Beyond Traditional Antimicrobials: A Caenorhabditis elegans Model for Discovery of Novel Anti-infectives

Abstract

The spread of antibiotic resistance amongst bacterial pathogens has led to an urgent need for new antimicrobial compounds with novel modes of action that minimize the potential for drug resistance. To date, the development of new antimicrobial drugs is still lagging far behind the rising demand, partly owing to the absence of an effective screening platform. Over the last decade, the nematode Caenorhabditis elegans has been incorporated as a whole animal screening platform for antimicrobials. This development is taking advantage of the vast knowledge on worm physiology and how it interacts with bacterial and fungal pathogens. In addition to allowing for in vivo selection of compounds with promising anti-microbial properties, the whole animal C. elegans screening system has also permitted the discovery of novel compounds targeting infection processes that only manifest during the course of pathogen infection of the host. Another advantage of using C. elegans in the search for new antimicrobials is that the worm itself is a source of potential antimicrobial effectors which constitute part of its immune defense response to thwart infections. This has led to the evaluation of effector molecules, particularly antimicrobial proteins and peptides (APPs), as candidates for further development as therapeutic agents. In this review, we provide an overview on use of the C. elegans model for identification of novel anti-infectives. We highlight some highly potential lead compounds obtained from C. elegans-based screens, particularly those that target bacterial virulence or host defense to eradicate infections, a mechanism distinct from the action of conventional antibiotics. We also review the prospect of using C. elegans APPs as an antimicrobial strategy to treat infections.

Keywords: Caenorhabditis elegans; anti-virulence; antimicrobial peptides; antimicrobials; immunomodulator.

FIGURE 1

Schematic diagram illustrating the possible outcomes of a Caenorhabditis elegans-based anti-infective screen. From a screen, the compounds can be categorized as those with possible toxic effect, potential anti-infective effect or no effect. The hits that promote the survival of C. elegans following an infection can either act as anti-microbial, anti-virulence, or immunoactivator/immunomodulator. The toxic effect of a compound can be further validated by exposing the worms to the particular compound in an uninfected condition where the worms are fed with dead Escherichia coli OP50.

FIGURE 2

Overview of the general experimental approaches used to characterize an anti-infective candidate. A potential anti-infective compound that does not directly target bacterial growth can either impair pathogen virulence and/or enhance/modulate host immune responses. Various strategies from both pathogen and host perspectives can be employed to further characterize the molecular mechanism(s) of the compound of interest. Whole-genome transcriptome profiles of the compound-treated pathogen or host can be generated to study the effect of the compound. To assess the effect on bacterial virulence, qualitative and quantitative biochemical tests as well as biofilm production and quorum sensing assays can be performed. Bacterial mutants are useful for target identification while GFP-expressing bacteria may assist in visualization of in vivo bacterial colonization. Electron microscopy can be used to observe the formation of biofilm or bacterial structures (e.g., flagella). In silico molecular docking provides a clue of the possible binding interference between a bacterial protein and the compound of interest. From the host perspective, the live bacteria can be recovered from the C. elegans intestine and enumerated. The readily available transgenic mutant strains are useful for host target identification and pathway elucidation.

FIGURE 3

Induction of APPs in C. elegans. When C. elegans encounters a single pathogen, it will secrete a mixture of APPs as part of its protective mechanism, specifically (A) caenopores/saposin-like proteins (SPPs), antibacterial factor proteins (ABFs) and invertebrate lysozymes (ILYSs) in the digestive tract, and (B) neuropeptide-like proteins (NLPs) at the epidermis. The dashed line arrow denotes the route of infection.