Efficacy of antimicrobial therapy in experimental rat pneumonia: effects of impaired phagocytosis

Combating Implant Infections: Shifting Focus from Bacteria to Host

The widespread use of biomaterials to support or replace body parts is increasingly threatened by the risk of implant-associated infections. In the quest for finding novel anti-infective biomaterials, there generally has been a one-sided focus on biomaterials with direct antibacterial properties, which leads to excessive use of antibacterial agents, compromised host responses, and unpredictable effectiveness in vivo. This review sheds light on how host immunomodulation, rather than only targeting bacteria, can endow biomaterials with improved anti-infective properties. How antibacterial surface treatments are at risk to be undermined by biomaterial features that dysregulate the protection normally provided by critical immune cell subsets, namely, neutrophils and macrophages, is discussed. Accordingly, how the precise modification of biomaterial surface biophysical cues, or the incorporation of immunomodulatory drug delivery systems, can render biomaterials with the necessary immune-compatible and immune-protective properties to potentiate the host defense mechanisms is reviewed. Within this context, the protective role of host defense peptides, metallic particles, quorum sensing inhibitors, and therapeutic adjuvants is discussed. The highlighted immunomodulatory strategies may lay a foundation to develop anti-infective biomaterials, while mitigating the increasing threat of antibacterial drug resistance.

Animal Models of Pneumococcal pneumonia

Streptococcus pneumoniae remains the most common bacterial pathogen causing lower respiratory tract infections and is a leading cause of morbidity and mortality worldwide, especially in children and the elderly. Another important aspect related to pneumococcal infections is the persistent rate of penicillin and macrolide resistance. Therefore, animal models have been developed to better understand the pathogenesis of pneumococcal disease and test new therapeutic agents and vaccines. This narrative review will focus on the characteristics of the different animal pneumococcal pneumonia models. The assessment of the different animal models will include considerations regarding pneumococcal strains, microbiology properties, procedures used for bacterial inoculation, pathogenesis, clinical characteristics, diagnosis, treatment, and preventive approaches.

Cefquinome-Loaded Microsphere Formulations in Protection against Pneumonia with Klebsiella pneumonia Infection and Inflammatory Response in Rats

PURPOSE

This study aimed to compare in vivo activity between cefquinome (CEQ)-loaded poly lactic-co-glycolic acid (PLGA) microspheres (CEQ-PLGA-MS) and CEQ injection (CEQ-INJ) against Klebsiella pneumonia in a rat lung infection model.

METHODS

Forty-eight rats were divided into control group (sham operated without infection and drug treatment), Klebsiella pneumonia model group (KPD + Saline), CEQ-PLGA-MS and CEQ-INJ therapy groups (KPD + CEQ-PLGA-MS and KPD + INJ, respectively). In the KPD + Saline group, rats were infected with Klebsiella pneumonia ATCC 10031. In the KPD + CEQ-PLGA-MS and KPD + INJ groups, infected rats were intravenously injected with 12.5 mg/kg body weight CEQ-PLGA-MS and CEQ-INJ, respectively.

RESULTS

Compared to CEQ-INJ treatment group, CEQ-PLGA-MS treatment further decreased the number of bacteria colonies (decreased to 1.94 lg CFU/g) in lung tissues and the levels of inflammatory cytokine including tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, IL-4 (p < 0.05 or p < 0.01) in bronchoalveolar lavage fluid at 48 h. Consistently, a significant decreases of scores of inflammation severity were showed at 48 h in the KPD + CEQ-PLGA-MS treatment group, compared to the KPD + CEQ-INJ treatment group.

CONCLUSION

Our results reveal that CEQ-PLGA-MS has the better therapeutic effect than CEQ-INJ for Klebsiella pneumonia lung infections in rats. The vehicle of CEQ-PLGA-MS as the promising alternatives to control the lung infections with the important pathogens.

Cefquinome-loaded microsphere formulations against Klebsiella pneumonia infection during experimental infections

The aim of this study was to prepare cefquinome-loaded polylactic acid microspheres and to evaluate their in vitro and in vivo characteristics and pharmacodynamics for the therapy of pneumonia in a rat model. Microspheres were prepared using a 0.7 mm two-fluid nozzle spray drier in one step resulting in spherical and smooth microspheres of uniform size (9.8 ± 3.6 μm). The encapsulation efficiency and drug loading of cefquinome were 91.6 ± 2.6% and 18.7 ± 1.2%, respectively. In vitro release of cefquinome from the microspheres was sustained for 36 h. Cefquinome-loaded polylactic acid microspheres as a drug delivery system was successful for clearing experimental Klebsiella pneumonia lung infections. A decrease in inflammatory cells and an inhibition of inflammatory cytokines TNF-α, IL-1β and IL-8 after microspheres treatment was found. Changes in cytokine levels and types are secondary manifestations of drug bactericidal effects. Rats were considered to be microbiologically cured because the bacterial load was less than 100 CFU/g. These results also indicated that the spray-drying method of loading therapeutic drug into polylactic acid microspheres is a straightforward and safe method for lung-targeting therapy in animals.

The immune response and antibacterial therapy

The host's immune defence mechanisms are indispensable factors in surviving bacterial infections. However, in many circumstances, the immune system alone is inadequate. Since the 1940s, the use of antibacterial therapy has saved millions of lives, improving the span and quality of life of individuals. Unfortunately, we are now facing an era where antibacterial agents are threatened by resistance. In addition to targeting bacteria, some antibacterial agents affect various aspects of the immune response to infection. Since many antibacterial drugs are failing in efficacy due to resistance, it has been strongly suggested that any synergy between these drugs and the immune response be exploited in the treatment of bacterial infections. This review explores the influence of antibacterial therapy on the immune response and new approaches that could exploit this interaction for the treatment of bacterial infections.

Low or high doses of cefquinome targeting low or high bacterial inocula cure Klebsiella pneumoniae lung infections but differentially impact the levels of antibiotic resistance in fecal flora

The combination of efficacious treatment against bacterial infections and mitigation of antibiotic resistance amplification in gut microbiota is a major challenge for antimicrobial therapy in food-producing animals. In rats, we evaluated the impact of cefquinome, a fourth-generation cephalosporin, on both Klebsiella pneumoniae lung infection and intestinal flora harboring CTX-M-producing Enterobacteriaceae. Germfree rats received a fecal flora specimen from specific-pathogen-free pigs, to which a CTX-M-producing Escherichia coli strain had been added. K. pneumoniae cells were inoculated in the lungs of these gnotobiotic rats by using either a low (10(5) CFU) or a high (10(9) CFU) inoculum. Without treatment, all animals infected with the low or high K. pneumoniae inoculum developed pneumonia and died before 120 h postchallenge. In the treated groups, the low-inoculum rats received a 4-day treatment of 5 mg/kg of body weight cefquinome beginning at 24 h postchallenge (prepatent phase of the disease), and the high-inoculum rats received a 4-day treatment of 50 mg/kg cefquinome beginning when the animals expressed clinical signs of infection (patent phase of the disease). The dose of 50 mg/kg targeting the high K. pneumoniae inoculum cured all the treated rats and resulted in a massive amplification of CTX-M-producing Enterobacteriaceae. A dose of 5 mg/kg targeting the low K. pneumoniae inoculum cured all the rats and averted an outbreak of clinical disease, all without any amplification of CTX-M-producing Enterobacteriaceae. These findings might have implications for the development of new antimicrobial treatment strategies that ensure a cure for bacterial infections while avoiding the amplification of resistance genes of human concern in the gut microbiota of food-producing animals.

Emergence of resistant Klebsiella pneumoniae in the intestinal tract during successful treatment of Klebsiella pneumoniae lung infection in rats

Antibiotic treatment of lung infections may lead to the emergence of resistance in the gut flora. Appropriate dosing regimens could mitigate this adverse effect. In gnotobiotic rats harboring intestinal Escherichia coli and Enterococcus faecium populations, a lung infection by Klebsiella pneumoniae was instigated with two different sizes of inoculum to represent an early or a late initiation of antibiotic treatment. The rats were treated with marbofloxacin, an expanded-spectrum fluoroquinolone, by a single-shot administration or a fractionated regimen over 4 days. Intestinal bacterial populations were monitored during and after treatment. At the infection site, bacterial cure without any selection of resistance was observed. Whatever the dosage regimen, fluoroquinolone treatment had a transient negative impact on the E. coli gut population but not on that of E. faecium. The intestinal flora was colonized by the pathogenic lung bacteria, and there was the emergence of intestine-resistant K. pneumoniae, occurring more often in animals treated with a single marbofloxacin dose than with the fractionated dose. Bacterial cure without resistance selection at the infection site with fluoroquinolone treatment can be linked to colonization of the digestive tract by targeted pulmonary bacteria, followed by the emergence of resistance.

Influence of inoculum size and marbofloxacin plasma exposure on the amplification of resistant subpopulations of Klebsiella pneumoniae in a rat lung infection model

We tested the hypothesis that the bacterial load at the infection site could impact considerably on the pharmacokinetic/pharmacodynamic (PK/PD) parameters of fluoroquinolones. Using a rat lung infection model, we measured the influence of different marbofloxacin dosage regimens on selection of resistant bacteria after infection with a low (10(5) CFU) or a high (10(9) CFU) inoculum of Klebsiella pneumoniae. For daily fractionated doses of marbofloxacin, prevention of resistance occurred for an area-under-the-concentration-time-curve (AUC)/MIC ratio of 189 h for the low inoculum, whereas for the high inoculum, resistant-subpopulation enrichment occurred for AUC/MIC ratios up to 756 h. For the high-inoculum-infected rats, the AUC/MIC ratio, C(max)/MIC ratio, and time within the mutant selection window (T(MSW)) were not found to be effective predictors of resistance prevention upon comparison of fractionated and single administrations. An index corresponding to the ratio of the time that the drug concentrations were above the mutant prevention concentration (MPC) over the time that the drug concentrations were within the MSW (T(>MPC)/T(MSW)) was the best predictor of the emergence of resistance: a T(>MPC)/T(MSW) ratio of 0.54 was associated with prevention of resistance for both fractionated and single administrations. These results suggest that the enrichment of resistant bacteria depends heavily on the inoculum size at the start of an antimicrobial treatment and that classical PK/PD parameters cannot adequately describe the impact of different dosage regimens on enrichment of resistant bacteria. We propose an original index, the T(>MPC)/T(MSW) ratio, which reflects the ratio of the time that the less susceptible bacterial subpopulation is killed over the time that it is selected, as a potentially powerful indicator of prevention of enrichment of resistant bacteria. This ratio is valid only if plasma concentrations achieve the MPC.

Animal models of Streptococcus pneumoniae disease

SUMMARY

Streptococcus pneumoniae is a colonizer of human nasopharynx, but it is also an important pathogen responsible for high morbidity, high mortality, numerous disabilities, and high health costs throughout the world. Major diseases caused by S. pneumoniae are otitis media, pneumonia, sepsis, and meningitis. Despite the availability of antibiotics and vaccines, pneumococcal infections still have high mortality rates, especially in risk groups. For this reason, there is an exceptionally extensive research effort worldwide to better understand the diseases caused by the pneumococcus, with the aim of developing improved therapeutics and vaccines. Animal experimentation is an essential tool to study the pathogenesis of infectious diseases and test novel drugs and vaccines. This article reviews both historical and innovative laboratory pneumococcal animal models that have vastly added to knowledge of (i) mechanisms of infection, pathogenesis, and immunity; (ii) efficacies of antimicrobials; and (iii) screening of vaccine candidates. A comprehensive description of the techniques applied to induce disease is provided, the advantages and limitations of mouse, rat, and rabbit models used to mimic pneumonia, sepsis, and meningitis are discussed, and a section on otitis media models is also included. The choice of appropriate animal models for in vivo studies is a key element for improved understanding of pneumococcal disease.

Experimental models of pulmonary infection

Experimental models of pulmonary infection are being discussed, focused on various aspects of good experimental design, such as choice of animal species and infecting strain, and route of infection/inoculation techniques (intranasal inoculation, aerosol inoculation, and direct instillation into the lower respiratory tract). In addition, parameters to monitor pulmonary infection are being reviewed such as general clinical signs, pulmonary-associated signs, complication of the pulmonary infection, mortality rate, and parameters after dissection of animals. Examples of pulmonary infection models caused by bacteria, fungi, viruses or parasites in experimental animals with intact or impaired host defense mechanisms are shortly summarized including key-references.

In vitro killing of erythromycin-exposed group A streptococci by polymorphonuclear leukocytes

After exposure to erythromycin, group A streptococci were tested for susceptibility to the antimicrobial activity of human peripheral blood neutrophils in the absence of the antibiotic. Bacterial susceptibility to phagocytic killing increased after prior exposure to supra-inhibitory levels of erythromycin for even as brief as three minutes. Extended exposure and higher concentrations of erythromycin increased phagocytic killing. Although the degree of sensitization varied in different strains of streptococci, all strains tested were significantly more susceptible to phagocytic killing after erythromycin exposure. Killing of erythromycin-treated bacteria that occurred in the absence of antibiotic was dependent upon internalization of the bacteria. Thus, the brief exposure of group A streptococci to inhibitory levels of erythromycin increases their susceptibility to phagocytic killing by peripheral blood neutrophils.

Relationships between alternative complement pathway activation, C-reactive protein, and pneumococcal infection

In the absence of specific antibody, opsonization of Streptococcus pneumoniae may be mediated by the alternative complement pathway (AP) or by C-reactive protein (CRP) via C1 binding. To determine the role of these mechanisms in pneumococcal (PNC) disease, we studied 19 patients with differing severities of PNC infection. C4 and CRP levels and zymosan-induced consumption of 50% hemolytic complement (CH50) were measured in specimens obtained acutely and then weekly. In patients with complicated illness, the modified mean CH50 in acute sera was 178 +/- 57 U/ml, significantly lower than the mean CH50 of 331 +/- 80 U/ml in patients with uncomplicated illness (P less than 0.05). The values of the two groups on a given day approximated each other on days 7, 14, and 23. Consumption of complement by zymosan was also lower in acute sera of patients with complicated illness, with a mean value of 19 +/- 18 U/ml compared with 58 +/- 30 U/ml in those with uncomplicated illness (P less than 0.05). This difference was also seen on day 7 (P less than 0.05). Disease involving lower-numbered PNC serotypes (less than 10) correlated with reduced availability of AP factors in acute sera, independent of illness severity. Mean CRP levels were inversely related to zymosan-induced complement activation in patients with complicated illness. These data suggest that in vivo depletion of AP factors is significantly greater in patients with complicated illness and is associated with high CRP levels. CRP may enhance AP activation via C3 convertase generation and function with it as a preantibody host defense mechanism.

Effect of amoxycillin and doxycycline on function of human granulocytes tested in vitro and on chemotaxis of granulocytes from rabbits given the two antibiotics

The effect of amoxycillin and doxycycline on human granulocyte function was studied in vitro. The antimicrobial agents were added to tests of chemotaxis, random motility, yeast phagocytosis, and killing, at progressing concentrations. The effect was also evaluated in vivo by measuring chemotaxis and random motility (agarose technique) of granulocytes from rabbits injected with these antibiotics. Amoxycillin was shown to have a slightly stimulating effect on chemotaxis, demonstrable only at the highest concentration tested (100 micrograms/ml), and no effect on the other variables. Doxycycline had a dose-related inhibitory effect on chemotaxis, random motility, and phagocytosis, and no effect on killing. Chemotaxis and random motility were slightly, but not significantly, stimulated in vivo when rabbits were given amoxycillin. Chemotaxis (but not random motility) was significantly impaired when the rabbits were given doxycycline.

Enhancement of the local inflammatory response to bacterial infection by muramyl dipeptide

The effect of the synthetic immuno-adjuvant compound, muramyl dipeptide (MDP), upon the local inflammatory response to experimental bacterial infection was assessed by histological examination. Within 24 h of the insertion of a bacteria-laden suture into the medial thigh musculature of mice treated with either MDP or placebo, an enhanced degree of polymorphonuclear leucocyte infiltration in the muscle around the suture was observed in the MDP-treated animals. The inflammatory response around a sterile suture was less intense in both treatment groups and specific correlation between the degree of local inflammation and the extent of bacteraemia developing in either group of animals was not noted. The extent of bacteraemia developing in either group of animals was not noted. The previously observed protection conferred by MDP against the local impact of bacterial challenge appears to be mediated in part by enhancement of the acute local inflammatory response.

Efficacy of continuous versus intermittent administration of penicillin G in Streptococcus pneumoniae pneumonia in normal and immunodeficient rats

An experimental Streptococcus pneumoniae pneumonia was used to study the influence of continuous versus intermittent administration of penicillin G on therapeutic efficacy in normal rats and in rats whose phagocytic capacities were impaired by decomplementation with cobra venom factor. Response to antibiotic treatment was evaluated with respect to numbers of bacteria in left lung, blood and pleural fluid. Penicillin treatment was started 36 h after bacterial inoculation, and continued for four days. With intermittent intramuscular administration of penicillin normal rats were cured after daily doses of 4 mg/kg at 12 h intervals, whereas decomplemented rats recovered only after daily doses of 100 or 102 mg/kg at 12 h or 8 h intervals, respectively. When penicillin was administered by way of continuous infusion, daily doses of 3.5 mg/kg were required for a cure of infections in both normal rats and in decomplemented rats. This treatment resulted in a constant level of 0.05 micrograms of penicillin per ml, which was slightly above the minimum bactericidal concentration for the infecting strain. These findings show that maintenance of bactericidal levels of penicillin were particularly important in curing severe infection in rats with impaired defense.

Complement and the host's defense against the pneumococcus

The complement system plays a critical role in resistance to Streptococcus pneumoniae. In the last two decades, a great deal of new knowledge has been generated on both the complement system and on the pneumococcus. Not only has this new knowledge helped produce a better understanding of how the complement system serves the host in its defense against S. pneumoniae, but in a more general fashion, the lessons learned from the interaction of complement and the pneumococcus have also provided insight into how the complement system functions in the defense of the host against a wide variety of other microorganisms. The following review will attempt to summarize current knowledge about the complement system, how it interacts at the molecular level with the pneumococcus, the biologic consequences of that interaction, and their significance in pneumococcal infections.

Impaired lung defenses against Staphylococcus aureus in mice with hereditary deficiency of the fifth component of complement

Mice of the C5-deficient DBA/2J and B10.D2/oSnJ inbred strains were aerosolized with Staphylococcus aureus, and the pulmonary clearance of bacteria was determined 4 h later. Both C5-deficient strains had a significantly decreased lung clearance of S. aureus compared with their genetically closest C5-sufficient relatives, DBA/1J and B10.D2/nSnJ strains, respectively. Serum hemolytic activity and pulmonary clearance of S. aureus were also investigated in F1 and F2 progenies (DBA/1J X DBA/2J and DBA/2J X B10.D2/oSnJ). Serum hemolytic activity was present in all F1 (DBA/1J X DBA/2J) mice, and their pulmonary clearance of S. aureus was no different from that of the C5-sufficient parents (DBA/1J). The absence of serum hemolytic activity (absence of C5) in all mice from F1 and F2 (DBA/2J X B10.D2/oSnJ) and 20% of the F2 (DBA/1J X DBA/2J) was related to a decreased lung clearance of S. aureus. These results are consistent with an autosomal recessive pattern of heredity for the murine abnormality in pulmonary clearance of S. aureus.

Therapeutic activities of cefazolin, cefotaxime, and ceftazidime against experimentally induced Klebsiella pneumoniae pneumonia in rats

The efficacies of several dosage schedules of cefazolin, cefotaxime, and ceftazidime, started 12 or 36 h after infection, were examined in experimental pneumonia caused by Klebsiella pneumoniae in rats. The therapeutic activities of the cephalosporins were compared with the antibacterial activities in vitro and the serum concentration curves. The course of experimental pneumonia was rapid and characterized by tissue necrosis. Response to antimicrobial treatment was evaluated with respect to mortality and numbers of bacteria in lung (left lobe), blood, and pleural fluid. When antibiotic treatment was started early, i.e., 12 h after bacterial inoculation, cefotaxime and ceftazidime were equally effective and superior to cefazolin. Eleven doses of 10 mg of cefotaxime or ceftazidime per kg or 11 doses of 60 mg of cefazolin per kg were required to improve the survival rate. With a delay in administration to 36 h after inoculation, the efficacy of the cephalosporins decreased markedly. In the three dosages tested, cefazolin was ineffective. Survival improved with the administration of nine doses of 60 mg of cefotaxime per kg or nine doses of 10 mg of ceftazidime per kg. These results are not in accordance with the ratio of in vitro activities of cefotaxime and ceftazidime or the serum concentration curves.

Comparative role of complement in pneumococcal and staphylococcal pneumonia

To evaluate the role of complement in pneumococcal and staphylococcal pneumonia, we decomplemented rats with cobra venom factor and inoculated them intratracheally with Staphylococcus aureus or type 25 pneumococci. S. aureus produced a patchy bronchopneumonia in normal Sprague-Dawley or Lewis rats, and decomplementation did not increase the severity of staphylococcal infection in either rat strain as judged by quantitative cultures of the lungs and blood at 6, 24, and 48 h after inoculation. In contrast, decomplementation markedly increased the severity of pneumonia caused by type 25 pneumococci in Sprague-Dawley and Lewis rats. In Sprague-Dawley rats, decomplementation significantly increased the number of bacteria in the lungs at 3, 6, and 24 h of infection. Bacteremia developed early in decomplemented Sprague-Dawley rats, but the higher pulmonary bacterial counts did not appear to be caused by bacteremic seeding of the lungs. Decomplemented Sprague-Dawley rats inoculated intravenously with pneumococci failed to develop the very high levels of bacteria in the lungs that were observed when the rats were inoculated intratracheally. Moreover, decomplemented Lewis rats inoculated intratracheally with pneumococci developed significantly increased numbers of pneumococci in the lungs early in infection (3 and 6 h) when they had no detectable bacteremia. These data indicate that in murine models complement plays a major protective role against type 25 pneumococci in the lung, whereas complement is not important to host defense in staphylococcal pneumonia.

The effect of exposure of Staphylococcus aureus to penicillin on susceptibility to the bactericidal activity of human leukocytes

The effect of brief exposure of staphylococci to penicillin upon susceptibility to leukocidal activity was studied. Antibiotic pretreated staphylococci were incubated with normal human leukocytes and viability of the bacteria was determined. Penicillin pretreated staphylococci were more susceptible to killing by leukocytes than untreated control bacteria. The extent of sensitization varied between different strains of staphylococci. When staphylococci were exposed simultaneously to leukocytes and penicillin, the bacteria were protected from the lethal action of penicillin. This study demonstrates that phagocytosed staphylococci are protected from penicillin, but prior exposure to the antibiotic increases their susceptibility to the activity of leukocytes.

Pulmonary opsonins in Klebsiella pneumoniae pneumonia in rats

Klebsiella pneumoniae was inoculated intrabronchially into rats, and bronchoalveolar lavage fluid and sera were obtained during the ensuring pneumonia. Klebsiellae recovered by lavage were not maximally coated with C3, as judged by studies with fluorescent antibody, whereas the organisms could be coated fully with C3 by a brief incubation in rat serum. The levels of C1 and C3 in lavage fluid obtained during infection were only a small fraction of the levels in the serum, and klebsiellae were not opsonized during incubation with concentrated lavage fluid. Systemic decomplementation did not affect the severity of K. pneumoniae pneumonia, as judged by the measurement of lung weight or by the numbers of klebsiellae in the lungs, but decomplemented rats had a larger number of klebsiellae in the blood at 24 h of infection than did controls. There were fewer klebsiellae in the lungs 4 h after inoculation of preopsonized organisms than after inoculation of organisms which were incubated in control (heat-inactivated) sera. These studies indicate that the concentration of complement and heat-labile opsonins within the alveoli is lower than that in the systemic circulation and is not adequate for effective opsonization of klebsiellae.

Efficacy of antimicrobial therapy in experimental rat pneumonia: antibiotic treatment schedules in rats with impaired phagocytosis

Pneumococcal pneumonia in rats with intact host defense mechanisms could be successfully cured by penicillin. The efficacy of this antibiotic therapy was lost in cobra venom factor-treated rats which had selectively impaired phagocytic functions. In these animals the effect of penicillin therapy was improved by increasing the daily dose and the frequency of injections and by earlier initiation of the therapy. The efficacy of penicillin in the cobra venom facttor-treated rats was restored either by markedly increasing the daily dose of penicillin or by increasing the daily dose in combination with a reduced interval of the penicillin injections.