polymers Review Microbial Degradation of Rubber: Actinobacteria Ann Anni Basik 1,2, Jean-Jacques Sanglier 2, Chia Tiong Yeo 2 and Kumar Sudesh 1,* 1 Ecobiomaterial Research Laboratory, School of Biological Sciences, Universiti Sains Malaysia, Gelugor 11800, Penang, Malaysia; [email protected] 2 Sarawak Biodiversity Centre, Km. 20 Jalan Borneo Heights, Semengoh, Kuching 93250, Sarawak, Malaysia; [email protected] (J.-J.S.); [email protected] (C.T.Y.) * Correspondence: [email protected]; Tel.: +60-4-6534367; Fax: +60-4-6565125 Abstract: Rubber is an essential part of our daily lives with thousands of rubber-based products being made and used. Natural rubber undergoes chemical processes and structural modifications, while synthetic rubber, mainly synthetized from petroleum by-products are difficult to degrade safely and sustainably. The most prominent group of biological rubber degraders are Actinobacteria. Rubber degrading Actinobacteria contain rubber degrading genes or rubber oxygenase known as latex clearing protein (lcp). Rubber is a polymer consisting of isoprene, each containing one double bond. The degradation of rubber first takes place when lcp enzyme cleaves the isoprene double bond, breaking them down into the sole carbon and energy source to be utilized by the bacteria. Actinobacteria grow in diverse environments, and lcp gene containing strains have been detected from various sources including soil, water, human, animal, and plant samples. This review entails the occurrence, physiology, biochemistry, and molecular characteristics of Actinobacteria with respect to its rubber degrading ability, and discusses possible technological applications based on the activity of Actinobacteria for treating rubber waste in a more environmentally responsible manner. Citation: Basik, A.A.; Sanglier, J.-J.; Keywords: latex clearing protein; rubber; degradation; actinobacteria; distribution; diversity Yeo, C.T.; Sudesh, K. Microbial Degradation of Rubber: Actinobacteria. Polymers 2021, 13, 1989. https://doi.org/10.3390/ polym13121989 1. Rubber—Polyisoprenes 1.1. Natural Rubber (NR) Academic Editor: Abdel-Hamid A large fraction of the organic biomass on earth consists of biopolymers; such as I. Mourad polysaccharides, polyamino acids (proteins), polyconiferylalcohols (lignins), polyhydrox- yalkanoic acids (PHAs), and polyisoprenes (rubbers) [1]. Natural rubbers (NR) farmed Received: 28 April 2021 from Hevea brasiliensis Muell. Arg—comprising 99% of the world market—and Parthenium Accepted: 7 June 2021 argentatum (guayule rubber) are produced commercially [2]. NR has a (cis)-1,4-polyisoprene Published: 17 June 2021 as its polymer backbone. The backbone consists of isoprene units (C5H8) each contain- ing one double bond in the cis or trans configuration, while 3 trans-isoprene units are Publisher’s Note: MDPI stays neutral found at one end of the molecule followed by several hundred to a few thousand cis- with regard to jurisdictional claims in isoprene units (Figure1)[3] . Most rubber-accumulating plants—and there are more than published maps and institutional affil- 2000 dicotyledons, and some fungi known to do so [4]—synthesize the polymer with iations. the isoprene units in the cis-configuration [5]. Some species, such as Manilkara chicle or Palaquium gutta, however, synthesize the trans-polymer producing rubbers known as chicle or gutta-percha [6]. The NR latex of H. brasiliensis origin is composed of 25–35% (w/w) polyisoprene; Copyright: © 2021 by the authors. 1.0–1.8% (w/w) protein; 1–2% (w/w) carbohydrates; 0.4–1.1% (w/w) neutral lipids; 0.5–0.6% Licensee MDPI, Basel, Switzerland. (w/w) polar lipids; 0.4–0.6% (w/w) inorganic components; 0.4% (w/w) amino acids, amides, This article is an open access article etc.; and 50–70% (w/w) water (Figure1)[ 3]. The polymer is present in 3- to 5-µm so-called distributed under the terms and rubber particles, which are covered by a layer of proteins and lipids [7]. By virtue of its conditions of the Creative Commons molecular structure, NR latex is a crossed-linked polymeric material that is highly flexible Attribution (CC BY) license (https:// and extensible. creativecommons.org/licenses/by/ 4.0/). Polymers 2021, 13, 1989. https://doi.org/10.3390/polym13121989 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 27 Polymers 2021, 13, 1989 2 of 27 Figure 1. NR Polymer Chain. Figure 1.1.2. NR Polymer Production Chain. and Usage NR is an essential raw material that is used to manufacture up to 50,000 (99% commer- 1.2. Productioncially used and Usage NR) different rubber and latex products [8,9]. The discovery of NR is attributed NRto is thean essential Olmec (also raw known material as that the is “rubber used to people”) manufacture one of up the to first 50,000 major (99% civilizations com‐ in merciallywhat used is NR) known different as the rubber Gulf of and Mexico latex products today, around [8,9]. The 1600 discovery B.C [10]. of However, NR is at‐ the first tributedpractical to the Olmec application (also known in the rubber as the industry“rubber people”) was discovered one of inthe 1839 first when major Charles civiliza Goodyear‐ tions in accidentallywhat is known dropped as the rubberGulf of andMexico sulfur today, on a around hot stovetop 1600 B.C and [10]. discovered However, vulcanization, the first practicala chemical application transformation in the rubber that improvesindustry was NR’s discovered elastic properties in 1839 [11when]. At Charles the end of the Goodyear1800s, accidentally the automobile dropped industry rubber and sulfur the resulting on a hot need stovetop for tyres and drove discovered an upsurge vul‐ of the canization,then a nascentchemical rubber transformation industry [12 that]. Prior improves to conversion NR’s elastic into rubberproperties products, [11]. At latex the from the end of therubber 1800s, tree the undergoes automobile several industry manufacturing and the resulting processes need whereby for tyres chemicals drove an are up added‐ to surge ofact the as then preservatives, nascent rubber anticoagulants, industry [12]. vulcanizing Prior to conversion agents, and into antioxidants rubber products, [13]. latex from the rubber tree undergoes several manufacturing processes whereby chemicals 1.3. Synthetic Rubber (IR) are added to act as preservatives, anticoagulants, vulcanizing agents, and antioxidants [13]. In 1909, synthetic rubber or isoprene rubber (IR) was prepared by Fritz Hofmann [14], but due to their different properties, the usage of both NR and IR continued to increase 1.3. Synthetic2.6-fold Rubber and (IR) 1.6-fold respectively from 1990 to 2017 [15]. IR refers to an artificial elastomer, mainly synthesized from by-products of the petroleum refining process [16]. There are In 1909, synthetic rubber or isoprene rubber (IR) was prepared by Fritz Hofmann approximately 20 different chemical types of IR, with different grades of rubber in each of [14], but due to their different properties, the usage of both NR and IR continued to in‐ those chemical categories [17]. Many of the IR consist of a mixture of copolymers whereby, crease 2.6‐fold and 1.6‐fold respectively from 1990 to 2017 [15]. IR refers to an artificial specific properties such as high temperature resistance, good resistant to abrasion, strength, elastomer,etc., mainly are achieved synthesized by changing from by the‐products composition of the of petroleum the copolymers. refining Both process NR and [16]. IR differ in There aremicro-structure, approximately but 20 different both have chemical isoprene types as the of mainIR, with chain. different grades of rubber in each of those chemical categories [17]. Many of the IR consist of a mixture of copolymers whereby,1.4. specific Rubber properties Wastes—Mitigation such as high and temperature Drawbacks resistance, good resistant to abra‐ sion, strength,Knowledge etc, are achieved on the by fate changing of rubber the materials composition in nature of the is stillcopolymers. limited. Rate Both of NR decompo- and IR differsition in depends micro‐structure, on the type but of both rubber, have its isoprene composition, as the and main the chain. environment; rubber bands take up to a year, latex glove take several months to years, rubber boot soles (synthetic 1.4. Rubberrubber) Wastes—Mitigation take 50–80 years and and Drawbacks tyre takes up to 2000 years to decompose [18]. Sustainabil- Knowledgeity efforts on for the managing fate of rubber waste materials rubber in products nature is must still includelimited. self-remediationRate of decompo‐ through sition dependsbiodegradation on the type of rubberof rubber, (pre-consumer its composition, and post-consumer)and the environment; in waste rubber sites bands [19]. take up to a Approximatelyyear, latex glove 60% take of several the worldwide months to consumption years, rubber of boot rubber soles is (synthetic attributed to the rubber) takeglobal 50–80 tyre years manufacturing and tyre takes industry, up to 2000 while years the to remaining decompose rubber [18]. Sustainability consumption is used efforts forto manufacturemanaging waste a wide rubber variety products of products must include such as self rubber‐remediation boots, rubber through mulch, bio‐ rubber degradationbands, of andrubber more (pre [17‐consumer]. Tyre rubbers and post usually‐consumer) consist in of waste 40–50% sites rubber [19]. (styrene-butadiene Approximatelyrubber, NR, and60% butyl of the rubber), worldwide 25–40% consumption carbon black, of andrubber 10–15% is attributed low-molecular-weight to the global tyreadditives manufacturing [20]. There industry, is currently while no the sustainable remaining and rubber environmentally consumption friendly is used methodto of manufactureproperly a wide recycling variety used of products car tyres such into as new rubber ones, boots, or for rubber converting mulch, the rubber rubber bands, polymer into and moreother [17]. industrially Tyre rubbers useful usually organic consist compounds of 40–50% [21 rubber]. (styrene‐butadiene rubber, NR, and butylLandfill rubber), and 25–40% stockpiles: carbon Each black, year, and 1.5 10–15% billion low end-of-life‐molecular tyres‐weight (ELTs) additives enter the envi- [20].