Strain CF6-2 was spotted and grew on each plate containing the cells of the indicated strain at 20 ☌ for 3 days until a clear zone around CF6-2 colony formed. Predation of Gram-positive bacteria by strain CF6-2.Ī The killing effect of strain CF6-2 on eight Gram-positive marine bacteria on agar plates. Co-culturing of the two bacteria led to a strong reduction in the cell numbers of strain MCCC0423 over time and an increase in the cell numbers of strain CF6-2, indicating the ability of strain CF6-2 to prey on strain MCCC0423 cells for nutrients (Fig. We further tested the interaction of strain CF6-2 and a representative Gram-positive bacterium Staphylococcus warneri strain MCCC1A00423 (hereafter strain MCCC0423, from South China Sea sediments) in artificial seawater. 1) but could inhibit the growth of eight Gram-positive marine bacterial strains of seven different chemotypes of PG, forming clear zones around its colonies on agar plates (Fig. Strain CF6-2 had no effect on the growth of several representative Gram-negative marine bacteria (Supplementary Fig. To analyze whether strain CF6-2 has antagonistic interactions with other marine bacteria, the interaction of strain CF6-2 with a variety of Gram-positive and Gram-negative marine bacteria on agar plates was investigated. Strain CF6-2 kills Gram-positive bacteria via secreted products Bioinformatics analyses suggest that genes encoding pseudoalterin-like proteins are found in other marine bacteria. The released nutrients can then be utilized by strain CF6-2 for growth. strain CF6-2, a Gram-negative bacterium from a deep-sea sediment, can kill a wide range of Gram-positive bacteria by secreting a large quantity of the M23 metalloprotease pseudoalterin 14, 15, and thus degrading the PG in their cell wall. Three main classes of PG hydrolases have been described glycosidases cleave the bonds in the glycan strands, amidases cleave the bond between the glycan strand and the peptide chain, and endopeptidases cleave the bonds within the peptide chains 13. A certain degree of variation has been found either in the peptide stem, in the glycan strands, or in the position or composition of the peptide bridge 12. Structurally, PG is a network formed by linear glycan strands interconnected by peptide stems that are typically composed of amino acid residues l-Ala, d-Glu, l-Lys/diaminopimelate, and d-Ala, and are linked directly or through a short peptide bridge 12. Peptidoglycan (PG) is an important structural component of the bacterial cell wall, accounting for up to 90% of the cell wall in Gram-positive bacteria and 10% in Gram-negative bacteria. The cell wall is thus usually a key target of attack by hydrolytic enzymes for predatory bacteria to kill their prey. The strategies used by predatory bacteria to kill their prey bacteria vary from case to case however, they generally include: (i) epibiotic predation, in which predators consume the prey from the outside 9 (ii) endobiotic predation or direct invasion, in which an individual predatory cell secretes hydrolytic enzymes that perforate and modify the prey cell wall, in order to penetrate either into the periplasmic space or into the cytoplasm 9 or (iii) group attack, in which a quorum of predators produce hydrolytic enzymes/secondary metabolites to degrade the prey cells 9, 11. Predatory bacteria actively hunt and kill their prey and consume their macromolecules as nutrients 9, 10. It is unclear, however, whether Gram-positive marine bacteria can also be preyed on by bacteria. Like Gram-negative bacteria, Gram-positive bacteria are also widespread in the ocean 5– 7, comprising up to 14% and 25% of total bacterial cell counts in seawater and sediments, respectively 8. Arguably, the best studied predators of bacteria are Bdellovibrio and Bdellovibrio-like organisms (BALOs) that prey on a wide range of Gram-negative bacteria 4. While viruses and protists are known agents responsible for bacterial mortality, predatory bacteria may also contribute substantially to bacterial death, although there are only a few known examples from the ocean 2, 3. Predation on bacteria and the accompanying mortality are important mechanisms for bacterial population control and nutrient recycling. Bacteria are ubiquitous in marine ecosystems and play a vital role in nutrient recycling in the microbial loop 1.
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