Although recent studies have documented a high diversity of agroecosystem-inhabiting ants and reported the significant role of the ecosystem-engineering activities of ant colonies in soil fertility enhancement [1, 2], only scanty information is available on the association of ant colonies with plant growth-promoting bacteria (PGPB) 
The beneficial free-living bacteria present in soil that colonize the rhizosphere, the rhizoplane (root surface), or the plant root itself (within radicular tissues) and promote plant growth, are referred to as PGPB . These PGPB enhance plant growth via production of phytohormones, such as indole acetic acid (IAA), gibberlin, and cytokinin . The principal hormone IAA, produced by PGPB is responsible for physiological processes, such as cell division, root, and shoot development . The insoluble phosphates in soil are also made available to the plants by phosphate-solubilizing microorganisms by the processes of acidification, chelation, and exchange reactions [7, 8]. Also, the secondary metabolites known as siderophores released from PGPB can chelate Fe3+ (ferric) ions from the soil with higher affinity and provide this complex to the plants . Many plant growth-promoting (PGP) microbes also play an important role in suppressing phytopathogens, and this activity may be mediated via direct and/or indirect mechanisms [10, 11]. While the direct mechanisms involve the production of siderophores, antibiotic and lytic enzymes [12, 13], the indirect mechanisms involve induction of local and systemic resistance against phytopathogens  by production of phenols such as phenylalanine ammonia-lyase (PAL), and peroxidase (POX) which play an important role in plant disease resistance . The use of these microbe-based biofertilizers helps in reducing the use of chemical fertilizers, improves soil fertility and contributes to enhanced plant productivity .
Enriched microbiomes documented to occur within ant nests and the surrounding areas are often found to be distinct from those found in bare soil, indicating the role of ant colonies in influencing soil microbial communities . The richness and diversity of soil microbes are known to be affected by the differences in the organic amendments added to the soil . Hence, the availability of substrates favored by PGPB within the nest chambers and in the external debris pile may be one possible reason for the reported PGP attributes of ant nest debris soil [1, 2]. Ant nests are characterized by the abundance of organic matter brought to the nests by the returning foragers as food for the colony members . While the fecal matter is often deposited in fecal patches in specific locations within the nest , the uneaten matter is usually discarded at the external refuse piles. Under experimental field conditions, the yield of crop plants has been found to be significantly enhanced when grown in the nest debris pile soil of Pheidole latinoda, which preferentially colonizes cultivated areas [19, 2]. A meta-analysis by Offenberg and Damgaard,  indicates that ants may also be involved in the reduction of pathogen loads of the visited plants.
Many ground-nesting ant species, such as the seed harvester ants, construct extensively modified galleries in their nests, forage on the seeds of their preferred plant species and accumulate a huge amount of organic debris in the close vicinity of their nests . Recent studies reveal that both the nest chambers as well as the external refuse piles of plant-visiting, Camponotus compressus colonies harbor many PGP microbes . Diverse ant species are reported to inhabit highly disturbed, managed ecosystems, such as annual cropping systems . Therefore, elucidating the influence of ant colonies on soil fertility and plant growth is of far-reaching significance. Thus, it is essential to ascertain whether PGPB are associated with ant nests and to find if they also affect plant growth via their effect on soil-borne pathogens.
Only scanty information is available regarding the influence of seed harvesting ant colonies on the microbial richness and these mainly pertain to the influence of the microbes on soil properties. Trichomyrmex scabriceps (Mayr 1865) is a seed harvester ant species widely distributed in many parts of India  but surprisingly, very little is known about its ecology. The nests of the seed-harvesting, T. scabriceps colonies are commonly found in hard, compact soil, such as that typically found on unpaved paths of agroecosystems, gardens and, roadsides (personal observations). Its nests are characterized by the presence of seed husk piles, a single pile being present at the nest entrance rim of a nest. Colonies of T. scabriceps have the potential to play a crucial role in linking the above-ground resources with the belowground biota, such as microbial communities.
In the present study, we have focused on the PGP microbiota associated with colonies of the Asian harvester ant, T. scabriceps (Mayr 1865), which is widely reported from a number of tropical and subtropical countries, including India and Sri Lanka . Colonies of T. scabriceps commonly occur in arid and semiarid areas and the workers primarily forage on seeds. Our preliminary survey revealed the widespread occurrence of T. scabriceps colonies in our study area. Moreover, our field observations (since April, 2018) suggest that the main nests (each main nest being accompanied by 1-3 satellite nests) are fairly long-lived (at least > 3 years). The PGP activities of ant nest associated microorganisms were further analyzed by the seed biopriming (SBP) method. For the SBP study, seeds of chickpea (Cicer arietinum L.) were preferred since this is one of the largest produced pulse crops in semi-arid tropical countries, including India, where it comprises about 40% of the total pulse crop . India being the largest producer of chickpea contributes around 65% of the world's total production. Although a high number of plant pathogens have been reported to attack the chickpea crop, Sclerotium rolfsii is a well-known soil-borne fungal pathogen of chickpea crop in tropical and subtropical areas of the world . It attacks the crop at the early seedling stage (resulting in the collar rot disease) and causes 55–95% mortality of chickpea seedlings under favorable environmental conditions . The control of S. rolfsii in agriculture has been challenging, since the highly competitive saprophytic pathogen exhibits high survivability in dry climatic regions, occurs on a wide range of natural hosts and can persist in soil for prolonged periods even after several crop rotations . Consequently, we examined the antagonistic (in-vitro) activities of isolated bacteria against S. rolfsii.
We hypothesize that soil fertility enhancement and the PGP effects of ant nest soil are due to the presence of associated PGPB. Due to the ecosystem engineering activities of ant colonies the ant nest soil with its high organic matter content provides suitable conditions for the growth of the PGPB, which could be directly involved in enhancing plant growth and possibly even in the reduction of soil-borne fungal pathogens such as, S. rolfsii. Therefore, in the present study, we addressed the following 4 inter-related questions: (1) Are the seed harvester ant colonies associated with PGPB? (2) Is there any difference between the abundance of PGPB in the debris pile and the nest chambers of T. scabriceps? (3) What is the direct impact of the PGPB isolates on plant growth? (4) Do these bacteria play any role in plant disease resistance?