Methicillin-resistant Staphylococcus aureus (MRSA) infections pose a serious threat worldwide. MRSA is the predominant species isolated from medical-device-related biofilm infections and chronic wounds. Its ability to form biofilms grants it resistance to almost all antibiotics on the market. Answering the call for alternative treatments, our lab has been investigating the efficacy of 600 Da branched polyethylenimine (BPEI) as a β-lactam potentiator against bacterial biofilms. Our previous study showed promise against methicillin-resistant Staphylococcus epidermidis biofilms. This study extends our previous findings to eradicate a more virulent pathogen: MRSA biofilms. Microtiter minimum biofilm eradication concentration models, crystal violet assays, and electron microscopy images show synergistic effects between BPEI and ampicillin as a two-step mechanism: step one is the removal of the extracellular polymeric substances (EPS) to expose individual bacteria targets, and step two involves electrostatic interaction of BPEI with anionic teichoic acid in the cell wall to potentiate the antibiotic.

Carbapenem‐resistant Enterobacteriaceae (CRE) are emerging pathogens that cause variety of severe infections. CRE evade antibiotic treatments because these bacteria produce enzymes that degrade a wide range of antibiotics including carbapenems and β‐lactams. The formation of biofilms aggravates CRE infections, especially in a wound environment. These difficulties lead to persistent infection and non‐healing wounds. This creates the need for new compounds to overcome CRE antimicrobial resistance and disrupt biofilms. Recent studies in our lab show that 600 Da branched polyethyleneimine (BPEI) and its derivative PEG350‐BPEI can overcome antimicrobial resistance and eradicate biofilms in methicillin‐resistant S. aureus, methicillin‐resistant S. epidermidis, P. aeruginosa, and E. coli. In this study, the ability of 600 Da BPEI and PEG350‐BPEI to eradicate carbapenem‐resistant Enterobacteriaceae bacteria and their biofilms is demonstrated. We show that both BPEI and PEG350‐BPEI have anti‐biofilm efficacy against CRE strains expressing Klebsiella pneumoniae carbapenemases (KPCs) and metallo‐β‐lactamases (MBLs), such as New Delhi MBL (NDM‐1). Furthermore, our results illustrate that BPEI affects planktonic CRE bacteria by increasing bacterial length and width from the inability to proceed with normal cell division processes. These data demonstrate the multi‐functional properties of 600 Da BPEI and PEG350‐BPEI to reduce biofilm formation and mitigate virulence in carbapenem‐resistant Enterobacteriaceae.

Biofilm formation is an adaptive resistance mechanism that pathogens employ to survive in the presence of antimicrobials. Pseudomonas aeruginosa is an infectious Gramnegative bacterium whose biofilm allows it to withstand antimicrobial attack and threaten human health. Chronic wound healing is often impeded by P. aeruginosa infections and the associated biofilms. Previous findings demonstrate that 600 Da branched polyethylenimine (BPEI) can restore β-lactam potency against P. aeruginosa and disrupt its biofilms. Toxicity concerns of 600 Da BPEI are mitigated by covalent linkage with low-molecularweight polyethylene glycol (PEG), and, in this study, PEGylated BPEI (PEG350-BPEI) was found exhibit superior antibiofilm activity against P. aeruginosa. The antibiofilm activity of both 600 Da BPEI and its PEG derivative was characterized with fluorescence studies and microscopy imaging. We also describe a variation of the colony biofilm model that was employed to evaluate the biofilm disruption activity of BPEI and PEG-BPEI.