Type IVc Pili (T4cP), also known as Tad or Flp pili, are long thin microbial filaments, which are made-up of small-sized pilins. These appendages serve different functions in bacteria including attachment, biofilm formation, surface sensing, motility, and host colonization. Despite their relevant role in diverse microbial lifestyles, knowledge about T4cP in bacteria that establish symbiosis with legumes, collectively referred to as rhizobia, is still limited. Sinorhizobium meliloti contains two clusters of T4cP-related genes: flp-1 and flp-2, which are located on the chromosome and pSymA megaplasmid, respectively. Bundle-forming pili associated to flp-1 have been involved in competitive nodulation of alfalfa plants but the role of flp-2 remains elusive. In this work, we have performed a comprehensive bioinformatic analyses of T4cP genes in the highly competitive S. meliloti strain GR4 and investigated the role of its flp clusters in pilus biogenesis, motility and in the interaction with alfalfa. Single and double flp cluster mutants were constructed in the wild-type genetic background as well as in a flagellaless derivative strain. Our data demonstrate that both chromosomal and pSymA flp clusters are functional in pili biogenesis, and contribute to surface translocation and nodule formation efficiency of GR4. In this strain, the presence of flp-1 in the absence of flp-2 reduces competitiveness for nodule occupation.

Many bacteria have the ability to survive in challenging environments; however, they cannot grow on standard culture media, a phenomenon known as the Viable but Non-Culturable (VBNC) state. Bacteria commonly go into the VBNC state under nutrient-poor environments or under stressful conditions. This review explores the concept of the VBNC state, providing insights into the beneficial bacteria known to employ this strategy. The investigation covers different chemical and physical factors that can induce the latency state, cell features, and gene expression observed in cells in the VBNC state. The revision also covered the significance and applications of beneficial bacteria, methods for evaluating bacterial viability, the ability of bacteria to persist in environments associated with higher organisms, and the factors that facilitate the return to the culturable state. Knowledge about beneficial bacteria capable of entering the VBNC state remains limited, however, beneficial bacteria in this state could face adverse environmental conditions, and return to culturable state when conditions become suitable and continue to exert their beneficial effects. Likewise, this unique feature positions them as potential candidates for healthcare applications, such as the use of probiotic bacteria to enhance human health, applications in industrial microbiology for the production of prebiotics, functional foods, and in the beer and wine industry. Moreover, their use in formulations to increase crop yield and for bacterial bioremediation offers an alternative pathway to harness their beneficial attributes.