRAB), and Stenotrophomonas maltophilia infections.

METHODS

A panel of six infectious diseases specialists with expertise in managing antimicrobial-resistant infections formulated questions about the treatment of AmpC-E, CRAB, and S. maltophilia infections. Answers are presented as suggestions and corresponding rationales. In contrast to guidance in the previous document, published data on optimal treatment of AmpC-E, CRAB, and S. maltophilia infections are limited. As such, guidance in this document is provided as "suggested approaches" based on clinical experience, expert opinion, and a review of the available literature. Because of differences in the epidemiology of resistance and availability of specific anti-infectives internationally, this document focuses on the treatment of infections in the United States.

RESULTS

Preferred and alternative treatment suggestions are provided, assuming the causative organism has been identified and antibiotic susceptibility results are known. Approaches to empiric treatment, duration of therapy, and other management considerations are also discussed briefly. Suggestions apply for both adult and pediatric populations.

CONCLUSIONS

The field of antimicrobial resistance is highly dynamic. Consultation with an infectious diseases specialist is recommended for the treatment of antimicrobial-resistant infections. This document is current as of September 17, 2021 and will be updated annually. The most current versions of IDSA documents, including dates of publication, are available at www.idsociety.org/practice-guideline/amr-guidance-2.0/.

Antimicrobial Susceptibilities of Clinical Acinetobacter baumannii Isolates With Different Genotypes

BACKGROUND

The treatment of Acinetobacter baumannii infections is difficult. Carbapenems, sulbactam, and colistin are the most effective antibiotics.

OBJECTIVES

The aim of this study was to evaluate the susceptibilities of genotypically different A. baumannii isolates to sulbactam, amikacin, netilmicin, meropenem, tigecycline and colistin.

PATIENTS AND METHODS

Isolates from various clinical samples of patients with hospital-acquired infections that were identified by the VITEK 2 Compact system in our hospital's microbiology laboratory between January 2010 and March 2012 were included in the study. To determine genetic relatedness of the isolates, the rep-PCR method was used. The broth microdilution method was used for amikacin, netilmicin, meropenem and colistin, while E-test was used for sulbactam and tigecycline.

RESULTS

Among the 300 isolates, 30 were found to be genotypically different and were evaluated in terms of their antimicrobial susceptibilities. All isolates were susceptible to colistin. The susceptibility rates were 66.6%, 50%, 36.6%, 30%, and 10% for netilmicin, tigecycline, sulbactam, amikacin, and meropenem, respectively. For carbapenem resistant isolates, the susceptibility rates were 66.6%, 51.8%, 33.3%, and 25.9% for netilmicin, tigecycline, sulbactam, and amikacin, respectively. The sulbactam minimum inhibitory concentration (MIC) 50 and MIC 90 were 8 μg/mL and 12 μg/mL, respectively.

CONCLUSIONS

In this study, it was concluded that determining the cut-off value for MIC breakpoints for sulbactam alone has a critical impact on the susceptibility results.

Acinetobacter baumannii: human infections, factors contributing to pathogenesis and animal models

Acinetobacter baumannii has emerged as a medically important pathogen because of the increasing number of infections produced by this organism over the preceding three decades and the global spread of strains with resistance to multiple antibiotic classes. In spite of its clinical relevance, until recently, there have been few studies addressing the factors that contribute to the pathogenesis of this organism. The availability of complete genome sequences, molecular tools for manipulating the bacterial genome, and animal models of infection have begun to facilitate the identification of factors that play a role in A. baumannii persistence and infection. This review summarizes the characteristics of A. baumannii that contribute to its pathogenesis, with a focus on motility, adherence, biofilm formation, and iron acquisition. In addition, the virulence factors that have been identified to date, which include the outer membrane protein OmpA, phospholipases, membrane polysaccharide components, penicillin-binding proteins, and outer membrane vesicles, are discussed. Animal models systems that have been developed during the last 15 years for the study of A. baumannii infection are overviewed, and the recent use of these models to identify factors involved in virulence and pathogenesis is highlighted.

Innate immune responses to systemic Acinetobacter baumannii infection in mice: neutrophils, but not interleukin-17, mediate host resistance

Acinetobacter baumannii is a nosocomial pathogen with a high prevalence of multiple-drug-resistant strains, causing pneumonia and sepsis. The current studies further develop a systemic mouse model of this infection and characterize selected innate immune responses to the organism. Five clinical isolates, with various degrees of antibiotic resistance, were assessed for virulence in two mouse strains, and between male and female mice, using intraperitoneal infection. A nearly 1,000-fold difference in virulence was found between bacterial strains, but no significant differences between sexes or mouse strains were observed. It was found that microbes disseminated rapidly from the peritoneal cavity to the lung and spleen, where they replicated. A persistent septic state was observed. The infection progressed rapidly, with mortality between 36 and 48 h. Depletion of neutrophils with antibody to Ly-6G decreased mean time to death and increased mortality. Interleukin-17 (IL-17) promotes the response of neutrophils by inducing production of the chemokine keratinocyte-derived chemoattractant (KC/CXCL1), the mouse homolog of human IL-8. Acinetobacter infection resulted in biphasic increases in both IL-17 and KC/CXCL1. Depletion of neither IL-17 nor KC/CXCL1, using specific antibodies, resulted in a difference in bacterial burdens in organs of infected mice at 10 h postinfection. Comparison of bacterial burdens between IL-17a(-/-) and wild-type mice confirmed that the absence of this cytokine did not sensitize mice to Acinetobacter infection. These studies definitely demonstrate the importance of neutrophils in resistance to systemic Acinetobacter infection. However, neither IL-17 nor KC/CXCL1 alone is required for effective host defense to systemic infection with this organism.

High susceptibility to respiratory Acinetobacter baumannii infection in A/J mice is associated with a delay in early pulmonary recruitment of neutrophils

Acinetobacter baumannii is an important cause of both community-associated and nosocomial pneumonia, which have become increasingly difficult to treat because of the rapid development of resistance to multiple antibiotics. Despite its clinical importance, the pathogenesis of and host defense against respiratory A. baumannii infection remains largely unknown. To examine host factors that could contribute to the defense, we compared the susceptibilities of A/J and C57BL/6 mice to intranasal (i.n.) inoculation with A. baumannii. We found that A/J mice were significantly more susceptible to infection with higher mortality (P<0.05) and tissue bacterial burdens (P<0.01) as well as greater histopathology in the lung and spleen than C57BL/6 mice. More importantly, the high susceptibility of A/J mice was associated with a reduced local proinflammatory cytokine/chemokine (particularly IL-1beta, MIP-2 and TNF-alpha) responses and a significant delay and reduction in the early influx of neutrophils in the lung (P<0.05). Intranasal administration of neutrophil-inducing chemokine MIP-2 to A/J mice enhanced pulmonary neutrophil influx and partially restored host resistance to A. baumannii to a level comparable to the more resistant C57BL/6 mice. Our results imply that the early recruitment of neutrophils into the lung is critical for initiating an efficient host defense against respiratory A. baumannii infection.

Current control and treatment of multidrug-resistant Acinetobacter baumannii infections

Institutional outbreaks caused by Acinetobacter baumannii strains that have acquired multiple mechanisms of antimicrobial drug resistance constitute a growing public-health problem. Because of complex epidemiology, infection control of these outbreaks is difficult to attain. Identification of potential common sources of an outbreak, through surveillance cultures and epidemiological typing studies, can aid in the implementation of specific control measures. Adherence to a series of infection control methods including strict environmental cleaning, effective sterilisation of reusable medical equipment, attention to proper hand hygiene practices, and use of contact precautions, together with appropriate administrative guidance and support, are required for the containment of an outbreak. Effective antibiotic treatment of A baumannii infections, such as ventilator-associated pneumonia and bloodstream infections, is also of paramount importance. Carbapenems have long been regarded as the agents of choice, but resistance rates have risen substantially in some areas. Sulbactam has been successfully used in the treatment of serious A baumannii infections; however, the activity of this agent against carbapenem-resistant isolates is decreasing. Polymyxins show reliable antimicrobial activity against A baumannii isolates. Available clinical reports, although consisting of small-sized studies, support their effectiveness and mitigate previous concerns for toxicity. Minocycline, and particularly its derivative, tigecycline, have shown high antimicrobial activity against A baumannii, though relevant clinical evidence is still scarce. Several issues regarding the optimum therapeutic choices for multidrug-resistant A baumannii infections need to be clarified by future research.

Acinetobacter baumannii: emergence of a successful pathogen

Acinetobacter baumannii has emerged as a highly troublesome pathogen for many institutions globally. As a consequence of its immense ability to acquire or upregulate antibiotic drug resistance determinants, it has justifiably been propelled to the forefront of scientific attention. Apart from its predilection for the seriously ill within intensive care units, A. baumannii has more recently caused a range of infectious syndromes in military personnel injured in the Iraq and Afghanistan conflicts. This review details the significant advances that have been made in our understanding of this remarkable organism over the last 10 years, including current taxonomy and species identification, issues with susceptibility testing, mechanisms of antibiotic resistance, global epidemiology, clinical impact of infection, host-pathogen interactions, and infection control and therapeutic considerations.

Rat pneumonia and soft-tissue infection models for the study of Acinetobacter baumannii biology

Acinetobacter baumannii is a bacterial pathogen of increasing medical importance. Little is known about its mechanisms of pathogenesis, and safe reliable agents with predictable activity against A. baumannii are presently nonexistent. The availability of relevant animal infection models will facilitate the study of Acinetobacter biology. In this report we tested the hypothesis that the rat pneumonia and soft-tissue infection models that our laboratory had previously used for studies of extraintestinal pathogenic Escherichia coli were clinically relevant for A. baumannii. Advantages of these models over previously described models were that the animals were not rendered neutropenic and they did not receive porcine mucin with bacterial challenge. Using the A. baumannii model pathogen 307-0294 as the challenge pathogen, the pneumonia model demonstrated all of the features of infection that are critical for a clinically relevant model: namely, bacterial growth/clearance, an ensuing host inflammatory response, acute lung injury, and, following progressive bacterial proliferation, death due to respiratory failure. We were also able to demonstrate growth of 307-0294 in the soft-tissue infection model. Next we tested the hypothesis that the soft-tissue infection model could be used to discriminate between the inherent differences in virulence of various A. baumannii clinical isolates. The ability of A. baumannii to grow and/or be cleared in this model was dependent on the challenge strain. We also hypothesized that complement is an important host factor in protecting against A. baumannii infection in vivo. In support of this hypothesis was the observation that the serum sensitivity of various A. baumannii clinical isolates in vitro roughly paralleled their growth/clearance in the soft-tissue infection model in vivo. Lastly we hypothesized that the soft-tissue infection model would serve as an efficient screening mechanism for identifying gene essentiality for drug discovery. Random mutants of 307-0294 were initially screened for lack of growth in human ascites in vitro. Selected mutants were subsequently used for challenge in the soft-tissue infection model to determine if the disrupted gene was essential for growth in vivo. Using this approach, we have been able to successfully identify a number of genes essential for the growth of 307-0294 in vivo. In summary, these models are clinically relevant and can be used to study the innate virulence of various Acinetobacter clinical isolates and to assess potential virulence factors, vaccine candidates, and drug targets in vivo and can be used for pharmacokinetic and chemotherapeutic investigations.

The increasing role of Acinetobacter species as nosocomial pathogens

Among gram-negative organisms playing a significant role in nosocomial infections, Acinetobacter species have attracted increasing attention in intensive care units during the past two decades. Acinetobacter species are implicated in a wide spectrum of infections (eg, bacteremia, nosocomial pneumonia, urinary tract infections, secondary meningitis, superinfections in burn patients). One of the most striking features of Acinetobacter species is their extraordinary ability to develop multiple resistance mechanisms against major antibiotic classes. They have become resistant to broad-spectrum β-lactams (third-generation cephalosporins, carboxypenicillins, and increasingly to carbapenems); they produce a wide range of aminoglycoside-inactivating enzymes; and most strains are resistant to fluoroquinolones. In Acinetobacter nosocomial infections, the major problems confronting clinicians in intensive care units are related to the severity of Acinetobacter nosocomial infections and to resistance to major antibiotic classes of these organisms.

Colistin and rifampicin in the treatment of nosocomial infections from multiresistant Acinetobacter baumannii

INTRODUCTION

The increased incidence of nosocomial infections by multi-drug resistant Acinetobacter baumannii creates demand on the application of some combinations of older antimicrobials on that species. We conducted the present observational study to evaluate the efficacy of intravenous and aerosolized colistin combined with rifampicin in the treatment of critically patients with nosocomial infections caused by multiresistant A. baumannii.

PATIENTS AND METHODS

Critically ill patients with nosocomial infections caused by A. baumannii resistant to all antibiotics except colistin in a medical intensive care unit. Diagnosis of infection was based on clinical data and isolation of bacteria. The bacterial susceptibilities to colistin were tested. Clinical response to colistin+rifampicin was evaluated.

RESULTS

Twenty-six patients (43.58+/-18.29 years, Acute Physiology and Chronic Health Evaluation II Score (APACHE II): 6.35+/-2.99), of whom 16 cases of nosocomial pneumonia treated by aerosolized colistin (1x10(6) IU three times/day) associated with intravenous rifampicin (10 mg/kg every 12h), nine cases of bacteraemia treated by intravenous colistin (2x10(6)IU three times/day) associated with intravenous rifampicin (10 mg/kg every 12h) in which three cases associated with ventilator associated pneumonia and one case of nosocomial meningitis treated by intrathecal use of colistin associated with intravenous rifampicin. The clinical evolution was favourable for all ill patients. Concerning side effects, we have noticed a moderate hepatic cytolysis in three patients.

CONCLUSION

This is the first clinical report of colistin combined with rifampicin for treatment of A. baumannii infection. Despite the lack of a control group and the limited number of patients, the results seem to be encouraging.

Multidrug-resistant Acinetobacter infections: an emerging challenge to clinicians

OBJECTIVE

To review and evaluate clinically relevant epidemiology, microbiology, and clinical studies regarding the treatment of multidrug-resistant Acinetobacter infections.

DATA SOURCES

Pertinent literature was identified by a MEDLINE search (1966-September 2003) and through secondary bibliographies of pertinent articles.

STUDY SELECTION AND DATA EXTRACTION

All English-language articles identified from data sources were evaluated for clinical relevance.

DATA SYNTHESIS

Acinetobacter baumannii has emerged as a worldwide problem as a nosocomial pathogen in hospitalized patients. Acinetobacter spp. can cause a multitude of infections including pneumonia, bacteremia, meningitis, urinary tract infections, and skin and soft tissue infections, and the mortality associated with these infections is high. Isolates resistant to almost all commercially available antimicrobials have been identified, thus limiting treatment options. The development of new agents and reappraisal of older compounds (ie, polymyxins, ampicillin/sulbactam) are necessary as we consider the optimal treatment of these multidrug-resistant organisms.

CONCLUSIONS

There is no simple answer to the treatment of Acinetobacter infections. Eradication of Acinetobacter spp. requires adherence to good infection control practices and prudent antibiotic use, as well as effective antimicrobial therapy. Alternative therapies such as colistin, ampicillin/sulbactam, and tetracycline are potential options, but prospective, randomized, controlled trials are still lacking.

Efficacy of colistin versus beta-lactams, aminoglycosides, and rifampin as monotherapy in a mouse model of pneumonia caused by multiresistant Acinetobacter baumannii

The treatment of life-threatening infections due to carbapenem-resistant Acinetobacter baumannii has become a serious challenge for physicians worldwide. Often, only colistin shows in general good in vitro activity against these carbapenem-resistant strains, but its antibacterial efficacy in comparison with the antibiotics most used in clinical practice is not well known. We studied the efficacy of colistin versus those of imipenem, sulbactam, tobramycin, and rifampin in an experimental pneumonia model with immunocompetent mice. We used three strains of A. baumannii corresponding to the main clones (A, D, and E) involved in the outbreaks of our hospital, with different grades of resistance to imipenem (imipenem MICs of 1, 8, and 512 microg/ml, respectively) and to the other antibiotics. The MIC of colistin was 0.5 microg/ml for the three strains. Reduction of log(10) CFU/g in lung bacterial counts, clearance of bacteremia, and survival versus results with controls were used as parameters of efficacy. Imipenem and sulbactam (Deltalung counts: -5.38 and -4.64 log(10) CFU/ml) showed the highest level of bactericidal efficacy in infections by susceptible and even intermediate strains. Tobramycin and rifampin (-4.16 and -5.15 log(10) CFU/ml) provided good results against intermediate or moderately resistant strains, in agreement with killing curves and pharmacodynamics. On the contrary, colistin showed the weakest antibacterial effect among the antibiotics tested, both in killing curves and in the in vivo model (-2.39 log(10) CFU/ml; P < 0.05). We conclude that colistin did not appear as a good option for treatment of patients with pneumonia due to carbapenem-resistant A. baumannii strains. Other alternatives, including combinations with rifampin, may offer better therapeutic profiles and thus should be studied.