**1. Introduction**

Antimicrobial resistance (AMR) threatens the effective treatment, control and management of an increasing range of infections caused by viruses, bacteria, parasites and fungal pathogens [1]. Development of effective therapies to suppress human immunodeficiency virus (HIV) and hepatitis B virus (HBV) has been achieved with great success [2]. However, the medicines are not curative, and therefore more efforts in HIV and HBV drug discovery are directed toward longer-acting therapies or compounds with new mechanisms of action that could potentially lead to a cure or complete eradication of the viruses [2]. In 2010, an estimated 7–20% of people starting antiretroviral therapy (ART) globally had drug-resistant HIV [3]. Some countries have recently reported levels of 15% amongst those starting HIV treatment, and up to 40% among people re-starting therapy [3]. Of great concern are the high levels of viral resistance towards nucleoside reverse transcriptase inhibitors (NRTIs) as recently found in Kenyan children [4]. In 2015, the World Health Organization (WHO) recommended that everyone living with HIV should start on antiretroviral treatment. Hence, increased ART resistance is expected as more people start ART [3]. The United States Food and Drug Administration (FDA) approved anti-HIV drugs are classified into eight classes according to modes of action [5]. The common side effects of Abacavir® and other NRTIs are that they can cause life-threatening side effects, including a serious allergic reaction, a build-up of lactic acid in the blood, and hepatotoxicity [6,7]. In fact, each drug class of the FDA approved anti-HIV drugs has side effects,

and some are contra-indicated for co-infected patients with HIV and HBC or HCV [8]. On the other hand, all influenza A viruses circulating in humans were reported to be resistant to Amantadine® and Rimantadine®, two essential antivirals for treatment of epidemic and pandemic influenza A. However, the frequency of resistance to Oseltamivir® another antiviral with different mode of action for treating influenza A remains low at 1–2% [3]. Treatment failure of antivirals has been suggested to be caused by the emergence of recombinant viruses, drug resistance, and cell toxicity [9,10]. Compounds with a different mode of action can play an essential role in overcoming AMR. Viral disease such as influenza spreads fast and knows no borders, with the vast masses of people travelling all over the globe due to efficient transport systems. Hence, there is an urgent need for international collaboration to identify new antiviral agents with new modes of action and better efficacy.

Myxobacteria are well-known to be producers of biologically active secondary metabolites with novel carbon skeletons and new modes of action [11–13]. Many compounds isolated from myxobacteria have recently been found to have impressive antiviral activity. More so, some have been found to have an unusual broad-spectrum antiviral activity [11,12,14].

#### **2. Myxobacteria**

Myxobacteria are δ-proteobacteria belonging to the order *Myxococcales*. They are rod-shaped, Gram-negative bacteria that exhibit gliding motility and swarm on solid surfaces. Under nutrient-limiting conditions, they form species-specific fruiting bodies (Figure 1) [15]. Within these fruiting bodies, some vegetative cells convert to myxospores, which are desiccation-resistant and can survive over decades. Under appropriate conditions the spores germinate [16]. These soil-dwelling microorganisms have also been isolated from other habitats such as the bark of trees, oceans, freshwater lakes, and herbivore dung [15,17]. Myxobacteria have also been isolated from extreme environments such as desert soils [18]. Numerous unique classes of secondary metabolites have been isolated from myxobacteria, the majority of which are biogenetically derived from polyketide synthases (PKSs) and non-ribosomal peptide synthetases (NPRSs) or a hybrid of PKSs and NPRSs [12,15,19,20]. PKSs and NPRSs are enzymatic "assembly lines" of complex multi-step biosynthetic pathways for making compounds by catalysing the stepwise condensation of a starter unit with small monomeric building blocks [19]. The ability to produce unique metabolites is conferred by the creative biosynthetic pathways and the large genome of 9–14 Mb [21], consistent with the strengthening correlation between genome size and the extent of secondary metabolites produced [21–25]. In fact, the sequenced genomes of myxobacteria are the largest yet known from any bacterium [20–23,25]. In the last 35 years, over 100 new carbon skeleton secondary metabolites, with over 600 analogues, have been isolated from over 9000 strains of myxobacteria [12]. The metabolites exhibited antifungal, antibacterial, antimalarial, antitumor, and anti-immunomodulatory properties some with novel modes of action and have been reviewed extensively [12,20,26]. Microorganisms are valuable as producers of bioactive metabolites because they can be cultivated in bioreactors from as little as below 5 mL to large scales of over 100,000 L, making the production of natural products independent of season, locality, or climate [15]. Furthermore, conditions in a bioreactor are controllable to optimise production of the desired outcome. Particularly important as illustrated by Zeeck et al., 2002 in the 'OSMAC' (One Strain-Many Compounds) approach, which revealed that microorganisms do not exhaust their potential for producing metabolites under standard laboratory conditions [27].

**Figure 1.** Images of myxobacteria. (**A**–**C**): *Sorangium cellulosum*; (**A**): Fruiting bodies; (**B**): Swarming on agar plate; (**C**): Cells from the liquid medium under the light microscope. (**D**–**E**): Images of the producers ofthiangazole (**7**), phenalamide A1 (**8**) and phenoxan (**9**), from agar plates; (**D**): *Myxococcus stipitatus*; (**E**): *Polyangium* species; (**F**–**G**): *Angiococcus disciformis*(strain An d30) producer of myxochelins; (**F**): culture under the light microscope from liquid media; (**G**): culture on agar plate. Images provided by Joachim Wink (HZI Braunschweig).

#### **3. Secondary Metabolites from Myxobacteria with Antiviral Activity**
