QS is a bacterial cell-cell communication mechanism that regulates bacterial gene expression in a population density-dependent manner (Miller and Bassler, 2001). Pseudomonas aeruginosa utilizes four QS systems, LasI/LasR, RhlI/RhlR, PQS/MvfR and IQS, to activate expression of many virulence factors, including pyocyanin, pyoverdine, elastases, alkaline protease, lectins, rhamnolipids and exotoxin A, which promote bacterial invasion and impair host immune response (Whiteley et al., 1999; Rutherford and Bassler, 2012; Lee et al., 2013). Specifically, P. aeruginosa pyocyanin is redox-active phenazine that is not only involved in maintaining bacterial fitness and facilitating biofilm formation but also interferes host cellular functions, such as electron transport, cellular respiration, ciliary function and inflammatory response (Rada and Leto, 2013; Jayaseelan et al., 2014). Pyoverdine is the major siderophore of P. aeruginosa that obtains extracellular iron from environment and host proteins, important for bacterial growth and virulence (Meyer, 2000). The elastases produced by P. aeruginosa are capable of damaging host tissues and degrading plasma proteins, including immunoglobulins, coagulation factors and complement proteins (Wretlind and Pavlovskis, 1983). Alkaline protease not only damages host tissues but also facilitates bacterial immune evasion through proteolytic cleavage of monomeric flagellin, thus impairing the Toll-like receptor five signaling (Bardoel et al., 2012). Lectins are adhesion molecules located on bacterial outer membrane, which recognize and bind to host glycoconjugates, allowing the attachment of bacteria to host cell surface (Imberty and Varrot, 2008). Rhamnolipids are a class of glycolipid biosurfactants mainly produced by P. aeruginosa, and they have multiple functions involved in modification of surface properties, modulation of bacterial swarming motility, disruption of biofilm and alteration of epithelial tight junction (Zulianello et al., 2006; Alhede et al., 2014). Exotoxin A is a highly toxic virulence factor of P. aeruginosa that is secreted into the extracellular environment and inhibits host protein synthesis by catalyzing ADP-ribosylation of elongation factor 2 (Michalska and Wolf, 2015).

The LasI/LasR and RhlI/RhlR are two canonical LuxI/LuxR QS circuits in P. aeruginosa, which catalyze the synthesis of N-acyl homoserine lactone (AHL) autoinducers, N-(3-oxododecanoyl)-L-homoserine lactone (3O-C12-HSL) and N-butanoyl-L-homoserine lactone (C4-HSL), respectively (Waters and Bassler, 2005; Lee and Zhang, 2015; Moradali et al., 2017). Subsequently, the signal molecules 3O-C12-HSL and C4-HSL bind to their respective cognate transcriptional regulators LasR and RhlR, and activate expression of QS-controlled virulence genes (Lee and Zhang, 2015). The Las system regulates the expression of virulence factors, including exotoxin A, LasA protease, LasB elastase and alkaline protease, whereas the Rhl system regulates the expression of the pyocyanin, LasB elastase, alkaline protease, LecA and LecB lectins, and rhamnolipids (Pearson et al., 1997; Glessner et al., 1999; Winzer et al., 2000; Kariminik et al., 2017). Moreover, Las system has shown to positively regulate Rhl system (Lee and Zhang, 2015). By contrast, the third QS signal, Pseudomonas quinolone signal (PQS), structurally identified as 2-heptyl-3-hydroxy-4-quinolone, is synthesized through multiple operons, including pqsABCDE, phnAB and pqsH, and it binds to the receptor PqsR, also known as MvfR, to activate production of PQS itself and virulence factors, such as LasB elastase, rhamnolipids, lecA lectin and pyocyanin (Lee and Zhang, 2015; Lin et al., 2018). Furthermore, the PQS synthesis was found to be negatively regulated by Rhl system and positively regulated by Las system, whereas PQS positively regulated Rhl system (McKnight et al., 2000; Wade et al., 2005). The fourth QS signal, integrated quorum sensing system (IQS), structurally characterized to be 2-(2-hydroxyphenyl)-thiazole-4-carbaldehyde, is synthesized via a gene cluster ambBCDE, and it is responsible for integrating bacterial stress response with the QS network (Lee and Zhang, 2015). In addition, IQS was identified to be tightly controlled by Las system under rich culture conditions but is activated by phosphate limitation stress, subsequently upregulating Rhl and PQS systems (Lee et al., 2013). The P. aeruginosa QS systems were illustrated in Figure 2.

Many Chinese herbal medicines have been reported to possess anti-QS activities (Table 2) (Koh and Tham, 2011). Tanreqing injection has been widely used as an herbal formula for the treatment of viral pneumonia in China, which consists of the extracts from the flower bud of Lonicera japonica Thunb. (Caprifoliaceae; Lonicerae Japonicae Flos), the root of Scutellaria baicalensis Georgi (Lamiaceae; Scutellariae Radix), the fruit of Forsythia suspensa (Thunb.) Vahl (Oleaceae; Forsythiae Fructus), Pulvis Fellis Ursi (Bear bile), and Cornu Saigae Tataricae (Antelope’s Horn) (Liu et al., 2020). Moreover, the side effects of Tanreqing injection include anaphylaxis, drug eruption, nausea, vomiting and arrhythmia (Liu et al., 2020). A recent study by Yang et al. showed that the five components of Tanreqing played the major role in inhibiting the three P. aeruginosa systems, Las, Rhl and PQS, through repression of the upstream two-component systems GacS/GacA and PprA/PprB, leading to decreased expression of QS-regulated virulence genes in P. aeruginosa. Furthermore, the authors demonstrated a Caenorhabditis elegans-P. aeruginosa slow-killing assay and found that the survival rate of the C. elegans fed with Tanreqing-treated P. aeruginosa was significantly increased by 30% compared to those fed with untreated P. aeruginosa, suggesting that Tanreqing reduced the virulence of P. aeruginosa and protected C. elegans from killing (Yang W. et al., 2020). However, the original concentration of Tanreqing injection was not provided in this study. Wei et al. introduced a dry distillation oil prepared from a mixture of the leaf of Artemisia argyi H.Lév. & Vaniot (Compositae; Artemisiae argyi Folium), the root bark of Dictamnus dasycarpus Turcz. (Rutaceae, Dictamni radicis cortex) and the root of Solanum melongena L. (Solanaceae) with a mixing ratio of 1 : 1 : 2 (1 part of Artemisiae argyi Folium, 1 part of Dictamni radicis cortex, and 2 parts of the root of Solanum melongena L.) by heating in a flask with condenser tube at 350°C, and they found the oil was able to inhibit P. aeruginosa PQS system by interrupting the binding of the PQS receptor PqsR to its corresponding promoter pqsA (Wei et al., 2020). No side effects of the three TCM herbs have been reported. Qi Gui Yin is a mixture of Chinese herbal medicines, comprising of the root of Astragalus mongholicus Bunge (Leguminosae, Astragali Radix), the root of Angelica sinensis (Oliv.) Diels (Apiaceae, Angelicae sinensis Radix), the flower bud of L. japonica Thunb. (Caprifoliaceae; Lonicerae Japonicae Flos), Artemisia annua L. (Compositae), and the root and rhizome of Reynoutria japonica Houtt. (Polygonaceae; Polygoni Cuspidati Rhizoma et Radix) at a ratio of 12:3: 3:2:2, and it has been found to effectively eliminate antibiotic-resistant P. aeruginosa strains (Kong et al., 2015; Ding et al., 2021a). The safety of Qi Gui Yin decoction has been tested on rats, and no toxic and side effects were observed at a dose of 14.3 g for every kg of body weight for 13 weeks (Ding et al., 2021b). Ding et al. analyzed the protein expression profiles of the P. aeruginosa strains treated or not treated with Qi Gui Yin decoction, and found the QS-associated proteins, PhzA, PhzB, PhzM and MetQ1, were downregulated in Qi Gui Yin-treated strains. Further study demonstrated that the serum from Qi Gui Yin-treated rats could effectively reduce the resistance of P. aeruginosa to imipenem (Ding et al., 2021a). However, the mechanisms underlying the Qi Gui Yin-mediated QS inhibition were not specified in this study, and the metabolites in the serum of Qi Gui Yin-treated rats need to be further characterized. Sodium houttuyfonate is a bioactive compound derived from Houttuynia cordata Thunb. (Saururaceae), a well-known TCM botanical drug in East Asia. A study by Wu et al. reported that Sodium houttuyfonate was able to inhibit the expression of P. aeruginosa LasI and LasR, leading to impaired production of QS-regulated virulence factors, including pyocyanin and LasA (Wu et al., 2014). It is noteworthy that H. cordata Thunb. may cause allergic reactions in some people (Shingnaisui et al., 2018). Baicalin is an active compound purified from S. baicalensis Georgi, a famous TCM known for its antioxidant, anti-inflammatory and anticoagulant properties (Kong et al., 2014; Lee et al., 2015). Furthermore, baicalin has been found to inhibit the Las, Rhl and PQS systems of P. aeruginosa by downregulating the expression of QS regulatory genes, including lasI, lasR, rhlI, rhlR, pqsR and pqsA (Luo et al., 2017). However, S. baicalensis Georgi may cause stomach discomfort, diarrhoea and drug eruption in individual patients (Zhao et al., 2019). Andrographis paniculata (Burm.f.) Nees (Acanthaceae) is a medicinal plant widely used in many Asian countries, including China, India and Thailand, and it has been extensively applied in treatment of upper respiratory infections (Jiang et al., 2021), and it has no obvious toxic and side effects on human and animals (Jayakumar et al., 2013). Zhang et al. identified that the andrographolide compounds, andrographolide, 14-deoxyandrographolide, 14-deoxy-12-hydroxyandrographolide and neoandrographolide, from A. paniculata (Burm.f.) Nees suppressed the gene expression of LasR in the clinical isolates of P. aeruginosa PA22 and PA247 (Tan Lim et al., 2021). Additionally, Banerjee et al. indicated that the chloroform extract of A. paniculata (Burm.f.) Nees effectively decreased the expression of lasI, lasR, rhlI and rhlR by 61, 75, 41, and 44% in P. aeruginosa PAO1, respectively (Banerjee et al., 2017). QS has been found to regulate bacterial swarming motility, which is a flagella-driven movement of bacterial cells on a moist surface (Daniels et al., 2004). Koh et al. used swarming motility as an indicator of QS activity to screen the QS inhibitory effects of TCM plants, and they found that acetone and water (1:1 ratio) extracts from the seed of Prunus armeniaca L. (Rosaceae), the flower and root of Panax notoginseng (Burkill) F.H.Chen (Araliaceae), and the seed of Areca catechu L. (Arecaceae) impaired the swarming motility of P. aeruginosa PAO1 (Koh and Tham, 2011). However, the inhibitory mechanisms of these TCM botanical drugs were not addressed. Additionally, the side effects of P. armeniaca L. remain unknown, whereas P. notoginseng (Burkill) F.H.Chen has shown toxic effects on liver and kidney (Wang et al., 2016), and A. catechu L. may cause diarrhea, stomach discomfort and nausea (Samappito et al., 2012).

A biofilm is an aggregate of microorganisms in which cells adhere to each other on a static surface, and the adherent cells are embedded within a self-produced matrix of extracellular polymeric substances (EPS), consisting of polysaccharides, proteins, DNA and lipids (Donlan, 2002). Generally, biofilm formation can be divided into five steps: 1) initial attachment of planktonic bacteria to a surface; 2) microcolony formation; 3) formation of matrix by producing EPS; 4) maturation of biofilm into a three-dimensional structure; 5) detachment and dispersion of the biofilm (Figure 3)(Jamal et al., 2018; O'Toole et al., 2000). Bacteria in biofilm are more resistant to antimicrobial agents than planktonic cells because of the poor antibiotic penetration, reduced metabolic and growth rates, induction of adaptive stress responses, and formation of persister cells in biofilm microenvironment (Stewart, 2002; Hoiby et al., 2010). Importantly, approximate 80% of chronic infections are associated with biofilm formation (Jamal et al., 2018). Disruption of biofilm is critical for controlling persistent bacterial infections.

Pseudomonas aeruginosa is a major cause of chronic respiratory infections in CF patients, leading to declined pulmonary function and ultimate mortality (Lyczak et al., 2002). Formation of P. aeruginosa biofilm is regulated by QS systems, two-component regulatory systems, exopolysaccharides and bis-(3′–5′)-cyclic dimeric guanosine monophosphate (c-di-GMP)(Rasamiravaka et al., 2015). Two-component regulatory systems consist of a sensor kinase and a response regulator, which are essential for signaling transduction in bacteria in response to environment stimuli (Mikkelsen et al., 2011). The two-component regulatory systems GacS/GacA and RetS/LadS are responsible for regulation of P. aeruginosa biofilm formation (Rasamiravaka et al., 2015). The exopolysaccharides produced by P. aeruginosa include alginate, Pel and Psl, which contribute to biofilm development and stabilize biofilm architecture (Ghafoor et al., 2011). The c-di-GMP is a nucleotide second messenger that regulates plentiful cellular processes in bacteria, and high levels of c-di-GMP repress bacterial motility and induce production of exopolysaccharides, which promote P. aeruginosa biofilm formation (Ha and O'Toole, 2015).

TCM botanical drugs have manifested inhibitory effects on P. aeruginosa biofilm (Table 3)(Fu et al., 2017; Wang T. et al., 2019; Xu Z. et al., 2019). Herba patriniae, also known as Bai Jiang Cao, is a TCM for heat-clearing and detoxication, consisting of two species of Caprifoliaceae family, Patrinia scabiosifolia Link (Caprifoliaceae) and Patrinia villosa Juss. (Caprifoliaceae) (Gong et al., 2021). Fu et al. reported that the water extract of Herba patriniae was able to disrupt the biofilm formation and alter the biofilm structure of P. aeruginosa PAO1 through inhibition of exopolysaccharide production and biofilm-associated genes including algU, pslM, pelA, algA, and bdlA (Fu et al., 2017). However, the authors did not specify the species of Herba patriniae in this study. Additionally, the mild side effects including temporary leukopenia, dizziness and nausea could be found in the patients treated with inappropriate dose of Herba Patriniae (Gong et al., 2021). As mentioned earlier, sodium houttuyfonate from H. cordata Thunb. could inhibit P. aeruginosa QS systems (Wu et al., 2014). A study by Wang et al. identified that sodium houttuyfonate was able to penetrate P. aeruginosa biofilm and suppress biofilm dispersion by inhibiting the expression of the key biofilm regulator BdlA (Wang T. et al., 2019). Moreover, Wu et al. set up a rat model of P. aeruginosa biofilm infections in airway, and they found that sodium houttuyfonate destructed biofilm formation and the sodium houttuyfonate-treated rat displayed reduced symptoms and pulmonary inflammation (Wu et al., 2016). Zeng et al. examined the effects of several TCM chemical compounds on P. aeruginosa biofilm and found that the Baicalin and Baicalein from S. baicalensis Georgi, the Esculetin and Esculin from Fraxinus chinensis subsp. rhynchophylla (Hance) A.E.Murray (Oleaceae), and the Psoralen from Cullen corylifolium (L.) Medik (Leguminosae) could significantly reduce the mass of P. aeruginosa biofilm (Zeng et al., 2008). The mechanisms involved in the reduction of biofilm biomass by the three active compounds need to be further explored. Furthermore, the side effects of F. chinensis subsp. rhynchophylla (Hance) A.E.Murray and C. corylifolium (L.) Medik have not yet been clinically reported. Ligustrum sinense Lour. (Oleaceae) is an evergreen shrub used in TCM for their anti-oxidative, anti-tumor, and diuretic properties (Ouyang et al., 2003; Yang et al., 2013), and the side effects of this TCM are not clear. A previous study revealed that the water-soluble extract of L. sinense Lour. (4 mg/ml) could strongly decrease the biomass of P. aeruginosa biofilm, and 1.35 mg/ml of this extract displayed a synergistic inhibitory effect with gentamycin sulphate (2 mg/ml) on the growth of P. aeruginosa PAO1 (Yang et al., 2013). Shuanghuanglian is an antiviral and antibacterial TCM formula comprised of the flower bud of L. japonica Thunb., the root of S. baicalensis Georgi, and the fruit of F. suspensa (Thunb.) Vahl (Yan et al., 2013), and Shuanghuanglian injection has been reported to induce skin allergic reactions (Wang et al., 2010). Xu et al. demonstrated that the Lonicerin, a flavonoid from the Shuanghuanglian component L. japonica Thunb., inhibited the alginate secretion of P. aeruginosa by sequestering alginate secretion protein AlgE, thus leading to reduced P. aeruginosa biofilm formation (Xu Z. et al., 2019). Qingre Baidu mixture is a TCM formula composed of A. sinensis (Oliv.) Diels, Viola mandshurica W.Becker (Violaceae), the flower bud of L. japonica Thunb., Paeonia lactiflora pall. (Paeoniaceae), Salvia miltiorrhiza Bunge (Lamiaceae; Salviae miltiorrhizae radix et rhizome), F. suspensa (Thunb.) Vahl, the root of A. mongholicus Bunge, Gleditsia sinensis Lam. (Leguminosae) and Glycyrrhiza glabra L. (Leguminosae)(Zhang et al., 2020), and the adverse reactions of this TCM formula are unclear. Shan et al. applied Qingre Baidu mixture to a rat model of refractory wound through oral administration with a dose of 40 mg for every kg of body weight, and discovered that Qingre Baidu mixture could inhibit the biofilm formation of P. aeruginosa through downregulation of a self-inducer molecule AI2, important for regulation of biofilm development in P. aeruginosa (Shan et al., 2019). In addition, the QS inhibitory TCM Qi Gui Yin has exhibited downregulation of the expression of biofilm-related gene ArcA and IscU in P. aeruginosa (Ding et al., 2021a). The andrographolide compounds from A. paniculata (Burm.f.) Nees mentioned earlier could inhibit the biofilm formation of P. aeruginosa PA22 by 69–84% and PA247 by 23–56% at a concentration of 5 mg/ml (Tan Lim et al., 2021), and the chloroform extract of A. paniculata (Burm.f.) Nees inhibited the biofilm of P. aeruginosa PAO1 by 75.2% at a concentration of 1.25 mg/ml (Banerjee et al., 2017).

A variety of TCMs have shown bactericidal effects against respiratory pathogens by directly killing or inhibiting their growth (Ooi et al., 2006; Liu et al., 2007; Yamada et al., 2016), and some of them could enhance the in vitro antibacterial effects of antibiotics in treatment of MDR P. aeruginosa infections (Liu et al., 2007; Xu H. et al., 2019), possibly due to the enhanced stability of antibiotics by the reductive components of TCM (Gim et al., 2009; Chen et al., 2013; Matkowski et al., 2013; Sun W. et al., 2014; Chen et al., 2020), and the inhibitory effects of TCM on bacterial efflux pumps (Huang et al., 2015; Wang et al., 2018; Wang J. et al., 2019), suggesting that TCM could be a good alternative or complement for synthetic antibiotics. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values of TCM against P. aeruginosa were summarized in Table 4.

Pyrrosia petiolosa (Christ et Bar.) Ching (Polypodiaceae) is a pteridophyte used as a TCM for the treatment of nephritis and bronchitis without causing adverse reactions (Cheng et al., 2020), and the ethanol extract of P. petiolosa (Christ et Bar.) Ching has shown significant inhibitory effects on P. aeruginosa ATCC27853 with an MIC and an MBC both at 5 mg/ml (Cheng et al., 2014). Zingiber striolatum Diels (Zingiberaceae) is a perennial plant widely distributed in southern China, and it has been used for many medicinal purposes for its antioxidant and antimicrobial properties without causing side effects (Sharifi-Rad et al., 2017). Tian et al. characterized the chemical components of the essential oil extracted from the rhizomes of Z. striolatum Diels by hydrodistillation, including β-phellandrene, sabinene, β-pinene, geranyl linalool, terpinen-4-ol, a-pinene and crypton, and determined the MIC of the essential oil against P. aeruginosa to be 3.12 mg/ml (Tian et al., 2020). The TCM Pentaherb formula is composed of the flower bud of L. japonica Thunb., Mentha canadensis L. (Lamiaceae), the root bark of Paeonia × suffruticosa Andrews (Paeoniaceae), Atractylodes lancea (Thunb.) DC. (Compositae) and the bark of Phellodendron chinense C.K.Schneid (Rutaceae) with a mixture ratio of 2:1:2:2:2, and it has been shown to effectively treat atopic dermatitis without causing side effects (Hon et al., 2007). Hon et al. determined the MIC and MBC of the water extracts of the Pentaherb formula against P. aeruginosa ATCC 27853 to be 1 mg/ml and 125 mg/ml, respectively (Hon et al., 2018). Monochasma savatieri Franch. ex Maxim. (Orobanchaceae) is a perennial botanical drug widely used in TCM for treatment of many inflammatory diseases, and no toxic effects of this botanical drug have been documented (Gao H. et al., 2017). Liu et al. found that P. aeruginosa ATCC 27853 was sensitive to the phenylethanoid glycosides from M. savatieri Franch. ex Maxim., which had an MIC of 0.5 mg/ml and an MBC of 2 mg/ml (Liu et al., 2013). Furthermore, they demonstrated that the survival rates of the mice pretreated with phenylethanoid glycosides at a dose of 180 mg for every kg of body weight were significantly increased by 75% compared to untreated mice following P. aeruginosa intraperitoneal infections, and the pre-treatment of this drug could reduce the bacterial burden in the lung tissues of the P. aeruginosa lung-infected mice (Liu et al., 2013). Chimonanthus praecox (L.) Link (Calycanthaceae), also known as wintersweet, is a deciduous shrub used for treatment of coughs, rheumatic arthritis, throat wounds, dizziness, nausea and fever, and its side effects are not well-characterized (Kitagawa et al., 2016). The sesquiterpenoids isolated from the ethyl acetate extract of C. praecox (L.) Link stems and roots have been reported to possess antibacterial effects against P. aeruginosa ATCC 10145 with an MIC of 207.9–249.1 ug/ml (Lou et al., 2018). Mulberry is not only a fruit with a lot of nutrients but also a TCM comprising of many antioxidant and anti-inflammatory compounds (Huang et al., 2013), and the MBC of the flavonoids from the fruits of black mulberry Morus nigra L. (Moraceae) against P. aeruginosa was assessed to be 2 mg/ml (Chen et al., 2017). A research group from Taiwan discovered 10 ethanol extracts of Chinese herbal medicines including the root bark of Paeonia × suffruticosa Andrews, the ripe pericarp of Trichosanthes kirilowii Maxim. (Cucurbitaceae; Trichosanthis Pericarpium), the unripe fruit of Prunus mume (Siebold) Siebold & Zucc. (Rosaceae), Neolitsea cassia (L.). Kosterm. (Lauraceae; Ramulus Cinnamomi), the root and rhizome of Sophora tonkinensis Gagnep. (Leguminosae; Sophorae Tonkinensis radix et rhizome), the bulb of Iphigenia indica (L.) A.Gray ex Kunth (Colchicaceae), the fruit of F. suspensa (Thunb.) Vahl, Omphalia lapidescens Schroet (Leiwan), the bulb of Fritillaria thunbergii Miq. (Liliaceae; Fritillariae Bulbus), and the root of S. miltiorrhiza Bunge possessed a broad-spectrum of bactericidal activity against antibiotic-resistant P. aeruginosa strains. Among these herbal medicines, N. cassia (L.) Kosterm. was found to synergize with tetracycline, gentamycin and streptomycin to inhibit P. aeruginosa growth (Liu et al., 2007). Of note, S. tonkinensis Gagnep. may cause toxic effects including hematotoxicity, neurotoxicity, and immunotoxicity (Zhang et al., 2021), and the bulb of F. thunbergii Miq. exhibited toxicity to rats at the doses greater than 1 mg for every kg of body weight (Li et al., 2014). The side effects of other TCM botanical drugs listed above have not been reported. Fuzheng Jiedu Huayu formula consists of eight Chinese botanical drugs, including S. baicalensis Georgi, F. suspensa (Thunb.) Vahl, Echinops latifolius Tausch (Compositae), T. kirilowii Maxim., Panax quinquefolius L. (Araliaceae), P. scabiosifolia Link, and Coix lacryma-jobi var. ma-yuen (Rom.Caill.) Stapf (Poaceae) with a mixture ratio of 2.5:2.5:2.5:2.5:5:1:5:5 (Xu H. et al., 2019). Furthermore, the Fuzheng Jiedu Huayu formula decoction has been applied to treat elderly patients with pneumonia in clinical trials, and no obvious adverse reactions were manifested (Xu et al., 2018). Xu et al. revealed that the Fuzheng Jiedu Huayu decoction with an MIC of 200 mg/ml against P. aeruginosa combined with Imipenem/cilastatin sodium could enhance the in vitro bactericidal and bacteriostatic effects of Imipenem/cilastatin sodium on MDR P. aeruginosa (Xu H. et al., 2019).

A tightly controlled immune response ensures an effective host defense against microbial infection and maintenance of tissue homeostasis (Liu and Cao, 2016). Excessive immune response causes host tissue damage, septic shock and ultimately death (Bortolotti et al., 2018). On the other hand, deficient immune response results in chronic and persistent bacterial infections (Smith et al., 2009). Immunomodulation is an important function of TCM, which activates or suppresses the immune responses of a variety of immune cells, including macrophages (Jian et al., 2019), dendritic cells (Chen et al., 2006), T cells (Guo et al., 2015), B cells (Kawakita et al., 1987a) and NK cells (Hoffman et al., 2020). The immunomodulatory effects of TCM on host immunity in the context of P. aeruginosa infections have been evaluated in past decade (Table 5)(Kong et al., 2015; Hou et al., 2016; Li et al., 2017).

Qingfei Xiaoyan Wan, a TCM pill, consists of E. sinica Stapf, Gypsum Fibrosum, Pheretima (earthworm), the ripe fruit of Arctium lappa L. (Compositae; Arctii Fructus), the seed of Descurainia sophia (L.) Webb ex Prantl (Brassicaceae), Bovis Calculus Artifactus (Artificial ox bezoar), the seed of P. armeniaca L. and Cornu Saigae Tataricae (Hou et al., 2016), and it has been used in China for treatment of asthma and respiratory tract infections without induing obvious side effects (Zhao et al., 2013). A study by Hou et al. evaluated the effects of Qingfei Xiaoyan Wan suspended in distilled water on a mouse model of P. aeruginosa-induced acute pneumonia, and they demonstrated that the oral administration of Qingfei Xiaoyan Wan (18 g for every kg of body weight per day) for 1 week could reduce the P. aeruginosa PAK-induced lung inflammation by decreasing the production of cytokines (TNF-α and IL-6) and chemokine (RANTES) in lung tissues, significantly ameliorating lung injury. Importantly, the authors identified arctigenin, cholic acid, chlorogenic acid and sinapic acid to be the anti-inflammatory ingredients of Qingfei Xiaoyan Wan, which suppressed the expression of the regulatory proteins in PI3K/AKT and Ras/MAPK pathways (Hou et al., 2016). Xiao Chai Hu Tang is a 7-ingredient TCM consisting of the tuber of Pinellia ternata (Thunb.) Makino (Araceae), the fruit of Ziziphus jujuba Mill. (Rhamnaceae), and the root of S. baicalensis Georgi, Bupleurum chinense DC. (Apiaceae), Panax ginseng C.A.Mey. (Araliaceae), Glycyrrhiza glabra L. (Leguminosae) and Zingiber officinale Roscoe (Zingiberaceae), and it is widely used for treatment of respiratory, hepatic and gastrointestinal diseases (Hsu et al., 2006). However, Xiao Chai Hu Tang has been reported to cause hepatotoxicity due to overdose or long-term use (Gao X. et al., 2017). Kawakita et al. demonstrated that the water extract of Xiao Chai Hu Tang (100 mg for every kg of body weight) was able to protect mice from intraperitoneal infections with P. aeruginosa by promoting the accumulation of polymorphonuclear leukocytes and enhancing the phagocytic activity of macrophages (Kawakita et al., 1987b). The underlying mechanisms need to the further explored. Shufengjiedu capsule is an oral Chinese patent medicine that has been extensively utilized for treatment of respiratory infectious diseases and chronic obstructive pulmonary disease without causing serious adverse reactions (Xia et al., 2020). In the context of P. aeruginosa pulmonary infections, pretreatment of the power suspension from Shufengjiedu capsule with a dose of 0.09 g for every kg of body weight by intragastric gavage was reported to reduce the P. aeruginosa PAK-induced lung inflammation in mice through inhibition of ERK pathway and NF-κB activation, leading to decreased IL-6 production and alleviated lung injury (Li et al., 2017). Furthermore, the active components of Shufengjiedu capsule including verbenalin, phillyrin and emodin were identified to contribute to the regulation of ERK pathway (Li et al., 2017). Pseudomonas aeruginosa is one of the common pathogens isolated from chronic wounds, and the infections by P. aeruginosa affect wound healing (Serra et al., 2015). Shiunko ointment is a traditional botanic formula used for treatment of wounded skin without causing undesirable side effects in China and Japan (Higaki et al., 1999), and it has been found to accelerate the epithelization of wounded skin infected with P. aeruginosa (Huang et al., 2004). The mechanisms underlying the Shiunko-modulated epithelization possibly being through enhancing fibroblast proliferation and collagen production, and suppressing skin inflammation (Yan et al., 2015; Imai et al., 2019). In addition to inhibition of P. aeruginosa QS and biofilm formation, Qi Gui Yin decoction (9.9 g for every kg of body weight) was also identified to promote host immune response to P. aeruginosa infections by enhancing the antibody reactivity to β-lactamases, including VIM-1, SPM-1, and TEM-1, and manifested anti-inflammatory effects by reducing the levels of IL-1β and Th1/Th2 ratio in a rat model of abdominal infection (Kong et al., 2015). Qingre Baidu mixture was reported to promote angiogenesis and wound healing in a rat model of refractory wounds during P. aeruginosa infection by upregulating the expression of HIF-1α, HIF-2α and VEGF (Shan et al., 2019).