Adit B. Alreja

Background/Objectives: Serratia marcescens is an opportunistic pathogen that causes severe hospital-acquired infections, notable for its biofilm formation abilities and development of extensive antibiotic resistance. Here, we aim to evaluate the efficacy of bacteriophages, antibiotics, and antimicrobial peptides (BAP), alone and in combination, against fourteen multidrug-resistant (MDR) S. marcescens isolates sourced from hospitals and other environmental settings. Methods: S. marcescens was grown planktonically or in surface-associated biofilms, and biofilm biomass was measured via changes in absorbance and colony-forming units or live/death staining. Results: Combining bacteriophage with a low-dose cocktail of penicillin–streptomycin, kanamycin, and ciprofloxacin enhanced antimicrobial activity compared with antibiotics alone. Across the isolate panel, responses to BAP treatment varied according to determined antibiotic resistance profiles. The highly resistant AR-0517 isolate was selected for detailed mature biofilm analysis, where the BAP treatment reduced biofilm biomass by 97.8% and recoverable bacteria by 99.99%. Microscopy and viability assays further confirmed extensive biofilm disruption and bacterial killing. Conclusions: These findings demonstrate that simultaneous targeting of multiple bacterial pathways can enhance antimicrobial activity against MDR S. marcescens in vitro and support further evaluation of BAP as a potential strategy for biofilm-associated infections.

Serratia marcescens is an opportunistic pathogen that causes severe hospital-acquired infections, notable for its biofilm formation abilities and development of extensive antibiotic resistance. Here we evaluated the efficacy of bacteriophages, antibiotics, and antimicrobial peptides (BAP), alone and in combination, against fourteen multi-drug-resistant (MDR) S. marcescens isolates sourced from hospitals and other environmental settings in an in vitro biofilm model. Phage combination with a cocktail of sub-minimal inhibitory concentration (MIC) of penicillin-streptomycin, kanamycin, and ciprofloxacin, reduced biofilm biomass, however, complete decolonization was not achieved. Incorporating an antimicrobial peptide cocktail into this regimen eradicated 99.99% of multi-drug-resistant isolates grown planktonically or in surface-associated biofilms. Microscopy and viability assays confirmed extensive biofilm disruption and bacterial clearance without regrowth. These findings reveal that simultaneous interference of cell wall synthesis, protein translation, DNA replication, and membrane integrity can overcome S. marcescens antimicrobial defenses, establishing a multifaceted therapeutic framework for managing device-associated infections caused by MDR pathogens.

Background/Objectives: Biofilm formation by Acinetobacter baumannii contributes to its persistence in clinical settings and resistance to antibiotic treatment. This study aims to identify and characterize antimicrobials from lactic acid bacteria (LAB) using molecular and in silico approaches that can prevent and disrupt A. baumannii biofilms, assess their antimicrobial host range, and define their synergy with current antibiotics. Methods: Thirteen LAB isolates from the Human Microbiome Project were screened for anti-biofilm activity against A. baumannii. Conditioned media was further tested against six ESKAPE pathogens and three skin commensals. Lentilactobacillus hilgardii was selected for detailed study and antimicrobial peptide (AMP) prediction analysis due to limited toxicity toward commensals. In silico identified peptides were synthesized and tested individually and in combination with sub-MIC doses of an antibiotic. Results: Conditioned media from five LAB species (Lentilactobacillus hilgardii, Lentilactobacillus buchneri, Ligilactobacillus ruminis, Limosilactobacillus fermentum, and Limosilactobacillus antri) significantly inhibited A. baumannii biofilm formation and reduced biomass of mature biofilms. LAB-conditioned media also exhibited broad-spectrum activity against ESKAPE pathogens, though effects on commensals varied. Bioinformatically predicted AMPs from L. hilgardii inhibited planktonic A. baumannii growth but showed no direct biofilm activity even at high doses. However, AMPs were found to synergize with sub-MIC doses of meropenem against mature biofilms leading to decolonization. Conclusions: Our study provides a comprehensive platform for the discovery and characterization of AMPs and supports using commensal bacteria to reduce, prevent, and decolonize biofilms from pathogenic bacteria in community and nosocomial settings.

This study investigates the efficacy of conditioned media derived from lactic acid bacteria (LAB) in preventing and disrupting Acinetobacter baumannii biofilms. Of the 13 LAB isolates from the Human Microbiome Project tested, five distinct species Lentilactobacillus hilgardii, Lentilactobacillus buchneri, Ligilactobacillus ruminis, Limosilactobacillus fermentum, and Limosilactobacillus antri demonstrated significant prevention of A. baumannii biofilm formation and reduced the biomass of pre-formed mature biofilms. The study extended to the six ESKAPE pathogens and three skin commensals, demonstrating broad-spectrum antimicrobial activity of LAB conditioned media against ESKAPE pathogens, albeit with variable effects on skin commensals. L. hilgardii was characterized further due to its reduced toxicity against skin commensals. Fractionation studies indicated potential peptide involvement; thus, bioinformatic analysis was used to predict antimicrobial peptides (AMPs) within the L. hilgardii genome, which were synthesized and tested, showing effective inhibition of A. baumannii planktonic growth. However, these peptides did not affect biofilms at concentrations up to 30 µg/mL, although synergy with sub-MIC levels of meropenem significantly disrupted mature biofilms. Our study provides a comprehensive platform for the discovery and characterization of AMPs and support the notion of how commensals can be used to reduce, prevent and decolonize biofilms from pathogenic bacteria in community and nosocomial settings.

Streptococcus pneumoniae ( Spn ), a Gram-positive bacterium, is responsible for causing a wide variety of invasive infections. The emergence of multi-drug antibiotic resistance has prompted the search for antimicrobial alternatives. Phage-derived peptidoglycan hydrolases, known as endolysins, are an attractive alternative. In this study, an endolysin active against Spn , designated SP-CHAP, was cloned, produced, purified, biochemically characterized, and evaluated for its antimicrobial properties. Cysteine, histidine-dependent amidohydrolase/peptidase (CHAP) domains are widely represented in bacteriophage endolysins but have never previously been reported for pneumococcal endolysins. Here, we characterize the first pneumococcal endolysin with a CHAP catalytic domain. SP-CHAP was antimicrobial against all Spn serovars tested, including capsular and capsule-free pneumococci, and it was found to be more active than the most widely studied pneumococcal endolysin, Cpl-1, while not affecting various oral or nasal commensal organisms tested. SP-CHAP was also effective in eradicating Spn biofilms at concentrations as low as 1.56 µg/mL. In addition, a Spn mouse nasopharyngeal colonization model was employed, which showed that SP-CHAP caused a significant reduction in Spn colony-forming units, even more than Cpl-1. These results indicate that SP-CHAP may represent a promising alternative to combating Spn infections. IMPORTANCE: Considering the high rates of pneumococcal resistance reported for several antibiotics, alternatives are urgently needed. In the present study, we report a Streptococcus pneumoniae -targeting endolysin with even greater activity than Cpl-1, the most characterized pneumococcal endolysin to date. We have employed a combination of biochemical and microbiological assays to assess the stability and lytic potential of SP-CHAP and demonstrate its efficacy on pneumococcal biofilms in vitro and in an in vivo mouse model of colonization. Our findings highlight the therapeutic potential of SP-CHAP as an antibiotic alternative to treat Streptococcus pneumoniae infections.

The rise of multi-drug-resistant bacteria that cannot be treated with traditional antibiotics has prompted the search for alternatives to combat bacterial infections. Endolysins, which are bacteriophage-derived peptidoglycan hydrolases, are attractive tools in this fight. Several studies have already demonstrated the efficacy of endolysins in targeting bacterial infections. Endolysins encoded by bacteriophages that infect Gram-positive bacteria typically possess an N-terminal catalytic domain and a C-terminal cell-wall binding domain (CWBD). In this study, we have uncovered the molecular mechanisms that underlie formation of a homodimer of Cpl-1, an endolysin that targets Streptococcus pneumoniae. Here, we use site-directed mutagenesis, analytical size exclusion chromatography, and analytical ultracentrifugation to disprove a previous suggestion that three residues at the N-terminus of the CWBD are involved in the formation of a Cpl-1 dimer in the presence of choline in solution. We conclusively show that the C-terminal tail region of Cpl-1 is involved in formation of the dimer. Alanine scanning mutagenesis generated various tail mutant constructs that allowed identification of key residues that mediate Cpl-1 dimer formation. Finally, our results allowed identification of a consensus sequence (FxxEPDGLIT) required for choline-dependent dimer formation-a sequence that occurs frequently in pneumococcal autolysins and endolysins. These findings shed light on the mechanisms of Cpl-1 and related enzymes and can be used to inform future engineering efforts for their therapeutic development against S. pneumoniae.