浮游态和生物被膜态致病菌的光动力灭菌研究进展

李栋辉1,石玉刚1,2*

1 浙江工商大学食品与生物工程学院 杭州310035 2 浙江工商大学生物技术研究所 杭州 310035)

摘要 光动力灭菌是一种新兴的致病菌杀灭技术。它通过光源激发光敏剂,产生活性氧自由基,多靶点作用于细菌细胞膜和DNA,造成氧化损伤并最终导致细菌死亡。传统灭菌方式在应用于食品工业时面临诸多问题,如导致耐药性细菌的出现,难以杀灭生物被膜态细菌,造成食品营养和风味流失,应用成本过高等。光动力灭菌作为非热杀菌技术,不会引发食品的感官变化和营养流失,所需设备简单,不易引发细菌耐药性,并且对细菌(浮游态和生物被膜态)、真菌、甚至孢子都有较好的杀灭作用,近年来受到广泛关注。本文概述光动力灭菌的原理和传统杀菌方法的局限性,重点阐述光动力灭菌在杀灭浮游态细菌和生物被膜态细菌中的应用和研究进展。

关键词 光动力灭菌;生物被膜;光敏剂;食源性致病菌;食品工业

据世界卫生组织报道,有害微生物污染是食源性疾病的主要原因,全球每年约有6 亿人因食源性危害患病,42 万人死亡(包括12.5 万名儿童),造成的生产力和医疗费用损失达1 100 亿美元[1]。对食品采取适当的灭菌处理,可有效防止食源性致病菌及其产生的毒素对人体的伤害[2]。在食品加工运输过程中,有害微生物主要以浮游态(Planktonic bacteria)和生物被膜态(Biofilm)污染食品。生物被膜通常表现出比浮游细菌更强的环境适应能力。65%的微生物感染与生物被膜有关,尤其在涉及机械设备的领域[3]

光动力灭菌(Photodynamic inactivation,PDI)作为一种新兴的非热杀菌技术,具备安全高效、能耗低廉,对有害菌的选择性高,不易引发细菌耐药性,对食品原有风味影响小等诸多优点。PDI 通过光源激发光敏剂(Photosensitizer,PS),产生活性氧自由基(Reactive oxygen species,ROS)以达到杀灭微生物(如细菌、真菌、病毒等)的目的[4]。通过选择合适的光敏剂和光源,PDI 对多种细菌及其形成的生物被膜具有显著的杀灭作用,展现出广谱且高效的杀菌能力[3,5]

相比于热杀菌方法,PDI 不依赖高温,在节约能源、提高经济效益的同时,还能较好地保留食品本身的风味和营养。相比于化学杀菌剂(如抗生素),PDI 通过多靶点作用于微生物,不易产生耐药性细菌[4]。PDI 的优势在耐药性细菌中威胁凸显,因新型抗生素研发难度加大,故其在全民追求食品大健康的背景下具有巨大的吸引力。本文概述PDI 的原理,比较PDI 与其它传统抑菌剂的优劣,整理不同光敏剂介导的PDI 对浮游细菌和生物被膜态细菌的杀灭效果。

1 光动力灭菌技术

1.1 PDI 作用机理

PDI 分为3 个部分:氧、光源和光敏剂(PS),PS 在光源照射下将氧转化为ROS,ROS 包括单线态氧(1O2)、羟基自由基(·OH)、过氧化氢(H2O2)和超氧阴离子(O2-·)等。在光照下,处于基态(S0)的PS(0ps)具有稳定的电子结构,2 个电子自选相反。具有最高能量的电子吸收特定能量的光量子后,从最高占据分子轨道(Highest occupied molecular orbital,HOMO)跃迁至最低未占据分子轨道(Lowest unoccupied molecular orbital,LUMO)。PS 分子进入激发单重态(S1),形成单重激发态PS 分子(1ps*)[6]。部分1ps* 通过自旋轨道耦合激发至激发三重态(T1),形成三重激发态PS 分子(3ps*),这一过程称为系间窜越(Intersystem crossing,ISC),ISC过程和3ps*的形成是物质具备光敏特性的深层原因。1ps*和3ps*都可衰变回S0,而3ps* 寿命更长,得以参加各种化学反应[7-9]

3ps* 通过两种机制与氧反应:1)Type Ⅰ型反应的特征是电子转移,指3ps*上的1 个电子或氢离子转移至底物(如细胞膜或小分子),产生自由基阴离子或阳离子。例如,3ps*上的1 个电子转移到氧分子上形成O2-·,O2-·可以将细胞内的三价铁还原为二价铁,还可以被超氧化物歧化酶催化或通过第2 次电子转移形成H2O2。H2O2 对细胞膜的亲和性很强,能够进入细胞膜并在Fe2+的催化下发生芬顿反应,产生·OH。·OH 具有极高的标准还原电位(2.31 V),具有强大的氧化性,能够与邻近的化合物或细胞组分(包括有机污染物、生物分子等)发生非选择性反应[10]。2)Type Ⅱ型反应的特征是能量转移,指3ps*直接将能量传递给三重态氧分子(3O2),形成高活性的1O2[9]。除了Type Ⅰ型反应和Type Ⅱ型反应外,还有一些文献提到了Type III 型光化学机制。这些光敏反应与氧无关,如四环素也可能与核糖体蛋白之间形成光活化共价交联,阻止tRNAs 与氨基酰基mRNA 复合物结合,从而抑制蛋白质合成[11]。图1 展示了PDI 的作用机制。

图1 光动力技术作用机制
Fig.1 Mechanisms of photodynamic inactivation

Type Ⅰ型和Ⅱ型反应可以同时发生,主要受PS 特性、氧浓度以及PS 与底物亲和力的影响。一般情况下,低氧水平更有利于Type I 型反应,而Type II 型反应则需要较高的氧浓度。PDI 产生的ROS 可以同时作用于多种靶向生物分子位点,包括:1)与质膜上的脂质结合并破坏膜转运系统,导致细胞内容物泄露;2)使胞内酶(蛋白)失活,干扰细胞代谢;3)导致DNA 的氧化损伤,阻碍转录翻译等过程[12-13]

1.2 光敏剂

光敏剂(PS)是一种能吸收光能并将其传递给相邻分子的物质,最初被应用于医学领域[14]。理想的PS 应该具备以下优点:1)有较高的吸收效率;2)由T1 转移至S0 时释放合适的能量;3)有较高的3ps* 产率和较长的寿命;4)毒性低,可从人体内自然排除;5)获取方便,成本低[15-16]

至今,PS 的发展已至三代,第1 代PS 是血卟啉及其衍生物(Hematoporphyrin derivative,HpD)。1841 年,Scherer 通过去除血液中的铁离子后复溶于水而获得HpD。随后,卟吩姆钠(photofrinR)作为第1 种PS,于1990 年代被美国食品药品管理局批准使用[15]。HpD 在医疗领域得到广泛的应用,然而存在纯度低,靶向性差,对正常人体细胞具有较高毒性,光吸收强度低等缺陷而不适合食品领域[15]。第2 代PS 主要包括卟啉类、醌类等,具有结构明确、纯度高的特点。而第2 代PS 仍存在水溶性差,应用场景局限,生物利用率低等弊端,且存在潜在的安全问题[17]。为了减少PS 对健康细胞的损伤,提高PS 的靶向性、水溶性和生物利用度,第3 代PS 的开发成为近年来研究的热门领域[18-19]。其中,天然光敏剂(如核黄素[18,20]、金丝桃素[21]、姜黄素[22],酚酸类化合物[23-24]等),因其来源可靠,毒副作用低,光敏效率高等特点,最有潜力应用于食品领域。

核黄素,即维生素B2,可以从尿液中排出,存在于多种生物中。核黄素在UVA(~360 nm)和蓝光波段(~440 nm)有较高吸收峰,核黄素介导的PDI 具有广谱抑菌特性,对耐药性细菌同样有效[18,20]。金丝桃素是从贯叶金丝桃中提取的天然色素,具有抗菌、抗病毒等特性,在波长600 nm 处有较强的吸收峰,结合PDI 后对真菌具有显著的抑制作用[21]。姜黄素,主要从姜黄根茎中分离得到,具有广泛的抗病毒、抗炎、抗肿瘤、抗菌等生物活性,吸收波长主要在蓝光波段(405~435 nm)。姜黄素主要通过Tpye Ⅱ型反应生成1O2,破坏细胞膜,最终杀灭细菌[22]。酚酸及其酯类化合物广泛存在于自然界的各种植物中,本课题组长期研究其抑菌活性和抑菌机制,主要通过Type Ⅰ型反应形成·OH,对浮游细菌,已形成的和形成中的生物被膜都有显著的抑制作用[23-25]。表1 综述了目前广泛研究的各种PS,图2 列出了部分第3 代光敏剂的化学结构。

表1 光敏剂分类
Table 1 Classification of photosensitizers

图2 部分天然提取物光敏剂的结构式
Fig.2 Structural formula of partial natural extract photosensitizer

1.3 光源

根据光敏剂(PS)的吸收光谱选择合适的激发光源,理想的光源应在PS 的最大吸收波长区域提供辐射,PS 的最大吸收波长取决于化学结构和电子特征[30]。目前在PDI 技术中应用的光源主要是白炽灯、LED 灯和激光灯。LED 灯作为PDI 光源,具有驱动电压低,不存在重金属等有害物质,发射带窄,光谱成分纯净,从紫外到红外的宽发射范围等优点[31],在PDI 技术中得到了广泛的应用。表2归纳了目前应用于PDI 技术的部分光源。

表2 光源的分类
Table 2 Classification of light source

2 传统抑菌剂的局限性

长期以来,抗生素被广泛应用于食品、药品和饲料等领域,其大规模使用导致耐药性细菌的出现,带来了一场席卷全球的卫生危机[34]。在可预见的未来,随着抗生素数量和种类的增加,耐药性食源性细菌的数量也将不断增加,造成这种情况的原因有很多,如亚致死剂量的抗生素对细菌造成的选择压力等[35-36]

除了抗生素,传统化学抑菌剂还包括氯基消毒剂、H2O2、O3 和过氧乙酸等。氯基消毒剂曾被广泛应用,而进一步研究发现其可与有机分子反应,形成致癌和致突变的副产物(如三氯甲烷、卤代乙酸等)[37]。H2O2 具有环境友好(分解产物为H2O 和O2),广谱抑菌的优点,且可通过复配提高抑菌活性[38]。Ding等[39]发现H2O2 会造成桃子的褐变,导致外观及品质损失。同时,Philip等[40]和Paula等[41]的研究都显示单独H2O2 对生物被膜态细菌的杀灭作用十分有限。O3 作为强氧化剂,低浓度即具有显著的抗菌能力。而O3 是不稳定的,Khadre等[42]发现O3 在20 ℃的半衰期约20 min,将温度提升至25 ℃后半衰期缩短至2~4 min。同时,O3 极易与食物发生反应,导致感官劣变和营养流失。Keutgen等[43]发现O3 处理后的草莓出现颜色损失。此外,Swami等[44]发现O3 处理后苹果的维生素C 含量降低了约83%。过氧乙酸常用于消毒食品接触表面和工艺用水[45],分解产物为氧和乙酸,价格昂贵且乙酸会增加废水的有机物含量,进而提高污水处理的难度[46]

与传统化学抑菌剂对比,PDI 具有诸多显著优势:1)设备要求低;2)作用靶点多样,不易产生耐药性;3)不会影响食品自身风味,多种PS 自身还具有营养功能;4)通过对PS 的改性或制备负载PS 的材料,PDI 具有广泛的应用场景[47]。表3 总结了传统化学抑菌剂及PDI 的优缺点。

表3 传统化学抑菌剂和PDI 的优缺点
Table 3 Advantages and disadvantages of traditional chemical sterilizers and PDI

3 浮游菌的光动力灭菌研究

PDI 具有广谱的抑菌活性,可有效对抗革兰氏阳性菌(G+)和革兰氏阴性菌(G-),甚至对孢子也有效果[48]。由于细胞壁组成差异,PDI 对革兰氏阳性菌的杀灭作用通常更加明显。革兰氏阳性菌细胞壁由肽聚糖和磷壁酸组成,疏松多孔的细胞壁包围着细胞质膜,PS 可以轻松穿过菌膜并与胞内靶点结合。革兰氏阴性菌特有的外膜形成了物理和功能性屏障,带负电的脂多糖抑制中性或带负电的PS 向细胞内的渗透,阳离子的PS 则可能会与革兰氏阴性菌带负电荷的表面相互作用并最终失活,这使得大部分的PS 作用效率低下[49-51]。两者菌膜的差别见图3。

图3 革兰氏阴性菌膜和革兰氏阳性菌膜对比
Fig.3 Gram-negative and gram-positive bacterial membrane comparison

PDI 对于杀灭浮游菌的有效性已经在体外和食品基质上进行了充分评估,多种PS 除了直接应用于浮游菌,包含PS 的可食用薄膜、涂层以及包装材料都表现出对食源性致病菌的杀灭作用,表4 归纳了PDI 技术对浮游状态的食源性致病菌的抗菌活性的主要研究。Brovko等[52]报道孟加拉红、荧光桃红、中性吖啶黄和孔雀石绿对5 种微生物的PDI 作用。PDI 效果排序如下:单增李斯特菌(G+)>芽孢杆菌(G+)>沙门氏菌(G-)>大肠杆菌(G-)>酿酒酵母。值得注意的是,革兰氏阴性菌表现出更强的抵抗力,并且高浓度的孟加拉红在黑暗条件下也显示出一定的抑菌作用,而辐照增强了孟加拉红的抑菌活性。Penha等[53]研究姜黄素对4 种食源性细菌的PDI 效果,20 min 即可完全灭活嗜水气单胞菌,30 min 可完全灭活大肠杆菌和金黄色葡萄球菌。PDI 效果排序如下:嗜水气单胞菌(G-)>金黄色葡萄球菌(G+)>大肠杆菌(G-)>沙门氏菌(G-)>铜绿假单胞菌(G-)。Yassunaka等[54]研究赤藓红及其衍生物的抑菌效果,PDI 效果排序如下:金黄色葡萄球菌(G+)>嗜水气单胞菌(G-)>大肠杆菌(G-)>沙门氏菌(G-)>铜绿假单胞菌(G-)。铜绿假单胞菌(G-)显示出对PDI 普遍的抵抗能力,而嗜水气单胞菌(G-)普遍较易杀灭,体现不同细菌对PDI 的抵抗能力展现出较大差异。此外,赤藓红酯类衍生物比赤藓红具备更好的PDI 效果,本组在研究没食子酸及其酯类衍生物时得到类似的结果。

本课题组报道没食子酸及其酯类衍生物对金黄色葡萄球菌(G+)和大肠杆菌(G-)的光动力灭菌效果,OG 表现出最佳的PDI 活性。0.15 mmol/L OG 在UV-A【(8.254±0.18)mW/cm2】下照射5 min便显示出对金黄色葡萄球菌超过5 lg(CFU/mL)的消杀能力[25]。此外,OG 与环糊精形成的分子包合物通过静电纺丝与PDI 技术结合,并应用于中国大鲵的保鲜[24]

相似的,将PS 与其它材料结合并应用于食品包装材料的报道还有很多。Nikola[55]用环糊精包封金丝桃素,结合黄光LED(589 nm,100 mW)作用于腐生葡萄球菌(G+),研究显示,2.5 μmol/L 和5 μmol/L 的金丝桃素包合物分别显示出3.8 和4.1 lg(CFU/mL)的消杀。Su等[56]报道了一种包含核黄素的壳聚糖薄膜,在蓝光下【LED,(455±5)nm,1.9 mW/cm2】 照射2 h,可以完全灭活单增李斯特菌(G+)、副溶血弧菌(G-)和希瓦氏菌(G-),其中含1%核黄素的薄膜只需照射1.5 h 即可完成对副溶血弧菌(G-)的完全杀灭。

4 生物被膜的光动力灭菌研究

生物被膜(Biofilm)是由细菌群落分泌细胞外聚合物形成的独立的三维结构,细菌嵌入基质中并表达特定基因。生物被膜态细菌表现出许多与浮游细菌不同的特征,具备更强的耐受能力,原因主要是:1)基质造成物理阻隔,阻止抗菌剂渗透;2)细菌分泌多种抗生素降解酶,分解抗菌剂;3)多种细菌通过群体感应,增强整体耐受能力。因此,生物被膜态细菌表现出对多种抗菌剂的免疫能力,对营养物质缺乏和环境因素(如温度、pH 值)的耐受能力,对感染者免疫系统的抵抗能力[57-58]。生物被膜对食品工业的威胁来自各个方面,包括食品基质、设备表面(如传送带和管道等)、包装材料表面(如玻璃、聚苯乙烯等)。由于漫长的加工周期、复杂的工厂设施、丰富的营养物质和不彻底的卫生处理等原因,浮游微生物得以定植并形成生物被膜。表5 归纳了PDI 对生物被膜的抗菌活性的主要研究。

部分文献对同一PDI 体系杀灭浮游菌及生物被膜进行了对比研究。对于第1、2 代光敏剂,Luksiene等[59]用蓝紫光(LED,λ=400 nm)激 活ALA 杀灭蜡样芽孢杆菌(G+),杀菌效果对PS 浓度和光通量显示出依赖性,并且对芽孢也有效果。ALA 在相同条件下分别作用于浮游菌和生物被膜,两者菌落数分别下降了6.3 和4.0 lg(CFU/mL),生物被膜展现出比浮游菌更强的环境适应性。Castro等[60]将卟啉与壳聚糖结合后制备成卟啉-壳聚糖薄膜,在白光下照射48 h 后,生物被膜的形成被完全抑制。

对于第3 代光敏剂,En-Sheng等[61]对赤藓红介导的金黄色葡萄球菌生物被膜(G+)的PDI 进行评估,并比较了该PDI 体系对浮游菌和生物被膜的抑菌效果。在不锈钢表面培养24 h 以形成生物被膜后,将生物被膜用0.05 mmol/L 的赤藓红处理,并暴露于绿光【LED,λ(nm)=540±5,50 J/cm2】下。仅光照或赤藓红处理均对生物被膜无明显影响,而赤藓红介导的PDI 作用后,观察到活细胞被完全杀灭。Cossu等[62]将在聚苯乙烯表面形成的生物被膜用10 mmol/L GA 处理并用UV-A(λ=365 nm)照射60 min,代谢活性降低约70%。Hendrik等[63]研究一种表面涂有金丝桃素纳米制剂的薄膜,用黄光(LED,λ=589 nm,20 mW)照射12 min,腐生葡萄球菌(G+)菌落数降低4.3 lg(CFU/mL)。值得注意的是,协同超声显著增强了PDI 效果(下降6.8 lg(CFU/mL))。这可能是超声加强了氧与PS 结合的能力,从而提高了PDI 效率。有研究姜黄素对副溶血弧菌(G-)和单增李斯特菌(G+)的PDI 作用显示,用蓝光(LED,λ=455~460 nm,1.14 J/cm2)激活1.0 μmol/L 姜黄素,5 min 即可完全杀灭副溶血弧菌(G-)浮游细胞,20 μmol/L姜黄素辐照60 min 能完全杀灭生物被膜[64-66]。本课题组将OG 与蓝光(LED,λ=420 nm)结合,分别作用于副溶血弧菌(G-)浮游菌与生物被膜,发现在0.2 mmol/L OG 介导的PDI 处理15 min(191.7 J/cm2)后,浮游细胞即不可见,其生物被膜在60 min 内几乎被根除(383.4 J/cm2[67]。上述例子进一步说明,单一菌种形成的生物被膜比浮游菌更难以杀灭,表现在同样的抑菌效果需要更高的PS 浓度及光剂量。

在现实的生物被膜中,基质中往往存在不止一种细菌。Banerjee等[57]研究了核黄素介导的PDI对耐多药大肠杆菌(G-)和耐甲氧西林金黄色葡萄球菌(G+)的单一和混合浮游细菌,单一和混合生物被膜的影响。用蓝光(LED,λ=450 nm,40 W/m2)激活50 μmol/L 核黄素,大肠杆菌(G-)和金黄色葡萄球菌(G+)菌落数分别下降5 和4 lg(CFU/mL),混合浮游态细菌分别下降7 和5 lg(CFU/mL)。两种菌的单一生物被膜均只留存34%,混合生物被膜则仍留存50%。混合生物被膜明显更难清除,其原因可能是混合菌种的EPS 对光照的影响更大[68]。值得注意的是,对于单一生物被膜,大肠杆菌(G-)更易被杀灭,这与核黄素对浮游细胞的消杀作用一致。然而在混合培养的生物被膜中,大肠杆菌(G-)表现出了更高的抵抗力。原因可能是在混培中,大肠杆菌成为优势菌株而抑制其它菌的生长。

5 结语与展望

《“健康中国2030 规划”纲要》将“健康”视为促进人的全面发展的必然要求。“民以食为天”,食品安全关系千家万户的健康。因此,聚焦新型食源性致病菌杀灭手段研究,具有重要的理论和产业价值。PDI 凭借众多优势,在果蔬、轻加工食品、水产、肉类等领域具有巨大的发展潜力。光源和光敏剂的配合是PDI 实现高效杀菌的关键,ROS 的产生是PDI 发挥灭菌作用的直接原因。因此,疏松多孔的革兰氏阳性菌膜易于光敏剂和ROS 的渗透,普遍表现出更高的敏感性。

然而,目前PDI 技术在食品产业化应用中仍处于初级探索阶段,对于构建出能满足食品产业化需求的PDI 高效系统,仍面临以下关键问题及挑战:1)高效安全的光敏剂;2)光敏剂在可见光区域具有良好的光活性且兼具其它功能活性,突破单一作用模式;3)光敏剂与有害菌及其生物被膜间的强相互作用。为此,未来PDI 抗菌在食品领域中的研究方向可能聚焦于:

1)设计开发新型高效光敏剂,如具有模拟天然酶(纳米酶)催化活性的光响应纳米材料[71]。区别于传统的外源性天然有机小分子光敏剂,一些具有光催化、光热和异质结构的纳米材料,在光辐照下不仅能产生大量毒性ROS 并快速杀灭有害菌[72],而且还表现出良好的稳定性与重复利用潜能,有望发展成为下一代高效光敏剂。

2)继续扩展PDI 在食品产业中的应用模式,如将光敏剂与食品包装结合,通过分子包合技术、纳米智造等,将光动力杀菌与纳米包装技术偶联,实现高效与持续抗菌效果。

3)将PDI 技术与其它技术协同,如通过超声处理促进溶解氧与光敏剂的相互作用,应用纳米气泡技术破坏生物被膜的基质,通过生物表面活性剂影响细菌间相互作用并干预生物被膜的形成等[73]

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Research Progress on Photodynamic Inactivation of Planktonic and Biofilm Bacteria

Li Donghui1,Shi Yugang1,2*
1School of Food Science and Biotechnology,Zhejiang Gongshang University,Hangzhou 310035 2Institute of Food Microbiology,Zhejiang Gongshang University,Hangzhou 310035)

Abstract Photodynamic inactivation is a new pathogenic bacterium killing technology.It activates photosensitizers through light sources to generate reactive oxygen species that target bacterial cell membranes and DNA,causing oxidative damage and ultimately leading to bacterial death.Traditional sterilization methods are faced with many problems in the food industry,such as the emergence of drug-resistant bacteria,biofilm bacteria are difficult to kill,food nutrition and flavor loss,and the high cost to application.As a non-thermal sterilization technology,photodynamic inactivation does not cause sensory changes and nutrient loss of food,requires simple equipment and low energy consumption,does not cause bacterial resistance,and has a good killing effect on bacteria(planktonic bacteria and biofilm),fungi,and even spores,which has attracted widespread attention in recent years.This article reviewed the limitations of traditional antibacterial methods,the principle of photodynamic inactivation,and the research progress of photodynamic inactivation in killing planktonic bacteria and biofilm bacteria are emphasized.

Keywords photodynamic inactivation;biofilm;photosensitizer;foodborne pathogens;food industry

文章编号 1009-7848(2024)04-0431-13

DOI:10.16429/j.1009-7848.2024.04.041

收稿日期:2023-04-07

基金项目:浙江省基础公益研究项目(LGJ19C200001);浙江省教育厅科研项目(Y201016439);浙江省引智项目(Z20170407);一流学科建设国际合作研究项目(2017SICR109)

第一作者:李栋辉,男,硕士生

通信作者:石玉刚 E-mail:yugangshi@zjgsu.edu.cn