The earliest report on the degradation of rubber by microbes was published in 1914, when Söhngen and Fol utilized highly pure NR as the only carbon source to demonstrate the absorption of the hydrocarbon chain by microorganisms (Söhngen and Fol, 1914). Two Actinomycetes were discovered to form colonies on the purified rubber material. Spence and Van Niel (1936) first isolated bacteria capable of degrading rubber by employing latex overlay plates. The formation of translucent halos within the opaque agar layer indicated the beginning of colony formation. The clear zone isolation technique enables the detection of bacteria that secrete extracellular enzymes during the degradation of cis-1,4-polyisoprene. This method selects NR utilizing strains based on the formation of translucent halos (hyaline zones) around colonies on opaque NR agar plates, which can be directly observed by the naked eye after the degradation action of the strain. Members of these groups usually belong to the class of Actinomycetes that can form mycelium, such as Actinobacteria, Streptomyces, and Micromonospora. In contrast, the second group of microorganisms that degrade NR do not produce any clear zones, and only the normal growth of strains can be observed on NR agar plates without forming any clear circles around the colonies. Therefore, these strains need to be in direct contact with the rubber matrix to function. These bacteria attack the rubber matrix directly, form biofilms and bind to polymers, which begin to degrade at the cell surface. Examples of this group are Nocardia, Mycobacterium, and Gordonia (Gallert, 2000).

At present, researchers have found that some microorganisms with special origins have some biodegradation effect on NR, and the microbial degradation of NR has been studied by extracting strains from specific environments (Tables 1, 2). Actinomycete isolated from sludge at wastewater treatment plants could grow and multiply in liquid medium using NR as a carbon source, and it was confirmed that the isolated strains could reduce the molecular weight of NR thus achieving microbial degradation of NR; Gordonia and Actinomycete (Hiessl et al., 2012; Ali Shah et al., 2013) isolated from soil at rubber plantations were found to be effective in microbial degradation of NR; the use of insect laccase and manganese peroxidase secreted by Penicillium can also be used for the biodegradation of NR (Nayanashree and Thippeswamy, 2015b). It has been reported that Rhodococcus, the predominant species accountable for the degradation of NR, is capable of degrading organic pollutants and is therefore considered effective (Dey et al., 2020). Streptomyces have been studied by researchers to degrade non-vulcanized fresh latex and common vulcanized rubber products such as NR gloves, NR condoms and rubber car tires (Nanthini and Sudesh, 2017). Variovorax degrades ethylene glycol phthalate (Prasad, 2017) that can be used as a plasticizer for plastic and rubber products and is used as a component of NR products with improved strength. It is presumed that strains of this genus can also degrade NR. Pseudomonas has been shown to make use of NR hydrocarbons and subsequently can degrade NR (Roy et al., 2006).

The two major NR degrading enzymes rubber oxygenase (RoxA/RoxB) and latex clearing protein (Lcp) have been identified in Gram-negative and Gram-positive bacteria, respectively (Table 3). Among Gram-positive NR degrading bacteria, Lcp is responsible for the initial oxidative cleavage of cis-1,4-polyisoprene in the strains, first demonstrated in Streptomyces sp. K30. The Lcp of this strain was experimentally shown to be a type B cytochrome that cleaves cis-1,4-polyisoprene to a low molecular weight product containing isoprene with aldehyde and ketone termini. The resulting compound is further degraded to the corresponding acid by oxidoreductase OxiAB, which is encoded by the OxiAB gene and subsequently enters the β-oxidation pathway via the action of coenzyme A for eventual oxidative catabolism. Yikmis et al. found that transcription of Lcp increases in the presence of latex or synthetic cis-1,4-polyisoprene and that Lcp transported via the bisarginine transport (Tat) system is secreted (Jendrossek and Reinhardt, 2003; Yikmis et al., 2008; Imai et al., 2011; Imai et al., 2013; Sharma et al., 2018), thus confirming the role of Lcp in the degradation of NR. In Gram-negative NR degrading bacteria, including some mycobacteria, the rubber oxygenase RoxA is responsible for the degradation of cis-1,4-polyisoprene. RoxA, encoded by the roxA gene, is an exo-type and a highly representative rubber oxygenase that occupies the same important position as the Lcp. It was first isolated and confirmed in strain Xanthomonas sp. 35Y. RoxA was shown to be a key enzyme involved in the degradation of cis-1,4-polyisoprene in strain Xanthomonas sp. 35Y. RoxA first binds oxygen molecules and then cleaves the carbon-carbon double bond of cis-1,4-polyisoprene to produce isoprene with aldehyde and ketone groups at the end of the molecule (Braaz et al., 2004; Hiessl et al., 2014; Luo et al., 2014). RoxA is an extracellular dioxygenase that oxidize cis-1,4-polyisoprene to 12-oxo-4,8-dimethyltridecane-4,8-diene-1-aldehyde (ODTD) thereby degrading NR. Conversely, the rubber oxygenase RoxB is also involved in the degradation process, which degrades NR by endo-cleavage of cis-1,4-polyisoprene. It is known from the experimental results of Daisuke Kasai et al. that the LatA protein is a key enzyme for the degradation of NR in strain NBRC 109400 demonstrated by the high-throughput transposon insertion technique (Birke et al., 2013; Seidel et al., 2013; Kasai et al., 2017). The direct relatives of functional RoxB and RoxA, namely, LatA1 and LatA2, respectively, were reported to be essential for NR degradation in Rhizobacter gummiphilus NS21 (Linos et al., 2000; Ibrahim et al., 2006; Yikmis and Steinbüchel, 2012).

The rubber oxidases identified so far include the exo-type rubber oxygenase A. Nevertheless, the exo-type of reaction is accountable for its suboptimal degradation efficiency and hinders the rapid and efficient degradation of polymers. This is presumably due to the inefficient threading of the substrate chain into the enzyme (Seidel et al., 2013). The endo-type rubber oxygenase B is capable of cleaving polyisoprene to C15 oligopolyisoprene (ODTD) (Jendrossek and Birke, 2019). RoxB can endo-cleave the carbon-carbon double bond of polyisoprene to produce oligopolyisoprenes with varying degrees of polymerization (Birke et al., 2017), and this endo-type of rubber oxidase is more suitable for polyolefin degradation systems (Zhang et al., 2022). Gibson et al. (2020) screened Nocardioides WS12, which is capable of degrading isoprene, from soil near willow trees, and they found that the strain’s genome also contained genes related to the degradation of rubber and speculated that it might also possess the ability to degrade NR.

Researchers have shown through experimental studies and practical applications that the degradation ability of a single strain of bacteria for NR is limited, and the speed and efficiency are very restricted, while multiple microorganisms interacting with each other for a long period of time will give better results, so they have focused on bacterial consortia. The construction of complex consortia is no longer limited to the cultivation of a single microorganism (Nguyen et al., 2020; Castañeda Alejo et al., 2022; Joseph et al., 2022). Instead, a variety of microorganisms are co-cultured in a specific environment. Through the interactions between the strains, they directly or indirectly influence the micro-ecosystem maintaining a balance. The interrelationship is either mutually beneficial or competitive, and in general, multiple microorganisms work together to accelerate and enhance the rate of rubber degradation and stabilize their maximum metabolic functions.

Studies have shown that the degradation of pollutants by consortia is superior to that of purely cultured microorganisms (Table 4). The microorganisms in the functional consortia perform different metabolic functions and accomplish the degradation of pollutants while coping with the harsh living environment through collaboration among themselves (Araújo et al., 2020; Chen et al., 2021). There are two methods to construct functional consortia: one is the natural selection method, in which the soil and water contaminated by NR are mixed, and the suspension is extracted after static sedimentation, and NR samples are added to the suspension and incubated with iterative shaking until the weight of the NR samples no longer changes, and a naturally selected functional consortium is obtained in each generation of the culture (Nguyen et al., 2020). The other method is an artificial combination method, that is, the soil or water contaminated by NR is first screened for specificity, and the functional bacteria that can use NR as the only carbon source are screened for pure culture. A variety of bacteria with the ability to degrade NR are currently screened from the phylum Actinobacteria (Basik et al., 2021b). Later, the screened NR degrading bacteria were used for consortium construction, and the more efficient consortium for degrading NR was selected.

NR degrading bacteria screened so far have been broadly classified into two groups: the first group can produce hyaline rings on a solid medium with NR as the only carbon source; the second group can form biofilms and thus grow directly attached to the surface of NR (Hapuarachchi et al., 2016). It is important to note when constructing an artificial functional consortia: only a small fraction of microorganisms in the natural environment can grow on artificial media, so the conventional plating method alone cannot completely isolate the functional microorganisms found in an environment. These difficult to isolate microorganisms may exercise a synergistic role in the degradation of pollutants, and their absence reduces the efficiency of the functional consortia in the degradation of pollutants (Lee et al., 2013). Meanwhile, Nguyen et al. (2020) found that high microbial diversity of the consortia inhibits the degradation of NR by microorganisms. Therefore, to ensure the efficiency and to maintain the stability and harmony of functional consortia, microorganisms that are abundant in the initial environment should be incorporated alongside known functional microorganisms when constructing such consortia.

Currently, there are studies on consortia analysis usually combined with metagenomics sequencing. In 1998, Handelsman et al. (1998) introduced the concept of metagenomics in this scenario. Metagenomics refers to the advancement of individual single studies of microorganisms to study the sum of all microbes. Metagenomics does not require the isolation and pure culture of microorganisms. It no longer focuses only on the role of a single strain, overcoming the limitations of isolation and culture methods. Metagenomics enables the exploration of non-culturable microorganisms, which were previously limited by technical methods, greatly expanding the scope of microbial research (Riesenfeld et al., 2004). In addition, the significant advantages of metagenomics are the ability to identify microbial communities with low abundance, to make more genetic resources available, to emphasize the exploitation and use of non-culturable microorganisms, and to uncover microbial activities and functions at the level of the complete community.

In recent years, the investigation of rubber biodegradation mechanisms and microbial ecology has been facilitated by the increased use of genomics, metagenomics, transcriptomics, proteomics, and multiomics, made possible by the rapid development of high-throughput sequencing (Illumina, PacBio, and Nanopore). In a study by Nawong et al. (2018), single bacterial strains and mixed culture of selected bacteria were subjected to degradation experiments on NR gloves in a mineral salt medium (MSM). Following a 30-day incubation period under identical conditions, the bacteria exclusively utilized the NR glove as a carbon source, resulting in the polymer’s degradation. The experimental results demonstrated that the weight loss rate of NR gloves from mixed bacterial culture was much higher than that of NR degraded by individual strains, with the highest weight loss rate of 9.36% for single bacterial degradation and 18.82% for the constructed consortia. Nguyen et al. (2020) isolated and cultured a batch of bacteria from waste from a Vietnamese rubber processing plant and used NR film as the only carbon source in MSM medium. After 14 days of incubation, the highest weight loss of NR films reached 34% among the six groups of bacteria consortia constructed, which greatly improved the degradation of NR.

Furthermore, by monitoring the metabolic and dynamic relationships among the functional consortia, multi-omics analysis of synthesized functional consortia could serve as a guide for subsequent manipulations of population ratios and environmental parameters (Figure 2).

In the majority of recent investigations, it has been observed that the biodegradation process of NR has shown a rather slow rate, despite the successful identification and isolation of a limited number of functional bacteria capable of degrading this material (Bosco et al., 2018; Joseph et al., 2022; Sarkar et al., 2022; Cheng et al., 2023). The duration of microbial culture has shown a range of 30–120 days, but assessment methods such as weight loss or SEM have only identified little deterioration. The main limitation of existing research is the absence of consistent standards and dependable techniques for accurately quantifying the efficacy of deterioration. For this reason, establishing standardized procedures for quantifying the degradation of NR is essential for future research evaluating the biodegradability of microorganisms.

In addition, many chemical additives, including vulcanization accelerators, antioxidants, retarders, and their breakdown products, are utilized in industrial rubber products, including tires. These additives are essential in commonly used rubber materials because they prolong the material’s life and durability by avoiding breakdown (Altenhoff et al., 2019; Kawakami et al., 2022). Antioxidants were shown to be the most interfering with in vitro depolymerization, and the substance’s hydrophobic nature, as well as the huge amounts of additives present, prevent considerable biodegradation rates. For this reason, it is crucial to identify specific critical additives found in common rubber in order to enhance the microbial or enzymatic degradation process, for instance by facilitating or establishing the desired pretreatment of rubber materials. Conversely, the incorporation of additive-degrading microorganisms into a functional consortia could enhance degradation capability through synergy with other bacteria in the consortia. As a result of their low enzyme production efficiency and straightforward enzyme systems, single stains are incapable of effectively combating the complex rubber degradation process. It is recommended that future research endeavors concentrate on both the discovery of novel, efficient enzymes/genes for rubber degradation and the investigation of degradation mechanisms. Furthermore, the integration of genetic engineering and enzyme engineering methodologies may facilitate the development of microorganisms that produce rubber-degrading enzymes with enhanced activities. Temperature, ultraviolet light, dissolved oxygen, humidity, pH, and a variety of biotic and abiotic factors (microbial abundance and diversity, microbial enzymatic activity, etc.) influence the microbial rubber degradation process. In order to determine the optimal conditions for rubber biodegradation, it is necessary to take into account a variety of factors, such as environmental parameters and condition of the culture. Despite our increased knowledge of enzyme activity, we know little about rubber substrate enzyme action and the microbial, molecular, and environmental variables that affect it. In order to fully understand the microbial diversity, biofilm-formation mechanisms, and biodegradation capacity of rubber waste, multi-omics studies should be conducted.

Despite the severe environmental damage caused by conventional rubber disposal methods, it is intriguing that new rubber valorization technologies that are economically and ecologically viable are being proposed and developed to meet the growing amount of rubber waste. Rubber waste is valorized through energy recovery, recycling, upcycling, or biodegradation in order to produce new, viable products or dispose of it in an environmentally friendly manner. Incineration and landfilling are currently the prevailing and efficacious approaches for managing rubber and its byproducts. For instance, a portion of the heat produced during the incineration procedure can be recovered and harnessed to generate electricity or heat. Unfortunately, traditional recycling or direct incineration has many disadvantages, namely, the tedious process of collection, sorting and cleaning, as well as limited recycling time, excessive costs, emission of harmful residues, and unsuitability for use in other products. Instead, chemical or biological methods of converting rubber waste, which are polymeric molecules, into smaller molecules and transforming these molecules into new and valuable products are emerging as an attractive and more environmentally friendly route to valorization (Stevenson et al., 2008; Andler, 2020; Bosco and Mollea, 2021). By utilizing metagenomics sequencing, one can functionally annotate related genes and assess the diversity and relative abundance of microorganisms in a given community. It was disclosed that as the microbial diversity of artificially constructed consortia decreases, the number of strains capable of degrading NR increases, thereby enhancing the community’s efficiency (Castañeda Alejo et al., 2022).

Integrating microbial consortia with multi-omics might inspire a novel rubber waste bio-upcycling technique (Figure 3). Multi-omics will illuminate rubber biodegradation metabolic pathways and microbial community interactions. The synthesis and logical design of the composition and metabolic pathways of microbial consortiums would facilitate biotransformation into a greater variety of rubber biodegradation products, thereby expanding the product line of rubber waste upcycling.