AMPs compromise bacterial membrane integrity, thus enhancing the penetration and efficacy of antibiotics. Specifically, AMPs enable antibiotics that target intracellular processes to reach their sites of action more efficiently. This action is crucial against bacteria equipped with efflux pumps that actively expel antibiotic molecules. For instance, the synthetic peptide β-Ala-modified analogs of anoplin demonstrate significant membrane disruption, followed by enhanced antimicrobial potency and synergistic effects with conventional antibiotics against drug-resistant Pseudomonas aeruginosa, without prompting resistance development (Zhong et al., 2020).

Similarly, the antimicrobial peptide LL-37, when combined with colistin, showed strong synergy, drastically reducing the minimum inhibitory concentrations (MIC) against multidrug-resistant Escherichia coli. This highlights LL-37's role in membrane permeabilization and efflux pump circumvention (Morroni et al., 2021). In another study, pleurocidin was found to enhance antibiotic effectiveness by inducing hydroxyl radical formation, which contributes to membrane damage, as well as by disrupting bacterial cytoplasmic membranes, thereby promoting antibiotic entry (Choi and Lee, 2012).

Cathelicidin peptides also demonstrated such mechanisms by disrupting bacterial cell membranes and enhancing the bactericidal activity of co-administered antibiotics such as aureomycin against enteric pathogens (Liu et al., 2011). In similar synergistic interactions, the peptide coprisin not only exhibited intrinsic antimicrobial properties but also enhanced the activity of conventional antibiotics by facilitating their access to internal bacterial targets, crucial for combating biofilm-forming bacteria (Hwang et al., 2013).

The combined use of peptides PMAP-36 and PRW4 with aminoglycosides also showcased the role of membrane permeabilization, where these peptides enhance the intracellular delivery of antibiotics, contributing to a synergistic antibacterial effect (Wang et al., 2014).

These findings collectively highlight the critical role of AMPs in disrupting bacterial membranes, which not only enhances the efficacy of antibiotics against resistant strains but also broadens the therapeutic window of existing antimicrobial agents. This provides a robust strategy against infections that are difficult to treat with traditional antibiotics alone.

AMPs have demonstrated an ability not only for disrupting the biofilm matrix but also for inhibiting the formation of biofilms in the first place. As such, AMPs can facilitate the action of antibiotics by exposing otherwise protected bacteria within a biofilm structure, thus enhancing their susceptibility to treatments. This mechanism is particularly important in addressing persistent infections.

The peptide Cecropin A has been shown to disrupt uropathogenic E. coli biofilms, enhancing the efficacy of the antibiotic nalidixic acid, and leading to a synergistic clearance of infections without inducing resistance, a vital aspect in treating recurrent infections (Kalsy et al., 2020). Similarly, the peptide GVF27 targets biofilms formed by Burkholderia cepacia complex, known for its robust antibiotic resistance, enhancing the effects of traditional antibiotics like ciprofloxacin (Bosso et al., 2022).

In another study involving both in vitro and in vivo experiments with murine subcutaneous abscess model, the combination of synthetic peptides with meropenem and erythromycin significantly reduced infection sizes caused by ESKAPE pathogens (E. faecium, Staphylococcus aureus, K. pneumoniae, Acinetobacter baumannii, P. aeruginosa, and Enterobacter spp), which are notorious for their biofilm-forming capabilities and multidrug resistance (Pletzer et al., 2018).

Additionally, peptide Pt5-1c, when combined with vancomycin and streptomycin, has shown to not only disrupt biofilms but also restore antibiotic sensitivity in multidrug-resistant strains, offering a dual advantage (Duan et al., 2021). The LL-37 peptide has also been reported to significantly reduce biofilm formation and, in combination with colistin, shows enhanced bactericidal activity against multidrug-resistant bacteria (Morroni et al., 2021).

The AMP-antibiotic synergy against biofilms have also been observed citropin 1.1, which, when combined with rifampicin, shows enhanced activity against Rhodococcus equi biofilms (Giacometti et al., 2005). Furthermore, combinations of early generation antibiotics with AMPs have been effective against biothreat agents like Burkholderia mallei and Yersinia pestis, indicating the potential of these combinations in both clinical applications (Cote et al., 2020).

AMPs can also directly augment the potency of conventional antibiotics through mechanisms that modify bacterial metabolic processes and increase antibiotic sensitivity. For instance, the peptide CLP-19 was reported to synergistically enhance the effects of both bactericidal and bacteriostatic agents. It significantly reduces the adverse effects associated with antibiotic-induced endotoxin release, which is especially important in severe infections (Li et al., 2017).

Additionally, studies have shown that some AMP SPR741 can potentiate antibiotics by circumventing bacterial resistance mechanisms, such as efflux pumps in E. coli, thus enabling higher intracellular concentrations of antibiotics (Corbett et al., 2017).

In another example, SLAP-S25, a peptide incorporating non-natural amino acids, has a minimal antibacterial effect on its own but substantially increases the efficacy of a broad range of antibiotics against multidrug-resistant pathogens. This enhancement is due to its ability to act alongside antibiotics in overcoming bacterial defenses (Song et al., 2020).

Antibiotic azithromycin was shown to have enhance activity when used in combination with AMPs. Although traditionally not recommended for multidrug-resistant Gram-negative infections, azithromycin showed significant bactericidal activity when combined with colistin, pointing to potential against difficult-to-treat infections (Lin et al., 2015).

The combination of AMPs and conventional antibiotics not only holds promise in vitro but has also proven effective in vivo, significantly reducing infection levels and suggesting a viable strategy for enhancing the efficacy of existing antibiotics. This approach may help mitigate the development of antibiotic resistance and improve treatment outcomes for infections caused by resistant bacteria (Kampshoff et al., 2019).

Some studies have reported the role of AMPs in inhibiting antibiotic resistance through various mechanisms, which is crucial for managing drug-resistant infections (Maron et al., 2022). This offers mechanisms that traditional antibiotics cannot exploit. For instance, proline-rich AMP A3-APO, in combination with colistin, was shown to significantly improve treatment efficacy and hindered resistance in treated pathogens (Otvos et al., 2018). This synergy is also seen in other combinations, where AMPs and antibiotics together achieve a greater antimicrobial effect and reduce the likelihood of resistance development (Zharkova et al., 2019).

Further, the AMP Cecropin A disrupts uropathogenic E. coli biofilms and inhibits efflux pump activity, the two mechanisms that induce bacterial resistance. Cecropin A was shown to slow the emergence of resistance while clearing infections (Kalsy et al., 2020). Similarly, β-Ala modified peptides like Ano-1β and Ano-8β exhibit strong membrane disruption and are noted for their low propensity for resistance development compared to standard antibiotics (Zhong et al., 2020).

Moreover, novel AMPs like CSM5-K5 not only exhibit potent bactericidal activity but also restore antibiotic sensitivity in previously resistant strains. Such findings are crucial, as they show that AMPs can reverse resistance trends-a significant advantage in the current era of high antibiotic failure rates (Thappeta et al., 2020).

In in vivo settings, AMPs have been observed to enhance immune responses, a mechanism not tested in in vitro experiments. For example, the combination of PMAP-36 and tetracycline not only reduced bacterial load but also promoted the migration of monocytes/macrophages to the site of infection, significantly increasing survival in murine models (Tao et al., 2023).

Another unique in vivo mechanism involves the reduction of inflammation and cytokine production. Certain AMPs, when combined with antibiotics, have demonstrated a significant decrease in inflammatory markers and cytokine levels, contributing to better clinical outcomes. This effect is particularly beneficial in treating infections where inflammation exacerbates the disease process, such as in cystic fibrosis or sepsis (Aoki et al., 2009; Herrmann et al., 2010).

An important advantage of using AMPs in combination with antibiotics is the delayed emergence of bacterial resistance. In vivo studies demonstrate that AMPs can sustain antibiotic efficacy and reduce the evolution of resistant strains, providing a sustainable approach to managing bacterial infections. For instance, PMAP-36 has been shown to delay the development of resistance to tetracycline in porcine extraintestinal pathogenic E. coli, highlighting its potential as part of a combination therapy to extend the usefulness of existing antibiotics (Hu et al., 2015; Tao et al., 2023).

Beyond their antimicrobial action, some AMP-antibiotic combinations have been shown to possess wound healing and tissue-regenerative properties. Minardi et al. (2007) demonstrated that Tachyplesin III, when combined with piperacillin-tazobactam, prevented P. aeruginosa biofilm formation on ureteral stents in a rat model, highlighting the potential for preventing infections in patients with indwelling medical devices. Desbois et al. (2010) explored the synergistic effect of ranalexin with lysostaphin in wound and systemic infections, showing significant reduction in MRSA burden in both rabbit and mouse models.

Modifying the structure of AMPs is a fundamental approach to enhance their utility in combination therapies. Strategies such as amino acid substitution, cyclization, and the incorporation of non-natural amino acids are employed to improve stability, membrane penetration, and specificity. Such modifications are designed to optimize the interaction of AMPs with bacterial membranes, thereby enhancing their antimicrobial effectiveness while reducing potential cytotoxic effects on mammalian cells (Giacometti et al., 2005; Taheri-Araghi and Ha, 2007; Choi and Lee, 2012; Khara et al., 2015; Mittal et al., 2016; Nešuta et al., 2016).

Tailoring therapy to the specific pathogens and their resistance mechanisms can significantly enhance the efficacy of AMP-antibiotic combinations. This precision medicine approach involves selecting AMPs and antibiotics that complement each other's mechanisms of action and are effective against the resistance profiles of the target bacteria. Consideration of the local microenvironment at the infection site is crucial to this strategy. Although still in its infancy, such targeted approaches hold great promise for improving therapeutic outcomes (Yu et al., 2016; Zhu et al., 2022).

To further refine optimization strategies, high-throughput screening methods and computational modeling serve as essential tools. These techniques allow for the rapid identification of effective AMP-antibiotic pairs by predicting potential synergies based on the properties of the drugs and the characteristics of the target bacteria. This can significantly accelerate the development of effective combination therapies (Maisetta et al., 2009; Paula Jorge and Pereira, 2012; Mookherjee et al., 2020).

The use of bioinformatics tools and comprehensive databases that catalog information on AMPs supports the discovery and design of novel AMPs. These resources are invaluable for researchers seeking to develop new synergistic combinations that can be effectively integrated into clinical practice (Maisetta et al., 2009; Paula Jorge and Pereira, 2012).

These strategies represent a multifaceted approach to enhancing the synergy between AMPs and conventional antibiotics. Each method contributes to a deeper understanding and more effective application of these combinations in the battle against resistant infections. As research progresses, these optimization strategies will likely evolve, offering new avenues for combating antimicrobial resistance and optimizing therapeutic strategies.