The Mla pathway – a potential Achilles heel for gram-negative bacteria?

Researchers from the University of California San Diego have identified a potential new target against Pseudomonas aeruginosa infections – which could be applied to help treat all gram-negative infections.

It was found that by interfering with the Mla pathway in P. aeruginosa, the bacteria became more permeable, and more susceptible to immune components.

P. aeruginosa, an opportunistic gram-negative bacteria, is a significant health threat in hospitals, preferentially infecting immunocompromised patients. It’s associated with several infections such as urinary tract infections, bacteraemia and ventilator-associated pneumonia, and worryingly, is also linked with the ability to resist several classes of antibiotic.

A major contributing factor to this resistance is the bacteria’s outer membrane, a feature shared by all gram-negative bacteria, which protects them from harmful compounds, including antibiotics, and the immune system of the people they’ve infected.

Pseudomonas aeruginosa is an opportunistic bacteria which is a significant health threat in hospitals. Image credit:

The Mla pathway, consisting of seven proteins, is associated with maintaining the structure of the outer membrane in gram-negative bacteria. As new, alternative targets are desperately needed to try and account for the lack of new antibiotics against this type of organism, the research team set out to determine if this pathway could be exploited in the fight against P. aeruginosa.

They began by testing the susceptibility of a mutant strain of P. aeruginosa which couldn’t express the VacJ gene (a crucial component of the Mla pathway), against the antimicrobial peptide LL-37. This was then compared against the susceptibility of a wild-type strain of the bacteria which could express the gene.

It was found that the mutant strain was much more susceptible to LL-37 than the wild-type; while the concentration required to inhibit growth of the wild-type was 16 μg/ml, this value fell to 8 μg/ml for the mutant strain.

Was this increased weakness the result of an increase in outer membrane permeability, caused by the mutation? The team examined this by growing both the mutant and wild-type strains with NPN, which becomes fluorescent if it reaches the inner membrane of gram-negative bacteria, and then treating the bacteria with LL-37.

When the peptide was added, the fluorescence values showed that mutant P. aeruginosa strains became twice as permeable as their non-mutated counterparts.

In further studies, the researchers were also able to show that human blood and serum (which contain several components involved in killing bacteria, such as complement, platelets and antibodies) exhibited an increased ability to kill the mutant strain, over the wild-type.

Finally, when the team tested the strains in a mouse pneumonia model, they found that 40% of mice were alive after 3 days of infection – as compared to the wild-type, which led to 100% death in the same amount of time. When the blood of the infected mice was analysed, there was a 3-fold reduction in the bacteria load of the mutant strain, indicating the immune system of the mice was better able to destroy the bacteria.

New treatments which target the Mla pathway to render the bacteria more susceptible to antibiotics and patient immune systems could be extremely beneficial, since they would exploit a system used by all gram-negative bacteria. The paper also states that as this treatment would sensitise the bacteria, rather than killing them outright, it could go some way to limiting the extra, unintended damage to the microbiome caused by antibiotics.

The paper was published in the Journal of Molecular Medicine on August 26.

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