Antimicrobial activity of LysSS, a novel phage endolysin, against Acinetobacter baumannii and Pseudomonas aeruginosa

Molecular mechanism of action of imidazolium carbosilane dendrimers on the outer bacterial membrane - From membrane damage to permeability to antimicrobial endolysin

The outer bacterial membrane of drug-resistant bacteria is a significant barrier to many antimicrobials. Therefore, the development of new antibacterials primarily focuses on damaging the outer bacterial membrane of Gram-negative bacteria. Among many membrane-disrupting substances, the most promising are cationic dendritic systems. However, the mode of action may vary among different strains due to variations in the lipid compositions of the membrane. Here, we investigated the interaction of two types of cationic imidazolium carbosilane dendrimers: one with a single cationic group (methyl imidazolium) and the other with the same cationic group but attached to a functional group (a pendant pyridyl moiety), capable of establishing interactions with membranes through H-bonding or ion-dipole electrostatic interactions. We used different models of the outer membrane of Gram-negative bacteria - Escherichia coli, Pseudomonas aeruginosa, and Acinetobacter baumannii. Additionally, we assessed the combined effect of the dendrimers and the antibacterial endolysin on P. aeruginosa. Our results show that the mechanism of action depends on the type of dendrimer and the lipid composition of the membrane. We also demonstrate that the alteration of membrane fluidity and permeability to endolysin by the methyl imidazolium and pyridyl imidazolium dendrimers may play a more significant role in antimicrobial activity compared to membrane damage caused by positively charged dendrimers.

Assessing the synergistic potential of bacteriophage endolysins and antimicrobial peptides for eradicating bacterial biofilms

Phage-encoded endolysins have emerged as a potential substitute to conventional antibiotics due to their exceptional benefits including host specificity, rapid host killing, least risk of resistance. In addition to their antibacterial potency and biofilm eradication properties, endolysins are reported to exhibit synergism with other antimicrobial agents. In this study, the synergistic potency of endolysins was dissected with antimicrobial peptides to enhance their therapeutic effectiveness. Recombinantly expressed and purified bacteriophage endolysin [T7 endolysin (T7L); and T4 endolysin (T4L)] proteins have been used to evaluate the broad-spectrum antibacterial efficacy using different bacterial strains. Antibacterial/biofilm eradication studies were performed in combination with different antimicrobial peptides (AMPs) such as colistin, nisin, and polymyxin B (PMB) to assess the endolysin's antimicrobial efficacy and their synergy with AMPs. In combination with T7L, polymyxin B and colistin effectively eradicated the biofilm of Pseudomonas aeruginosa and exhibited a synergistic effect. Further, a combination of T4L and nisin displayed a synergistic effect against Staphylococcus aureus biofilms. In summary, the obtained results endorse the theme of combinational therapy consisting of endolysins and AMPs as an effective remedy against the drug-resistant bacterial biofilms that are a serious concern in healthcare settings.

Endolysins: a new antimicrobial agent against antimicrobial resistance. Strategies and opportunities in overcoming the challenges of endolysins against Gram-negative bacteria

Antibiotic-resistant bacteria are rapidly emerging, and the increasing prevalence of multidrug-resistant (MDR) Acinetobacter baumannii poses a severe threat to humans and healthcare organizations, due to the lack of innovative antibacterial drugs. Endolysins, which are peptidoglycan hydrolases encoded by a bacteriophage, are a promising new family of antimicrobials. Endolysins have been demonstrated as an effective therapeutic agent against bacterial infections of A. baumannii and many other Gram-positive and Gram-negative bacteria. Endolysin research has progressed from basic in vitro characterization to sophisticated protein engineering methodologies, including advanced preclinical and clinical testing. Endolysin are therapeutic agent that shows antimicrobial properties against bacterial infections caused by drug-resistant Gram-negative bacteria, there are still barriers to their implementation in clinical settings, such as safety concerns with outer membrane permeabilizers (OMP) use, low efficiency against stationary phase bacteria, and stability issues. The application of protein engineering and formulation techniques to improve enzyme stability, as well as combination therapy with other types of antibacterial drugs to optimize their medicinal value, have been reviewed as well. In this review, we summarize the clinical development of endolysin and its challenges and approaches for bringing endolysin therapies to the clinic. This review also discusses the different applications of endolysins.

Identification and characterization of the capsule depolymerase Dpo27 from phage IME-Ap7 specific to Acinetobacter pittii

Among the Acinetobacter genus, Acinetobacter pittii stands out as an important opportunistic infection causative agent commonly found in hospital settings, which poses a serious threat to human health. Recently, the high prevalence of carbapenem-resistant A. pittii isolates has created significant therapeutic challenges for clinicians. Bacteriophages and their derived enzymes are promising therapeutic alternatives or adjuncts to antibiotics effective against multidrug-resistant bacterial infections. However, studies investigating the depolymerases specific to A. pittii strains are scarce. In this study, we identified and characterized a capsule depolymerase, Dpo27, encoded by the bacteriophage IME-Ap7, which targets A. pittii. A total of 23 clinical isolates of Acinetobacter spp. were identified as A. pittii (21.91%, 23/105), and seven A. pittii strains with various K locus (KL) types (KL14, KL32, KL38, KL111, KL163, KL207, and KL220) were used as host bacteria for phage screening. The lytic phage IME-Ap7 was isolated using A. pittii 7 (KL220) as an indicator bacterium and was observed for depolymerase activity. A putative tail fiber gene encoding a polysaccharide-degrading enzyme (Dpo27) was identified and expressed. The results of the modified single-spot assay showed that both A. pittii 7 and 1492 were sensitive to Dpo27, which was assigned the KL220 type. After incubation with Dpo27, A. pittii strain was susceptible to killing by human serum; moreover, the protein displayed no hemolytic activity against erythrocytes. Furthermore, the protein exhibited sustained activity across a wide pH range (5.0-10.0) and at temperatures between 20 and 50°C. In summary, the identified capsule depolymerase Dpo27 holds promise as an alternative treatment for combating KL220-type A. pittii infections.

Combined effect of SAR-endolysin LysKpV475 with polymyxin B and Salmonella bacteriophage phSE-5

Endolysins are bacteriophage (or phage)-encoded enzymes that catalyse the peptidoglycan breakdown in the bacterial cell wall. The exogenous action of recombinant phage endolysins against Gram-positive organisms has been extensively studied. However, the outer membrane acts as a physical barrier when considering the use of recombinant endolysins to combat Gram-negative bacteria. This study aimed to evaluate the antimicrobial activity of the SAR-endolysin LysKpV475 against Gram-negative bacteria as single or combined therapies, using an outer membrane permeabilizer (polymyxin B) and a phage, free or immobilized in a pullulan matrix. In the first step, the endolysin LysKpV475 in solution, alone and combined with polymyxin B, was tested in vitro and in vivo against ten Gram-negative bacteria, including highly virulent strains and multidrug-resistant isolates. In the second step, the lyophilized LysKpV475 endolysin was combined with the phage phSE-5 and investigated, free or immobilized in a pullulan matrix, against Salmonella enterica subsp. enterica serovar Typhimurium ATCC 13311. The bacteriostatic action of purified LysKpV475 varied between 8.125 μg ml-1 against Pseudomonas aeruginosa ATCC 27853, 16.25 μg ml-1 against S. enterica Typhimurium ATCC 13311, and 32.50 μg ml-1 against Klebsiella pneumoniae ATCC BAA-2146 and Enterobacter cloacae P2224. LysKpV475 showed bactericidal activity only for P. aeruginosa ATCC 27853 (32.50 μg ml-1) and P. aeruginosa P2307 (65.00 μg ml-1) at the tested concentrations. The effect of the LysKpV475 combined with polymyxin B increased against K. pneumoniae ATCC BAA-2146 [fractional inhibitory concentration index (FICI) 0.34; a value lower than 1.0 indicates an additive/combined effect] and S. enterica Typhimurium ATCC 13311 (FICI 0.93). A synergistic effect against S. enterica Typhimurium was also observed when the lyophilized LysKpV475 at ⅔ MIC was combined with the phage phSE-5 (m.o.i. of 100). The lyophilized LysKpV475 immobilized in a pullulan matrix maintained a significant Salmonella reduction of 2 logs after 6 h of treatment. These results demonstrate the potential of SAR-endolysins, alone or in combination with other treatments, in the free form or immobilized in solid matrices, which paves the way for their application in different areas, such as in biocontrol at the food processing stage, biosanitation of food contact surfaces and biopreservation of processed food in active food packing.

Functional domains of Acinetobacter bacteriophage tail fibers

A rapid increase in antimicrobial resistant bacterial infections around the world is causing a global health crisis. The Gram-negative bacterium Acinetobacter baumannii is categorized as a Priority 1 pathogen for research and development of new antimicrobials by the World Health Organization due to its numerous intrinsic antibiotic resistance mechanisms and ability to quickly acquire new resistance determinants. Specialized phage enzymes, called depolymerases, degrade the bacterial capsule polysaccharide layer and show therapeutic potential by sensitizing the bacterium to phages, select antibiotics, and serum killing. The functional domains responsible for the capsule degradation activity are often found in the tail fibers of select A. baumannii phages. To further explore the functional domains associated with depolymerase activity, tail-associated proteins of 71 sequenced and fully characterized phages were identified from published literature and analyzed for functional domains using InterProScan. Multisequence alignments and phylogenetic analyses were conducted on the domain groups and assessed in the context of noted halo formation or depolymerase characterization. Proteins derived from phages noted to have halo formation or a functional depolymerase, but no functional domain hits, were modeled with AlphaFold2 Multimer, and compared to other protein models using the DALI server. The domains associated with depolymerase function were pectin lyase-like (SSF51126), tailspike binding (cd20481), (Trans)glycosidases (SSF51445), and potentially SGNH hydrolases. These findings expand our knowledge on phage depolymerases, enabling researchers to better exploit these enzymes for therapeutic use in combating the antimicrobial resistance crisis.

Antimicrobial peptide thanatin fused endolysin PA90 (Tha-PA90) for the control of Acinetobacter baumannii infection in mouse model

BACKGROUND

This study addresses the urgent need for infection control agents driven by the rise of drug-resistant pathogens such as Acinetobacter baumannii. Our primary aim was to develop and assess a novel endolysin, Tha-PA90, designed to combat these challenges.

METHODS

Tha-PA90 incorporates an antimicrobial peptide (AMP) called thanatin at its N-terminus, enhancing bacterial outer membrane permeability and reducing host immune responses. PA90 was selected as the endolysin component. The antibacterial activity of the purified Tha-PA90 was evaluated using an in vitro colony-forming unit (CFU) reduction assay and a membrane permeability test. A549 cells were utilized to measure the penetration into the cytosol and the cytotoxicity of Tha-PA90. Finally, infection control was monitored in A. baumannii infected mice following the intraperitoneal administration of Tha-PA90.

RESULTS

Tha-PA90 demonstrated remarkable in vitro efficacy, completely eradicating A. baumannii strains, even drug-resistant variants, at a low concentration of 0.5 μM. Notably, it outperformed thanatin, achieving only a < 3-log reduction at 4 μM. Tha-PA90 exhibited 2-3 times higher membrane permeability than a PA90 and thanatin mixture or PA90 alone. Tha-PA90 was found within A549 cells' cytosol with no discernible cytotoxic effects. Furthermore, Tha-PA90 administration extended the lifespan of A. baumannii-infected mice, reducing bacterial loads in major organs by up to 3 logs. Additionally, it decreased proinflammatory cytokine levels (TNF-α and IL-6), reducing the risk of sepsis from rapid bacterial lysis. Our findings indicate that Tha-PA90 is a promising solution for combating drug-resistant A. baumannii. Its enhanced efficacy, low cytotoxicity, and reduction of proinflammatory responses render it a potential candidate for infection control.

CONCLUSIONS

This study underscores the significance of engineered endolysins in addressing the pressing challenge of drug-resistant pathogens and offers insights into improved infection management strategies.

Engineering strategies and challenges of endolysin as an antibacterial agent against Gram-negative bacteria

Bacteriophage endolysin is a novel antibacterial agent that has attracted much attention in the prevention and control of drug-resistant bacteria due to its unique mechanism of hydrolysing peptidoglycans. Although endolysin exhibits excellent bactericidal effects on Gram-positive bacteria, the presence of the outer membrane of Gram-negative bacteria makes it difficult to lyse them extracellularly, thus limiting their application field. To enhance the extracellular activity of endolysin and facilitate its crossing through the outer membrane of Gram-negative bacteria, researchers have adopted physical, chemical, and molecular methods. This review summarizes the characterization of endolysin targeting Gram-negative bacteria, strategies for endolysin modification, and the challenges and future of engineering endolysin against Gram-negative bacteria in clinical applications, to promote the application of endolysin in the prevention and control of Gram-negative bacteria.

Characterization of a Straboviridae phage vB_AbaM-SHI and its inhibition effect on biofilms of Acinetobacter baumannii

Acinetobacter baumannii (A. baumannii) is a popular clinical pathogen worldwide. Biofilm-associated antibiotic-resistant A. baumannii infection poses a great threat to human health. Bacteria in biofilms are highly resistant to antibiotics and disinfectants. Furthermore, inhibition or eradication of biofilms in husbandry, the food industry and clinics are almost impossible. Phages can move across the biofilm matrix and promote antibiotic penetration. In the present study, a lytic A. baumannii phage vB_AbaM-SHI, belonging to family Straboviridae, was isolated from sauce chop factory drain outlet in Wuxi, China. The DNA genome consists of 44,180 bp which contain 93 open reading frames, and genes encoding products morphogenesis are located at the end of the genome. The amino acid sequence of vB_AbaM-SHI endolysin is different from those of previously reported A. baumannii phages in NCBI. Phage vB_AbaM-SHI endolysin has two additional β strands due to the replacement of a lysine (K) (in KU510289.1, NC_041857.1, JX976549.1 and MH853786.1) with an arginine (R) (SHI) at position 21 of A. baumannii phage endolysin. Spot test showed that phage vB_AbaM-SHI is able to lyse some antibiotic-resistant bacteria, such as A. baumannii (SL, SL1, and SG strains) and E. coli BL21 strain. Additionally, phage vB_AbaM-SHI independently killed bacteria and inhibited bacterial biofilm formation, and synergistically exerted strong antibacterial effects with antibiotics. This study provided a new perspective into the potential application value of phage vB_AbaM-SHI as an antimicrobial agent.

A synthetic biology approach to assemble and reboot clinically relevant Pseudomonas aeruginosa tailed phages

The rise in the frequency of antibiotic resistance has made bacterial infections, specifically Pseudomonas aeruginosa, a cause for greater concern. Phage therapy is a promising solution that uses naturally isolated phages to treat bacterial infections. Ecological limitations, which stipulate a discrete host range and the inevitable evolution of resistance, may be overcome through a better understanding of phage biology and the utilization of engineered phages. In this study, we developed a synthetic biology approach to construct tailed phages that naturally target clinically relevant strains of Pseudomonas aeruginosa. As proof of concept, we successfully cloned and assembled the JG024 and DMS3 phage genomes in yeast using transformation-associated recombination cloning and rebooted these two phage genomes in two different strains of P. aeruginosa. We identified factors that affected phage reboot efficiency like the phage species or the presence of antiviral defense systems in the bacterial strain. We have successfully extended this method to two other phage species and observed that the method enables the reboot of phages that are naturally unable to infect the strain used for reboot. This research represents a critical step toward the construction of clinically relevant, engineered P. aeruginosa phages.IMPORTANCEPseudomonas aeruginosa is a bacterium responsible for severe infections and a common major complication in cystic fibrosis. The use of antibiotics to treat bacterial infections has become increasingly difficult as antibiotic resistance has become more prevalent. Phage therapy is an alternative solution that is already being used in some European countries, but its use is limited by the narrow host range due to the phage receptor specificity, the presence of antiviral defense systems in the bacterial strain, and the possible emergence of phage resistance. In this study, we demonstrate the use of a synthetic biology approach to construct and reboot clinically relevant P. aeruginosa tailed phages. This method enables a significant expansion of possibilities through the construction of engineered phages for therapy applications.

Antibacterial and Anti-Biofilm Efficacy of Endolysin LysAB1245 against a Panel of Important Pathogens

Infections caused by antibiotic-resistant bacteria pose a significant global challenge. This study explores the antibacterial effects of a bacteriophage-derived endolysin, LysAB1245, against important pathogens, including Acinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Staphylococcus aureus. We determined the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) for all tested isolates. A time-kill study was conducted to evaluate the reduction in bacterial survival following treatment with LysAB1245. Additionally, the effects of LysAB1245 on P. aeruginosa K1455 and methicillin-resistant S. aureus (MRSA) NPRC 001R-formed biofilms were investigated. The MIC and MBC of LysAB1245 against all the tested isolates ranged from 4.68 to 9.36 µg/mL and 4.68 to 18.72 µg/mL, respectively. The time-kill study demonstrated more than a 4 log CFU/mL (99.99%) reduction in bacterial survival within 6 h of LysAB1245 treatment at 2MIC. LysAB1245 (1/8-1/2MIC) treatment significantly reduced biofilms formed by P. aeruginosa and MRSA in a concentration-dependent manner. Furthermore, scanning electron and confocal laser scanning microscopy confirmed the potential inhibition effects on 3-day established biofilms formed on abiotic surfaces upon treatment with LysAB1245 at 2MIC. The findings indicate that endolysin LysAB1245 could be employed as a new alternative therapeutic antibacterial and anti-biofilm agent for combating biofilm-related infections.

Characterization of a novel bacteriophage endolysin (LysAB1245) with extended lytic activity against distinct capsular types associated with Acinetobacter baumannii resistance

Capsular polysaccharides are considered as major virulence factors associated with the ability of multidrug-resistant (MDR) Acinetobacter baumannii to cause severe infections. In this study, LysAB1245, a novel bacteriophage-encoded endolysin consisting of a lysozyme-like domain from phage T1245 was successfully expressed, purified, and evaluated for its antibacterial activity against distinct capsular types associated with A. baumannii resistance. The results revealed a broad spectrum activity of LysAB1245 against all clinical MDR A. baumannii isolates belonging to capsular type (KL) 2, 3, 6, 10, 47, 49, and 52 and A. baumannii ATCC 19606. At 2 h following the treatment with 1.7 unit/reaction of LysAB1245, more than 3 log reduction in the numbers of bacterial survival was observed. In addition, LysAB1245 displayed rapid bactericidal activity within 30 min (nearly 3 log CFU/mL of bacterial reduction). Thermostability assay indicated that LysAB1245 was stable over a broad range of temperature from 4 to 70°C, while pH sensitivity assay demonstrated a wide range of pH from 4.5 to 10.5. Furthermore, both minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of LysAB1245 against all MDR A. baumannii isolates and A. baumannii ATCC 19606 were 4.21 μg/mL (0.1 unit/reaction). Conclusively, these results suggest that LysAB1245 possesses potential application for the treatment of nosocomial MDR A. baumannii infections.

Medical Device-Associated Healthcare Infections: Sterilization and the Potential of Novel Biological Approaches to Ensure Patient Safety

Healthcare-associated infections caused by multi-drug-resistant pathogens are increasing globally, and current antimicrobial options have limited efficacy against these robust species. The WHO details the critically important bacterial and fungal species that are often associated with medical device HAIs. The effective sterilization of medical devices plays a key role in preventing infectious disease morbidity and mortality. A lack of adherence to protocol and limitations associated with each sterilization modality, however, allows for the incidence of disease. Furthermore, issues relating to carcinogenic emissions from ethylene oxide gas (EtO) have motivated the EPA to propose limiting EtO use or seeking alternative sterilization methods for medical devices. The Food and Drug Administration supports the sterilization of healthcare products using low-temperature VH2O2 as an alternative to EtO. With advances in biomaterial and medical devices and the increasing use of combination products, current sterilization modalities are becoming limited. Novel approaches to disinfection and sterilization of medical devices, biomaterials, and therapeutics are warranted to safeguard public health. Bacteriophages, endolysins, and antimicrobial peptides are considered promising options for the prophylactic and meta-phylactic control of infectious diseases. This timely review discusses the application of these biologics as antimicrobial agents against critically important WHO pathogens, including ESKAPE bacterial species.

Bacteriophage endolysin Ply113 as a potent antibacterial agent against polymicrobial biofilms formed by enterococci and Staphylococcus aureus

Antibiotic resistance in Enterococcus faecium, Enterococcus faecalis, and Staphylococcus aureus remains a major public health concern worldwide. Furthermore, these microbes frequently co-exist in biofilm-associated infections, largely nullifying antibiotic-based therapy. Therefore, it is imperative to develop an efficient therapeutic strategy for combating infections caused by polymicrobial biofilms. In this study, we investigated the antibacterial and antibiofilm activity of the bacteriophage endolysin Ply113 in vitro. Ply113 exhibited high and rapid lytic activity against E. faecium, E. faecalis, and S. aureus, including vancomycin-resistant Enterococcus and methicillin-resistant S. aureus isolates. Transmission electron microscopy revealed that Ply113 treatment led to the detachment of bacterial cell walls and considerable cell lysis. Ply113 maintained stable lytic activity over a temperature range of 4-45°C, over a pH range of 5.0-8.0, and in the presence of 0-400 mM NaCl. Ply113 treatment effectively eliminated the mono-species biofilms formed by E. faecium, E. faecalis, and S. aureus in a dose-dependent manner. Ply113 was also able to eliminate the dual-species biofilms of E. faecium-S. aureus and E. faecalis-S. aureus. Additionally, Ply113 exerted potent antibacterial efficacy in vivo, distinctly decreasing the bacterial loads in a murine peritoneal septicemia model. Our findings suggest that the bacteriophage endolysin Ply113 is a promising antimicrobial agent for the treatment of polymicrobial infections.

Phage-induced bacterial morphological changes reveal a phage-derived antimicrobial affecting cell wall integrity

In a looming post-antibiotic era, antibiotic alternatives have become key players in the combat against pathogens. Although recent advances in genomic research allow scientists to fully explore an organism's genome in the search for novel antibacterial molecules, laborious work is still needed in order to dissect each individual gene product for its antibacterial activity. Here, we exploited phage-induced bacterial morphological changes as anchors to explore and discover a potential phage-derived antimicrobial embedded in the phage genome. We found that, upon vibriophage KVP40 infection, Vibrio parahaemolyticus exhibited morphological changes similar to those observed when treated with mecillinam, a cell wall synthesis inhibitor, suggesting the mechanism of pre-killing that KVP40 exerts inside the bacterial cell upon sieging the host. Genome analysis revealed that, of all the annotated gene products in the KVP40 genome that are involved in cell wall degradation, lytic transglycosylase (LT) is of particular interest for subsequent functional studies. A single-cell morphological analysis revealed that heterologous expression of wild-type KVP40-LT induced similar bacterial morphological changes to those treated with the whole phage or mecillinam, prior to cell burst. On the contrary, neither the morphology nor the viability of the bacteria expressing signal-peptide truncated- or catalytic mutant E80A- KVP40-LT was affected, suggesting the necessity of these domains for the antibacterial activities. Altogether, this research paves the way for the future development of the discovery of phage-derived antimicrobials that is guided through phage-induced morphological changes.

Bacteriophages and their derived enzymes as promising alternatives for the treatment of Acinetobacter baumannii infections

Nosocomial infections with the opportunistic bacterium Acinetobacter baumannii pose a severe challenge to clinical treatment, which is aggravated by the increasing occurrence of multi-drug resistance, especially resistance to carbapenems. The use of phage therapy as an alternative and supplement to the current antibiotics has become an important research topic in the post-antibiotic era. This review summarizes in vivo and in vitro studies on phage therapy against multi-drug-resistant A. baumannii infection that have used different approaches, including treatment with a single phage, combination with other phages or non-phage agents, and administration of phage-derived enzymes. We also briefly discuss the current challenges of phage-based therapy as well as promising approaches for the treatment of A. baumannii infection in the future.

A comprehensive review of the applications of bacteriophage-derived endolysins for foodborne bacterial pathogens and food safety: recent advances, challenges, and future perspective

Foodborne diseases are caused by food contaminated by pathogenic bacteria such as Escherichia coli, Salmonella, Staphylococcus aureus, Listeria monocytogenes, Campylobacter, and Clostridium, a critical threat to human health. As a novel antibacterial agent against foodborne pathogens, endolysins are peptidoglycan hydrolases encoded by bacteriophages that lyse bacterial cells by targeting their cell wall, notably in Gram-positive bacteria due to their naturally exposed peptidoglycan layer. These lytic enzymes have gained scientists' interest in recent years due to their selectivity, mode of action, engineering potential, and lack of resistance mechanisms. The use of endolysins for food safety has undergone significant improvements, which are summarized and discussed in this review. Endolysins can remove bacterial biofilms of foodborne pathogens and their cell wall-binding domain can be employed as a tool for quick detection of foodborne pathogens. We explained the applications of endolysin for eliminating pathogenic bacteria in livestock and various food matrices, as well as the limitations and challenges in use as a dietary supplement. We also highlight the novel techniques of the development of engineering endolysin for targeting Gram-negative bacterial pathogens. In conclusion, endolysin is safe and effective against foodborne pathogens and has no adverse effect on human cells and beneficial microbiota. As a result, endolysin could be employed as a functional bio-preservative agent to improve food stability and safety and maintain the natural taste of food quality.

Isolation, characterization, and genome analysis of a broad host range Salmonella phage vB_SenS_TUMS_E4: a candidate bacteriophage for biocontrol

Salmonella enteritidis is one of the most important foodborne pathogens that cause numerous outbreaks worldwide. Some strains of Salmonella have become progressively resistant to antibiotics, so they could represent a critical threat to public health and have led to the use of alternative therapeutic approaches like phage therapy. In this study, a lytic phage, vB_SenS_TUMS_E4 (E4), was isolated from poultry effluent and characterized to evaluate its potential and efficacy for bio-controlling S. enteritidis in foods. Transmission electron microscopy revealed that E4 has a siphovirus morphotype, with an isometric head and non-contractile tail. Determining the host range showed that this phage can effectively infect different motile as well as non-motile Salmonella enterica serovars. The biological characteristics of E4 showed that it has a short latent period of about 15 min and a large burst size of 287 PFU/cell, and is also significantly stable in a broad range of pHs and temperatures. The E4 whole genome contains 43,018 bp and encodes 60 coding sequences (CDSs) but no tRNA genes. Bioinformatics analysis revealed that the genome of E4 lacks any genes related to lysogeny behavior, antibiotic resistance, toxins, or virulence factors. The efficacy of phage E4 as a bio-control agent was assessed in various foodstuffs inoculated with S. enteritidis at 4°C and 25°C, and the resulting data indicated that it could eradicate S. enteritidis after a very short time of 15 min. The findings of the present study showed that E4 is a hopeful candidate as a bio-control agent against S. enteritidis and has the potential to be used in various foodstuffs.

LysP53 activity against Salmonella and its application in decontamination of Salmonella on fresh romaine lettuce

Salmonella is a zoonotic pathogen that is commonly associated with foodborne disease outbreaks. This study found that a newly identified Gram-negative lysin LysP53 had good activity against a wide range of Salmonella, including Salmonella Newington, Salmonella Typhimurium, and Salmonella Dublin. Without the help of an outer membrane permeabilizer, 4 μM LysP53 could reduce 97.6% of planktonic Salmonella Enteritidis and 90% of the bacteria in biofilms. Moreover, LysP53 was highly thermostable because it maintained >90% activity even after exposure to temperatures up to 95 °C. Although high concentrations of salts could reduce the activity, LysP53 was found safe for oral gavage of mice without affecting body weights and cytokines in sera and able to reduce 90% of Salmonella Enteritidis loads on fresh romaine lettuce after 30 min of treatment. Because of its good activity against a wide range of bacteria, thermal stability, safe for oral administration, LysP53 could be used as a biocontrol agent for reducing bacterial loads in fresh vegetable food. KEY POINTS: • Lysin LysP53 has high bactericidal activity against Salmonella. • LysP53 is thermostable even at high temperature of up to 95 °C. • LysP53 can be used for topical decontamination of Salmonella on vegetables.

Biological Characterization of Pseudomonas fluorescens Phage Pf17397_F_PD1 and Its Application in Food Preservation

In order to explore the application prospects of phages for controlling bacterial contamination, a lytic phage Pf17397_F_PD1 (Later abbreviated as PD1) was isolated from fish guts using Pseudomonas fluorescens ATCC 17397 as the host bacterium. The phage displayed short latency (18 min), long lysis period (212 min), and high lysis volume (1.47 × 102 PFU/each cell). It displayed wide temperature (30-70°C) and pH (4-11) tolerance. Genomic comparison revealed a maximum sequence identity of 48.65% between phage PD1 and other identified phages, indicating that PD1 was a new phage. The phage PD1 significantly inhibited the growth of P. fluorescens in milk and grass carp at 4°C and 25°C. Compared to the negative control, bacterial levels in milk stored at 25°C for 48 h were reduced by 2.71 log CFU/mL and 2.84 log CFU/mL at the multiplicity of infection (MOI) of 100 and 1,000, respectively. In contrast, when grass carp were stored at 25°C for 24 h, the bacterial load was reduced by 1.28 log CFU/g and 2.64 log CFU/g compared to the control (MOI of 100 and 1,000). When the phage was applied for preservation of grass carp blocks, total volatile salt nitrogen (TVB-N) values of phage-treated samples increased by 6.8 mg/100 g and 7.5 mg/100 g at MOI of 100 and 1,000, respectively, after 7 days of storage, which was significantly lower than that of the control group (15.83 mg/100 g). This study showed that phage PD1 was a good natural biological antimicrobial agent against P. fluorescens ATCC 17397.

Combined antimicrobial effect of phage-derived endolysin and depolymerase against biofilm-forming Salmonella Typhimurium

This study was designed to evaluate the antimicrobial activity of phage-derived endolysin (LysPB32) and depolymerase (DpolP22) against planktonic and biofilm cells of Salmonella Typhimurium (STKCCM). Compared to the control, the numbers of STKCCM were reduced by 4.3 and 5.9 log, respectively, at LysPB32 and LysPB32 + DpolP22 in the presence of polymyxin B (PMB) after 48-h incubation at 37 °C. LysPB32 + DpolP22 decreased the relative fitness (0.8) and the cross-resistance of STKCCM to chloramphenicol (CHL), cephalothin (CEP), ciprofloxacin (CIP), and tetracycline (TET) in the presence of PMB. The MICtrt/MICcon ratios of CHL, CEP, CIP, PMB, and TET were between 0.25 and 0.50 for LysPB32 + DpolP22 in the presence of PMB. These results suggest that the application of phage-encoded enzymes with antibiotics can be a promising approach for controlling biofilm formation on medical and food-processing equipment. This is noteworthy in that the application of LysPB32 + DpolP22 could increase antibiotic susceptibility and decrease cross-resistance to other antibiotics.

Phage Interactions with the Nervous System in Health and Disease

The central nervous system manages all of our activities (e.g., direct thinking and decision-making processes). It receives information from the environment and responds to environmental stimuli. Bacterial viruses (bacteriophages, phages) are the most numerous structures occurring in the biosphere and are also found in the human organism. Therefore, understanding how phages may influence this system is of great importance and is the purpose of this review. We have focused on the effect of natural bacteriophages in the central nervous system, linking them to those present in the gut microbiota, creating the gut-brain axis network, as well as their interdependence. Importantly, based on the current knowledge in the field of phage application (e.g., intranasal) in the treatment of bacterial diseases associated with the brain and nervous system, bacteriophages may have significant therapeutic potential. Moreover, it was indicated that bacteriophages may influence cognitive processing. In addition, phages (via phage display technology) appear promising as a targeted therapeutic tool in the treatment of, among other things, brain cancers. The information collected and reviewed in this work indicates that phages and their impact on the nervous system is a fascinating and, so far, underexplored field. Therefore, the aim of this review is not only to summarize currently available information on the association of phages with the nervous system, but also to stimulate future studies that could pave the way for novel therapeutic approaches potentially useful in treating bacterial and non-bacterial neural diseases.

PlyKp104, a Novel Phage Lysin for the Treatment of Klebsiella pneumoniae, Pseudomonas aeruginosa, and Other Gram-Negative ESKAPE Pathogens

Klebsiella pneumoniae and Pseudomonas aeruginosa are two leading causes of burn and wound infections, pneumonia, urinary tract infections, and more severe invasive diseases, which are often multidrug resistant (MDR) or extensively drug resistant. Due to this, it is critical to discover alternative antimicrobials, such as bacteriophage lysins, against these pathogens. Unfortunately, most lysins that target Gram-negative bacteria require additional modifications or outer membrane permeabilizing agents to be bactericidal. We identified four putative lysins through bioinformatic analysis of Pseudomonas and Klebsiella phage genomes in the NCBI database and then expressed and tested their intrinsic lytic activity in vitro. The most active lysin, PlyKp104, exhibited >5-log killing against K. pneumoniae, P. aeruginosa, and other Gram-negative representatives of the multidrug-resistant ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, K. pneumonia, Acinetobacter baumannii, P. aeruginosa, and Enterobacter species) without further modification. PlyKp104 displayed rapid killing and high activity over a wide pH range and in high concentrations of salt and urea. Additionally, pulmonary surfactants and low concentrations of human serum did not inhibit PlyKp104 activity in vitro. PlyKp104 also significantly reduced drug-resistant K. pneumoniae >2 logs in a murine skin infection model after one treatment of the wound, suggesting that this lysin could be used as a topical antimicrobial against K. pneumoniae and other MDR Gram-negative infections.

Characterization and Engineering Studies of a New Endolysin from the Propionibacterium acnes Bacteriophage PAC1 for the Development of a Broad-Spectrum Artilysin with Altered Specificity

The emergence of multidrug-resistant (MDR) bacteria has risen rapidly, leading to a great threat to global public health. A promising solution to this problem is the exploitation of phage endolysins. In the present study, a putative N-acetylmuramoyl-L-alanine type-2 amidase (NALAA-2, EC 3.5.1.28) from Propionibacterium bacteriophage PAC1 was characterized. The enzyme (PaAmi1) was cloned into a T7 expression vector and expressed in E. coli BL21 cells. Kinetics analysis using turbidity reduction assays allowed the determination of the optimal conditions for lytic activity against a range of Gram-positive and negative human pathogens. The peptidoglycan degradation activity of PaAmi1 was confirmed using isolated peptidoglycan from P. acnes. The antibacterial activity of PaAmi1 was investigated using live P. acnes cells growing on agar plates. Two engineered variants of PaAmi1 were designed by fusion to its N-terminus two short antimicrobial peptides (AMPs). One AMP was selected by searching the genomes of Propionibacterium bacteriophages using bioinformatics tools, whereas the other AMP sequence was selected from the antimicrobial peptide databases. Both engineered variants exhibited improved lytic activity towards P. acnes and the enterococci species Enterococcus faecalis and Enterococcus faecium. The results of the present study suggest that PaAmi1 is a new antimicrobial agent and provide proof of concept that bacteriophage genomes are a rich source of AMP sequences that can be further exploited for designing novel or improved endolysins.

Antibacterial activity of vB_AbaM_PhT2 phage hydrophobic amino acid fusion endolysin, combined with colistin against Acinetobacter baumannii

Phage lytic enzymes are promising antimicrobial agents. In this study, an endolysin derived from vB_AbaM_PhT2 (vPhT2), was identified. This endolysin represented the conserved lysozyme domain. Recombinant endolysin (lysAB- vT2) and hydrophobic fusion endolysin (lysAB-vT2-fusion) were expressed and purified. Both endolysins showed lytic activity against bacterial crude cell wall of Gram-negative bacteria. The MIC of lysAB-vT2-fusion was 2 mg/ml corresponding to 100 µM, while the MIC of lysAB-vT2 was more than 10 mg/ml (400 µM). Combination of lysAB-vT2-fusion with colistin, polymyxin B or copper was synergistic against A. baumannii (FICI value as 0.25). Antibacterial activity of lysAB-vT2-fusion plus colistin at the fractional inhibitory concentrations (FICs) revealed that it can inhibit Escherichia coli, Klebsiella pneumoniae and various strains of extremely drug-resistant A. baumannii (XDRAB) and phage resistant A. baumannii. The lysAB- vT2-fusion still retained its antibacterial activity after incubating the enzyme at 4, 20, 40 and 60 °C for 30 min. The lysAB-vT2-fusion could inhibit the mature biofilm, and incubation of lysAB-vT2-fusion with T24 human cells infected with A. baumannii led to a partial reduction of LDH release from T24 cells. In summary, our study highlights the antimicrobial ability of engineered lysAB-vT2-fusion endolysin, which can be applied for the control of A. baumannii infection.

Acinetobacter baumannii Bacteriophage: Progress in Isolation, Genome Sequencing, Preclinical Research, and Clinical Application

Acinetobacter baumannii (A. baumannii) is a common nosocomial pathogen associated with serious clinical challenges owing to its rapidly increasing resistance to antibiotics. Due to their high host specificity and easy access to the natural environment, bacteriophages (phages) may serve as good antibacterial agents. Phage therapy has been successfully used to treat antibiotic-resistant A. baumannii infections. As a fundamental step before phage therapy, the characterization and sequencing of A. baumannii phages have been well studied. Until October 2022, 132 A. baumannii phages have been sequenced and studied, with their genomes ranging from 4 to 234 kb, and we summarize the characterized and sequenced A. baumannii phages. This review is a current and short overview that does not go into detail on the A. baumannii phages. In addition, preclinical studies and clinical applications of A. baumannii phages are also included.

Therapeutic potential of bacteriophage endolysins for infections caused by Gram-positive bacteria

Gram-positive (G⁺) bacterial infection is a great burden to both healthcare and community medical resources. As a result of the increasing prevalence of multidrug-resistant G⁺ bacteria such as methicillin-resistant Staphylococcus aureus (MRSA), novel antimicrobial agents must urgently be developed for the treatment of infections caused by G⁺ bacteria. Endolysins are bacteriophage (phage)-encoded enzymes that can specifically hydrolyze the bacterial cell wall and quickly kill bacteria. Bacterial resistance to endolysins is low. Therefore, endolysins are considered promising alternatives for solving the mounting resistance problem. In this review, endolysins derived from phages targeting G⁺ bacteria were classified based on their structural characteristics. The active mechanisms, efficacy, and advantages of endolysins as antibacterial drug candidates were summarized. Moreover, the remarkable potential of phage endolysins in the treatment of G⁺ bacterial infections was described. In addition, the safety of endolysins, challenges, and possible solutions were addressed. Notwithstanding the limitations of endolysins, the trends in development indicate that endolysin-based drugs will be approved in the near future. Overall, this review presents crucial information of the current progress involving endolysins as potential therapeutic agents, and it provides a guideline for biomaterial researchers who are devoting themselves to fighting against bacterial infections.

The applications of animal models in phage therapy: An update

The rapid increase in antibiotic resistance presents a dire situation necessitating the need for alternative therapeutic agents. Among the current alternative therapies, phage therapy (PT) is promising. This review extensively summarizes preclinical PT approaches in various in-vivo models. PT has been evaluated in several recent clinical trials. However, there are still several unanswered concerns due to a lack of appropriate regulation and pharmacokinetic data regarding the application of phages in human therapeutic procedures. In this review, we also presented the current state of PT and considered how animal models can be used to adapt these therapies for humans. The development of realistic solutions to circumvent these constraints is critical for advancing this technology.

Acinetobacter Baumannii Phages: Past, Present and Future

Acinetobacter baumannii (A. baumannii) is one of the most common clinical pathogens and a typical multi-drug resistant (MDR) bacterium. With the increase of drug-resistant A. baumannii infections, it is urgent to find some new treatment strategies, such as phage therapy. In this paper, we described the different drug resistances of A. baumannii and some basic properties of A. baumannii phages, analyzed the interaction between phages and their hosts, and focused on A. baumannii phage therapies. Finally, we discussed the chance and challenge of phage therapy. This paper aims to provide a more comprehensive understanding of A. baumannii phages and theoretical support for the clinical application of A. baumannii phages.

Identification of Three Campylobacter Lysins and Enhancement of Their Anti-Escherichia coli Efficacy Using Colicin-Based Translocation and Receptor-Binding Domain Fusion

The emergence of multidrug-resistant Escherichia coli, which poses a major threat to public health, has motivated the development of numerous alternative antimicrobials. Lysins are bacteriophage- and bacterium-derived peptidoglycan hydrolases that represent a new antibiotic treatment targeting bacterial cell walls. However, the bactericidal effect of native lysins on Gram-negative bacteria is restricted by the presence of an outer membrane. Here, we first evaluated the antibacterial activity of three Campylobacter-derived lysins (Clysins) against E. coli. To improve their transmembrane ability and antibacterial activities, six engineered Clysins were constructed by fusing with the translocation and receptor-binding (TRB) domains from two types of colicins (colicin A [TRBA] and colicin K [TRBK]), and their biological activities were determined. Notably, engineered lysin TRBK-Cly02 exhibited the highest bactericidal activity against the E. coli BL21 strain, with a reduction of 6.22 ± 0.34 log units of cells at a concentration of 60.1 μg/mL, and formed an observable inhibition zone even at a dose of 6.01 μg. Moreover, TRBK-Cly02 killed E. coli dose dependently and exhibited the strongest bactericidal activity at pH 6. It also exhibited potential bioactivity against multidrug-resistant E. coli clinical isolates. In summary, this study identified three lysins from Campylobacter strains against E. coli, and the enhancement of their antibacterial activities by TRB domains fusion may allow them to be developed as potential alternatives to antibiotics. IMPORTANCE Three lysins from Campylobacter, namely, Clysins, were investigated, and their antibacterial activities against E. coli were determined for the first time. To overcome the restriction of the outer membrane of Gram-negative bacteria, we combined the TRB domains of colicins with these Clysins. Moreover, we discovered that the Clysins fused with TRB domains from colicin K (TRBK) killed E. coli more effectively, and this provides a new foundation for the development of novel bioengineered lysins by employing TRBK constructs that target outer membrane receptor/transport systems. One of the designed lysins, TRBK-Cly02, exhibited potent bactericidal efficacy against E. coli strains and may be used for control of multidrug-resistant clinical isolates. The results suggest that TRBK-Cly02 can be considered a potential antibacterial agent against pathogenic E. coli.

Characterisation and sequencing of the novel phage Abp95, which is effective against multi-genotypes of carbapenem-resistant Acinetobacter baumannii

Acinetobacter baumannii has become one of the most challenging conditional pathogens in health facilities. It causes various infectious diseases in humans, such as wound or urinary tract infections and pneumonia. Phage therapy has been used as an alternative strategy for antibiotic-resistant A. baumannii infections and has been approved by several governments. Previously, we have reported two potential phage therapy candidates, Abp1 and Abp9, both of which are narrow-host-range phages. In the present study, we screened and isolated 22 A. baumannii bacteriophages from hospital sewage water and determined that Abp95 has a wide host range (29%; 58/200). The biological and genomic characteristics and anti-infection potential of Abp95 were also investigated. Abp95 belongs to the Myoviridae family, with a G+C content of 37.85% and a genome size of 43,176 bp. Its genome encodes 77 putative genes, none of which are virulence, lysogeny, or antibiotic resistance genes. Abp95 was found to accelerate wound healing in a diabetic mouse wound infection model by clearing local infections of multidrug-resistant A. baumannii. In conclusion, the lytic phage Abp95, which has a wide host range, demonstrates potential as a candidate for phage therapy against multiple sequence types of carbapenem-resistant A. baumannii.

Diverse infective and lytic machineries identified in genome analysis of tailed coliphages against broad spectrum multidrug-resistant Escherichia coli

The emergence of multidrug-resistant (MDR) E. coli with deleterious consequences to the health of humans and animals has been attributed to the inappropriate use of antibiotics. Without effective antimicrobials, the success of modern medicine in treating infections would be at an increased risk. Bacteriophages could be used as an alternative to antibiotics for controlling the dissemination of MDR bacteria. However, before their use, the bacteriophages have to be assessed for the safety aspect. In this study, three broad host range highly virulent coliphage genomes were sequenced, characterized for infective and lytic potential, and checked for the presence of virulence and resistance genes. The genome sequencing indicated that coliphages ϕEC-S-21 and ϕEC-OE-11 belonged to Myoviridae, whereas coliphage ϕEC-S-24 belonged to the Autographiviridae family derived from the Podoviridae family. The genome size of the three coliphages ranged between 24 and 145 kb, with G + C content ranging between 37 and 51%. Coding sequences (CDS) ranged between 30 and 251 amino acids. The CDS were annotated and the proteins were categorized into different modules, viz., phage structural proteins, proteins associated with DNA replication, DNA modification, bacterial cell lysis, phage packaging, and uncharacterized proteins. The presence of tRNAs was detected only in coliphage ϕEC-OE-11. All three coliphages possessed diverse infective and lytic mechanisms, viz., lytic murein transglycosylase, peptidoglycan transglycosylase, n-acetylmuramoyl-l-alanine amidase, and putative lysozyme. Furthermore, the three coliphage genomes showed neither the presence of antibiotic resistance genes nor virulence genes, which makes them desirable candidates for use in phage therapy-based applications.

Bactericidal and Anti-Biofilm Activity of the FtsZ Inhibitor C109 against Acinetobacter baumannii

In the last few years, Acinetobacter baumannii has ranked as a number one priority due to its Multi Drug Resistant phenotype. The different metabolic states, such as the one adopted when growing as biofilm, help the bacterium to resist a wide variety of compounds, placing the discovery of new molecules able to counteract this pathogen as a topic of utmost importance. In this context, bacterial cell division machinery and the conserved protein FtsZ are considered very interesting cellular targets. The benzothiadiazole compound C109 is able to inhibit bacterial growth and to block FtsZ GTPase and polymerization activities in Burkholderia cenocepacia, Pseudomonas aeruginosa, and Staphylococcus aureus. In this work, the activity of C109 was tested against a panel of antibiotic sensitive and resistant A. baumannii strains. Its ability to inhibit biofilm formation was explored, together with its activity against the A. baumannii FtsZ purified protein. Our results indicated that C109 has good MIC values against A. baumannii clinical isolates. Moreover, its antibiofilm activity makes it an interesting alternative treatment, effective against diverse metabolic states. Finally, its activity was confirmed against A. baumannii FtsZ.

Engineering of lysin by fusion of antimicrobial peptide (cecropin A) enhances its antibacterial properties against multidrug-resistant Acinetobacter baumannii

Most clinical isolates of Acinetobacter baumannii, a nosocomial pathogen, are multidrug-resistant (MDR), fueling the search for alternative therapies. Bacteriophage-derived endolysins have potent antibacterial activities and are considered as alternatives to antibiotics against A. baumannii infection. Gram-negative bacteria possess outer lipid membrane that prevents direct contact between the endolysins and the cell wall. We hypothesized that the fusion of antimicrobial peptide (AMP) with endolysin could help to reduce bacterial endolysin resistance and increase antimicrobial activity by membrane permeability action. Accordingly, we fused cecropin A, a commonly used AMP, with the N-terminus of AbEndolysin, which enhances the bactericidal activity of the chimeric endolysin. The bactericidal activity of cecropin A-fused AbEndolysin increased by at least 2–8 fold for various MDR A. baumannii clinical isolates. The in vitro bactericidal activity results also showed higher bacterial lysis by the chimeric endolysin than that by the parental lysin. The engineered AbEndolysin (eAbEndolysin) showed synergistic effects with the beta-lactam antibiotics cefotaxime, ceftazidime, and aztreonam, and an additive effect with meropenem and imipenem. eAbEndolysin had no cytotoxic effect on A549 cell line and rescued mice (40% survival rate) from systemic A. baumannii infection. Together, these findings suggest the potential of lysin therapy and may prompt its use as an alternative to antibiotics.

Novel antimicrobial agents for combating antibiotic-resistant bacteria

Antibiotic therapy has become increasingly ineffective against bacterial infections due to the rise of resistance. In particular, ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) have caused life-threatening infections in humans and represent a major global health threat due to a high degree of antibiotic resistance. To respond to this urgent call, novel strategies are urgently needed, such as bacteriophages (or phages), phage-encoded enzymes, immunomodulators and monoclonal antibodies. This review critically analyses these promising antimicrobial therapies for the treatment of multidrug-resistant bacterial infections. Recent advances in these novel therapeutic strategies are discussed, focusing on preclinical and clinical investigations, as well as combinatorial approaches. In this 'Bad Bugs, No Drugs' era, novel therapeutic strategies can play a key role in treating deadly infections and help extend the lifetime of antibiotics.

Biological Control of Acinetobacter baumannii: In Vitro and In Vivo Activity, Limitations, and Combination Therapies

The survival, proliferation, and epidemic spread of Acinetobacter baumannii (A. baumannii) in hospital settings is associated with several characteristics, including resistance to many commercially available antibiotics as well as the expression of multiple virulence mechanisms. This severely limits therapeutic options, with increased mortality and morbidity rates recorded worldwide. The World Health Organisation, thus, recognises A. baumannii as one of the critical pathogens that need to be prioritised for the development of new antibiotics or treatment. The current review will thus provide a brief overview of the antibiotic resistance and virulence mechanisms associated with A. baumannii’s “persist and resist strategy”. Thereafter, the potential of biological control agents including secondary metabolites such as biosurfactants [lipopeptides (surfactin and serrawettin) and glycolipids (rhamnolipid)] as well as predatory bacteria (Bdellovibrio bacteriovorus) and bacteriophages to directly target A. baumannii, will be discussed in terms of their in vitro and in vivo activity. In addition, limitations and corresponding mitigations strategies will be outlined, including curtailing resistance development using combination therapies, product stabilisation, and large-scale (up-scaling) production.

Antibacterial Effects of Recombinant Endolysins in Disinfecting Medical Equipment: A Pilot Study

Nosocomial infections caused by multidrug-resistant (MDR) bacteria are severe life-threatening factors. Endolysins (lysins) degrade the bacterial cell wall peptidoglycan and may help control pathogens, especially MDR bacteria prevalent in hospital settings. This study was conducted to verify the potential of lysin as disinfectant to kill bacteria contaminating medical devices that cause hospital infections. Eight catheters removed from hospitalized patients were collected and tested for their ability to kill bacteria contaminating the catheters using two lysins, LysSS and CHAP-161. Catheter-contaminating bacterial species were isolated and identified by 16s rRNA sequencing. From the eight catheters, bacteria were cultured from seven catheters, and five bacterial species (Bacillus megaterium, Bacillus muralis, Corynebacterium striatum, Enterococcus faecium, and Staphylococcus epidermidis) were identified. LysSS could inhibit catheter-contaminating bacteria, including C. striatum and S. epidermidis, compared with untreated controls but could not inhibit the growth of E. faecium. CHAP-161 showed more bactericidal effects than LysSS, but could not inhibit the growth of S. epidermidis. This study showed the potential of lysin as an alternative disinfectant for hazardous chemical disinfectants used in hospitals.

Combination Effect of Engineered Endolysin EC340 With Antibiotics

Bacteriophage lysins, also known as endolysins or murein hydrolases, are hydrolytic enzymes produced by bacteriophages during the final stage of the lytic cycle to enable cleavage through the host's cell wall, thus allowing the phages to burst out of their host bacteria after multiplication inside them. When applied externally to Gram-negative bacteria as recombinant proteins, lysins cannot easily reach the cell wall due to the presence of an outer membrane (OM). In this study, endolysin EC340 obtained from phage PBEC131 infecting Escherichia coli was engineered for improved OM permeability and increased activity against Gram-negative bacteria. The engineered endolysin, LNT113, was tested for potential synergistic effects with standard-of-care antibiotics. A synergistic effect was demonstrated with colistin, while an additive effect was seen with meropenem, tigecycline, chloramphenicol, azithromycin, and ciprofloxacin. Neither ceftazidime nor kanamycin showed any synergy or additive effects with the LNT113 endolysin. Moreover, synergy and additive effects could not be generalized by antibiotic class, OM traverse mechanism, molecular weight, or the bactericidal nature of each antibiotic tested.

Non-Antibiotics Strategies to Control Salmonella Infection in Poultry

Salmonella spp. is a facultative intracellular pathogen causing localized or systemic infections, involving economic and public health significance, and remains the leading pathogen of food safety concern worldwide, with poultry being the primary transmission vector. Antibiotics have been the main strategy for Salmonella control for many years, which has allowed producers to improve the growth and health of food-producing animals. However, the utilization of antibiotics has been reconsidered since bacterial pathogens have established and shared a variety of antibiotic resistance mechanisms that can quickly increase within microbial communities. The use of alternatives to antibiotics has been recommended and successfully applied in many countries, leading to the core aim of this review, focused on (1) describing the importance of Salmonella infection in poultry and the effects associated with the use of antibiotics for disease control; (2) discussing the use of feeding-based (prebiotics, probiotics, bacterial subproducts, phytobiotics) and non-feeding-based (bacteriophages, in ovo injection, vaccines) strategies in poultry production for Salmonella control; and (3) exploring the use of complementary strategies, highlighting those based on -omics tools, to assess the effects of using the available antibiotic-free alternatives and their role in lowering dependency on the existing antimicrobial substances to manage bacterial infections in poultry effectively.

Intrinsic Antimicrobial Peptide Facilitates a New Broad-Spectrum Lysin LysP53 to Kill Acinetobacter baumannii In Vitro and in a Mouse Burn Infection Model

Antimicrobial resistance-related infections of Gram-negative pathogens pose a huge threat to global public health. Lysins, peptidoglycan hydrolases from bacteriophages, are expected as an alternative weapon against drug-resistant bacteria. In the present study, we report a new lysin LysP53 from Acinetobacter baumannii phage 53. Bioinformatic analysis revealed that LysP53 contains a positively charged N-terminal region and a putative peptidase catalytic domain. In vitro biochemical experiments showed that LysP53 is active against multiple antibiotic-resistant Gram-negative bacteria, including A. baumannii, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Escherichia coli, with a reduction of 5 logs in viable A. baumannii number after exposure to 100 μg/mL LysP53 for 1 h. Further studies showed that LysP53 contains a functional antimicrobial peptide, i.e., N-terminal 33 aa, with a comparable spectrum of activity to LysP53. In an A. baumannii-associated mouse model of burn infection, a single dose of 14 μg/mouse LysP53 (57.6 μM) showed higher decolonization efficacy than 4 μg/mouse minocycline- (874 μM; p < 0.05) and buffer-treated groups (p <0.001), leading to a bacterial reduction of 3 logs. Our findings collectively establish that LysP53 could be a promising candidate in the treatment of topical infections caused by multiple Gram-negative pathogens.

Discovering the Potentials of Four Phage Endolysins to Combat Gram-Negative Infections

Endolysin-based therapeutics are promising antibacterial agents and can successfully supplement the existing antibacterial drugs array. It is specifically important in the case of Gram-negative pathogens, e.g., ESKAPE group bacteria, which includes Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species, and are highly inclined to gain multiple antibiotic resistance. Despite numerous works devoted to the screening of new lytic enzymes and investigations of their biochemical properties, there are significant breaches in some aspects of their operating characteristics, including safety issues of endolysin use. Here, we provide a comprehensive study of the antimicrobial efficacy aspects of four Gram-negative bacteria-targeting endolysins LysAm24, LysAp22, LysECD7, and LysSi3, their in vitro and in vivo activity, and their biological safety. These endolysins possess a wide spectrum of action, are active against planktonic bacteria and bacterial biofilms, and are effective in wound and burn skin infection animal models. In terms of safety, these enzymes do not contribute to the development of short-term resistance, are not cytotoxic, and do not significantly affect the normal intestinal microflora in vivo. Our results provide a confident base for the development of effective and safe candidate dosage forms for the treatment of local and systemic infections caused by Gram-negative bacterial species.

Engineering a lysin with intrinsic antibacterial activity (LysMK34) with cecropin A enhances its antibacterial properties against Acinetobacter baumannii

Bacteriophage-encoded lysins are increasingly reported as alternatives to combat Acinetobacter baumannii infections for which limited therapeutic options are available. Some lysins such as LysMK34 have a C-terminal amphipathic helix allowing them to penetrate the otherwise impermeable outer membrane barrier. Another approach to kill Gram-negative pathogens with lysins relies on fusion of a peptide with outer membrane permeabilizing properties to the lysin. In this work, we aimed to leverage the intrinsic antibacterial activity of LysMK34 by fusing the peptide cecropin A to its N-terminus via a linker of three Ala-Gly repeats, resulting in eLysMK34. The engineered lysin has an improved antibacterial activity compared to the parental lysin LysMK34 in terms of minimum inhibitory concentration (0.45 - 1.2 μM), killing rate and killing extent. eLysMK34 has an at least two-fold increased activity against stationary-phase cells and the bactericidal effect becomes less dependent on the intracellular osmotic pressure. Particularly colistin-resistant strains become highly susceptible to eLysMK34 and enhanced antibacterial activity is observed in complement-deactivated human serum. These observations demonstrate that fusion of a lysin with intrinsic antibacterial activity with a selected outer membrane permeabilizing peptide is a useful strategy to further improve the in vitro antibacterial properties of such lysins. Importance Phage lysins are a new class of enzyme-based antibiotics that increasingly gain interest. Lysins kill cells through rapid degradation of the peptidoglycan layer, resulting in sudden osmotic lysis. Whereas Gram-positive bacteria are readily susceptible to the action of lysins, Gram-negative bacteria are naturally resistant as the outer membrane protects their peptidoglycan layer. This work reveals that fusing an outer membrane permeabilizing peptide to a lysin with intrinsic antibacterial activity results in a superior lysin that shows improved robustness in its antibacterial activity, including against the most worrisome colistin-resistant strains A. baumannii.

Endolysin, a Promising Solution against Antimicrobial Resistance

Antimicrobial resistance (AMR) is a global crisis for human public health which threatens the effective prevention and control of ever-increasing infectious diseases. The advent of pandrug-resistant bacteria makes most, if not all, available antibiotics invalid. Meanwhile, the pipeline of novel antibiotics development stagnates, which prompts scientists and pharmacists to develop unconventional antimicrobials. Bacteriophage-derived endolysins are cell wall hydrolases which could hydrolyze the peptidoglycan layer from within and outside of bacterial pathogens. With high specificity, rapid action, high efficiency, and low risk of resistance development, endolysins are believed to be among the best alternative therapeutic agents to treat multidrug resistant (MDR) bacteria. As of now, endolysins have been applied to diverse aspects. In this review, we comprehensively introduce the structures and activities of endolysins and summarize the latest application progress of recombinant endolysins in the fields of medical treatment, pathogen diagnosis, food safety, and agriculture.

Efficacy of the Endolysin-Based Antibacterial Gel for Treatment of Anaerobic Infection Caused by Fusobacterium necrophorum

Abscess formation is a common complication of severe life-threatening infections caused by obligate anaerobes. Fusobacterium necrophorum is among the frequently detected anaerobic pathogens from clinical specimens associated with liver abscesses, skin and soft tissue infections, or oral abscesses. The antimicrobial therapy for this kind of infection needs to be optimized. Here, we examined the possibility of treating F. necrophorum-induced abscess wound infections with candidate therapeutics based on three endolysins with activity against a broad spectrum of aerobe Gram-negative pathogens. Antibacterial gel containing three Gram-negative bacteria-targeting endolysins, LysAm24, LysAp22, and LysECD7, was formulated for topical use. Abscess formation was induced in rabbits with F. necrophorum and caused systemic infection. The survival and lifespan of the animals, general parameters, and biochemical and hematological blood tests were analyzed to assess the effectiveness of the gel treatment for the wound infection. The administration of the investigated gel twice per day for 5 days resulted in less acute inflammation, with decreased leukocytes and segmented neutrophils in the blood, retardation of infection progression, and an almost two-fold increase in the lifespan of the animals compared to the placebo group. The results indicate that endolysin-based therapy is an effective approach to treat anaerobic bacterial infections. The use of endolysins as independent pharmaceuticals, or their combination with antibiotics, could significantly reduce the development of complications in infectious diseases caused by sensitive bacterial species.

Current Status of Endolysin-Based Treatments against Gram-Negative Bacteria

The prevalence of multidrug-resistant Gram-negative bacteria is a public health concern. Bacteriophages and bacteriophage-derived lytic enzymes have been studied in response to the emergence of multidrug-resistant bacteria. The availability of tRNAs and endolysin toxicity during recombinant protein expression is circumvented by codon optimization and lower expression levels using inducible pET-type plasmids and controlled cultivation conditions, respectively. The use of polyhistidine tags facilitates endolysin purification and alters antimicrobial activity. Outer membrane permeabilizers, such as organic acids, act synergistically with endolysins, but some endolysins permeate the outer membrane of Gram-negative bacteria per se. However, the outer membrane permeation mechanisms of endolysins remain unclear. Other strategies, such as the co-administration of endolysins with polymyxins, silver nanoparticles, and liposomes confer additional outer membrane permeation. Engineered endolysins comprising domains for outer membrane permeation is also a strategy used to overcome the current challenges on the control of multidrug-resistant Gram-negative bacteria. Metagenomics is a new strategy for screening endolysins with interesting antimicrobial properties from uncultured phage genomes. Here, we review the current state of the art on the heterologous expression of endolysin, showing the potential of bacteriophage endolysins in controlling bacterial infections.

Characterization of a Novel Phage ΦAb1656-2 and Its Endolysin with Higher Antimicrobial Activity against Multidrug-Resistant Acinetobacter baumannii

Acinetobacter baumannii is a nosocomial pathogen, which is a problem worldwide due to the emergence of a difficult-to-treat multidrug-resistant A. baumannii (MDRAB). Endolysins are hydrolytic enzymes produced by a bacteriophage that can be used as a potential therapeutic agent for multidrug-resistant bacterial infection in replacing antibiotics. Here, we isolated a novel bacteriophage through prophage induction using mitomycin C from clinical A. baumannii 1656-2. Morphologically, ΦAb1656-2 was identified as a Siphoviridae family bacteriophage, which can infect MDRAB. The whole genome of ΦAb1656-2 was sequenced, and it showed that it is 50.9 kb with a G + C content of 38.6% and 68 putative open reading frames (ORFs). A novel endolysin named AbEndolysin with an N-acetylmuramidase-containing catalytic domain was identified, expressed, and purified from ΦAb1656-2. Recombinant AbEndolysin showed significant antibacterial activity against MDRAB clinical strains without any outer membrane permeabilizer. These results suggest that AbEndolysin could represent a potential antimicrobial agent for treating MDRAB clinical isolates.

Sustained Release of a Streptococcus pneumoniae Endolysin from Liposomes for Potential Otitis Media Treatment

Local delivery of antimicrobials for otitis media treatment would maximize therapeutic efficacy while minimizing side effects. However, drug transport across the tympanic membrane in the absence of a delivery system is challenging. In this study, the MSlys endolysin was encapsulated in deformable liposomes for a targeted treatment of S. pneumoniae, one of the most important causative agents of otitis media. MSlys was successfully encapsulated in liposomes composed of l-alpha-lecithin and sodium cholate (5:1) or l-alpha-lecithin and PEG2000 PE (10:1), with encapsulation efficiencies of about 35%. The PEGylated and sodium cholate liposomes showed, respectively, mean hydrodynamic diameters of 85 and 115 nm and polydispersity indices of 0.32 and 0.42, both being stable after storage at 4 °C for at least one year. Both liposomal formulations showed a sustained release of MSlys over 7 days. Cytotoxicity studies against fibroblast and keratinocyte cell lines revealed the biocompatible nature of both MSlys and MSlys-loaded liposomes. Additionally, the encapsulated MSlys showed prompt antipneumococcal activity against planktonic and biofilm S. pneumoniae, thus holding great potential for transtympanic treatment against S. pneumoniae otitis media.

Acinetobacter defence mechanisms against biological aggressors and their use as alternative therapeutic applications

Several Acinetobacter strains are important nosocomial pathogens, with Acinetobacter baumannii being the species of greatest worldwide concern due to its multi-drug resistance and the recent appearance of hyper-virulent strains in the clinical setting. Colonisation of this environment is associated with a multitude of bacterial factors, and the molecular features that promote environmental persistence in abiotic surfaces, including intrinsic desiccation resistance, biofilm formation and motility, have been previously addressed. On the contrary, mechanisms enabling Acinetobacter spp. survival when faced against other biological competitors are starting to be characterised. Among them, secretion systems (SS) of different types, such as the T5bSS (Contact-dependent inhibition systems) and the T6SS, confer adaptive advantages against bacterial aggressors. Regarding mechanisms of defence against bacteriophages, such as toxin-antitoxin, restriction-modification, Crispr-Cas and CBASS, among others, have been identified but remain poorly characterised. In view of this, we aimed to summarise the present knowledge on defence mechanisms that enable niche establishment in members of the Acinetobacter genus. Different proposals are also described for the use of some components of these systems as molecular tools to treat Acinetobacter infections.

Phage Therapy in Livestock and Companion Animals

The irrational use of antibiotics has led to a high emergence of multi-drug resistant (MDR) bacteria. The traditional overuse of antibiotics in the animal feed industry plays a crucial role in the emergence of these pathogens that pose both economic and health problems. In addition, antibiotics have also recently experienced an increase to treat companion animal infections, promoting the emergence of MDR bacteria in pets, which can reach humans. Phages have been proposed as an alternative for antibiotics for the treatment of livestock and companion animal infections due to their multiple advantages as adaptative drugs, such as their ability to evolve, to multiply at the site of infections, and their high specificity. Moreover, phage-derived enzymes may also be an interesting approach. However, the lack of regulation for this type of pharmaceutical hinders its potential commercialization. In this review, we summarize the main recent studies on phage therapy in livestock and companion animals, providing an insight into current advances in this area and the future of treatments for bacterial infections.