It has been recognised that there are only a relatively few species of microorganisms in common use in laboratories which may be sources of antibiotics.
A large team, headed by Prof. Kim Lewis of Northeastern University, Boston, Massachusetts, USA, used novel culture techniques to extend the opportunities to isolate microbes ("uncultured bacteria"?) from their natural environment - still mostly soil samples.
This was achieved after burying sample cells in a variety of places for some time - several weeks.
Retrieving the iChip from the mud ...
The sample cells - iChips (isolation chips, not microchips and not Apple products!) consisted of 3 layers of plates with about three hundred matched perforations, screwed together to enclose 2 layers of a "semipermeable" film. Before burial, the central section was dipped in a liquid sample soil suspension (of a suitable dilution), likely to contain bacteria from the particular environment.
Each perforation forms a small diffusion chamber sandwiched between the membranes. These chambers allow for a free exchange of chemicals with the external environmment by diffusion, while restricting the movement of cells and contamination/interaction with other microbes.
The components of the iChip and its preparation
a inoculation by immersion in supension from source
b central section and possible single bacterial cells
c exploded view showing 4 membrane discs
The small size should result in each perforation potentially containing (different?) microbial colonies, each derived from individual cells in the initial inoculum permitting the isolation of many isolates in pure culture. It has been suggested that continuous cultivation in diffusion chambers adapts some microorganisms for growth under otherwise prohibitive conditions in the laboratory.
These colonies could then be transferred to normal laboratory media (agar in Petri dishes) in the ordinary way.
A large number of microbial species were obtained, which could then be cultured and further examined in the normal way in the laboratory,
but specific growth factors were found to be important. Some bacteria were found to depend on other bacterial species: they require siderophores produced by other species in order to solubilise and access iron in the surrounding medium.
From several thousand isolates, 25 antibiotic-producing strains were identified as potentially useful.
The most promising one of these has been named teixobactin, derived from a newly described bacterium (within the class β-proteobacteria) provisionally named Eleftheria terrae.
Teixobactin consists of a chain of 11 amino acids, the last 4 of which are looped back forming a ring structure. At each end of the molecule are
unusual substituted amino-acids: N-methyl-D-phenylalanine and enduracididine (not the usual amino acids in proteins). It is a depsipeptide - a molecule that has both peptide and ester linkages in proximity in the same amino acid chain.
The two NRPS (non-ribosomal protein synthesis genes reponsible for this have been shown to code for 7 amino acids which are assembled into a straight peptide chain, following the methylation of the first one (phenylalanine), and for the other section of 4 amino acids which then undergoes lactonisation to form a loop.
Teixobactin's mode of action is said to be "resistant to bacterial resistance". This is a contentious claim to some, but it certainly differs in action from most other antibiotics which rely on products of protein synthesis.
Instead it damages lipid components of the bacterial cell wall, killing a range of bacteria.
Teixobactin prevents these bacteria from forming their cell wall, by binding to the precursors of 2 lipids which are components of the cell wall - peptidoglycan and teichoic acid.
Teixobactin is active against Gram positive bacteria, notably Staphylococcus aureus: MRSA, Enterococci, and also Mycobacterium tuberculosis. It has proved successful against known mutant strains of these organisms which are resistant to other antibiotics.
Gram positive bacteria have a cell wall consisting of a thick peptidoglycan layer with projecting teichoic and
phosphoteichoic acids (LTA in the graphic on the right) on the outside of a cell membrane which is a phospholipid bilayer.
The cell wall complex of Mycobacterium tuberculosis - which is not reactive to the Gram stain - also contains peptidoglycan, but over 60% of the mycobacterial cell wall is composed of complex lipids.
Teixobactin is not active against Gram negative bacteria, however. These organisms have a cell wall with an extra outer layer
(a phospholipid bilayer with projecting lipopolysaccharides,and a periplasmic space beneath this).
Human cells have cell membranes based on lipids but they have a different molecular structure, so there is a potential therapeutic advantage when used to treat bacterial diseases. Preliminary testing with mice has shown that teixobactin is not toxic to mammalian cells.
However it has been suggested that the amino acid/peptide/protein nature of teixobactin may cause an allergic response in some users.
There is still some time before clinical trials and production on a commercial scale.