Two-component signaling in the virulence of Staphylococcus aureus: a silkworm larvae-pathogenic agent infection model of virulence

Alternatives to animal models to study bacterial infections

Animal testing has made a significant and unequalled contribution to important discoveries and advancements in the fields of research, medicine, vaccine development, and drug discovery. Each year, millions of animals are sacrificed for various experiments, and this is an ongoing process. However, the debate on the ethical and sensible usage of animals in in vivo experimentation is equally important. The need to explore and adopt newer alternatives to animals so as to comply with the goal of reduce, refine, and replace needs attention. Besides the ever-increasing debate on ethical issues, animal research has additional drawbacks (need of trained labour, requirement of breeding area, lengthy protocols, high expenses, transport barriers, difficulty to extrapolate data from animals to humans, etc.). With this scenario, the present review has been framed to give a comprehensive insight into the possible alternative options worth exploring in this direction especially targeting replacements for animal models of bacterial infections. There have been some excellent reviews discussing on the alternate methods for replacing and reducing animals in drug research. However, reviews that discuss the replacements in the field of medical bacteriology with emphasis on animal bacterial infection models are purely limited. The present review discusses on the use of (a) non-mammalian models and (b) alternative systems such as microfluidic chip-based models and microdosing aiming to give a detailed insight into the prospects of these alternative platforms to reduce the number of animals being used in infection studies. This would enlighten the scientific community working in this direction to be well acquainted with the available new approaches and alternatives so that the 3R strategy can be successfully implemented in the field of antibacterial drug research and testing.

Bacteriophage Therapy for Staphylococcus Aureus Infections: A Review of Animal Models, Treatments, and Clinical Trials

Staphylococcus aureus (S. aureus) is a common and virulent human pathogen causing several serious illnesses including skin abscesses, wound infections, endocarditis, osteomyelitis, pneumonia, and toxic shock syndrome. Antibiotics were first introduced in the 1940s, leading to the belief that bacterial illnesses would be eradicated. However, microorganisms, including S. aureus, began to develop antibiotic resistance from the increased use and abuse of antibiotics. Antibiotic resistance is now one of the most serious threats to global public health. Bacteria like methicillin-resistant Staphylococcus aureus (MRSA) remain a major problem despite several efforts to find new antibiotics. New treatment approaches are required, with bacteriophage treatment, a non-antibiotic strategy to treat bacterial infections, showing particular promise. The ability of S. aureus to resist a wide range of antibiotics makes it an ideal candidate for phage therapy studies. Bacteriophages have a relatively restricted range of action, enabling them to target pathogenic bacteria. Their usage, usually in the form of a cocktail of bacteriophages, allows for more focused treatment while also overcoming the emergence of resistance. However, many obstacles remain, particularly in terms of their effects in vivo, necessitating the development of animal models to assess the bacteriophage efficiency. Here, we provide a review of the animal models, the various clinical case treatments, and clinical trials for S. aureus phage therapy.

Five major two components systems of Staphylococcus aureus for adaptation in diverse hostile environment

Staphylococcus aureus is an eminent and opportunistic human pathogen that can colonize in the intestines, skin tissue and perineal regions of the host and cause severe infectious diseases. The presence of complex regulatory network and existence of virulent gene expression along with tuning metabolism enables the S. aureus to adopt the diversity of environments. Two component system (TCS) is a widely distributed mechanism in S. aureus that permit it for changing gene expression profile in response of environment stimuli. TCS usually consist of transmembrane histidine kinase (HK) and cytosolic response regulator. S. aureus contains totally 16 conserved pairs of two component systems, involving in different signaling mechanisms. There is a connection among these regulatory circuits and they can easily have effect on each other's expression. This review has discussed five major types of TCS in S. aureus and covers the recent knowledge of their virulence gene expression. We can get more understanding towards staphylococcal pathogenicity by getting insights about gene regulatory pathways via TCS, which can further provide implications in vaccine formation and new ways for drug design to combat serious infections caused by S. aureus in humans.

Efficacy of FimA antibody and clindamycin in silkworm larvae stimulated with Porphyromonas gulae

Objective: Porphyromonas gulae, a major periodontal pathogen in animals, possesses fimbriae that have been classified into three genotypes (A, B, C) based on the diversity of fimA genes encoding fimbrillin protein (FimA). P. gulae strains with type C fimbriae were previously shown to be more virulent than other types. In this study, we further examined the host toxicity mediated by P. gulae fimbriae by constructing recombinant FimA (rFimA) expression vectors for each genotype and raised antibodies to the purified proteins.

Methods and Results: All larvae died within 204 h following infection with P. gulae type C at the low-dose infection, whereas type A and B did not. Among fimA types, the survival rates of the larvae injected with rFimA type C were remarkably decreased, while the survival rates of the larvae injected with rFimA type A and type B were greater than 50%. Clindamycin treatment inhibited the growth of type C strains in a dose-dependent manner, resulting in an increased rate of silkworm survival. Finally, type C rFimA-specific antiserum prolonged the survival of silkworm larvae stimulated by infection with P. gulae type C strain or injection of rFimA type C protein.

Conclusion: These results suggested that type C fimbriae have high potential for enhancement of bacterial pathogenesis, and that both clindamycin and anti-type C rFimA-specific antibodies are potent inhibitors of type C fimbriae-induced toxicity. This is the first report to establish a silkworm infection model using P. gulae for toxicity assessment.

Animal infection models using non-mammals

The use of non-human animal models for infection experiments is important for investigating the infectious processes of human pathogenic bacteria at the molecular level. Mammals, such as mice and rabbits, are also utilized as animal infection models, but large numbers of animals are needed for these experiments, which is costly, and fraught with ethical issues. Various non-mammalian animal infection models have been used to investigate the molecular mechanisms of various human pathogenic bacteria, including Staphylococcus aureus, Streptococcus pyogenes, and Pseudomonas aeruginosa. This review discusses the desirable characteristics of non-mammalian infection models and describes recent non-mammalian infection models that utilize Caenorhabditis elegans, silkworm, fruit fly, zebrafish, two-spotted cricket, hornworm, and waxworm.

Polysaccharides of a fermented food, natto, suppress sucrose-induced hyperglycemia in an in vivo evaluation system and inhibit glucose uptake by human intestinal cells

Natto is a well-known traditional Japanese food produced by fermenting soybeans with Bacillus subtilis var natto. Here we found that the water-soluble viscous fraction of natto inhibits sucrose- or glucose-induced hyperglycemia in silkworms. The water-soluble viscous fraction treated with DNase I, RNase A, and proteinase K, followed by phenol extraction also suppressed sucrose-induced hyperglycemia in silkworms. The enzyme-treated polysaccharide fraction of natto inhibits glucose uptake by Caco-2 cells, human intestinal epithelial cells. These findings suggest that the polysaccharide components of natto selected on the basis of their suppressive effects on sucrose-induced hyperglycemia in silkworms inhibit glucose uptake by human intestinal cells.

Enterococcus faecalis YM0831 suppresses sucrose-induced hyperglycemia in a silkworm model and in humans

Hyperglycemia caused by excessive intake of sucrose leads to lifestyle-related diseases such as diabetes. Administration of a lactic acid bacterial strain to mice suppresses sucrose-induced hyperglycemia, but evidence for a similar effect in humans is lacking. Here we show that Enterococcus faecalis YM0831, identified using an in vivo screening system with silkworms, suppressed sucrose-induced hyperglycemia in humans. E. faecalis YM0831 also suppressed glucose-induced hyperglycemia in silkworms. E. faecalis YM0831 inhibited glucose uptake by the human intestinal epithelial cell line Caco-2. A transposon insertion mutant of E. faecalis YM0831, which showed decreased inhibitory activity against glucose uptake by Caco-2 cells, also exhibited decreased inhibitory activity against both sucrose-induced and glucose-induced hyperglycemia in silkworms. In human clinical trials, oral ingestion of E. faecalis YM0831 suppressed the increase in blood glucose in a sucrose tolerance test. These findings suggest that E. faecalis YM0831 inhibits intestinal glucose transport and suppresses sucrose-induced hyperglycemia in humans.

Immune responses to bacterial and fungal infections in the silkworm, Bombyx mori

The silkworm Bombyx mori, an economically important insect that is usually reared indoors, is susceptible to various pathogens, including bacteria, fungi, viruses, and microsporidia. As with other insects, the silkworm lacks an adaptive immune system and relies solely on innate immunity to defend itself against infection. Compared to other intensively studied insects, such as the fruit fly and tobacco hornworm, the principal immune pathways in the silkworm remain unclear. In this article, we review the literature concerning silkworm immune responses to bacteria and fungi and present our perspectives on future research into silkworm immunity.

An in vivo invertebrate evaluation system for identifying substances that suppress sucrose-induced postprandial hyperglycemia

Sucrose is a major sweetener added to various foods and beverages. Excessive intake of sucrose leads to increases in blood glucose levels, which can result in the development and exacerbation of lifestyle-related diseases such as obesity and diabetes. In this study, we established an in vivo evaluation system using silkworms to explore substances that suppress the increase in blood glucose levels caused by dietary intake of sucrose. Silkworm hemolymph glucose levels rapidly increased after intake of a sucrose-containing diet. Addition of acarbose or voglibose, α-glycosidase inhibitors clinically used for diabetic patients, suppressed the dietary sucrose-induced increase in the silkworm hemolymph glucose levels. Screening performed using the sucrose-induced postprandial hyperglycemic silkworm model allowed us to identify some lactic acid bacteria that inhibit the increase in silkworm hemolymph glucose levels caused by dietary intake of sucrose. The inhibitory effects of the Lactococcus lactis #Ll-1 bacterial strain were significantly greater than those of different strains of lactic acid bacteria. No effect of the Lactococcus lactis #Ll-1 strain was observed in silkworms fed a glucose diet. These results suggest that the sucrose diet-induced postprandial hyperglycemic silkworm is a useful model for evaluating chemicals and lactic acid bacteria that suppress increases in blood glucose levels.

Fluorescence imaging for a noninvasive in vivo toxicity-test using a transgenic silkworm expressing green fluorescent protein

In drug development, the toxicity of candidate chemicals must be carefully examined in an animal model. Here we developed a live imaging technique using silkworms for a noninvasive toxicity test applicable for drug screening. Injection of carbon tetrachloride, a tissue-injuring chemical, into transgenic silkworms expressing green fluorescent protein (GFP) induced leakage of GFP from the tissues into the hemolymph. The leakage of GFP was suppressed by pre-administration of either cimetidine, a cytochrome P450 inhibitor, or N-acetyl cysteine, a free-radical scavenger. The transgenic silkworm was made transparent by feeding a diet containing chemicals that inhibit uric acid deposition in the epithelial cells. In the transparent silkworms, GFP fluorescence in the fat body could be observed from outside the body. Injection of salicylic acid or iron sulfate, tissue-injuring chemicals, into the transparent silkworms decreased the fluorescence intensity of the GFP in the fat body. These findings suggest that the transparent GFP-expressing silkworm model is useful for evaluating the toxicity of chemicals that induce tissue injury.

Diabetic silkworms for evaluation of therapeutically effective drugs against type II diabetes

We previously reported that sugar levels in the silkworm hemolymph, i.e., blood, increase immediately (within 1 h) after intake of a high-glucose diet, and that the administration of human insulin decreases elevated hemolymph sugar levels in silkworms. In this hyperglycemic silkworm model, however, administration of pioglitazone or metformin, drugs used clinically for the treatment of type II diabetes, have no effect. Therefore, here we established a silkworm model of type II diabetes for the evaluation of anti-diabetic drugs such as pioglitazone and metformin. Silkworms fed a high-glucose diet over a long time-period (18 h) exhibited a hyperlipidemic phenotype. In these hyperlipidemic silkworms, phosphorylation of JNK, a stress-responsive protein kinase, was enhanced in the fat body, an organ that functionally resembles the mammalian liver and adipose tissue. Fat bodies isolated from hyperlipidemic silkworms exhibited decreased sensitivity to human insulin. The hyperlipidemic silkworms have impaired glucose tolerance, characterized by high fasting hemolymph sugar levels and higher hemolymph sugar levels in a glucose tolerance test. Administration of pioglitazone or metformin improved the glucose tolerance of the hyperlipidemic silkworms. These findings suggest that the hyperlipidemic silkworms are useful for evaluating the hypoglycemic activities of candidate drugs against type II diabetes.

The Role of Two-Component Signal Transduction Systems in Staphylococcus aureus Virulence Regulation

Staphylococcus aureus is a versatile, opportunistic human pathogen that can asymptomatically colonize a human host but can also cause a variety of cutaneous and systemic infections. The ability of S. aureus to adapt to such diverse environments is reflected in the presence of complex regulatory networks fine-tuning metabolic and virulence gene expression. One of the most widely distributed mechanisms is the two-component signal transduction system (TCS) which allows a pathogen to alter its gene expression profile in response to environmental stimuli. The simpler TCSs consist of only a transmembrane histidine kinase (HK) and a cytosolic response regulator. S. aureus encodes a total of 16 conserved pairs of TCSs that are involved in diverse signalling cascades ranging from global virulence gene regulation (e.g. quorum sensing by the Agr system), the bacterial response to antimicrobial agents, cell wall metabolism, respiration and nutrient sensing. These regulatory circuits are often interconnected and affect each other's expression, thus fine-tuning staphylococcal gene regulation. This manuscript gives an overview of the current knowledge of staphylococcal environmental sensing by TCS and its influence on virulence gene expression and virulence itself. Understanding bacterial gene regulation by TCS can give major insights into staphylococcal pathogenicity and has important implications for knowledge-based drug design and vaccine formulation.

Evaluating efficacy of bacteriophage therapy against Staphylococcus aureus infections using a silkworm larval infection model

Silkworm larva has recently been recognized as an alternative model animal for higher mammals to evaluate the effects of antibiotics. In this study, we examined the efficacy of the bacteriophage (phage) therapy, which harnesses phages as antibacterial agents, against Staphylococcus aureus infections, using the silkworm larval infection model. Two newly isolated staphylococcal phages, S25-3 and S13', were used as therapeutic phage candidates. They were assigned to two different lytic phage genera, Twort-like and AHJD-like viruses, based on their morphologies and the N-terminal amino acid sequences of the major capsid proteins. Both had a broad host range and strong lytic activity and showed preservative quality. Administration of these phages alone caused no adverse effects in the silkworm larvae. Moreover, the viruses showed life-prolonging effects in the silkworm larval infection model 10 min, 6 h, 12 h, and 24 h following infection. Such phage effects in the silkworm larval model were almost paralleled to the therapeutic efficacies in mouse models. These results suggest that phages S25-3 and S13' are eligible as therapeutic candidates and that the silkworm larval model is valid for the evaluation of phage therapy as well as mouse models.

Evaluation of Staphylococcus aureus virulence factors using a silkworm model

Previous studies have indicated that the silkworm model is useful for identifying virulence genes of Staphylococcus aureus, a human pathogenic bacterium. Here we examined the scope of S. aureus virulence factors that can be evaluated using the silkworm model. Gene-disrupted mutants of the agr locus, arlS gene and saeS gene, which regulate the expression of cell surface adhesins and hemolysins, exhibited attenuated virulence in silkworms. Mutants of the hla gene encoding α-hemolysin, the hlb gene encoding β-hemolysin, and the psmα and psmβ operons encoding cytolysins, however, showed virulence in silkworms indistinguishable from that of the parent strain. Thus, these S. aureus cytolysins are not required for virulence in silkworms. In contrast, the gene-disrupted mutants of clfB, fnbB and sdrC, which encode cell-wall-anchored proteins, attenuated S. aureus virulence in silkworms. In addition, the mutant of the srtA gene encoding sortase A, which anchors cell-wall proteins, showed attenuated virulence in silkworms. These findings suggest that the silkworm model can be used to evaluate S. aureus cell-wall proteins and regulatory proteins as virulence factors.

Investigation of the Staphylococcus aureus GraSR regulon reveals novel links to virulence, stress response and cell wall signal transduction pathways

The GraS/GraR two-component system has been shown to control cationic antimicrobial peptide (CAMP) resistance in the major human pathogen Staphylococcus aureus. We demonstrated that graX, also involved in CAMP resistance and cotranscribed with graRS, encodes a regulatory cofactor of the GraSR signaling pathway, effectively constituting a three-component system. We identified a highly conserved ten base pair palindromic sequence (5' ACAAA TTTGT 3') located upstream from GraR-regulated genes (mprF and the dlt and vraFG operons), which we show to be essential for transcriptional regulation by GraR and induction in response to CAMPs, suggesting it is the likely GraR binding site. Genome-based predictions and transcriptome analysis revealed several novel GraR target genes. We also found that the GraSR TCS is required for growth of S. aureus at high temperatures and resistance to oxidative stress. The GraSR system has previously been shown to play a role in S. aureus pathogenesis and we have uncovered previously unsuspected links with the AgrCA peptide quorum-sensing system controlling virulence gene expression. We also show that the GraSR TCS controls stress reponse and cell wall metabolism signal transduction pathways, sharing an extensive overlap with the WalKR regulon. This is the first report showing a role for the GraSR TCS in high temperature and oxidative stress survival and linking this system to stress response, cell wall and pathogenesis control pathways.

An invertebrate hyperglycemic model for the identification of anti-diabetic drugs

The number of individuals diagnosed with type 2 diabetes mellitus, which is caused by insulin resistance and/or abnormal insulin secretion, is increasing worldwide, creating a strong demand for the development of more effective anti-diabetic drugs. However, animal-based screening for anti-diabetic compounds requires sacrifice of a large number of diabetic animals, which presents issues in terms of animal welfare. Here, we established a method for evaluating the anti-diabetic effects of compounds using an invertebrate animal, the silkworm, Bombyx mori. Sugar levels in silkworm hemolymph increased immediately after feeding silkworms a high glucose-containing diet, resulting in impaired growth. Human insulin and 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), an AMP-activated protein kinase (AMPK) activator, decreased the hemolymph sugar levels of the hyperglycemic silkworms and restored growth. Treatment of the isolated fat body with human insulin in an in vitro culture system increased total sugar in the fat body and stimulated Akt phosphorylation. These responses were inhibited by wortmannin, an inhibitor of phosphoinositide 3 kinase. Moreover, AICAR stimulated AMPK phosphorylation in the silkworm fat body. Administration of aminoguanidine, a Maillard reaction inhibitor, repressed the accumulation of Maillard reaction products (advanced glycation end-products; AGEs) in the hyperglycemic silkworms and restored growth, suggesting that the growth defect of hyperglycemic silkworms is caused by AGE accumulation in the hemolymph. Furthermore, we identified galactose as a hypoglycemic compound in jiou, an herbal medicine for diabetes, by monitoring its hypoglycemic activity in hyperglycemic silkworms. These results suggest that the hyperglycemic silkworm model is useful for identifying anti-diabetic drugs that show therapeutic effects in mammals.

Inhibitory role for D-alanylation of wall teichoic acid in activation of insect Toll pathway by peptidoglycan of Staphylococcus aureus

Pathogenic bacteria mitigate host immunity to establish infections, but the mechanism of this bacterial action has not been fully elucidated. To search for cell wall components that modulate innate immune responses in host organisms, we examined Staphylococcus aureus mutants, which were deficient in components of the cell wall, for pathogenicity in Drosophila. A mutation of dltA, which is responsible for the D-alanylation of teichoic acids, brought about an increase in the survival rate of adult flies that had received a septic infection with the bacteria. The growth of dltA-deficient S. aureus in adult flies was less efficient than that of the parental strain. The level of mRNA of Toll pathway-dependent antimicrobial peptides was higher in flies infected with the dltA mutant than that observed after the infection with the parental strain. The defective phenotype associated with the mutation of dltA, reduced pathogenicity and growth, was not evident in flies lacking the Toll pathway. Finally, a fraction of peptidoglycan prepared from the dltA mutant induced the expression of mRNA of a Toll-dependent antimicrobial peptide in flies and was bound by peptidoglycan recognition protein-SA in vitro more effectively than that obtained from the parental strain, and this difference was lost after the removal of wall teichoic acid from peptidoglycan. Taken together, we conclude that D-alanylated wall teichoic acid of S. aureus mitigates a Toll-mediated humoral response in Drosophila interfering with the recognition of peptidoglycan by a pattern recognition receptor.

A promising broad spectrum antimicrobial red fluorescent protein present in silkworm excreta

The purified silkworm excretory red fluorescent protein (SE-RFP) has exhibited a potent broad spectrum antimicrobial activity. The anti-microbial assays of purified SE-RFP against several pathogenic bacterial (both Gram positive and Gram negative) and fungal strains were performed by agar cup plate method. The minimum inhibitory concentration (MIC) of SE-RFP against pathogenic bacteria and fungi was evaluated by agar dilution technique. The SE-RFP has exhibited highest activity (lowest minimum inhibitory concentration and largest zone of inhibition) against Staphylococcus aureus and Candida albicans among the tested bacteria and fungi, respectively. For the first time, we are reporting here the bioactivity of a red fluorescent protein purified from the silkworm excreta against clinically important bacteria and fungi. The bioactive SE-RFP has two absorption peaks at 280 and 603 nm and, it has exhibited fluorescence emission peaks at 334 and 619 nm upon exciting at 280 and 603 nm, respectively. The SE-RFP being an aqua-soluble, economically feasible and eco-friendly protein, it can therefore be used for the practical applications as an effective antimicrobial agent.

Auxiliary role for D-alanylated wall teichoic acid in Toll-like receptor 2-mediated survival of Staphylococcus aureus in macrophages

We previously reported that Staphylococcus aureus avoids killing within macrophages by exploiting the action of Toll-like receptor 2 (TLR2), which leads to the c-Jun N-terminal kinase (JNK)-mediated inhibition of superoxide production. To search for bacterial components responsible for this event, a series of S. aureus mutants, in which the synthesis of the cell wall was interrupted, were screened for the level of JNK activation in macrophages. In addition to a mutant lacking the lipoproteins that have been suggested to act as a TLR2 ligand, two mutant strains were found to activate the phosphorylation of JNK to a lesser extent than the parental strain, and this defect was recovered by acquisition of the corresponding wild-type genes. Macrophages that had phagocytosed the mutant strains produced more superoxide than those engulfing the parental strain, and the mutant bacteria were more efficiently killed in macrophages than the parent. The genes mutated, dltA and tagO, encoded proteins involved in the synthesis of D-alanylated wall teichoic acid. Unlike a cell wall fraction rich in lipoproteins, D-alanine-bound wall teichoic acid purified from the parent strain by itself did not activate JNK phosphorylation in macrophages. These results suggest that the d-alanylated wall teichoic acid of S. aureus modulates the cell wall milieu for lipoproteins so that they effectively serve as a ligand for TLR2.

Evaluation of target specificity of antibacterial agents using Staphylococcus aureus ddlA mutants and D-cycloserine in a silkworm infection model

The availability of a silkworm larva infection model to evaluate the therapeutic effectiveness of antibiotics was examined. The 50% effective doses (ED50) of D-cycloserine against the Staphylococcus aureus ddlA mutant-mediated killing of larvae were remarkably lower than those against the parental strain-mediated killing of larvae. Changes in MICs and ED50 of other antibiotics were negligible, suggesting that these alterations are d-cycloserine selective. Therefore, this model is useful for selecting desired compounds based on their therapeutic effectiveness during antibiotic development.