Antimicrobial Effects of Probiotics and Novel Probiotic-Based Approaches for Infectious Diseases

Abstract

Probiotics are live microorganisms, which confer health benefits on host when administered in adequate amounts. Probiotics exert their beneficial effects by maintenance flora healthy, enhancement of mucosal barrier integrity and modulation of immune responses. Antimicrobial substances including bacteriocins, hydrogen peroxide, organic acids, and short-chain fatty acids (SCFAs) produced by probiotics allow them to inhibit mucosal and epithelial adherence of pathogens and compete for limiting resources, thus suppress the growth of bacterial and fungal pathogens. Probiotics effect the colonization of fungal pathogen Candida to host surfaces, suppress Candida growth and biofilm development in vitro. Clinical results have shown that some probiotics can reduce oral, vaginal, and enteric colonization of Candida, alleviate clinical signs and symptoms, and potentially reduce the incidence of invasive fungal infection. Therefore, probiotics may be potential antifungals for prevention and treatment of candidiasis.

1. Introduction

Probiotics are “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host,” which was defined by the Food and Drug Organization of the United Nations (FAO) and World Health Organization (WHO) [1, 2, 3]. Probiotics should have some fundamental characteristics, such as human origin, nonpathogenic in nature, resistance to destruction by technical processing, acid and bile tolerances, adequate adherence and colonization on epithelial surfaces, antagonistic activity against pathogens, regulation of immune response, and influence human metabolic activities [4, 5, 6, 7].

Bacteria belonging to the genera Lactobacillus and Bifidobacterium are the most frequently used probiotics. Besides, Enterococcus, Streptococcus, Saccharomyces, and Bacillus are also commonly used probiotics (representative species are listed in Table 1). The administration of probiotics has been confirmed as an alternative biological approach to combat bacterial and fungal pathogens in the oral cavity, GI tract, and urogenital system [4, 5, 7, 8, 9, 10, 11, 12, 13, 14]. It has been reported that probiotics could reduce Candida, which cause fungal infections in different organ systems of the human body and prevent bacterial infectious diseases [9, 10, 15]. Probiotics were capable of preventing cancers [16], modulating blood pressure [17, 18], and repressing cholesterol levels [19]. Recently, species of Akkermansia muciniphila, Eubacterium hallii, and Faecalibacterium prausnitzii are identified as new potential probiotics because of their great benefits to the microbial metabolic networks and human health, especially the effects on correcting the imbalance of gut microbiota composition [7, 20, 21, 22]. A combination of probiotics with traditional treatment has been thought to be a potential approach for treatment of certain diseases.

Genera	Species
Lactobacillus	Lactobacillus rhamnosus, Lactobacillus casei, Lactobacillus plantarum, Lactobacillus acidophilus, Lactobacillus reuteri, Lactobacillus paracasei, Lactobacillus sporogenes, Lactobacillus lactis, Lactobacillus helveticus, and Lactobacillus fermentium
Lactococcus	Lactococcus lactis, Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. diacetylactis, and Lactococcus Lactis subsp. cremoris
Bifidobacterium	Bifidobacterium longum, Bifidobacterium bifidum, Bifidobacterium bifidus, and Bifidobacterium lactis
Enterococcus	Enterococcus faecalis and Enterococcus faecium
Saccharomyces	Saccharomyces cerevisiae and Saccharomyces boulardii
Streptococcus	Streptococcus thermophiles
Bacillus	Bacillus coagulans and Bacillus subtilis
Others	Akkermansia muciniphila, Eubacterium hallii, and Faecalibacterium prausnitzii

Table 1.

Representative microbe commonly considered as probiotics.

It is noteworthy that health benefits of probiotic bacteria are strain specific, which cannot be generalized to other strains, not even the same species, although some properties may be common for different strains because of the similarities in the metabolism of ecological functionality [5, 6]. Thus, the selection of certain probiotics for therapeutic purposes should be targeted for specific pathogens. Probiotics effects are dose specific [5, 6]. It has been suggested that a daily intake of 10⁶–10⁹ colony-forming units (CFUs) of probiotic microorganisms is the minimum effective dose for therapeutic purposes [5, 6, 8].

A number of probiotics are currently commercially available, and they have been categorized into single-strain or multi-strain/multispecies products [7, 23, 24]. Multi-strain/multispecies probiotics exhibited better effects than single-strain probiotics. The multispecies probiotic consortium VSL#3 (Streptococcus thermophilus, Eubacterium faecium, Bifidobacterium breve, Bifidobacterium infantis, Bifidobacterium longum, Lactobacillus acidophilus, Lactobacillus plantarum, Lactobacillus casei, and Lactobacillus delbrueckii subsp. bulgaricus) was proven more effective than single-strain probiotics for the treatment of ulcerative colitis [23]. The multispecies probiotic consortium, Ecologic AAD (Bifidobacterium bifidum W23, Bifidobacterium lactis W18, Bifidobacterium longum W51, Enterococcus faecium W54, Lactobacillus acidophilus W37 and W55, Lactobacillus paracasei W72, Lactobacillus plantarum W62, Lactobacillus rhamnosus W71, and Lactobacillus salivarius W24), combined with amoxicillin, could reduce diarrhea-like bowel movements, while the single strain could not [25]. Thus, the combination-specific probiotic effects from diverse strains can lead to synergistic effects.

Among the most frequently used probiotics, the genera Lactobacillus, Bifidobacterium, Lactococcus, and Saccharomyces have been included in the category of “generally regarded as safe” (GRAS) [4, 6]; however, other probiotic organisms such as Enterococcus, Bacillus, and Streptococcus are not generally regarded as safe. Since probiotics have been applied in food production, disease treatment, and others, it is important to undergo safety evaluation of probiotics before human consumption.

In this chapter, we briefly review the mechanisms of action of probiotics, the safety concern of probiotics, and their potentials for prevention and treatment of diseases. Here, we discuss the application of probiotics in the fungal Candida-infected and invasion candidiasis.

2. Probiotics mechanism of action

Probiotics mechanism of action is with important differences among different species and strain, examples are listed in Table 2.

Mechanism of action	Probiotics	Study outcomes	References
Maintenance flora healthy by reduction the growth and colonization of pathogens	L. rhamnosus GG, L. casei Shirota, L. reuteri SD2112 and L. brevis CD2	L. rhamnosus GG showed the strongest inhibitory activity in fructose and glucose medium against C. albicans, followed by L. casei Shirota, L. reuteri SD2112 and L. brevis CD2	[26]
	L. plantarum, commercial preparation LactoLevure®	Increased survival of mice infected by multidrug resistant P. aeruginosa and E. coli	[27]
	B. breve, L. casei (randomized controlled trial, RCT)	Levels of beneficial organic acids significantly increased in the gut, and the incidences of infectious (pneumonia and bacteremia) complications were significantly lower in the probiotic group	[32]
	Synbiotic (Lactobacillus, Bifidobacterium, and galactooligosaccharides) for 8 weeks (RCT)	Acetic acid concentration significantly increased (100 times), pH value decreased, Gram-negative rod (1/10) in the gut decreased, and P. aeruginosa decreased in the probiotic group	[33]
	Multi-strain synbiotic for 7 days (RCT)	Synbiotic group had lower pathogenic bacteria (43% versus 75%) and multiple organisms (39% versus 75%) in nasogastric aspirates than controls	[34]
	B. lactis Bb12 for 7–21 days (RCT)	Probiotic group had great higher counts of Bifidobacterium (P = 0.001) and lower counts of Enterobacteriaceae (P = 0.015) and Clostridium spp. (P = 0.014) than in placebo group	[35]
	L. casei subsp. rhamnosus for 6 weeks (RCT)	Colonization of Candida in gut was reduced in probiotic group (P = 0.01)	[28]
Enhancement of mucosal barrier integrity	L. plantarum 299v for 8 days (RCT)	Bacterial translocation in mesenteric lymph nodes and liver was reduced to 0 and 12%, respectively	[29]
	Microencapsulated Bifidobacteria	Bacterial translocation to mesenteric lymph nodes was reduced by encapsulated Bifidobacteria (P < 0.05)	[30]
	VSL#3 (RCT)	Decreased incidence of bacterial translocation in VSL#3 group than in water group (8% versus 50%; P = 0.03)	[31]
Immune modulation	VSL#3 (Lactobacillus, Bifidobacterium, and S. thermophilus) for 7 days (RCT)	Reduced acute physiology and chronic health evaluation II score; reduced sequential organ failure assessment, IL-6, procalcitonin, and protein	[36]
	L. plantarum 299v (RCT)	Late attenuating effect (after 15 days), serum IL-6 levels reduced	[37]

Table 2.

Mechanism of action of probiotics.

2.1. Maintenance flora healthy by reduction the growth and colonization of pathogens

The ability of probiotics to establish in the gastrointestinal (GI) tract, maintain flora healthy, and reduce the growth of pathogens and colonization is enhanced by their ability to eliminate competitors. Probiotic strains release different antimicrobial molecules such as organic acids, hydrogen peroxide (Н₂О₂), and antimicrobial peptide bacteriocins into the intestinal environment to limit the growth of bacterial and fungal pathogens [6, 39, 40, 41, 42, 43].

Lactic acid and acetic acid are the main metabolites formed by lactic acid bacteria (LAB). Both lactic acid and acetic acid could result in acidity environment and thus inhibit the growth of various microorganisms. Acetic acid has a broader spectrum of antimicrobial activity when compared to lactic acid. Moreover, it is known that a synergistic effect exists between the two acids: mixtures of acetic and lactic acids suppress the growth of the pathogenic enteric bacterium Salmonella typhimurium [44].

LAB can also produce Н₂О₂, the antimicrobial activity of which is linked to the strong oxidizing effect. Hydrogen peroxide showed a bactericidal effect on most pathogens when in combination with lactoperoxidase-thiocyanate milk system [45]. L. johnsonii NCC933 and L. gasseri KS120.1 killed enteric uropathogenic and vaginosis-associated pathogens due to the production of lactic acid and hydrogen peroxide [46].

Bacteriocins are ribosomally synthesized antimicrobial peptides, which have broad spectrum of inhibitory effect against Gram-positive and Gram-negative bacteria, viruses, and fungi [47, 48, 49, 50]. L. plantarum 2.9, a bacteriocinogenic strain, inhibited a set of foodborne pathogens including B. cereus, E. coli O157:H7, and S. enterica [51]. Bacteriocin-producing strains identified in our lab, e.g., L. plantarum ZJ316, L. plantarum LZ95, L. plantarum ZJ008, and L. plantarum ZJ005, showed antimicrobial activity against various pathogens in vitro such as S. aureus, E. coli, S. enterica, L. monocytogenes, and C. albicans [42, 52, 53, 54].

2.2. Enhancement of mucosal barrier integrity

Probiotics have been shown to improve barrier function and the mechanisms of barrier function including alteration of tight junction protein expression and/or localization, induction of mucus secretion, increased production of cytoprotective molecules such as heat-shock proteins, inhibition of apoptosis of epithelial cells, and promoting cell survival [29, 55, 56]. They compete with pathogens and prevent their invasion through the epithelium by the ability of adherence to the intestinal epithelium and mucus. L. plantarum has been shown to enhance mucosal barrier by adhering to the mucosal membrane and reducing Gram-negative bacteria [29]. Probiotics also compete for limiting resources, thus suppressing the growth of bacterial and fungal pathogens. The probiotic E. coli Nissle 1917 is able to effectively take up multiple limited environmental irons and simultaneously competitively inhibit the growth of other intestinal microbes and pathogens [57].

Furthermore, butyrate, a short-chain fatty acid (SCFA), could reduce bacterial translocation, improve the organization of tight junctions, modulate intestinal motility in addition to being an energy source for colonocytes, and maintain the integrity of the intestinal epithelium [29, 30, 31, 58, 59, 60]. E. hallii is an important anaerobic butyrate producer resident in our gut, which influences the intestinal metabolic balance and enhances the host-gut microbiota homeostasis [61]. Thus, the administration of probiotics with butyrate-producing bacteria, in particular, could be an effective way to achieve health benefits.

2.3. Immune modulation

Probiotics are reported to enhance phagocytic activity of granulocytes and cytokine excretion in lymphocytes, increase immunoglobulin-secreting cells, and attenuate inflammasome activation. They are able to affect cells involved in immune responses, including epithelial cells, dendritic cells (DCs), T cells, regulatory T (Treg) cells, monocytes/macrophages, immunoglobulin A (IgA)-producing B cells, and natural killer cells [62, 63].

Probiotic bacteria have an effect on intestinal DCs, which have the ability to recognize and respond to different bacteria by linking the innate immune system to the adaptive immune response and to develop T- and B-cell responses. Badia et al. found that the immunomodulatory role of S. boulardii in the DCs prior to infection was related to the upregulation of tumor necrosis factor alpha (TNFα) and C─C chemokine receptor type 7 mRNAs, which might make the DCs more effective in antagonizing bacteria [64, 65]. Smith et al. reported that S. boulardii stimulated the production of cytokines TNFα, IL-1, IL-12, IL-6, and IL-10 in DCs and also induced high levels of costimulatory molecules CD80 and CD86, thus modulated the immune system and led to an efficient clearing of enteropathogenic bacteria from the blood stream coupled with a faster cytokine response [65, 66].

Probiotics also influence intestinal epithelial cells through interaction with Toll-like receptors (TLRs) and downregulate the expression of NF-κB and proinflammatory cytokines [67, 68]. This effect is supported by the following studies: the supernatant of probiotic Faecalibacterium prausnitzii inhibited the NF-κB pathway in vitro and in vivo and showed protective effects in different models such as dinitrobenzene sulfate (DNBS)-induced colitis model and dextran sodium sulfate (DSS)-induced colitis [69]; the probiotic strain L. rhamnosus GG prevented cytokine-induced apoptosis in intestinal epithelial cells [70]; and L. rhamnosus GR-1 reduced the adhesion of E. coli by promoting TLR2 and NOD1 synergism and attenuating ASC-independent NLRP3 inflammasome activation [71].

3. Probiotic as antifungals for prevention and treatment of candidiasis

Candida is an opportunistic pathogen, causing mucosal infections including infections in the oral cavity, oropharynx, esophagus, and vagina, and potentially life-threatening systemic candidiasis. Candida albicans is the most common fungal pathogen in humans responsible for causing superficial as well as deep invasive candidiasis, which are essentially caused by Candida biofilms attached to body surfaces. Other Candida species such as Candida tropicalis, Candida guilliermondii, Candida krusei, and Candida glabrata are less frequently isolated in healthy and diseased humans [72, 73, 74]. Probiotics are known to reduce Candida infection in different organs and are generally considered to be beneficial for overall health. They appear to assist the host combat the pathogen by suppressing filamentation formation and reducing biofilm development, the mechanism of which may be related to expression of genes associated with biofilm formation and filamentation in Candida species. In vitro and in vivo studies have demonstrated the role of probiotics in the prevention of Candida colonization and invasive candidiasis [38, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86].

3.1. In vitro evidences: probiotics in prevention/treatment of Candida infections

Several in vitro studies have addressed the antifungal effects of probiotics against Candida isolated from the human oral cavity, GI tract, and genitourinary tract [77, 78, 79, 80, 81, 86, 87]. The probiotics that have been investigated against Candida species include Lactobacillus (e.g., L. rhamnosus, L. plantarum, L. fermentum, L. acidophilus, L. paracasei, L. johnsonii, and L. salivarius), Bifidobacterium (e.g., B. bifidum and B. infantis), Saccharomyces (e.g., S. boulardii), and Streptococcus (e.g., S. thermophilus). Table 3 shows candidacidal activity of probiotic strains in different studies. C. albicans appears to be more susceptible to the antifungal effect of Lactobacillus than C. pseudotropicalis [81], and the probiotics exhibited growth inhibitory activities against C. glabrata, C. krusei, and C. parapsilosis [79, 87].

Probiotics	Target pathogen	Study outcome	References
14 strains: L. fermentum, L. rhamnosus, L. plantarum, and L. acidophilus	C. albicans and C. pseudotropicalis	All probiotics inhibited the growth of C. albicans by H2O2 production and alternative mechanism	[81]
S. boulardii	C. albicans SC5314	S. boulardii inhibited the affecting hyphae formation, Candida adhesion, and biofilm formation by capric acid production	[87]
L. paracasei IMC 502	C. glabrata, C. krusei, C. parapsilosis, and C. tropicalis	High activity toward Candida strains except C. glabrata and C. tropicalis	[79]
L. plantarum ATCC 8014 and L. johnsonii enriched or not with SeNPs	C. albicans ATCC 14053	Strong inhibition of C. albicans by supernatant of selenium-enriched Lactobacillus spp.	[86]
L. acidophilus, L. rhamnosus, L. salivarius, B. bifidum, S. thermophiles, and B. infantis	C. albicans 10341	Significant inhibitory effect on biofilm formation and reduce viability of Candida	[80]
L. rhamnosus GR-1 and L. reuteri RC-14	C. albicans SC5314	Visible inhibition zones of fungal C. albicans by probiotic treatment; low pH environment caused by lactic acid and the H2O2 production may be anti-Candida factors	[77]
L. acidophilus ATCC 4356	C. albicans ATCC 18804	Reduce growth of C. albicans cells by 45.1%	[78]
L. casei subsp. rhamnosus	Candida spp.	80 preterm neonates with a very low birth weight: probiotic reduced incidence and intensity of enteric colonization by Candida spp. (RCT)	[28]
L. rhamnosus GG, L. rhamnosus LC705, P. freudenreichii subsp. shermanii JS	Candida spp.	276 elderly people: probiotic intervention reduced the risk of high yeast counts by 75% and the prevalence of hyposalivation (RCT)	[76]
L. rhamnosus GR-1 and L. reuteri RC-14	Candida spp.	55 women: probiotics significant reduced vaginal discharge, itching, and/or burning vaginal feeling, dyspareunia, and/or dysuria, and reduced the presence of Candida spp. (RCT)	[82]
L. acidophilus, L. rhamnosus, B. longum, B. bifidum, S. boulardii, and S. thermophilus	Candida spp.	150 children (aged 3 month to 12 year) on broad-spectrum antibiotics for at least 48 h: probiotic therapy avoided a significant increase in the number of patients colonized by Candida spp., significantly reduced the presence of Candida in the urine (RCT)	[83]
L. bulgaricus, B. longum, and S. thermophilus	Candida spp.	65 patients with Candida-associated stomatitis: detection rate of Candida spp. was reduced in the probiotic group; significant relief of clinical signs and symptoms after probiotic administration (RCT)	[84]
L. acidophilus, B. lactis, B. longum, and B. bifidum	Candida spp.	112 preterm neonates (gestational age < 37 wk and birth weight < 2500 g): probiotics may reduce enteral fungal colonization and invasive fungal sepsis in low-birth-weight neonates (RCT)	[75]
L. reuteri DSM 17938 and L. reuteri ATCC PTA 5289	Candida spp.	215 elderly people (aged 60–102 y): significant reduction of Candida cells in saliva and plaque (RCT)	[85]

Table 3.

Probiotics in prevention/treatment of Candida infections.

However, the mechanisms involved in antifungal activity of probiotics against Candida remain unclarified. Strus et al. found that Lactobacillus strains could inhibit the growth of C. albicans to a certain degree and their anticandidal activity related to H₂O₂ production [81]. Murzyn et al. reported that S. boulardii was able to secrete active compounds, mainly capric acid, reduced the expression of hwp1, ino1, and csh1 genes that encode virulence factors in C. albicans SC5314 cells, and inhibited filamentation of C. albicans and its mycelial development [87]. Therefore, it is likely that the antimicrobial molecules, organic acids, and Н₂О₂ produced by probiotic are major factors to limit growth of fungal pathogen Candida. This idea was supported by the research of Köhler et al. They demonstrated that low pH environment caused by lactic acid and the H₂O₂ production of L. rhamnosus GR-1 and L. reuteri RC-14 strains played important role in their inhibition activity to C. albicans SC5314. Moreover, L. rhamnosus GR-1 and L. reuteri RC-14 inhibited genes associated with C. albicans biofilm formation [87]. This result, together with the findings in Murzyn et al. study, shed light on a novel approach for uncovering the molecular mechanisms of the probiotic effect by using gene expression and related technology.

3.2. In vivo evidences: probiotics in prevention/treatment of Candida infections

In vivo studies, especially RCTs, have also been performed to substantiate the antifungal activity of probiotics in humans. These studies mostly focus on the sites of oral cavity, GI tract, and urogenital tract, which are susceptible to Candida infections (Table 3).

The elderly are a group particularly susceptible to oral candidiasis, because of frequent usage of dentures, hyposalivation, and their weakened immune status. Researches by Hatakka et al. and Kraft-Bodi et al. have shown that the daily consumption of food with L. reuteri DSM17938, L. reuteri ATCC PTA 5289, and L. rhamnosus GG ATCC 53103 significantly reduced the high yeast counts in saliva and biofilms in the elderly [76, 85]. The removal of biofilms by the use of probiotics that reduce the oral burden of Candida could play a major role in preventing oral candidiasis in denture wearers.

For the urogenital tract, chronic vulvovaginal candidiasis (VVC) is the most common candidiasis disease and impacts the life quality of thousands of women around the world. Researches on the effect of probiotics in the treatment and prophylaxis of VVC have been performed [82]. Martinez et al., in an RCT involving 55 women, demonstrated that the administration of L. rhamnosusGR-1 and L. reuteri RC-14 significantly reduced the presence of Candida and therefore reduced the vaginal discharge, itching, and/or burning vaginal feeling, dyspareunia, and/or dysuria [82].

For the GI tract, Candida species are common inhabitants of GI tract. Dysbiosis of GI tract may lead to candidal overgrowth and possible invasive infections, especially in infants. Hence, immunocompromised children, especially preterm neonates with low birth weight, have been the target population of a large number of studies to evaluate the prevention or/and treatment potentials of probiotics to Candida infections [28, 75, 83]. Manzoni et al., in an RCT involving 80 very low birth weight (VLBW) neonates, demonstrated that orally administered L. casei subsp. rhamnosus significantly reduced incidence and intensity of enteric colonization by Candida [28]. Another RCT, by Roy et al., found L. acidophilus, B. lactis, B. longum, and B. bifidum reduced enteral fungal colonization and invasive fungal sepsis in 112 preterm neonates (gestational age < 37 wk and birth weight < 2500 g) [75].

Together, both the laboratory studies and clinical studies showed that probiotics could prevent Candida colonization by inhibiting adhesion, filamentation, and biofilm formation, and therefore supplementation of probiotics could be a potential approach for reducing Candida colonization and invasive candidiasis.

4. Safety of probiotics

Although most commercially available probiotic strains are generally regarded as safe and none of the clinical studies mentioned above were reported to have adverse effects directly related to probiotics, there are some concerns regarding the safety of probiotics, including potential of bacteremia and/or endocarditis occurrence, toxicity to the gastrointestinal tract, and transfer of antibiotic resistance [4].

4.1. Potential of bacteremia and/or endocarditis occurrence

Lactic acid bacteria, including Bifidobacterium, have been reported to cause bacteremia as well as endocarditis [88, 89, 90, 91, 92]. Cannon et al. described that L. rhamnosus caused liver abscess, lactobacillemia, and infective endocarditis in a few case studies, and also the occurrence of Lactobacillus sepsis was directly linked with the ingestion of probiotic supplements, especially among immunocompromised patients and those with endocarditis [89]. Kunz et al. found two premature infants with short gut syndrome developed Lactobacillus bacteremia while taking Lactobacillus GG supplements. However, the risk of infection due to Lactobacilli is extremely rare. Statistic data from surveillance in Finland suggest that there was no increase in Lactobacillus bacteremia during 1990–2000, and Lactobacilli were isolated in 0.02% of all blood cultures [93].

4.2. Toxicity to the gastrointestinal tract

The role of probiotics on gastrointestinal physiology suggests a theoretical possibility that the production of metabolites might be undesirable and also might lead to malabsorption due to deconjugation of bile salts. These might increase the risk of colon cancer; however, there is no epidemiologic or clinical evidence to support this hypothesis [94, 95].

4.3. Transfer of antibiotic resistance

Another major safety concern of theoretical importance is genetic transfer of antibiotic resistance from probiotic strains to pathogenic cells in the gastrointestinal tract [96, 97]. Plasmids with antibiotic-resistance genes, including genes encoding resistance to tetracycline, erythromycin, chloramphenicol, and macrolide-lincosamide-streptogramin, have been found in L. plantarum, L. fermentum, L. acidophilus, and L. reuteri strains. L. plantarum 5057 exhibited tetracycline resistance, and L. lactis was with streptomycin, tetracycline, and chloramphenicol resistances [98, 99, 100]. Although the transfer of native Lactobacillus plasmids is quite rare, there are some cases, e.g., the antibiotic-resistance plasmids from Lactococcus species could transfer to Leuconostoc species and Pediococcus species.

With respect to the potential risks of probiotics, it is important to conduct population-based surveillance for safety concern.

5. Conclusions

Probiotics have the ability to restore the imbalance of intestinal microbiota and could act as both prophylactic and adjunctive therapy against candidiasis. Antifungal effect of probiotics is likely due to their interference with Candida biofilm development and hyphal differentiation. Safety may be of concern in application, as probiotic strains may, although quite rarely, cause bacteremia, fungemia, and sepsis. Well-designed RCTs are required to address these issues before the routine use of probiotics is recommended.

Acknowledgments

This project was funded by the National Key Research and Development Program of China (2016YFD0400400), the National Science Foundation of China (31601449), the International Science and Technology Cooperation Program of China (2013DFA32330), the Natural Science Foundation of Zhejiang Province (LY16C200002), and the Food Science and Engineering—the most important discipline of Zhejiang Province (2017SIAR202).