Interstrain transfer of the large pathogenicity island (PAPI-1) of Pseudomonas aeruginosa

The large Pseudomonas aeruginosa pathogenicity island PAPI-1 of strain PA14 is a cluster of 108 genes that encode a number of virulence features. We demonstrate that, in a subpopulation of cells, PAPI-1 can exist in an extrachromosomal circular form after precise excision from its integration site within the 3' terminus of the tRNA(Lys) gene. Circular PAPI-1 can reintegrate into either of the two tRNA(Lys) genes, including the one that was used for integration of small pathogenicity island PAPI-2 in strain PA14. The excision requires PAPI-1-encoded integrase, a member of the tyrosine recombinase family. PAPI-1 Soj contains the conserved domains of proteins that are related to chromosome and plasmid partition. soj plays a role in maintaining PAPI-1 and mutations in soj result in the loss of PAPI-1 from P. aeruginosa. We further demonstrate that, during coculture, the PAPI-1-containing strains are able to transfer it into P. aeruginosa recipient strains that do not harbor this island naturally. After transfer, PAPI-1 integrates into either of the two tRNA(Lys) genes. PAPI-1 encompasses many features of mobile elements, including mobilization and maintenance modules. Together with the virulence determinants, PAPI-1 plays an important role in the evolution of P. aeruginosa, by expanding its natural habitat from soil and water to animal and human infections.     

Analysis of Pseudomonas aeruginosa diguanylate cyclases and phosphodiesterases reveals a role for bis-(3'-5')-cyclic-GMP in virulence

The opportunistic pathogen Pseudomonas aeruginosa is responsible for systemic infections in immunocompromised individuals and chronic respiratory disease in patients with cystic fibrosis. Cyclic nucleotides are known to play a variety of roles in the regulation of virulence-related factors in pathogenic bacteria. A set of P. aeruginosa genes, encoding proteins that contain putative domains characteristic of diguanylate cyclases (DGCs) and phosphodiesterases (PDEs) that are responsible for the maintenance of cellular levels of the second messenger bis-(3'-5')-cyclic dimeric GMP (c-di-GMP) was identified in the annotated genomes of P. aeruginosa strains PAO1 and PA14. Although the majority of these genes are components of the P. aeruginosa core genome, several are located on presumptive horizontally acquired genomic islands. A comprehensive analysis of P. aeruginosa genes encoding the enzymes of c-di-GMP metabolism (DGC- and PDE-encoding genes) was carried out to analyze the function of c-di-GMP in two disease-related phenomena, cytotoxicity and biofilm formation. Analysis of the phenotypes of DGC and PDE mutants and overexpressing clones revealed that certain virulence-associated traits are controlled by multiple DGCs and PDEs through alterations in c-di-GMP levels. A set of mutants in selected DGC- and PDE-encoding genes exhibited attenuated virulence in a mouse infection model. Given that insertions in different DGC and PDE genes result in distinct phenotypes, it seems likely that the formation or degradation of c-di-GMP by these enzymes is in highly localized and intimately linked to particular targets of c-di-GMP action.     

Role of the quorum-sensing system in experimental pneumonia due to Pseudomonas aeruginosa in rats

The virulence of Pseudomonas aeruginosa is partly controlled by the las quorum-sensing system. A rat model of acute pneumonia was used to investigate the pathophysiological impact of this system by comparing the virulence of the wild-type virulent laboratory strain PAO1 with that of its lasR-deleted mutant PAOR. In comparison with PAO1, PAOR was avirulent after an instillation of 106 cfu (mortality rates, 72 versus 0%, respectively; p < 0.0001). A ten-fold higher inoculum slightly increased the mortality rate induced by PAOR (25%), which remained lower than that induced by PAO1 (75%, p = 0.0001). In addition, with both inocula lung and bronchoalveolar lavage bacterial counts were significantly lower in rats infected with PAOR than with PAO1 (p 相似文献   

The Pseudomonas aeruginosa genome: how do we use it to develop strategies for the treatment of patients with cystic fibrosis and Pseudomonas infections?

In the 2 years since the complete sequence of Pseudomonas aeruginosa strain PAO1 was published, at least 200 papers have been published describing research that made use of the PAO1 genome sequence. Some of this research included genome-wide studies of gene expression or the effect of mutation on bacterial functions such as biofilm formation; this type of global analysis would not have been possible without the availability of the sequence. As a result of these and other, more traditional, research studies, there is a wealth of new knowledge about the physiology of this pathogen. This raises the possibility of new strategies for the treatment of patients with P. aeruginosa infection, either by novel antibiotics or by drugs targeting bacterial functions essential for survival and virulence in the human host.     

Secretion of the toxin ExoU is a marker for highly virulent Pseudomonas aeruginosa isolates obtained from patients with hospital-acquired pneumonia

Overall, hospital-acquired pneumonia (HAP) caused by Pseudomonas aeruginosa is associated with high attributable mortality. Although the intrinsic virulence of P. aeruginosa undoubtedly contributes to this phenomenon, it is unclear whether all strains share this property or whether only a subpopulation of strains are capable of causing such severe disease. In this study, the virulence of 35 P. aeruginosa isolates obtained from patients with HAP by use of a cytolytic cell-death assay, an apoptosis assay, and a mouse model of pneumonia. The virulence of individual isolates differed significantly from one to another in each of these assays. Increased virulence was associated with the secretion of ExoU, a toxin transported by the P. aeruginosa type III secretion system. Secretion of ExoS or ExoY, 2 other proteins transported by this system, was not consistently associated with increased virulence. Together, these findings suggest that secretion of ExoU is a marker for highly virulent strains of P. aeruginosa.     

Cyanide produced by human isolates of Pseudomonas aeruginosa contributes to lethality in Drosophila melanogaster

Some Pseudomonas aeruginosa strains are cyanogenic, and cyanide may contribute to the bacterium's virulence. Using human isolates of P. aeruginosa, we have shown that Drosophila melanogaster suspended above cyanogenic strains become motionless and develop bradycardia and that flies injected with cyanogenic bacterial strains die more rapidly than those injected with noncyanogenic strains. Flies exposed to cyanogenic strains had high cyanide and low adenosine triphosphate (ATP) concentrations in body extracts, and treatment with a cyanide antidote equalized survival of flies injected with cyanogenic and noncyanogenic strains. P. aeruginosa PAO1 strain with a mutation in the hydrogen cyanide synthase gene cluster was much less toxic to flies than the parental cyanogenic strain or 2 knock-in strains. Transgenic flies overexpressing rhodanese, which detoxifies cyanide by converting it to thiocyanate, were resistant to cyanide and the increased virulence of cyanogenic strains. We conclude that D. melanogaster is a good model for studying cyanide produced by P. aeruginosa.     

In vivo regulation of virulence in Pseudomonas aeruginosa associated with genetic rearrangement.

A chronic pulmonary infection model was used to induce conversion to the mucoid phenotype by Pseudomonas aeruginosa PAO. At 6 months after initial inoculation, organisms isolated from infected lungs demonstrated the mucoid phenotype. Significant decreases (P less than .01) were seen in the levels of exotoxin A, exoenzyme S, phospholipase C, and pyochelin produced by the mucoid P. aeruginosa PAO rat lung isolates that returned to parental levels after reversion to the nonmucoid phenotype. In addition, lipopolysaccharide of the mucoid PAO lung isolates failed to react with serotype B-specific antibody in contrast to the original PAO and the revertant PAO organisms. Digestion of chromosomal DNA and hybridization with P. aeruginosa virulence factor-specific probes demonstrated that conversion to the mucoid phenotype was associated with rearrangement of chromosomal DNA upstream of the exotoxin A gene. Analysis of DNA from revertant organisms revealed hybridization patterns identical to the original PAO organism.     

Effects of azithromycin on clinical isolates of Pseudomonas aeruginosa from cystic fibrosis patients

There is considerable interest in the use of azithromycin for the treatment of lung disease in patients with cystic fibrosis (CF). Although its mechanism of action as an inhibitor of bacterial protein synthesis has been well-established, it is less clear how azithromycin ameliorates the lung disease associated with Pseudomonas aeruginosa, which is considered to be resistant to the drug. We tested the effects of azithromycin on clinical isolates (CIs) from CF patients and compared them with laboratory reference strains to establish how this drug might interfere with the production of bacterial virulence factors that are relevant to the pathogenesis of airway disease in CF patients. Azithromycin inhibited P aeruginosa PAO1 protein synthesis by 80%, inhibiting bacterial growth and the expression of immunostimulatory exoproducts such as pyocyanin, as well as the gene products necessary for biofilm formation. In contrast, the effects of azithromycin on CIs of P aeruginosa were much more variable, due in large part to their slow growth and limited exoproduct expression. Culture supernatants for two of three clinical strains induced appreciable CXCL8 expression from cultured epithelial cells. Azithromycin treatment of the organisms inhibited 65 to 70% of this induction; azithromycin had no direct effect on the ability of either normal cells or CF epithelial cells to produce CXCL8. Azithromycin does decrease the P aeruginosa synthesis of immunostimulatory exoproducts and is likely to be most effective against planktonic, actively growing bacteria. This effect is less predictable against CIs than the prototypic strain PAO1.     

The roles of the quorum-sensing system in the release of extracellular DNA, lipopolysaccharide, and membrane vesicles from Pseudomonas aeruginosa

Biofilms play an important role in the establishment of chronic infection caused by Pseudomonas aeruginosa. It has been suggested that membrane vesicles (MVs) are released into the surrounding medium during normal growth and might supply the bacterial extracellular DNA that is required for early biofilm formation, as MVs released from the bacterial outer membrane are suspected to be the source of extracellular DNA. MVs possess lipopolysaccharide (LPS), extracellular DNA, and several hydrolytic enzymes. It is well known that the quorum-sensing (QS) system is important in controlling virulence factors in P. aeruginosa and biofilm formation. In the current study, we investigated extracellular LPS and DNA in the supernatants of culture solutions from PAO1, the wild-type P. aeruginosa, and those of QS mutants. As compared to that of las QS mutants, the amount of LPS and DNA released was significantly higher in PAO1 and in las QS mutants complemented with N-(3-oxododecanoyl) homoserine lactone. Our study indicated that the QS is among the regulators involved in the release of extracellular DNA and LPS. It is possible that these extracellular components are supplied from MVs. Investigation of the mechanism of biofilm formation is of particular interest, as it may be useful for designing treatments for severe P. aeruginosa infection.     

Conservation of genome content and virulence determinants among clinical and environmental isolates of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a ubiquitous environmental bacterium capable of causing a variety of life-threatening human infections. The genetic basis for preferential infection of certain immunocompromised patients or individuals with cystic fibrosis by P. aeruginosa is not understood. To establish whether variation in the genomic repertoire of P. aeruginosa strains can be associated with a particular type of infection, we used a whole-genome DNA microarray to determine the genome content of 18 strains isolated from the most common human infections and environmental sources. A remarkable conservation of genes including those encoding nearly all known virulence factors was observed. Phylogenetic analysis of strain-specific genes revealed no correlation between genome content and infection type. Clusters of strain-specific genes in the P. aeruginosa genome, termed variable segments, appear to be preferential sites for the integration of novel genetic material. A specialized cloning vector was developed for capture and analysis of these genomic segments. With this capture system a site associated with the strain-specific ExoU cytotoxin-encoding gene was interrogated and an 80-kb genomic island carrying exoU was identified. These studies demonstrate that P. aeruginosa strains possess a highly conserved genome that encodes genes important for survival in numerous environments and allows it to cause a variety of human infections. The acquisition of novel genetic material, such as the exoU genomic island, through horizontal gene transfer may enhance colonization and survival in different host environments.     

Comparison of the complete genome sequences of Pseudomonas syringae pv. syringae B728a and pv. tomato DC3000

The complete genomic sequence of Pseudomonas syringae pv. syringae B728a (Pss B728a) has been determined and is compared with that of P. syringae pv. tomato DC3000 (Pst DC3000). The two pathovars of this economically important species of plant pathogenic bacteria differ in host range and other interactions with plants, with Pss having a more pronounced epiphytic stage of growth and higher abiotic stress tolerance and Pst DC3000 having a more pronounced apoplastic growth habitat. The Pss B728a genome (6.1 Mb) contains a circular chromosome and no plasmid, whereas the Pst DC3000 genome is 6.5 mbp in size, composed of a circular chromosome and two plasmids. Although a high degree of similarity exists between the two sequenced Pseudomonads, 976 protein-encoding genes are unique to Pss B728a when compared with Pst DC3000, including large genomic islands likely to contribute to virulence and host specificity. Over 375 repetitive extragenic palindromic sequences unique to Pss B728a when compared with Pst DC3000 are widely distributed throughout the chromosome except in 14 genomic islands, which generally had lower GC content than the genome as a whole. Content of the genomic islands varies, with one containing a prophage and another the plasmid pKLC102 of Pseudomonas aeruginosa PAO1. Among the 976 genes of Pss B728a with no counterpart in Pst DC3000 are those encoding for syringopeptin, syringomycin, indole acetic acid biosynthesis, arginine degradation, and production of ice nuclei. The genomic comparison suggests that several unique genes for Pss B728a such as ectoine synthase, DNA repair, and antibiotic production may contribute to the epiphytic fitness and stress tolerance of this organism.     

Complete genome sequence and analysis of Wolinella succinogenes

To understand the origin and emergence of pathogenic bacteria, knowledge of the genetic inventory from their nonpathogenic relatives is a prerequisite. Therefore, the 2.11-megabase genome sequence of Wolinella succinogenes, which is closely related to the pathogenic bacteria Helicobacter pylori and Campylobacter jejuni, was determined. Despite being considered nonpathogenic to its bovine host, W. succinogenes holds an extensive repertoire of genes homologous to known bacterial virulence factors. Many of these genes have been acquired by lateral gene transfer, because part of the virulence plasmid pVir and an N-linked glycosylation gene cluster were found to be syntenic between C. jejuni and genomic islands of W. succinogenes. In contrast to other host-adapted bacteria, W. succinogenes does harbor the highest density of bacterial sensor kinases found in any bacterial genome to date, together with an elaborate signaling circuitry of the GGDEF family of proteins. Because the analysis of the W. succinogenes genome also revealed genes related to soil- and plant-associated bacteria such as the nif genes, W. succinogenes may represent a member of the epsilon proteobacteria with a life cycle outside its host.     

2型猪链球菌人源致病株07NJH06的分离和病原生物学鉴定

目的系统分析鉴定南京地区脑膜炎型猪链球菌感染病人脑脊液分离株07NJH06病原生物学特性。方法采集患者脑脊液,进行细菌分离和培养,观察菌体显微形态,通过免疫凝集试验和PCR方法对分离物进行种属鉴定及毒力因子基因携带情况分析,纸片法测定菌株药物敏感性,采用小白鼠和仔猪模型评估分离菌株的致病特性。结果从病人脑脊液分离到1株革兰阳性链球菌,经免疫学、生物化学及分子生物学方法系统鉴定为2型猪链球菌（命名为07NJH06）,毒力基因型为cps2J＋mrp＋epf＋sly＋gdh＋fbp＋srtA＋,未获得含有完整毒力岛PAI89k的证据,该岛两侧边缘反向重复区靶基因以及岛内二元调控系统SalK/SalR均未检出,但针对岛内3个独立的镶嵌结构区代表基因进行扩增,其中2个结构区代表基因（05SSU0942和05SSU0965）得到特异性扩增;药敏试验显示,07NJH06株对青霉素、万古霉素等抗生素敏感,对四环素、头孢噻肟钠等抗生素耐药;动物感染试验显示,07NJH06株对小鼠和猪仔有致病性,毒力较国内暴发流行强致病株05ZYH33弱,与国际参考株毒力相当。结论07NJH06株为致病性2型猪链球菌,基本生物学特性、主要毒力基因型与既往国内暴发现场分离株相似,但不含有完整的毒力岛PAI89k基因结构,系统掌握该菌遗传信息对于阐明国内强毒株遗传进化规律有重要价值。     

Pseudomonas aeruginosa vasculitis and bacteremia following conjunctivitis: a simple model of fatal pseudomonas infection in neutropenia.

During attempts to create a realistic model of fatal bacteremia due to Pseudomonas aeruginosa during immunosuppression, it was found that the invasive as well as the disseminated phase of infection could be mimicked by gentle instillation of 10(8) colony-forming units of P. aeruginosa into the intact conjunctival sac of agranulocytic rabbits. Within 48 hr animals developed conjunctivits leading to severe necrotizing vasculitis and fatal bacteremia. Twelve of 26 strains from patients with P. aeruginosa infections were virulent, causing death in 50%--100% of animals. Nine (75%) of 12 isolates from blood but only two (15%) of 13 isolates from sputum and urine were highly lethal. Neither proteolytic enzyme production nor serum resistance alone accounted for virulence. No infection developed in animals and normal leukocyte counts or in neutropenic animals given Escherichia coli, Klebsiella pneumoniae, or non-aeruginosa pseudomonads. A rare vasculitic lesion was observed in animals inoculated with Serratia marcescens. This model, which illustrates the distinctive features of P. aeruginosa infection, is so simple and reproducible that it should be useful for evaluation of the efficacy of drugs and immunization against Pseudomonas in the compromised host.     

A simple alfalfa seedling infection model for Pseudomonas aeruginosa strains associated with cystic fibrosis shows AlgT (sigma-22) and RhlR contribute to pathogenesis

A sensitive plant infection model was developed to identify virulence factors in nontypeable, alginate overproducing (mucoid) Pseudomonas aeruginosa strains isolated from cystic fibrosis (CF) patients with chronic pulmonary disease. Nontypeable strains with defects in lipopolysaccharide O-side chains are common to CF and often exhibit low virulence in animal models of infection. However, 1,000 such bacteria were enough to show disease symptoms in the alfalfa infection. A typical mucoid CF isolate, FRD1, and its isogenic mutants were tested for alfalfa seedling infection. Although defects in the global regulators Vfr, RpoS, PvdS, or LasR had no discernable effect on virulence, a defect in RhlR reduced the infection frequency by >50%. A defect in alginate biosynthesis resulted in plant disease with >3-fold more bacteria per plant, suggesting that alginate overproduction attenuated bacterial growth in planta. FRD1 derivatives lacking AlgT, a sigma factor required for alginate production, were reduced >50% in the frequency of infection. Thus, AlgT apparently regulates factors in FRD1, besides alginate, important for pathogenesis. In contrast, in a non-CF strain, PAO1, an algT mutation did not affect its virulence on alfalfa. Conversely, PAO1 virulence was reduced in a mucA mutant that overproduced alginate. These observations suggested that mucoid conversion in CF may be driven by a selection for organisms with attenuated virulence or growth in the lung, which promotes a chronic infection. These studies also demonstrated that the wounded alfalfa seedling infection model is a useful tool to identify factors contributing to the persistence of P. aeruginosa in CF.     

Identification of protective and broadly conserved vaccine antigens from the genome of extraintestinal pathogenic Escherichia coli

Extraintestinal pathogenic Escherichia coli (ExPEC) are a common cause of disease in both mammals and birds. A vaccine to prevent such infections would be desirable given the increasing antibiotic resistance of these bacteria. We have determined the genome sequence of ExPEC IHE3034 (ST95) isolated from a case of neonatal meningitis and compared this to available genome sequences of other ExPEC strains and a few nonpathogenic E. coli. We found 19 genomic islands present in the genome of IHE3034, which are absent in the nonpathogenic E. coli isolates. By using subtractive reverse vaccinology we identified 230 antigens present in ExPEC but absent (or present with low similarity) in nonpathogenic strains. Nine antigens were protective in a mouse challenge model. Some of them were also present in other pathogenic non-ExPEC strains, suggesting that a broadly protective E. coli vaccine may be possible. The gene encoding the most protective antigen was detected in most of the E. coli isolates, highly conserved in sequence and found to be exported by a type II secretion system which seems to be nonfunctional in nonpathogenic strains.     

Clinical response to azithromycin in cystic fibrosis correlates with in vitro effects on Pseudomonas aeruginosa phenotypes

A 6-month clinical trial of azithromycin (AZM) in American cystic fibrosis (CF) patients with chronic Pseudomonas aeruginosa infection showed clinical improvement without significant reduction in bacterial density. Sub-inhibitory AZM has been hypothesized to affect P. aeruginosa virulence, partly contributing to the mechanism of action of AZM. To correlate bacterial phenotypes of P. aeruginosa isolates with clinical response to AZM in CF patients. Pre-treatment P. aeruginosa isolates from subjects randomized to AZM in the US trial were characterized for bacterial phenotypes: AZM minimal inhibitory concentration (MIC), mucoidy, and baseline and AZM effects on twitching and swimming motility, and production of pyocyanin, protease and phospholipase C (PLC). Initial analyses of a subset of subjects identified phenotypes most strongly associated with FEV(1) response and pulmonary exacerbation. These phenotypes were subsequently characterized and tested in isolates from subjects of the complete AZM cohort. Exploratory analyses of the initial subset suggested that the MIC and in vitro change in PLC and swimming motility with AZM were the strongest candidates among the bacterial phenotypes. When tested, only the change in PLC was significantly correlated with the change in FEV(1) (P=0.05), and occurrence and time to pulmonary exacerbation (both P=0.02). In the complete cohort, change in PLC continued to show significant correlation with FEV(1) response (P=0.006), but not exacerbation. The in vitro effect of AZM on PLC correlates with FEV(1) response to AZM. This suggests that AZM anti-virulence effects may be predictive of clinical response and play a role in the mechanism of action of AZM in CF patients.     

16株军团菌全基因组比较及重要毒力因子分析北大核心CSCD

目的运用生物信息学方法,分析军团菌参考菌株全基因组序列,了解军团菌基因组特征,比较不同菌株间的毒力因子、耐药基因以及种系发生,为进一步研究军团菌基因组流行病学、遗传进化提供数据。方法选择NCBI公共数据库中基因组序列完整、背景清楚、不同来源(临床、外环境)和不同种的代表性军团菌16株,运用开源基因组数据分析平台RAST、MAUVE、VFDB、CARD等进行生物信息学分析。结果16株参考菌株基因组大小在(3.13~4.23)Mbp,编码基因(CDSs)3018~3954个,划分为265~387个亚系统。同种军团菌基因组结构基本相同,非嗜肺军团菌(非LP)不同菌株之间基因组结构差异大,同时存在较多非保守区域。嗜肺军团菌血清I型(LP1)和米克戴德军团菌(L.micdadei)各属于1个种系进化主分枝,LP1细分为3个小分枝。临床分离的LP1携带的毒力因子类别和数量最多。菌株YU237缺失Dot/Icm T4BSS分泌系统,其余参考菌株都具有分泌系统的4种毒力因子,临床分离的LP1携带的T4BSS效应蛋白毒力因子数量为31~35个,其它参考菌株3~5个。16株参考菌株都携带有adeF耐药基因,部分菌株携带有LpeA、LpeB、FEZ-1耐药基因。结论不同种的军团菌基因组存在差异,而种内相对保守;相对于临床分离株,外环境非LP不同菌株基因组种间差异大。LP1军团菌存在遗传分化现象。毒力因子种类和数量跟LP1军团菌的致病力相关,特别是T4BSS效应蛋白。全基因组生物信息分析技术,可以对可能的耐药基因进行预测,为临床治疗提供辅助参考,同时可以解析病原菌的遗传和功能特征。     

信号系统在大鼠肺部铜绿假单胞菌感染中的作用及与致病因子的关系

目的研究密度感应信号系统(QS)在铜绿假单胞菌(PA)致大鼠肺部感染中的作用。方法将PA制成PA藻酸盐包裹体,建立大鼠肺部感染模型,SD大鼠258只,随机分为3组;比较野生型PA菌株PAO1及QS基因缺失的PA变异株PAO1-JP2大鼠肺部感染模型中细菌学、病理学的不同,从而评价2株菌在大鼠肺部感染中的致病性差异。用刚果红法测定该2株细菌弹性蛋白酶的活性;用免疫印迹法检测外毒素A的表达。结果感染后第14天、28天,PAO1-JP2肺组织匀浆菌落计数分别为(9．6±3．3)lgCFU／g及(4．2±3．1)lgCFU／g,显著低于PAO1组的(11．3±2．8)lgCFU／g及(9．1±1．5)lgCFU／g(P<0．05)。感染后第7、14及28天,PAO1-JP2感染组大鼠肺部病理变化指数及肺组织大体病理评分、肺部脓肿／肉芽肿的平均直径及肺组织显微病理评分均显著低于PAO1组。体外实验结果表明,PAO1-JP2菌株弹性蛋白酶活性(吸光度值)为0．35±0．03,高于PAO1菌株的0．02±0．00。免疫印迹实验可见PAO1株在相对分子质量73 000处显示棕色条带,PAO1-JP2株未见棕色条带。结论QS系统缺损后由于某些致病因子如弹性蛋白酶、外毒素不能表达,其导致的肺部感染程度有所减低。