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Antimicrobial Agents and Chemotherapy, June 2004, p. 2251-2259, Vol. 48, No. 6
0066-4804/04/$08.00+0 DOI: 10.1128/AAC.48.6.2251-2259.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Second Department of Internal Medicine,1 Department of Histology and Cell Biology, School of Medicine, Nagasaki University,4 Department of Pathology, Nagasaki University Hospital, Nagasaki,3 Second Department of Internal Medicine, Medical School, Oita University, Oita, Japan2
Received 17 August 2003/ Returned for modification 31 October 2003/ Accepted 17 February 2004
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Several Japanese studies have demonstrated the therapeutic benefits of long-term therapy with macrolides, such as erythromycin (ERY), clarithromycin (CAM), roxithromycin (RXM), and azithromycin (AZM), in DPB patients (3, 6, 13, 17, 19, 20, 22). Recently, two studies from western countries demonstrated similar effects of AZM therapy in CF patients (1, 29). Macrolides are common antibiotics used in patients with respiratory infections but are regarded to have weak or no activity against P. aeruginosa, since the maximum concentrations of macrolide antibiotics in serum and sputum are below the MICs for this organism and never inhibit the proliferation of P. aeruginosa (23).
Several mechanisms for the therapeutic benefits of macrolides, both in vitro and in vivo, have been proposed; these include the effect of ERY on neutrophil function (7); the effects of ERY, CAM, RXM, and AZM on interleukin-8 production (9, 10, 21, 24); the effect of ERY on tracheal secretions (26); the effects of ERY, CAM, and AZM on the biofilm produced by P. aeruginosa (14); the inhibition of quorum-sensing P. aeruginosa by AZM (27); and even direct P. aeruginosa reduction by ERY, CAM, and AZM (28). Although all of these mechanisms have been discussed, the precise mechanism is not yet clear.
Recently, several studies with experimental animal models demonstrated the potential of CAM in combination with an antipseudomonal agent for the treatment of acute (11) and chronic (31) respiratory infections caused by mucoid-producing P. aeruginosa. However, to our knowledge, no studies with experimental animal models have demonstrated the clearance of organisms by ERY alone, especially in chronic respiratory infections caused by P. aeruginosa.
In the present study, we investigated the clinical effects of long-term ERY therapy in our established murine model of chronic bronchial P. aeruginosa infection.
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Preparation of tubes precoated with bacteria. P. aeruginosa NUS10, a clinical mucoid isolate from the sputum of a DPB patient at Nagasaki University Hospital, was cultured on a Mueller-Hinton II agar (Becton Dickinson Microbiology Systems, Cockeysville, Md.) plate for 24 h. The bacteria were suspended in saline, harvested by centrifugation (3,000 x g, 4°C, 10 min), resuspended in sterile saline, and adjusted to 109 CFU/ml, as estimated by turbidimetry. The intubation tubes, disposable sterile plastic cutdown intravenous catheters (3 French, 1.0-mm diameter; Atom Co., Tokyo, Japan) cut to 3.0-mm lengths and with a few slits made at the proximal end to prevent clogging by airway secretions, were immersed in the bacterium-saline suspensions for 3 days at 37°C. To count bacterial numbers, the bacteria were detached from the tubes by using a concussion machine for 5 min. Only viable bacteria were counted. The number of bacteria at 3 days after incubation and before intubation was 6.32 ± 0.57 log10 CFU/tube (mean and standard deviation [SD]; n = 10).
Experimental model of chronic airway infection. Chronic airway infection was induced in mice by using the method described previously by Yanagihara et al. (32, 33). Briefly, the blunted end of the inner needle of an intravenous catheter (Angiocath; Becton Dickinson Vascular Access, Sandy, Utah) was inserted through the oral cavity of fully anesthetized mice, with the outer sheath and the attached tube at the tip. The tube was advanced through the vocal cords into the trachea. The inner needle was pulled out, and the outer sheath was gently pushed to place the precoated tube into the main bronchus. After intubation, the infected mice were allowed to recover, eat, and drink spontaneously. The infection was restricted to the lungs.
Drug administration. ERY (Sigma Chemical Co., St. Louis, Mo.) was dissolved in sterile water immediately before use. The MIC of the agent was determined by the agar dilution technique with Mueller-Hinton II agar plates and an inoculum size of 104 CFU per spot. The MIC of ERY for P. aeruginosa mucoid isolate NUS10 was >400 µg/ml. Treatment commenced from day 80 after intubation with a tube carrying P. aeruginosa or a sterile tube. After this 80-day interval, 84 mice were allocated into two groups. The first group was treated with ERY (10 mg/kg of body weight/day), and the other group was treated with saline as the control. Both groups of mice were sacrificed and examined after 10, 20, 40, and 80 days of administration of ERY or saline once a day by intraperitoneal injection. For this study, the treatment drug and its dosage were based on the DPB clinical practice guidelines of the Japanese Ministry of Health and Welfare. According to those details, ERY is the first-choice drug for low-dose macrolide therapy because of its low rate of side effects, and the usual therapeutic dose of ERY (400 to 600 mg/person/day) is almost equal to those used in this study (10 mg/kg/day).
Bacteriologic and histopathologic examinations. After treatment, the animals were sacrificed by severing the axillary artery under general anesthesia, and the lungs were excised under aseptic conditions. For bacteriologic analysis, both lungs were homogenized, including the implanted tube, to avoid the detachment of biofilm from the outer surface of the tube by picking it up from the murine bronchus, and cultured quantitatively. The infected tube and the infected lung tissue were separately cultured in a previous study (33). In that study, Yanagihara et al. showed that the counts of viable bacteria isolated from the lung tissue and the tube removed from the murine bronchus were approximately similar, at 105 CFU per specimen. Bacterial enumeration was performed for four mice in each group by serially diluting samples on Mueller-Hinton II agar plates, incubating the plates at 37°C in air overnight, and then counting colonies on the plates to estimate the CFU in the lungs of the mice.
For histopathologic examination, lung specimens from four mice in each group were fixed in 10% formalin buffer. For scanning electron microscopy, the tube was removed from two mice in each group and cut longitudinally. These specimens were fixed for 2 h at 4°C with 2% glutaraldehyde in 0.1 M phosphate buffer, followed by refixation for 2 h at 4°C in 1% osmium acid in the same buffer, dehydration in a series of aqueous ethanol solutions (50 to 100%), and freeze-drying. The specimens were coated with platinum-palladium by using an ion sputter and observed by using a JSM-35C scanning electron microscope (JEOL, Tokyo, Japan).
Statistical analysis. Data were expressed as the mean and standard deviation. Differences between groups were examined for statistical significance by using the unpaired Student's t test. A P value of less than 0.05 denoted the presence of a statistically significant difference.
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TABLE 1. Counts of viable P. aeruginosa NUS10 in lungs after treatment with ERY
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FIG. 1. Pulmonary changes in mice just 80 days after inoculation of P. aeruginosa (A and B), after 80 days of saline treatment (C and D), and after 80 days of ERY treatment (E and F) (hematoxylin-eosin stain; magnifications: A, C, and E, x17.4, and B, D, and F, x174). The airways (asterisks) are filled with neutrophils. Mice treated with saline showed the progression of disease, including the further destruction of the epithelium (D, arrows). ERY-treated mice showed the recovery of columnar ciliated cells (F, arrows).
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FIG. 2. Interface between the inoculation tube (arrowheads) and the lining epithelia (asterisks) in saline- and ERY-treated mice (hematoxylin-eosin stain; magnification, ca. x392). (A and B) Mice treated with saline (A) and with ERY (B) showed similar features after 20 days. (C and D) After 40 days of treatment, mice treated with saline (C) and with ERY (D). (E and F) After 80 days of treatment, mice treated with saline (E) and with ERY (F).
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FIG. 3. Scanning electron microscopy of the biofilm on the inner space of the inoculation tube (longitudinal section) in mice that received 40 days of saline treatment after inoculation with P. aeruginosa (A, B, and C) and 40 days of ERY treatment (D, E, and F). Bars, 100 µm (A and D) and 10 µm (B, C, E, and F). The arrows in panel A show the smooth surface of the biofilm covering the inner wall of the tube. The inflammatory cells (arrowheads) are separated from the bacteria (arrow) embedded deeply in the multilayer biofilm (asterisks) (C). The arrows in panel E show the surface of the base layer of the biofilm. In mice treated with ERY, the inflammatory cells (arrowheads) were mixed with the biofilm (F).
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FIG. 4. Scanning electron microscopy of the biofilm on the inner space of the inoculation tube (longitudinal section) in mice that received 80 days of saline treatment after inoculation with P. aeruginosa (A, B, and C) and 80 days of ERY treatment (D, E, and F). Bars, 100 µm (A and D), 10 µm (B and E), and 1 µm (C and F). The arrows show the smooth surface of the biofilm, not the inner wall of the tube (A). Mice treated with ERY showed a very thin, single-layer biofilm, and the inner wall of the tube can be seen (arrows in D and arrowheads in E).
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In addition to previous reports showing the anti-inflammatory effects of certain macrolides, Tateda et al. reported that AZM inhibited quorum-sensing P. aeruginosa (27) and that AZM, ERY, and CAM at clinically achievable concentrations resulted in direct P. aeruginosa reduction (28). Fujii et al. also reported the disappearance of P. aeruginosa in cultures of sputum samples from DPB patients after 12 months of ERY therapy (3). Therefore, we speculate that certain macrolides have some antipseudomonal activities.
P. aeruginosa is one of the most important bacterial pathogens in patients with chronic pulmonary diseases such as CF (15) and DPB (5). It is well known that P. aeruginosa forms a bacterial biofilm, and biofilms have been detected not only on various biomaterial objects in the body, such as catheters, pacemakers, and the surface of artificial organs, but also on a variety of living and inert surfaces within the human body (12, 14, 16); biofilms are often observed on the airway surfaces of patients with DPB, CF, and bronchiectasis (14). Morphologically, the biofilm produced by P. aeruginosa tended to form a large mass on the affected portion of the airway surface, and there the bacteria were associated with secreted mucus and host cell debris (as shown in Fig. 4A, B, and C). The dense and tightly structured biofilm was characterized by resistance to attack by a wide range of antibacterial agents and humoral or cellular host defense mechanisms (14). Consequently, despite treatment with potent antibiotics, P. aeruginosa infection in the lungs of such patients typically leads to death by respiratory failure or other complications. Biofilm bacteria are a major concern for clinicians in the treatment of chronic infectious diseases.
As in a previous study (34) with the same murine model of chronic infection with mucoid-producing P. aeruginosa NUS10, in the present study we started treatment with ERY or saline 80 days after infection. Yanagihara et al. (34) demonstrated serial changes in the concentrations of proinflammatory cytokines measured up to 60 days after the induction of respiratory infection. They showed that significant increases in the tumor necrosis factor alpha and interleukin 1ß concentrations at 7 days after inoculation of the tube with P. aeruginosa were still seen even at 60 days postinfection. Although increases in the concentrations of gamma interferon and interleukin 2 up to 30 days after inoculation were moderate, the concentrations showed a significant increase 60 days later. Based on these results, we speculate that stabilization of these cytokines needs at least 60 days. Therefore, we started ERY treatment 80 days after inoculation to avoid any effects of the above changes in proinflammatory cytokines.
Using the same animal model as that described in the present study, Yanagihara et al. previously reported that the mean number of viable NUS10 bacteria recovered from the lungs was 105 to 106 CFU/specimen throughout the entire year (32). In the present study, we demonstrated that long-term administration of ERY alone could reduce the number of viable P. aeruginosa without the use of other antipseudomonal agents. To our knowledge, this is the first report to show the time course of morphologic changes in the biofilm structure of P. aeruginosa in situ in association with the administration of ERY alone. Moreover, we noted that P. aeruginosa exposed to ERY for 80 days required 48 h, twice as long as nontreated bacteria, to form apparent colonies. This result suggests that prolonged exposure to ERY may inhibit the replication of P. aeruginosa. Thus, we speculate that the antipseudomonal effects of prolonged exposure to ERY (reductions in biofilm formation and number of viable P. aeruginosa) lead to improved intrabronchial clearing by phagocytic cells and by the mucociliary transportation system after thinning of the biofilm in vivo (Fig. 1F). Although the ERY therapy did not change the presence of peribronchial dense lymphocytic infiltration and atelectatic changes, the recovery of ciliated bronchial cells may have a positive effect on mucociliary transportation and the clearance of debris. These results support the use of long-term macrolide therapy in the treatment of DPB.
With respect to bacterial biofilm, Yasuda et al. (35), using a rat model of experimentally induced subcutaneous infection with the presence of biofilm formed by P. aeruginosa, reported that the quantities of alginate and hexose in which bacterial biofilm had been formed clearly decreased in a dose-dependent manner after treatment with CAM. Tateda et al. (27, 28) also reported that the abilities of certain macrolides (AZM, ERY, and CAM) to reduce the viability of P. aeruginosa following prolonged incubation might be associated with the inhibition of protein synthesis by P. aeruginosa (28). Furthermore, they demonstrated that AZM inhibited quorum-sensing P. aeruginosa and might reduce the production of quorum-sensing-dependent extracellular virulence factors, elastase and rhamnolipids, by reducing the production of both the 3-oxo-C12-homoserine lactone (3-oxo-C12-HSL) and C4-HSL autoinducers by P. aeruginosa (27). Recently, Favre-Bonté et al. (2) showed that the inhibition of biofilm formation by P. aeruginosa observed with AZM could be complemented with the exogenous autoinducers 3-oxo-C12-HSL and C4-HSL. They suggested that AZM, because of its ability to block cell-to-cell signaling by reducing both 3-oxo-C12-HSL and C4-HSL formation, not only leads to a reduction in the total amount of biofilm formed but also affects its differentiation (2).
Certain macrolides, such as ERY, CAM, RXM, and AZM, are known to exhibit good penetration and distribution into organs and tissues not only in animals but also in humans (8, 25, 36). Tateda et al. (28) also demonstrated AZM accumulation in strain PAO-1 in a time-dependent manner, and such intracellular accumulation of this macrolide was responsible for bactericidal activity through inhibition of the protein synthesis of P. aeruginosa. They suggested that macrolide exposure induces changes in the cell surface structures of bacteria, which may in turn facilitate macrolide entry and allow the antibiotic to accumulate within the bacterial cell. We therefore speculate that both the antibacterial and the anti-quorum-sensing effects of ERY in the present study are due to the high intracellular concentrations of ERY within P. aeruginosa following long-term administration.
In conclusion, we report the antipseudomonal effect of long-term ERY therapy in a murine model of chronic infection with mucoid-producing P. aeruginosa. In the present study, prolonged treatment with ERY suppressed biofilm formation as part of the virulence of P. aeruginosa and led to a significant decrease in the number of viable bacteria. We speculate that prolonged exposure to this macrolide is one of the critical factors for its antipseudomonal effect. Based on the results presented here, we speculate that the efficacy of this macrolide therefore is the result not only of its suggested anti-inflammatory effect but also, at least in part, its antipseudomonal activity.
This study was supported in part by grants-in-aid for scientific research on priority areas (14021089) and scientific research (13670605) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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