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近年來(lái)，為了應(yīng)對(duì)“禁抗、限抗”給畜牧業(yè)帶來(lái)的挑戰(zhàn)，大量科研工作者致力于開(kāi)發(fā)新型抗生素替代物的研究。抗菌肽(antimicrobial peptides，AMPs)廣泛存在于動(dòng)植物和微生物等有機(jī)體中，是一類能夠幫助機(jī)體抵抗外界病原菌的小分子物質(zhì)[1]，同時(shí)對(duì)多種病原菌、病毒、腫瘤、寄生蟲(chóng)等均具有殺滅作用。其作用機(jī)制主要通過(guò)物理方式破壞細(xì)胞膜完整性, 從而發(fā)揮殺菌作用，且不易產(chǎn)生耐藥性[2-3]，有望成為抗生素替代物之一。有研究表明，在動(dòng)物飼糧中添加AMPs能夠抑制病菌繁殖，有效改善動(dòng)物腸道菌群結(jié)構(gòu)，提高生產(chǎn)性能[4]。大量的研究已經(jīng)表明，AMPs在畜牧業(yè)上的應(yīng)用必將為畜牧業(yè)的發(fā)展帶來(lái)新的推動(dòng)力。但由于天然AMPs穩(wěn)定性低、毒性高以及應(yīng)用成本較高，很少有成熟的抗菌肽產(chǎn)品應(yīng)用于畜牧業(yè)或臨床中[5-6]。因此，優(yōu)化天然抗菌肽序列，開(kāi)發(fā)具有抗菌、抗生物膜、抗酶解、抗炎、耐鹽等功能的抗菌肽成為新的研究熱點(diǎn)。近些年，研究者們提出了多種工程肽的設(shè)計(jì)思路，例如將天然抗菌肽作為模板進(jìn)行截短或雜合[7-11]；再者就是利用結(jié)構(gòu)功能關(guān)系或氨基酸間的作用力(例如π-π作用力、氫鍵等)來(lái)構(gòu)建具有特殊結(jié)構(gòu)和功能的抗菌肽[12-18]。
但是，隨著對(duì)抗菌藥物研究的深入，人們逐漸認(rèn)識(shí)到廣譜的抗菌藥物在發(fā)揮作用的同時(shí)也會(huì)對(duì)非致病菌群造成影響，致使有害菌群的大量定植，引起微生物菌群分布不平衡[19-21]，進(jìn)而引發(fā)動(dòng)物疾病，影響生產(chǎn)性能。同時(shí)，長(zhǎng)期飼喂廣譜抗菌藥物極有可能引發(fā)多種致病性微生物的抗藥性，使得藥物對(duì)這類細(xì)菌感染的預(yù)防和治療效果不佳，造成二次感染和藥物殘留，進(jìn)而危害人類健康。因此，高特異性的抗菌肽具有很大的開(kāi)發(fā)潛力。在以往的研究中發(fā)現(xiàn)乳酸菌素肽除了其殺傷結(jié)構(gòu)域外，還含有一段靶向結(jié)構(gòu)域，這增加了其對(duì)目標(biāo)細(xì)菌的識(shí)別和殺滅作用。Eckert等[22]基于這一發(fā)現(xiàn)，第一次提出靶向抗菌肽(specifically targeted antimicrobial peptide, STAMP)的概念，在廣譜抗菌肽中引入靶向肽段，將其轉(zhuǎn)化為能夠選擇性殺菌的“智能”化合物。這類“智能”化合物在維持微生物區(qū)系穩(wěn)態(tài)和組織細(xì)胞健康的同時(shí)極大限度地發(fā)揮著針對(duì)性抗菌作用。在減少藥物副作用，保護(hù)食品中的益生菌群，保持微生態(tài)平衡的基礎(chǔ)上實(shí)現(xiàn)靶向消除癌細(xì)胞、靶向殺滅病原菌，靶向識(shí)別感染部位的目的。為此，如何獲得高效安全的靶向抗菌肽成為了當(dāng)下抗菌肽開(kāi)發(fā)的重要研究課題，為使得抗菌肽同時(shí)具備抗菌和靶向的雙重功能，近年來(lái)研究者們從結(jié)構(gòu)功能關(guān)系和開(kāi)發(fā)天然抗菌肽等方向出發(fā)，賦予抗菌肽主動(dòng)識(shí)別目標(biāo)菌群的能力，或是利用微環(huán)境改變提高抗菌肽活性，設(shè)計(jì)出一系列具有靶向功能的工程肽，為后人開(kāi)發(fā)新型靶向抗菌肽奠定基礎(chǔ)。
1 融合型靶向抗菌肽的設(shè)計(jì)融合型靶向抗菌肽可分為三個(gè)功能區(qū)(如圖 1A)：靶向結(jié)構(gòu)域(targeting region)、連接結(jié)構(gòu)域(linker region)和殺菌結(jié)構(gòu)域(killing region)。靶向結(jié)構(gòu)域通常能夠在微生物菌群中特異性識(shí)別目標(biāo)菌群(如圖 1B、C)，一般不具有抗菌作用。連接結(jié)構(gòu)域通常由柔性氨基酸，如甘氨酸(glycine，G)和絲氨酸(serine，S)組成，連接兩個(gè)功能區(qū)，避免由于氨基酸的相互作用干擾兩個(gè)獨(dú)立的功能區(qū)發(fā)揮作用。殺菌結(jié)構(gòu)域通常是一條廣譜抗菌肽，例如大腸菌素等。
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A.融合型靶向抗菌肽的組成；B、C.靶向結(jié)構(gòu)域識(shí)別目標(biāo)菌群A.Composition of fusion-targeted antimicrobial peptides; B, C. Targeting region recognizes targeted bacteria圖 1 融合型靶向抗菌肽的結(jié)構(gòu)Fig. 1 Structure of fusion specially targeted antimicrobial peptides |
脂多糖(lipopolysaccharide, LPS)僅存在于革蘭陰性菌外層細(xì)胞膜中[23]。由于LPS中不飽和脂肪酸的存在，阻擋親脂類抗生素滲透進(jìn)入磷脂雙分子層中[24]，容易誘發(fā)對(duì)諸如利福平、克林霉素、紅霉素等親脂類抗生素的耐藥。同時(shí)Lázár等[25]通過(guò)利用集成方法測(cè)定臨床分離菌株對(duì)不同作用機(jī)制的抗菌肽的敏感性，證實(shí)外膜上LPS的含量也是影響陽(yáng)離子抗菌肽發(fā)揮作用的主要原因之一。因此LPS被認(rèn)為是設(shè)計(jì)抗革蘭陰性菌抗菌肽的理想靶標(biāo)。Muhle和Tam[26]構(gòu)建半胱氨酸β-折疊框架模擬LPS的結(jié)合位點(diǎn)，設(shè)計(jì)所得的抗菌肽序列及其衍生物對(duì)革蘭陰性細(xì)菌Escherichia coli(E. coil)具有顯著的殺菌效果(最小抑菌濃度20 nmol·L-1)，抗菌活性高于對(duì)革蘭陽(yáng)性細(xì)菌Staphylococcus aureus (S. aureus)選擇性200倍。同時(shí)利用LPS結(jié)合基序也能夠拓寬抗菌肽的抗菌譜。常見(jiàn)的與LPS有結(jié)合作用的蛋白主要有脂多糖結(jié)合蛋白(LPS-binding protein, LBP)、殺菌/通透性增加蛋白(bactericidal/permeability-increasing protein, BPI)、血清淀粉樣蛋白P(serum amyloid P component, SAP)及中性粒細(xì)胞分泌的抗菌蛋白等[27]，如表 1所示是LPS結(jié)合蛋白中包含的LPS結(jié)合肽段。Kim等[28]根據(jù)天然的LPS結(jié)合基序中氨基酸出現(xiàn)的頻率和位置構(gòu)建了LPS結(jié)合肽段，將其連接到抗菌肽GNU7的N端，結(jié)果發(fā)現(xiàn)LPS結(jié)合肽明顯提高了GNU7對(duì)革蘭陰性菌的殺滅活性，拓寬GNU7的抗菌譜。但由于LPS只存在于革蘭陰性細(xì)菌中，靶向LPS不適用于針對(duì)革蘭陽(yáng)性菌靶向藥物開(kāi)發(fā)。
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表 1 常見(jiàn)的LPS結(jié)合蛋白中LPS結(jié)合肽段的氨基酸序列[27-28]Table 1 Comparison of LPS-binding region of common LPS-binding protein[27-28] |
信息素是由多種生物分泌的信號(hào)分子。在細(xì)菌的世界里，它們不僅能夠檢測(cè)細(xì)胞密度并發(fā)出信號(hào)，還能夠與細(xì)胞膜上的受體蛋白結(jié)合進(jìn)行信號(hào)分子的傳遞、誘導(dǎo)或抑制信號(hào)分子傳遞到細(xì)胞內(nèi)，調(diào)節(jié)細(xì)菌基因的表達(dá)、細(xì)菌內(nèi)酶的釋放或毒力因子的產(chǎn)生，調(diào)控微生物群體的生理過(guò)程，例如生物膜的形成[29-32]。與分泌高絲氨酸內(nèi)酯的革蘭陰性菌不同，革蘭陽(yáng)性菌分泌的信息素多是寡肽。為此，將信息素作為抗菌肽的“靶向”識(shí)別區(qū)域設(shè)計(jì)針對(duì)革蘭陽(yáng)性菌的抗菌藥物具有廣闊的應(yīng)用前景。Qiu等[33]以大腸桿菌素(能夠通過(guò)破壞細(xì)胞膜磷脂雙分子層來(lái)殺滅大腸桿菌及相關(guān)種屬細(xì)菌)作為殺菌結(jié)構(gòu)域，以金黃色葡萄球菌信息素AgrD1作為靶向結(jié)構(gòu)域，信息素與目標(biāo)菌特異性受體識(shí)別后將細(xì)菌素靶向投遞至目標(biāo)菌群中，發(fā)揮選擇性殺菌的作用。隨后Qiu等[34]又將大腸桿菌素與腸球菌分泌的包含7個(gè)氨基酸殘基的信息素cCf 10進(jìn)行融合，成功制備了一種針對(duì)耐萬(wàn)古霉素糞腸球菌融合靶向抗菌肽。Mao等[35]同樣利用金黃色葡萄球菌信息素AgrD1作為靶向區(qū)域，與廣譜抗菌肽plectasin進(jìn)行融合，構(gòu)建針對(duì)耐甲氧西林金黃色葡萄球菌(methicillin-resistant Staphylococcus aureus, MRSA)的靶向抗菌肽。細(xì)菌分泌的信息素是一類理想的靶向結(jié)構(gòu)域，將其與廣譜抗菌肽融合能夠?qū)崿F(xiàn)對(duì)不同細(xì)菌靶向殺滅的目的。但革蘭陰性菌分泌的高絲氨酸內(nèi)酯構(gòu)建靶向抗菌肽具有合成成本高、合成過(guò)程復(fù)雜等缺點(diǎn)，因此這類方法只適用于設(shè)計(jì)靶向革蘭陽(yáng)性菌的抗菌肽。
1.3 抗體型抗菌肽的設(shè)計(jì)抗體類藥物是靶向型抗菌藥物中的一種，這種藥物一直以靶向性和高效性著稱，靶向細(xì)菌表面侵襲力物質(zhì)的抗體藥物主要是調(diào)動(dòng)宿主免疫系統(tǒng)的補(bǔ)體固定和調(diào)理吞噬殺傷(opsonophagocytic killing，OPK)來(lái)清除致病菌[36]。例如靶向金黃色葡萄球菌表面抗原的抗體類藥物在金黃色葡萄球菌和表皮葡萄球菌細(xì)胞壁錨定蛋白的IgG1(rF1)和靶向金黃色葡萄球菌自溶酶亞基氨基葡萄糖苷酶的單抗的基礎(chǔ)上，設(shè)計(jì)的靶向金黃色葡萄球菌抗體藥物[37]；Horn等[38]則開(kāi)發(fā)了一種靶向銅綠假單胞菌表面抗原的抗體藥物，其靶點(diǎn)是O11血清型銅綠假單胞菌脂多糖。該單抗在體外表現(xiàn)出OPK活性并能夠減少細(xì)菌感染數(shù)量，增加感染模型中動(dòng)物的存活率。Touti等[39]利用靶向識(shí)別的單克隆抗體與抗菌肽相結(jié)合，設(shè)計(jì)合成了一種靶向E. coil的抗體-抗菌肽結(jié)合物，實(shí)現(xiàn)了靶向與殺菌功能的結(jié)合。但由于這種蛋白類藥物的生產(chǎn)成本高，化學(xué)合成效率低以及穩(wěn)定性低等問(wèn)題，使其很難應(yīng)用于生產(chǎn)中。Eckert等[22]則對(duì)單克隆抗體肽段進(jìn)行篩選，篩選出靶向銅綠假單胞菌(Pseudomonas spp)的短肽，將其與廣譜抗菌肽novispirin G10結(jié)合，設(shè)計(jì)出靶向銅綠假單胞菌的小分子肽類藥物。雖然現(xiàn)有的抗菌抗體藥物還有一定的限制條件，例如蛋白類藥物容易引起機(jī)體免疫原性反應(yīng)，但是抗菌抗體藥物可能在不久的將來(lái)改變預(yù)防和治療細(xì)菌感染的整體格局。
1.4 噬菌體肽庫(kù)篩選型抗菌肽的設(shè)計(jì)噬菌體展示技術(shù)(phage display)是將外源基因整合到噬菌體核酸中，表達(dá)的外源蛋白質(zhì)與噬菌體的衣殼蛋白融合，再利用噬菌體篩選技術(shù)將具有特異性識(shí)別功能的多肽篩選出來(lái)[40]。噬菌體展示技術(shù)已被廣泛應(yīng)用于蛋白質(zhì)的研究中，用于開(kāi)發(fā)多肽和蛋白質(zhì)藥物，利用蛋白質(zhì)之間的相互作用篩選具有靶向功能的多肽等[41]。Anandakumar等[42]構(gòu)建隨機(jī)肽噬菌體庫(kù)，篩選特異性靶向白色念珠菌(Candida albicans，C. albicans)的肽段。McCarthy等[43]突破了噬菌體展示技術(shù)只能呈現(xiàn)由天然氨基酸組成的肽的限制，在噬菌體展示多肽中引入2-乙酰苯基硼酸，誘導(dǎo)肽段與細(xì)菌細(xì)胞表面的動(dòng)態(tài)共價(jià)結(jié)合，篩選對(duì)臨床耐藥金黃色葡萄球菌和鮑曼不動(dòng)桿菌具有高度特異性的結(jié)合序列。因此，噬菌體展示技術(shù)可以用于設(shè)計(jì)靶向抗菌肽，但這類抗菌肽的靶向機(jī)制尚不明確。
融合型靶向抗菌肽的設(shè)計(jì)思路是將靶向肽段與殺菌肽段進(jìn)行融合，使其各自發(fā)揮功能，這類抗菌肽一般肽鏈相對(duì)較長(zhǎng)，成本較高。
2 根據(jù)抗菌肽的結(jié)構(gòu)功能參數(shù)設(shè)計(jì)靶向抗菌肽大多數(shù)靶向抗菌肽通常是在廣譜抗菌肽的N端或C端引入一段具有靶向功能的肽段，通過(guò)靶向肽段與目標(biāo)菌受體蛋白識(shí)別，將殺菌肽段投遞至目標(biāo)菌細(xì)胞膜表面發(fā)揮靶向抗菌作用。但氨基酸之間的相互作用可能會(huì)改變肽鏈的結(jié)構(gòu)，使得靶向功能或者殺菌功能減弱甚至喪失。因此，研究者們開(kāi)始從抗菌肽的結(jié)構(gòu)功能關(guān)系出發(fā)設(shè)計(jì)靶向抗菌肽。
抗菌肽的結(jié)構(gòu)功能參數(shù)(包括兩親性、疏水性、正電荷數(shù)、二級(jí)結(jié)構(gòu)以及氨基酸的組成)對(duì)抗菌肽的生物學(xué)活性起決定性的作用。Mishra等[44]對(duì)抗菌肽數(shù)據(jù)庫(kù)(the Antimicrobial Peptide Database，APD)中3 014條天然抗菌肽序列進(jìn)行比對(duì)后發(fā)現(xiàn)，靶向革蘭陽(yáng)性菌的抗菌肽一般具有低陽(yáng)離子性和高疏水性的特性，而靶向革蘭陰性菌的抗菌肽一般是具有低疏水性、高陽(yáng)離子性的特性，部分序列還富含脯氨酸。因此，調(diào)整抗菌肽的結(jié)構(gòu)功能參數(shù)能夠直接影響抗菌肽的抗菌譜。Zhu等[45]通過(guò)對(duì)螺旋肽RI16的疏水面中心位置氨基酸進(jìn)行替換，獲得具有靶向銅綠假單胞菌的抗菌肽序列；Wang等[13]用柔性配對(duì)的甘氨酸(glycine，Gly)替代序列中的丙氨酸(alanine，Ala)，也獲得了對(duì)革蘭陰性菌具有較好殺傷效果的窄譜肽。Shang等[46]用色氨酸(tryptophan, Trp)替代蛙皮膚抗菌肽temporin-1CE中不同位置的疏水性氨基酸，結(jié)果表明Trp位置改變能夠影響抗菌肽與LPS的結(jié)合能力，影響抗菌譜。
上述方法通過(guò)單個(gè)氨基酸的替換實(shí)現(xiàn)了靶向的目的，但這類靶向抗菌肽的設(shè)計(jì)方法并不系統(tǒng)。Mishra和Wang[47]利用APD更加系統(tǒng)地分析天然抗菌肽庫(kù)中抗金黃色葡萄球菌抗菌肽的基本結(jié)構(gòu)參數(shù)，根據(jù)肽鏈長(zhǎng)度、總電荷數(shù)和疏水性等關(guān)鍵參數(shù)進(jìn)行分類，篩選出概率最大的參數(shù)值，根據(jù)這些參數(shù)進(jìn)行靶向金黃色葡萄球菌抗菌肽的設(shè)計(jì)。Chou等[48]將APD篩選技術(shù)與抗菌肽模板相結(jié)合，利用APD篩選出的氨基酸，根據(jù)對(duì)稱結(jié)構(gòu)原則進(jìn)行排列，同時(shí)引入能夠與細(xì)菌細(xì)胞膜發(fā)生強(qiáng)烈相互作用的Trp，成功構(gòu)建一系列靶向革蘭陰性菌的窄譜肽。抗菌肽庫(kù)篩選策略保留了天然抗菌肽中殺滅某種特定菌株應(yīng)具備的結(jié)構(gòu)參數(shù)，更有針對(duì)性地進(jìn)行正電荷和疏水性的排布，與其他設(shè)計(jì)方法相比成功的概率較大(圖 2)。
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圖 2 數(shù)據(jù)庫(kù)篩選技術(shù)Fig. 2 Database filtering technology |
酶觸前體藥物活性釋放系統(tǒng)(directed enzyme prodrug therapy, DEPT)是在靶細(xì)胞或是靶器官中利用酶活性局部激活前體藥物，釋放藥物活性[49]。Li等[50]研制了一種新型細(xì)菌感染診斷探針(PPA-PLGVRG-Van)，包括信號(hào)分子(pyropheophorbide-α, PPA)、酶應(yīng)答肽連接體(明膠酶酶切位點(diǎn)PLGVRG)和靶向配體(vancomycin, Van)，Van與革蘭陽(yáng)性菌細(xì)胞壁通過(guò)多重氫鍵進(jìn)行結(jié)合，陽(yáng)性菌過(guò)度表達(dá)明膠酶選擇性地切斷PLGVRG連接體，減少了信號(hào)分子PPA分子的空間位阻，引發(fā)疏水結(jié)構(gòu)域PPA自組裝，超分子聚集體能有效地增強(qiáng)光聲信號(hào)，以實(shí)現(xiàn)對(duì)細(xì)菌感染部位的實(shí)時(shí)成像，準(zhǔn)確識(shí)別并判斷細(xì)菌感染部位。Wang等[51]將明膠酶識(shí)別序列作為連接體，連接抗菌肽HHC36生物相容性肽RGD，構(gòu)建具有細(xì)菌選擇性的生物材料，這種設(shè)計(jì)策略降低了HHC36的細(xì)胞毒性，同時(shí)賦予其生物選擇性，為今后制備智能多功能材料提供理論基礎(chǔ)(圖 3)。Qi等[52]同樣利用明膠酶識(shí)別序列開(kāi)發(fā)了一種殼聚糖肽。在細(xì)菌感染微環(huán)境中，陽(yáng)性菌分泌明膠酶切斷明膠酶識(shí)別位點(diǎn)，剝離聚乙二醇(polyethylene glycol, PEG)保護(hù)層，破壞疏水/親水平衡。通過(guò)殼聚糖的鏈相互作用，自發(fā)地促進(jìn)納米粒子重組為纖維結(jié)構(gòu)發(fā)揮抗菌作用。利用酶觸反應(yīng)激發(fā)抗菌肽分子抗菌活性是設(shè)計(jì)靶向抗菌肽的新型策略，其有效地克服了傳統(tǒng)靶向抗菌肽設(shè)計(jì)方法的弊端，同時(shí)這種策略也可以有效地應(yīng)用于設(shè)計(jì)檢測(cè)細(xì)菌感染等新型高分子材料中，具有不錯(cuò)的開(kāi)發(fā)前景。
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A.蛋白酶陰性菌未分泌蛋白酶，前體肽中和活性中心的抗菌活性；B.蛋白酶陽(yáng)性菌分泌蛋白酶，酶切前體肽，活性中心暴露發(fā)揮抗菌作用A.No protease was secreted by protease-negative bacteria, and pre-peptides decrease the antibacterial activity of active center; B. Protease-positive bacteria secrete proteases, which cleave pre-peptides, and the peptide shows the targeted antimicrobial activity圖 3 蛋白酶激活性抗菌肽的作用機(jī)制[51]Fig. 3 Mechanism of protease-activated AMPs[51] |
另一個(gè)能夠影響抗菌肽活性的環(huán)境因素是pH。機(jī)體內(nèi)存在許多酸性環(huán)境，例如口腔、陰道、創(chuàng)傷口等。pH的改變能夠引起抗菌肽結(jié)構(gòu)發(fā)生改變從而影響抗菌活性。Xiong等[53]將谷氨酸(陰離子基)和叔胺(陽(yáng)離子基)交替排列，利用酸性環(huán)境下谷氨酸質(zhì)子化的特性，使得帶有正電荷的叔胺在胃中酸性環(huán)境下聚合形成α螺旋，殺滅幽門(mén)螺旋桿菌(Helicobacter pylori，H. pylori)，克服了臨床中質(zhì)子泵抑制劑(proton pump inhibitor, PPI)與抗生素聯(lián)合治療帶來(lái)的副作用。Song等[54]同樣利用谷氨酸(glutamate, Glu)在酸性環(huán)境下質(zhì)子化的特性打破Glu和賴氨酸(lysine, Lys)之間的靜電吸引靶向殺滅腫瘤細(xì)胞。這種pH靶向策略不同于傳統(tǒng)的腫瘤治療藥物，能夠大大降低對(duì)正常組織細(xì)胞的毒副作用。另外，富含組氨酸(histidine, His)的抗菌肽在低pH環(huán)境中也能表現(xiàn)出良好的抗菌活性。His是一種堿性氨基酸，但由于其等電點(diǎn)較低，正電性小于賴氨酸和精氨酸。通常在酸性環(huán)境下才表現(xiàn)出正電性。自然界中存在許多富含His的小肽類物質(zhì)，如凝血酶衍生的C-末端肽，能夠在皮膚局部酸性炎癥環(huán)境中抑制細(xì)菌生長(zhǎng)，加速傷口愈合[55]。一些研究者們認(rèn)為低pH環(huán)境中組氨酸肽正電性增加，增加其與陰離子細(xì)菌表面之間靜電吸引力，實(shí)現(xiàn)pH調(diào)控活性的目的(圖 4)[56]。然而，Li等[57]用苯丙氨酸(phenylalanine, Phe)和His設(shè)計(jì)了一系列酸激活的抗菌肽(acid-activated peptides, AAPs)，結(jié)果發(fā)現(xiàn)，低pH環(huán)境下組氨酸肽抗菌活性增強(qiáng)不僅僅是由于細(xì)菌細(xì)胞膜的靜電吸引力的增強(qiáng)，而是由于干擾質(zhì)子轉(zhuǎn)運(yùn)蛋白而發(fā)揮抗菌作用。Dennison等[58]和Lai等[59]用His和異亮氨酸(isoleucine, Ile)開(kāi)發(fā)了一系列抗酶解活性肽，結(jié)果發(fā)現(xiàn)在酸性環(huán)境中靶向革蘭陰性細(xì)菌，他們推測(cè)這種靶向機(jī)制可能是由于革蘭陽(yáng)性菌膜上的pH依賴性變化引起的。
4 靶向抗菌肽的應(yīng)用靶向抗菌肽在延續(xù)了廣譜抗菌肽低耐藥性的同時(shí)增強(qiáng)了抗細(xì)菌感染的靶向性，使得靶向抗菌肽在臨床和畜牧生產(chǎn)中表現(xiàn)出了巨大的應(yīng)用前景。眾所周知，現(xiàn)階段通常依靠殺菌漱口水和口服廣譜抗生素來(lái)治療齲齒，但是廣譜的抗菌藥物能夠破壞整個(gè)口腔細(xì)菌微生態(tài)平衡，使得變異鏈球菌(Streptococcus mutans，S. mutan)和共生細(xì)菌之間平等競(jìng)爭(zhēng)，導(dǎo)致S. mutan在牙齒表面大量定植，感染口腔，形成齲齒。因此，開(kāi)發(fā)針對(duì)S. mutan的靶向抗菌藥物能夠在消除S. mutan的同時(shí)建立健康的口腔生物系統(tǒng)，能夠長(zhǎng)期有效地保護(hù)牙齒免受齲齒威脅[19]。在畜牧生產(chǎn)中，哺乳動(dòng)物的腸道內(nèi)存在著大量以細(xì)菌為主的共生微生物群落。這些腸道菌群能夠促進(jìn)腸道組織發(fā)育，如雙歧桿菌、乳酸菌等有益菌合成動(dòng)物機(jī)體生長(zhǎng)發(fā)育所必需的維生素；促進(jìn)動(dòng)物機(jī)體對(duì)多種離子的吸收；為宿主提供多種動(dòng)物機(jī)體自身不具備的酶與生化反應(yīng)的底物；分解動(dòng)物機(jī)體不易消化的多糖、寡聚糖以及糖蛋白質(zhì)等物質(zhì)[60-62]。但畜牧生產(chǎn)中抗生素的濫用，使得腸道內(nèi)對(duì)藥物敏感的有益菌群受到不同程度的威脅。另外，當(dāng)腸道正常菌群受到抗生素的影響后，天然免疫中的重要模式識(shí)別受體Toll樣受體基因表達(dá)水平下降，其下游的適應(yīng)性免疫功能降低，影響外周免疫器官的正常功能，從而導(dǎo)致機(jī)體免疫功能下降[63]。同時(shí)母體抗生素的使用也會(huì)破壞子代建立正常腸道菌群，造成腸道微生態(tài)紊亂，菌群失衡。
開(kāi)發(fā)靶向抗菌肽替代抗生素作為飼料添加劑，能夠在維持體內(nèi)菌群穩(wěn)態(tài)的同時(shí)有效發(fā)揮抗菌作用，提高生產(chǎn)性能?！熬珳?zhǔn)”靶向已經(jīng)為疾病治療和藥物開(kāi)發(fā)提供新的思路。而多肽類藥物為靶向抗菌藥物的研究與開(kāi)發(fā)提供了新的起點(diǎn)。靶向抗菌肽在未來(lái)有望成為藥物的有效傳遞載體，例如用于鑒定細(xì)菌感染或是靶向消除腫瘤的藥物，促進(jìn)藥物濃集于靶器官、靶組織、靶細(xì)胞或細(xì)胞內(nèi)結(jié)構(gòu)，避免藥物在非特異性健康組織中積聚，實(shí)現(xiàn)靶向治療、靶向殺滅的目的[64]。本文總結(jié)了近年來(lái)靶向抗菌藥物的設(shè)計(jì)思路，以期在更多疾病的靶向治療中發(fā)揮更大的作用，為將來(lái)靶向抗菌肽的開(kāi)發(fā)與設(shè)計(jì)提供思路。
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